-1- First evidence for the two-proton decay of 45 Fe M. Pfützner1 , E. Badura2 , C. Bingham3 , B. Blank4 , M. Chartier5 , H. Geissel2 , J. Giovinazzo4 , L.V. Grigorenko2 , R. Grzywacz1 , M. Hellström2 , Z. Janas1 , J. Kurcewicz1 , A.S. Lalleman4 , C. Mazzocchi2 , I. Mukha2 , G. Münzenberg2 , C. Plettner2 , E. Roeckl2 , K. Rykaczewski1,6 , K. Schmidt7 , R.S. Simon2 , M. Stanoiu8 and J.-C. Thomas4 1 Warsaw University, Poland; 2 GSI, Darmstadt, Germany; 3 University of Tennessee, Knoxville, USA; 4 CEN Bordeaux-Gradignan, France; 5 University of Liverpool, UK; 6 ORNL, Oak Ridge, USA; 7 University of Edinburgh, UK; 8 GANIL, Caen, France In an experiment performed at the GSI Fragment Separator (FRS) in July 2001, the decay of 45 Fe has been investigated in order to establish whether it proceeds by simultaneous emission of two protons from the ground state, as suggested by theoretical predictions [1, 2], or by conventional β + decay. This extremely neutron-deficient nucleus was produced in the fragmentation of a 650 MeV/nucleon 58 Ni beam impinging on a 4 g/cm2 Be target. The average beam intensity was about 4×108 ions/s. In order to reduce the number of contaminant ions transmitted through the FRS, an aluminum degrader was mounted at the first FRS focus in addition to the standard achromatic degrader at the middle focal plane. Ions reaching the final focus were identified in-flight using the Bρ-TOF-∆E method. The time-of-flight (TOF) was determined by means of 3 scintillator detectors located at the second, third and final focal planes, respectively. The energy loss (∆E) was measured with an ionization chamber. After identification, the ions were slowed down in an aluminum degrader of variable thickness and implanted into a telescope of eight Si detectors, each 60 mm in diameter and 300 µm thick. The telescope was mounted inside a 30 cm long NaI(Tl) detector, composed of six crystals forming a barrel with inner and outer diameters of 8 cm and 40 cm, respectively. The NaI detector allowed to provide efficient discrimination against β + events which are accompanied by at least two annihilation photons. The probability to detect at least one γ photon following the β + decay of 49 Ni and 50 Co was measured to be 93 %. Decay signals from the Si detectors were processed using specially developed preamplifiers with a fast-reset function, allowing to block the input circuit by an external logical pulse. In this way, low-energy decay signals (≈ 1 MeV) could be detected already a few microseconds after the implantation of a heavy ion which is accompanied by a release of up to 1 GeV in the same detector. A detailed description of the applied detection setup is given in Ref.[3]. During ≈140 hours, six ions of 45 Fe were implanted in the Si telescope. Five of these implantation events could be correlated with decay signals occurring later in the same detector, see Table 1. (Ion number 4 was implanted into a detector which decay spectroscopy setup was temporarily malfunctioning.) In one decay (event 3), the energy of about 10 MeV was released and a coincident photon was observed in the NaI detector. This event is consistent with the β-delayed proton emission scenario. In each of the other four decay events, a completely different pattern is observed: the energy of ≈1 MeV is released in the Si detector where the ion was stopped, with no other signal in coincidence. Such a pattern is expected if 45 Fe decays by the emission of two protons from the ground state. A detailed analysis of possible sources of background radiation was undertaken. It was found that the probability to observe a random background-induced decay signal, with about 1 MeV and occurring within 10 ms after implantation of a heavy ion, was smaller than 10−3 . This leads us to conclude that the decay signals observed to be correlated with implanted 45 Fe ions do represent the decay of this nuclide. The half-life estimated by a applying a maximumlikelihood method to the five data points is T1/2 = 3.2+2.6 −1.0 ms. Our data suggest that 45 Fe decays predominantly (≈ 80 %) by the emission of two protons with the total energy of 1.1 ± 0.1 MeV. The measured decay energy is in very good agreement with estimated Q-values for 2p emission: (1.154 ± 0.094) MeV [1] and (1.279±0.181) MeV [2]. The half-life is found to be in reasonable agreement with the prediction of a rigorous three-body model of 2p radioactivity, developed by Grigorenko et al. [4], which yields T1/2 = 13 ms for Q = 1.1 MeV. A more detailed description of the data analysis and discussion of the results has been published in Ref.[5]. The results of another 45 Fe decay study, performed at GANIL, support our conclusions [6]. Table 1: Implantation detector, decay energy and decay time recorded for each of the six 45 Fe events observed (events are ordered chronologically). Event Detector E [keV] T [ms] 1 2 3 4 5 6 4 3 5 5 2 2 1000 ± 120 990 ± 130 10010 ± 100 1150 ± 100 1200 ± 100 0.644 5.276 3.395 1.196 12.617 References [1] B.A. Brown, Phys. Rev. C 43, R1513 (1991). [2] E. Ormand, Phys. Rev. C 53, 214 (1996). [3] M. Pfützner et al., Nucl. Instr. and Meth. A 493, 155 (2002). [4] L.V. Grigorenko et al., Phys. Rev. C 64, 054002 (2001). [5] M. Pfützner et al., Eur. Phys. J. A 53, 279 (2002). [6] J. Giovinazzo et al., Phys. Rev. Lett. 89, 102501 (2002). -2- Beta-delayed proton decay of a high-spin isomer of 94 Ag I. Mukha1 , C. Plettner1 , J. Döring1 , L. Batist2 , A. Blazhev1 , H. Grawe1 , C. Hoffman3 , Z. Janas4 , R. Kirchner1 , M. La Commara5 , C. Mazzocchi1 , E. Roeckl1 , S. L. Tabor3 and M. Wiedeking3 1 Gesellschaft für Schwerionenforschung mbH, D-64291 Darmstadt, Germany; 2 St. Petersburg Nuclear Physics Institute, RU-188350 Gatchina, Russia; 3 Florida State University, Tallahassee, FL-32306, USA; 4 Warsaw University, PL-00681 Warsaw, Poland; 5 Universita di Napoli ”Federico II”, I-80126 Napoli, Italy In the β + /EC decay of the N=Z nucleus 94 Ag, preliminary evidence for a long-lived (T1/2 =0.3(2) s), high-spin (I≥17) isomer in 94 Ag, in addition to the known (7+ ) isomer, was obtained in a previous GSI-ISOL experiment [1]. The two 94 Ag levels were studied by measuring β − γ coincidences, which yielded also information about excited states in the daughter nucleus 94 Pd. The existence of the (I≥17) isomer has been confirmed in a recent GSI-ISOL experiment, where both β-delayed γ rays [2] and protons were detected. The 94 Ag nuclei were produced by using the fusion-evaporation reaction 58 Ni(40 Ca,p3n). The β + /EC-delayed proton decay of 94 Ag was measured by recording β−p, p−γ, β−p−γ and p−γ − γ coincidences with high-granularity arrays of germanium and silicon detectors [3]. The spectrum of γ rays measured in coincidence with protons is displayed in Fig. 1. This spectrum shows more and the proton separation energy in 94 Pd amounts to 4.47 MeV, the end-point of the proton spectrum of about 5 MeV gives an estimate of at least 15 MeV for the highest excitation energy of 94 Pd states fed in β decay of the 94 Ag isomer (Iπ ≥17+ ). As can be seen from the comparatively modest intensity of the β component in Fig. 2, the dominant β-decay mechanism leading to proton emission is electron-capture, which apparently holds for the decay of both 94 Ag isomers. A remarkable feature of the observed decay pattern is a preferred feeding of negativeparity states at high excitation energies in 93 Rh, which points to odd orbital momenta of the emitted protons assuming even parity for the parent states [1]. Figure 2: Protons measured in coincidence with the 698 keV γ-transition in 93 Rh which stems from a 5.69 MeV level in this nucleus. Figure 1: Energy spectrum of γ rays from 94 Ag measured in coincidence with protons. Energies of known transitions in 93 Rh are indicated in keV. than 20 peaks matching the γ de-excitation pattern of high-spin states in 93 Rh, which are known from in-beam γ-spectroscopy [4]. Thus we have observed the feeding of high-spin states in 93 Rh, populated by proton emission from excited states of 94 Pd. This gives further evidence for the β decay of a spin-gap isomer in 94 Ag with I≥17 and T1/2 =0.5(1) s, with the latter result being deduced from the time distribution of the p−γ data. The proton energy spectrum measured in coincidence with the known 698 keV γ-transition in 93 Rh is shown in Fig. 2. It has a maximum at an energy around 3 MeV, with the low-energy bump corresponding to β particles. As the 698 keV γ-transition (see Fig. 1) indicates a population of the 5.69 MeV, 33/2− state in 93 Rh (thus discriminating against p-γ events from the decay of the (7+ ) isomer), All in all, we have obtained experimental evidence for a I≥17, T1/2 =0.5(1) s isomer in 94 Ag, whose β-decay energy is at least 15 MeV. This state is characterized by the highest spin ever observed for β-decaying nuclei. Shellmodel calculations have predicted numerous spin-gap isomers in the proton-neutron (p1/2 , g9/2 ) model space [5], but so far only the Iπ =(21/2+ ) state in 95 Pd was observed [6]. These calculations fail to predict a high-spin isomer for 94 Ag, with the closest candidate 21+ , which represents the highest spin in the model space [1]. Only inclusion of excitations up to 3p-3h of the 100 Sn core can reproduce this type of isomerism [7]. References [1] [2] [3] [4] [5] [6] [7] M. La Commara et al., Nucl. Phys. A708, 167 (2002) C. Plettner et al., to be published I. Mukha et al.,, contribution to this report H. A. Roth et al., J. Phys. G 21, L1 (1995) K. Ogawa, Phys. Rev. C28, 958 (1983) E. Nolte and H. Hick, Z. Phys. A305, 289 (1982) F. Nowacki, private communication, and Nucl. Phys. A704, 223c (2002) -3- Beta-decay studies of neutron-deficient Sn isotopes M. Karny1 , Z. Janas1 , L. Batist2 , J. Döring3 , I. Mukha3 , C. Plettner3 , A. Banu3 , A. Blazhev3 , F. Becker3 , W. Brüchle3 , T. Faestermann4 , M. Górska3 , H. Grawe3 , A. Jungclaus5 , M. Kavatsyuk3 , O. Kavatsyuk3 , R. Kirchner3 , M. La Commara6 , S. Mandal3 , C. Mazzocchi3 , A. PÃlochocki1 , E. Roeckl3 , M. Romoli7 , M. Schädel3 , R. Schwengner8 and J. Żylicz1 1 4 University of Warsaw, Poland; 2 St. Petersburg Nuclear Physics Institute, Russia; 3 GSI, Darmstadt, Germany; Technische Universität München, Germany; 5 IEM CSIC, and Universidad Autónoma de Madrid, Spain; 6 Universitá di Napoli, Italy; 7 INFN Napoli, Italy; 8 Forschungszentrum Rossendorf, Germany Studies of nuclei in the 100 Sn region offer the possibility to test nuclear models describing structure and decay properties of nuclei in which protons and neutrons occupy identical orbitals near a double shell closure. An insight into the structure of nuclei close to 100 Sn can be gained by studying their β decay which is dominated by πg9/2 →νg7/2 Gamow-Teller (GT) transitions. An attractive feature of such nuclei is that most of the GT strength lies within the QEC -value window. Such a concentration of strength has recently been observed in a series of light indium and silver isotopes (see [1] and references therein). 100 Sn has been predicted to decay by one GT transition to a single 1+ 1p-1h state in 100 In at an excitation energy of about 1.8 MeV, while the closest even-even neighbours of 100 Sn, 98 Cd and 102 Sn, show a spreading of the GT strength over a number of 1+ states in the daughter nucleus. In the recent experiments at the GSI-ISOL facility we used the FEBIAD-B3C ion sources with the addition of CS2 [2] for the mass separation of SnS+ ions. In this way routinely about 60% of the intensity of the Sn+ beam was shifted to the SnS+ molecular side-band, where the strong suppression of contaminants [2] cleaned the beams from In, Cd, and Pd isobars. Only the strongly produced activities of Ag were traced in on-line experiments, the suppression of which was 4-5 times lower than the anticipated value of 104 found in off-line studies. The latter effect may be due to an operation of the ion source at lower temperature compared to the off-line measurement, which was made to enhance the SnS+ intensity. We measured β-γ-γ decay properties of 101−105 Sn with two complementary set-ups, namely (i) the Total Absorption Spectrometer (TAS) for the measurement of GT strength distributions and (ii) an array of germanium detectors (including a FZR-Cluster and two GSI-Clover detectors) operated in coincidence with silicon β-detectors. β-delayed protons of 101 Sn were measured by using ∆E-E silicon telescopes. The intensities of the mass-separated 101−105 Sn beams were obtained from the experimental decay properties. By using a 40 particlenA 58 Ni beam, a 3 mg/cm2 50 Cr target, and a catcher of ZrO2 fibers inside the FEBIAD source, we reached secondary beam intensities given in Table 1. These values are about one to two orders of magnitude higher than those obtained by a previous FRS experiment [3]. The data on the 102 Sn decay collected with the high resolution set-up (ii) as well as spectra obtained by the small germanium detector in the TAS confirm the main features of the 102 Sn decay scheme proposed by Stolz [3] on the basis of an FRS experiment. The main difference is that we do not confirm the 53 keV transition (see Fig. 1), which was previously [3] placed at the very bot- Figure 1: Low energy γ-ray spectrum obtained for mass 102. An arrow shows the expected position of the 53 keV line which remained unobserved in this experiment. Figure 2: Beta-gated TAS spectrum of 102 Sn from the experiment (solid line) and from a GEANT simulation based on the modified level scheme of Ref. [3] (dashed line). tom of the 102 In level scheme. Removing this transition from the decay scheme may affect the spin assignment of the ground state of 102 In. Figure 2 presents the β-gated TAS spectrum of 102 Sn after subtraction of contributions from daughter activities which were determined in separate measurements. The maximum occurring at a recorded TAS-energy of about 2.5 MeV is interpreted as being due to the πg9/2 →νg7/2 GT resonance at a 102 In excitation energy of about 1.5 MeV. In summary, the development of SnS+ beams at the GSI ISOL allowed us to study in detail the β-decay properties of 101−105 Sn. These data, in particular those on β-γ-γ coincidences, are under evaluation. An extrapolation of the experimental beam intensities, yields 8 atoms/h for 100 Sn indicating that the measurement of β-delayed γ rays of this nucleus will indeed be a very challenging task. References [1] C. Plettner et al., Phys. Rev. C66, 044319 (2002) [2] R. Kirchner, Nucl. Instr. and Meth. B, in print, and www.gsi.de/annrep2001 (page 211) [3] A. Stolz, Ph.D. Thesis, TU München (2001) Table 1: Measured ISOL rates of the 101−105 Sn isotopes. 101 103 104 105 Isotope Sn 102 Sn Sn Sn Sn atoms/min 2.4 31 1.4 · 103 3.0 · 104 2.0 · 105 -4- Beta decay of 103 Sn O. Kavatsyuk1,2 , M. Kavatsyuk1,2 , J. Döring1 , L. Batist3 , A. Banu1 , F. Becker1 , A. Blazhev1,4 , W. Brüchle1 , T. Faestermann5 , M. Górska1 , H. Grawe1 , Z. Janas6 , A. Jungclaus7 , M. Karny6 , R. Kirchner1 , M. La Commara8 , S. Mandal1 , C. Mazzocchi1 , I. Mukha1 , C. Plettner1 , A. Plochocki4 , E. Roeckl1 , M. Romoli8 , M. Schädel1 , R. Schwengner9 and J. Żylicz6 1 GSI, Darmstadt, Germany; 2 Kiev National University, Ukraine; 3 St. Petersburg Nuclear Physics Institute, Russia; 4 University of Sofia, Bulgaria; 5 Technische Universität München, Germany; 6 University of Warsaw, Poland; 7 Instituto Estructura de la Materia, CSIC, and Departamento de Fisica Teórica, UAM Madrid, Spain; 8 Universitá di Napoli, Italy; 9 Forschungszentrum Rossendorf, Germany 1356 100 Counts 740 643 726 50 720 1397 1077 821 1428 0 600 800 1000 1200 Energy [keV] 1400 Figure 1: Gamma-ray spectrum obtained for mass A=103+32 in coincidence with positrons. The strongest lines are marked by their energies in keV. 50 Counts 50 776 60 1022 10 T1/2=7.0(6) s 5 0 4 8 12 Time [s] Counts Doubly closed-shell nuclei and neighbouring isotopes/isotones provide a sensitive test ground for the nuclear shell model. 100 Sn is the heaviest doubly-magic N=Z nucleus, located at the proton drip line, where protons and neutrons occupy identical shell-model orbitals. The overlap of their wave functions is large, which further causes a strong proton-neutron interaction to be expected. Beta decay in this region is dominated by an allowed GamowTeller (GT) transformation πg9/2 → νg7/2 , which in the decay of an even-even nucleus populates the I π = 1+ GT resonance. For an odd-neutron parent nucleus the coupling of this resonance to the unpaired nucleon can be studied. This provides a test of the residual interaction via the β-delayed γ-ray spectroscopy. Measurements of β-delayed γ-rays and protons were performed at the GSI-ISOL facility for 101,103,105 Sn. It was essential for this experiment to efficiently suppress the isobaric indium, cadmium, silver and palladium contaminants by using the novel sulphurisation technique [1]. The β-delayed γ-ray spectra were measured with an array of high-resolution germanium detectors (17 crystals) in grow-in mode as well as with the Total Absorption Spectrometer (TAS) in decay mode. Moreover, ∆E-E telescope was used to record β-delayed protons [2, 3]. Further experimental details are given in refs. [1-3]. We report on the new data for the β decay of 103 Sn. In Fig. 1 the β-gated γ-ray spectrum for 103 Sn, taken at mass A=103+32 with the germanium array, is shown. The 720, 726 and 740 keV lines are known to belong to the decay of the 103 In daughter activity [4]. The 1077 keV line has been identified by in-beam spectroscopy [5] to represent the 11/2+ → 9/2+ transition in 103 In. The data shown in Fig. 1 yield the first evidence for β-delayed γ-rays of 643, 821, 1077, 1356, 1397 and 1428 keV, which are preliminarily assigned to the decay of 103 Sn. The TAS spectrum gated by protons is shown in Fig. 2. The 776 and 776+1022 keV lines correspond to the 2+ state in 102 Cd fed by β-delayed protons after a EC and β + -decay of 103 Sn, respectively. Based on these data, a β + /EC ratio of 0.06 for the proton emission to the 2+ state in 102 Cd was estimated. A corresponding ratio of 0.6 for the proton emission to the ground state in 102 Cd was obtained using a proton-γ anti-coincidence condition. The QEC value of 103 Sn was preliminarily determined from these β + /EC ratios and the average energy of β-delayed protons [6] to be 7.5±0.5 MeV. The half-life of 103 Sn, obtained from the β-delayed proton time distribution, is shown in the inset of Fig. 2. The result of the fit being T1/2 = 7.0 ± 0.6 s is in agreement 40 30 16 20 10 0 776+1022 500 1000 1500 Energy [keV] Figure 2: TAS spectrum taken in coincidence with βdelayed protons from the 103 Sn decay. The inset shows the time characteristic of β-delayed protons. with the previously measured values of 7±3 s [6], 7.5±1.5 s [7] and in disagreement with 8.7±0.6 s [8] by two standard deviations. References [1] [2] [3] [4] [5] [6] R. Kirchner, Proc. Conf. to EMIS-14, NIM A, in print M. Karny et al., contr. GSI Sci. Rep. 2002 I. Mukha et al., contr. GSI Sci. Rep. 2002 J. Szerypo et al., Z. Phys. A 359 (1997) 117 J. Kownacki et al., Nucl. Phys. A627 (1997) 239 P. Tidemand-Petersson et al., Z. Phys. A 302 (1981) 343 [7] K. Rykaczewski, Report GSI-95-09, 1995 [8] A. Stolz, Ph.D. Thesis, TU München (2001) -5- Probing neutron-rich In and Cd nuclei with isomer spectroscopy M. Hellströma,b , M.N. Minevab , A. Blazheva,c , H.J. Boardmand , J. Ekmanb , J. Gerla , K. Gladnishkic,e , H. Grawea , R. Paged , Zs. Podolyáke , D. Rudolphb and the GSI-FRS-Isomer collaborationa Lund University, Sweden; c University of Sofia, Bulgaria; e University of Surrey, UK d University of Liverpool, UK; interpret, and it is also not known whether the observed γ-ray represents the primary isomeric transition. In analogy to 125 Cd, the observed γ-ray could be a hindered M2 19/2+-to-15/2−transition. 126 In: The decay of this previously unreported isomer exhibits a strong γ-ray at 244 keV, with a half-life of 29(2) µs, as well as two weaker 614 and 865 keV transitions with as of yet undetermined lifetimes. We tentatively interpret this as the primary isomeric M2 (E3) transition connect−1 + ingthe 1− member of the πg−1 9/2 νh11/2 multiplet with the 3 ground state. Evidence for other members ofthis negativeparity multiplet comes from the proposed (8− )β-decaying state that has previously been observed[2]. 130 In: A single delayed γ-transition of 389 keV with a half-life T1/2 <6 µs was observed in coincidence with the implanted 130 In ions, apparently associated with the decay of a previously unreported isomer. The delayed transition we observe could connect a previously unobserved member of theπg9/2 νh11/2 multiplet with the 1− ground state or another level with the same configuration. References [1] M.N. Mineva et al., Eur. Phys. J. A11 (2001) 9. [2] L. Spanier et al., Nucl.Phys. A474 (1987) 359. Counts / 2 keV bin Some thirty years after it was opened up to observation, the region around doubly magic 132 Sn still remains the object of intense interest as the shell structure and effective residual interactions in this part of the nuclidic chart are intimately related to many important issues. The very different predictions obtained by various theoretical approaches, e.g., for the proposed quenching of shell-closures for extremely neutron rich nuclei, is a clear indication that the detailed understanding of neutron-rich systems far from stability is far from complete. To obtain more experimental information about excited states in nuclei “southwest” of 132 Sn, we have performed an experiment aimed at searching for relatively long-lived (0.1-100 µs) isomeric states and studying their decay using high-resolution γray spectroscopy. Ref. [1] describes the in-flight fission fragment isomer spectroscopy method in more detail. The isotopes of interest were produced directly in projectile fission of 732 MeV/nucleon 238 U on a 208 Pb target, and subsequently separated and identified event-by-event using the fragment separator FRS. At the FRS focal plane, the transmitted ions were slowed down and subsequently implanted in a plastic catcher viewed by six segmented Clover-type detectors, with which delayed γ-rays emitted by the implanted ions were detected. The energy and time of all ”first hits” in the Ge detectors within an 80 µs interval following the implantation of an ion were recorded together with the particle identification information for the respective ion. This allowed the construction of heavy iongated γ-ray energy-time and energy-energy matrices. The data presented here were obtained during an effective measurement time of 8 hours with the FRS optimized for the transmission of 130 Sn. In the following, we briefly discuss some of the preliminary results obtained so far. In those cases where the observed properties are difficult to explain by systematics alone, on-going realistic shell model calculations will hopefully aid our interpretation. 125 Cd: Two strong coincident γ-transitions of 720 and 743 keV with similar intensity follow the decay of this previously unknown isomer with a half-life of 14(2) µs. Two much weaker delayed γ-transitions keV, with as of yet undetermined lifetimes, are also observed.Comparing with the A=120-130 tin isotopes, where the lowest-lying excited state is alternately 2+ or 15/2−, a possible interpretation of this decay would be a cascade starting with a hindered M2 transition deexciting a 19/2+ isomer via an 15/2− level down to a known (11/2− ) state. 127 Cd: One significant delayed γ-transition at 820 keV was observed in coincidence with the implanted 127 Cd ions. The very low statistics have thus far only allowed placing the limit 1 µs<T1/2 <10 µs on its half-life. In addition, a number of weaker delayed γ-rays may also be present. The origin of this previously unknown isomer is difficult to 720 100 Cd-125 743 * 50 486 * * 665 0 0 200 400 600 800 1000 1200 1400 Gamma-ray energy [keV] 15 Counts / 2 keV bin b Cd-127 10 342 710 5 738 820 0 0 200 400 600 800 1000 1200 1400 Gamma-ray energy [keV] Counts / 2 keV bin GSI Darmstadt, Germany; 400 244 In-126 300 201 200 * 100 * 0 0 200 400 614 600 * 865 800 1000 1200 1400 Gamma-ray energy [keV] 25 Counts / 2 keV bin a In-130 20 389 15 10 5 0 0 200 400 600 800 1000 1200 1400 Gamma-ray energy [keV] Figure 1: Heavy-ion-gated delayed (1-60 µs after prompt) γ-ray spectra of selected isomers. Asterisks label background activities. -6- Direct mass measurement of short-lived fission products at FRS-ESR C. Scheidenberger1 , F. Attallah1 , K. Beckert1 , P. Beller1 , F. Bosch1 , D. Boutin1 , T. Faestermann2 , B. Franczak1 , B. Franzke1 , H. Geissel1,3 , M. Hausmann1,4 , M. Hellström1 , E. Kaza1 , O. Klepper1 , H.-J. Kluge1 , R. Koyama5 , C. Kozhuharov1 , K.-L. Kratz6 , Yu. A. Litvinov1,7 , L. Maier2 , M. Matos1 , G. Münzenberg1,6 , F. Nolden1 , Yu.N. Novikov1,7 , T. Ohtsubo5 , A. Ostrowski6 , A. Ozawa8 , B. Pfeiffer6 , M. Portillo1 , V. Shishkin1 , J. Stadlmann1,3 , M. Steck1 , K. Sümmerer1 , T. Suzuki5 , D. J. Vieira4 , S. Watanabe5 , H. Weick1 , M. Winkler1,3 , H. Wollnik4 , T. Yamaguchi1 1 GSI, 2 TU München, 3 Univ. Giessen, 4 Los Alamos Nat. Lab., 5 Niigata Univ., 6 Univ. Mainz, 7 NPI St. Petersburg, 8 RIKEN Direct mass measurements are key experiments for the exploration of unknown territory in the chart of nuclei. Neutron-rich nuclei are of special interest because they play an important role in stellar nucleosynthesis, which progresses along the r-process path through the area of neutron-rich nuclei. Many masses of neutron-rich nuclei, produced by fission of an uranium primary beam, have been investigated in a recent FRS-ESR experiment employing isochronous mass spectrometry (IMS) [1]. The nuclei of interest were produced and separated with the FRS, and the ESR was used as a high-resolution time-of-flight mass spectrometer. A peculiarity of the fission kinematics of relativistic projectiles can be used in order to inject efficiently the most neutron-rich isotopes and suppressing at the same time the much more abundantly produced isotopes closer to the stability line. This can be understood from Fig. 1. The figure Figure 2: Revolution-time spectrum of fission fragments in the ESR, obtained with Isochronous Mass Spectrometry. The observed zink-(Z=30) and tin-isotopes (Z=50) are labeled. 79 Zn, 81 Zn und 135 Sn are the isotopes of these elements, whose masses are so far unknown. ments and masses is covered, and thus a large m/q range (∆(m/q)/(m/q) ' 13 %) can be investigated simultaneously. The mass of many nuclides, which is so far only known from theoretical predictions, is determined for the first time in this experiment. A preliminary overview is given in Fig. 3. These new mass data are of particular in- Figure 1: Result of a MOCADI simulation [2]: energy distribution of 135 Sn50+ fission fragments as a function of the spatial angle relative to the direction of the incident primary beam. shows the kinetic energy of 135 Sn50+ fission fragments as a function of the spatial angle when leaving the production target at the entrance of the FRS. In the center-of-mass system the fission fragments are spatially isotropically distributed and both fragments share unequally the available energy (which is the energy equivalent of the mass difference between the projectile and the sum of both fragments). In the laboratory frame, the fission fragments cover a wide range of kinetic energies, depending on the angle of emission relative to the direction of the primary beam. The fragments emitted in forward direction leave the target with a velocity, which is up to 6 % larger than that of the primary beam. Optimizing the FRS-ESR settings on these ’fast’ fragments, the neutron-rich isotopes are preferably transmitted and the less neutron-rich fragments, which are ’slower’, are suppressed. Fig. 2 shows the corresponding mass spectrum for a setting, which is optimized for 135 Sn50+ . The spectrum was accumulated during ≈ 50 hours, with a primary beam intensity of constantly 2 · 109 uranium ions per pulse (every 15 s a pulse was fast extracted from the SIS). A large number of nuclides is observed, a large range of ele- Figure 3: Part of a schematic chart of nuclei. Atomic masses, which have been determined for the first time in this experiment with respect to [3] are indicated. terest in nuclear astrophysics, where Q-values, neutronseparation energies, and half-lives are needed for nuclearreaction network calculations, which aim at modeling the true r-process path in nucleosynthesis and the understanding of the observed elemental and isotopic abundances in the solar system [4]. The data in the vicinity of closed shells (N = 50, 82, Z = 28, 50) and especially in the vicinity of double shell closures will allow the investigation of the isospin dependence of shell effects and possible new phenomena like shell quenching. [1] M. Hausmann et al., Hyp. Int. 132, 291 (2001). [2] N. Iwasa et al., NIM B 126, 284 (1997). [3] G. Audi et al., Nucl. Phys. A 624, 1 (1997). [4] K.-L. Kratz et al., Astrophys. J. 403, 216 (1993). -7- The Gamow-Teller resonance populated in β-decay of odd-odd nuclei above 146Gd E. Nácher1 , B. Rubio1 , A. Algora1,2 , D. Cano-Ott1 , J. L. Taı́n1 , A. Gadea1,3 , L. Batist4 , J. Döring5 , R. Kirchner5 , I. Mukha5 , E. Roeckl5 , C.Plettner5 and M. Gierlik6 1 IFIC, Valencia; 2 MTA ATOMKI, Debrecen; 3 LNL, Legnaro; The nuclei around 146 Gd represent the unique cases in the nuclide chart where the Gamow-Teller (GT) transition πh11/2 → νh9/2 is accessible in β-decay. Among these nuclei, the odd-odd N=83 cases are of special interest for different reasons. In general they have two β-decaying isomers: one has spin-parity 2− , with an even number of πh11/2 particles in the main configuration, whereas the other one is a 9+ state with an odd number of πh11/2 particles. Another reason for studying such nuclei is that the 2− isomer can be produced cleanly, i.e. without contamination from the 9+ activity. Correspondingly, the 9+ contribution can be obtained by subtraction. Different experiments were carried out at the GSI On-Line Mass-Separator with the aim of a systematic study of the β-decay of odd-odd N=83 nuclei. The β-decays of the isotope of interest were studied using a Total Absorption Spectrometer (TAS), a special device to detect entire cascades rather than individual gamma-rays. This has proved to be the best tool to reliably extract the GT strength (BGT ) from experimental β-decay data. PNPI, Gatchina; 152 Tm 9 0.04 150 0.57 [1] Tb 2 0.19 [1] 148 − n h 11/2 =0 0.10 + Ho 9 n h 11/2 =3 0.09 0.56 + n h 11/2 =5 1.66 − Ho 2 n h 11/2 =2 6 Univ. Warsaw [1] D. Cano-Ott, PhD Thesis, Univ. de Valencia (2000). [2] E. Nácher et al. GSI Sci. Report 2001, 8. [3] G. F. Bertsch Nucl. Phys. A354, 157C (1981). − 150 GSI, Darmstadt; References Tm 2 n h 11/2 =4 1.14 5 presented here for the first time. As can be seen in the figure, in the low-spin cases the BGT distribution is concentrated in a very narrow prominent peak at about 4.5 MeV, the GT resonance. This kind of decay occurs when a proton pair in the h11/2 orbital is broken, and one of the protons decays populating four quasi-particle (4qp) states in the daughter nucleus. On the other hand, the decay of the high-spin states has two different components: the decay of the paired particle as in the low-spin case, and the decay of the unpaired πh11/2 proton that can populate only one 2qp state in the daughter nucleus. The ratio between the BGT value related to the 4qp states (the resonance) and to the 2qp state grows rapidly with the increase of the number of πh11/2 protons. This ratio is always bigger than what one would expect from an extreme single-particle approach. This indicates that a sizeable part of the BGT that should go to the 2qp state is shifted to higher-lying levels in the daughter nucleus. This is a common phenomenon already observed in (p,n) reactions which has been interpreted theoretically [3]. The advantage of this work is that the effect is observed systematically in several decays and can therefore be properly quantified. In Fig. 1 we present the BGT distribution for the six cases studied in this work. Each of them represents a different occupancy number of the πh11/2 orbital starting all the way from 0 protons (148 Tb 2− ) to 5 protons (152 Tm 9+ ). The decay of 150 Ho was the first of these cases measured with the TAS at GSI. The analysis and results are discussed in [1]. The decay of 148 Tb was presented together with the former in [2]. Results on the decay of 152 Tm are 152 4 148 Tb 9 + n h 11/2 =1 0.14 Fig. 1 Gamow-Teller strength distributions observed in the decay of odd-odd N=83 nuclei. 2 The numbers in the graphs represent integrated BGT values in gA /4π units. -8- Beta-decay of 156 Tm measured by total absorption spectroscopy E. Nácher1 , B. Rubio1 , A. Gadea1,2 , D. Cano-Ott1 , J. L. Taı́n1 , A. Algora1,3 , L. Batist4 , R. Collatz5 , M. Hellström5 , Z. Hu5 , Z. Janas6 , M. Karny6 , R. Kirchner5 , F. Moroz4 , E. Roeckl5 , K. Rykaczewski6 and V. Wittman4 1 IFIC, Valencia; 2 LNL, Legnaro; 3 MTA ATOMKI, Debrecen; The β decay of the 2− ground-state of 156 Tm (N=87, Z=69) is characterized by transitions to the Gamow-Teller resonance in 156 Er. A reliable measurement of this resonance requires the use of a total absorption spectrometer for detecting the entire γ-ray cascades following β decay. Two different experiments were carried out at GSI to perform such a measurement. In the first experiment we used a total absorption spectrometer developed at the PNPI, St. Petersburg (PNPI-TAgS). Details of this experiment as well as the analysis of the data and the results are given in Ref. [1]. In the second experiment an improved total absorption spectrometer (TAS) [2], was used. In this report we present the results obtained by using the TAS and compare them with the previous work [1]. The PNPI-TAgS represented a big step forwards at the time it was designed [3]. It was the first large size NaI spectrometer which covered a solid angle of almost 4π around the source and for the first time a beta detector was placed inside such a spectrometer. This has the triple advantage of suppressing the room background, selecting the β + component and defining the emission direction of the positrons, thus avoiding their penetration into the crystals of the spectrometer. In comparison, the TAS presents further improvements: 1. It consists of one single NaI crystal (and a plug detector) with bigger volume than the two NaI crystals that formed the PNPI-TAgS, which corresponds to a sizeable increase in detection efficiency. 2. It has a better energy resolution, mainly due to the use of better photo-multiplier tubes. 3. It has, in addition to beta detectors, a small germanium detector which allows to select the EC component of the decay by demanding coincidences with characteristic X-rays and to suppress isobaric contaminants. In the PNPI-TAgS the EC component of the decay was obtained as the difference between singles and β + -coincident spectra. Fig. 1 shows the B(GT) distributions obtained for the Tm decay from the two experiments. Both analyses 2 yielded the same total B(GT) value of 0.48 gA /4π. However there are two main differences in the experimental B(GT) distributions. One is that the TAS data show a more detailed structure of the resonance. The other one is the small shift of about 80 keV observed for the centroid of the resonance (see Fig. 1). Both these features are related to differences in the detectors and the data treatment. The measurement performed with the PNPI-TAgS was analyzed using the “peel-off” algorithm developed at PNPI [4] to deconvolute the measured spectra, whereas 156 4 PNPI, Gatchina; 5 GSI, Darmstadt; 6 IEP, Warsaw TAS (+ EM algorithm) PNPI-TAgS (+ peel-off algorithm) ∼80 keV Figure 1: Gamow-Teller strength distribution in the decay of 156 Tm obtained with the TAS (upper panel), and with the PNPI-TAgS (lower panel). The algorithm to analyze the data was different in both cases. in the case of the TAS measurement the ExpectationMaximization method adapted to the TAS problem at IFIC [5] was applied. For both analyses the response function of the detector was calculated by means of Monte Carlo simulations. In the case of the PNPI-TAgS analysis the simulations were performed assuming that the light produced in the scintillator is proportional to the γ-ray energy. In reality it is not proportional but roughly linear with the energy. This was taken into account to calculate the TAS response function and explains the ≈80 keV shift. It is also interesting to compare the B(GT) distribution of the 156 Tm decay presented here with that of the 152 Tm 2− decay discussed in [6]. We observe that the spreading width of the resonance is wider as we increase the number of neutrons. This is probably due to the increase of 2p-2h and 3p-3h states that become accessible in the final state. References [1] A. Gadea et al., Proc. Int. Conference on “The Future of Nuclear Spectroscopy”, Crete, Greece. Ed. W. Gelletly et al., 162 (1993). [2] M. Karny et al., NIM B 126, 411 (1997). [3] A.A. Bykov et al., Izv. AN SSSR, ser. fis. 44, 15 (1980). [4] V.D. Wittman, private communication. [5] D. Cano-Ott, PhD Thesis, Universidad de Valencia (2000), and J.L. Tain et al., in preparation. [6] E. Nácher et al., contribution to this GSI Sci. Rep. 2002. -9- New isotope 184 Bi and decay spectroscopy of 185,186,188,190 Bi A.N. Andreyev1 , D. Ackermann2,4 , F. P. Heßberger2 , S. Hofmann2 , M.Huyse3 , B. Kindler2 , I. Kojouharov2 , B. Lommel2 , G. Münzenberg2 , R.D. Page1 , K. Van de Vel3 and P. Van Duppen3 1 Department of Physics, Oliver Lodge Laboratory, University of Liverpool, Liverpool L69 7ZE, UK; 2 Gesellschaft fur Schwerionenforschung, Planckstrasse 1, 64291 Darmstadt, Germany; 3 Instituut voor Kern- en Stralingsfysica, University of Leuven, Celestijnenlaan 200 D, B-3001 Leuven, Belgium; 4 Inst. f. Physik, Johannes Gutenberg-University, Staudingerweg 7, 55099 Mainz meric states in 184 Bi with half-lives of T1/2 =13(2) ms and 6.5(1.5) ms (see table 1). The α branching ratio of the daughter 180 Tl nucleus was deduced for the first time as bα (180 Tl)=(2-12)%. Full discussion of the data will be given in [1]. 185 Figure 1: Data obtained in the 93 Nb(94 Mo,3n)184 Bi reaction at Elab (94 Mo)=444 MeV in front of the target. Part a) shows α1 decays recorded in the PSSD between the beam pulses and correlated with recoils within 60 ms. Part b) α shows a spectrum of α1 decays from the recoil-184 Bi →1 180 β + /EC α T l → 180 Hg →2 176 P t analysis with a condition that the recoil-α1 events from a) are further correlated within 10 s with the 6120 keV decay of 180 Hg, being the daughter of 180 Tl after its dominant β + /EC decay; Part c) shows a spectrum of α2 decays from the [recoil-α1 (184 Bi)]-α2 analysis, with a gate on the Eα1 =7120-7550 keV region of fig. 1a. The new nuclide 184 Bi was identified and decay properties of the neutron-deficient 185,186,188,190 Bi isotopes were re-studied in detail in experiments at the velocity filter SHIP. To produce 184−186 Bi nuclei the 2n,3n- channels of the symmetric reactions 94,95 Mo+93 Nb→187,188 Bi∗ were used, while the pxn- channel of the 50,52 Cr+142 Nd→192,194 Po∗ reactions was exploited to study 188,190 Bi. The evaporation residues were separated in-flight and subsequently identified on the basis of the recoil-α, recoil-α-α and recoil-α-γ analysis and excitation function measurements. 184 Bi Fig. 1 shows spectra used for the identification of 184 Bi, to which three groups of α decays at about 7120-7350 keV, 7445(25) keV and at 7730-7850 keV were assigned (see Fig.1a). They were attributed to the decays of two iso- Bi The particle (α and proton) decay of 185 Bi was previously studied by Davids et al. [2] and Poli et al. [3]. In total a few tens of proton and α decays were observed (deduced proton branching ratio bp =85(6)% [3]), which were attributed to the decay of the 1/2+ oblate intruder state. The observation of only one isomeric state in 185 Bi was in contrast to the decay of all heavier odd-mass 187−207 Bi nuclei in which two long-lived isomeric states are known: a nearly spherical 9/2− ground state coexisting with an excited oblate intruder 1/2+ state. In order to search for the decay of a possible second isomeric state and to improve statistical uncertainty of the previous measurements we re-investigated the decay of 185 Bi. In our experiment the statistics was increased by about one order of magnitude within about half the beam time as compared to [3]. The spectrum of proton decays of 185 Bi obtained in our work is shown in Fig.2. The data analysis is in progress. By correlating the proton decays of 185 Bi with the known 6626 keV α decay of the daughter nucleus 184 Pb an α-branching ratio of bα (184 Pb)=80(15)% was deduced, which is in disagreement with the value of bα (184 Pb)=23(14)% from [3]. The reduced α width δα2 (184 Pb)=48(10) keV, deduced from our data using the Rasmussen formalism [4], fits well to the systematics of reduced widths for the neighbouring even-even Pb nuclei. In contrast, the value of δα2 (184 Pb)=13(8) keV from [3] results in an unexpected kink in the otherwise smooth reduced αwidth systematics. 186 Bi To produce 186 Bi nuclei a 93 Nb(95 Mo,2n)186 Bi reaction at Elab (95 Mo)=419 MeV was used. The total number of 186 Bi nuclei collected was at least fifty times higher than in the previous works [5, 6]. Along with the possibility of measuring α-γ coincidences and excitation functions this allowed us to perform a more detailed study of this nucleus (see table 1). As in the heavier odd-odd 188−196 Bi isotopes, the existence of two α-decaying isomeric states with tentative spin and parity assignments of (10− ) and (3+ ) was confirmed. A number of new γ transitions in the daughter isotope 182 Tl were observed. Partial data are reported in this contribution, while the full account will be given in [1]. - 10 Table 1: Summary of our results on 184,186 Bi. For 186 Bi the previous data from [6] are also shown. Column 1 shows the final half-life of each isomer, deduced from our work, while the values for each individual decay (where deduced) are shown in column 3. The uncertainty of the γ-energy values is 1 keV. Assignment , T1/2 , Iπ 186m1 Bi, 9.8(4) ms , (10− ) 186m2 Bi , 14.8(8) ms, (3+ ) 184m1 184m2 Bi, 13(2) ms Bi, 6.6(1.5) msc Eα keV 7263(5) 7369(10) 7070-7230a 7080(15) 7120(15) 7152(15) 7226(15) 7120-7350b 7194(20) 7220(15) 7445(25) 7730-7850 Our Data T1/2 ms 9.8(4) 14.8(8) Ref. [6] Eα T1/2 keV ms 7261(20) 9.8(13) Coincident γ’s keV 108.5(5) E1 87, 98, 133, 215, 238, 276, 281, 371, 380, 444, 520 520 133 444 238 13(2) 14+6 −4 +1.9 4.6−1.3 +3.0 8.1−2.2 +3.0 6.7−2.2 7158(20) 15.0(17) 124 449 a - complex structure with contributions from many α decays. The next four lines show the α decays with a well-defined α-decay energy only. Due to uncertainty in the half-life determination these decays may belong to different isomers in 186 Bi. They are shown in Table 1 separately only to provide information on the α-γ coincidence relations. b - complex structure with contributions from many α decays. c - based on the summed statistics for the shorter-lived groups at 7445(25) keV, 7730-7850 keV and 7220(15)-449(1) keV 188,190 Bi Based on α-γ coincidence relations considerably improved data on the fine structure α decays of 188,190 Bi were obtained. For both of these nuclei a total number of α decays observed was about two orders of magnitude greater compared to previous work [7]. In particular, a (10− ) isomeric state, being presumably a member of the [π1h9/2 × ν1i13/2 ]2− −11− proton-neutron multiplet was identified in 184 Tl. The excitation energy of the state fits well to the systematics of 10− intruder states in the heavier odd-odd mass Tl nuclei. Full account of the data will be given in [8]. References Figure 2: Proton spectrum measured in the Nb(95 Mo,3n)185 Bi reaction. Shown are the proton decays recorded in the PSSD and correlated with recoils within 300 µs after implantation. 93 [1] A.N. Andreyev et al., to be submitted to Eur. Phys. J., A, (2003). [2] C.N. Davids et al., Phys. Rev. Lett. 76, 592 (1996). [3] G.L. Poli et al., Phys. Rev. C, 63, 044304 (2001). [4] J. O. Rasmussen, Phys. Rev. 113, 1593 (1959). [5] J. Schneider, GSI report GSI-84-3 (1984), unpublished. [6] J. Batchelder et al., Z. Phys. A. A357, 121 (1997). [7] P. Van Duppen et al., Nucl. Phys. A529, 268 (1991). [8] A.N. Andreyev et al., to be submitted to Eur. Phys. J. A, (2003). - 11 - Fine Structure in the α - Decay of 215 Ac to 211 Fr P. Kuusiniemi1, F.P. Heßberger1, S. Hofmann1, I. Kojouharov1, and D.Ackermann1,2 1 Ges. für Schwerionenforschung mbH, Darmstadt, Germany; 2Johannes Gutenberg Universität Mainz, Mainz, Germany taken into account, since for the relatively large γ-energies for most of the transitions, one expects small internal conversion coefficients. This is also evidenced by a small number of Fr K X-rays associated with 215Ac α-decay. The by far largest fraction of X-rays is associated with the 505.9 keV level and by some fraction also with the 633.1 keV and 739.2 keV levels. Therefore α-intensities feeding these levels are probably underestimated and thus their HF’s overestimated. Table 1: The data attributed to the α-decay of 215Ac. The isotope 215Ac was first identified by Valli et al. [1], who reported a half-life of T1/2 = 0.17(1) s and an α energy of Eα = 7602 keV. In a recent study on decay properties of Ac – isotopes close to N = 126 indication for fine structure in the α-decay of 215Ac was observed [2]. To confirm these results and to obtain further information on its decay properties a more detailed investigation was performed. 215Ac was produced by the reaction 209 Bi(12C,6n)215Ac at bombarding energies Elab = 85 MeV and 109 MeV. It was separated from the projectile beam by SHIP and afterwards implanted into a 16-strip – Sidetector, which was used to measure the α-particle energies. -rays emitted in coincidence with α-particles were measured with a four-fold segmented Ge-Clover detector. The results are listed in table 1. Our proposed spin and parity assignments for the daughter nucleus 211Fr are based on comparisons with the N = 124 isotones 207Bi and 209At, α-decay hindrance factors, an observed (tentative) M1 transition and known 9/2−ground state, and (11/2−)- and (13/2−)-levels at 583.2(2) keV and 652.62(10) keV, respectively [3]. Hindrance factors (HF) in table 1 are calculated employing the equation HF = δ 2gs / δ 2ex, where δ 2gs is the average reduced α-decay width of 214Ra and 216Th (data are taken from [3]) and δ 2ex is the reduced width for the decay of interest. Both reduced widths are calculated according to the method of Rasmussen [4]. In figure 1 we compare our proposed partial level scheme for the daughter nucleus 211Fr with those of neighbouring N = 124 isotones. In the α-intensities losses due to summing α-particle and conversion electron signals were not 634.3(1) 613.0(2) 466.5(2) α α 0 213Ac α − 395.8(1) 99.57(7) 1.3 7108(10) 505.9(2) 0.007(4) a 7108(10) 110.1(4) 7029(10) a transition − 30 (7/2 ) → 9/2− 350 (5/2−)a → 9/2− − 11 (5/2−)a →(7/2−)a 583.2(1) − 0.12(1) 6978(10) 633.1(2) 0.007(4)a 120 (5/2−)a → 9/2− 6978(10) 237.2(4) 652.6(2) − 11 (5/2−)a→ (7/2−)a 6960(10) − 0.07(1) 13 (7/2−)a → 9/2− − (7/2−)a → (7/2−) 7211(10) 0.20(2) 6877(10) 739.2(4) 0.026(14) 6877(10) 342.6(5) − a − a (11/2−) → 9/2− (13/2−) → 9/2− tentative. References: [1] [2] [3] [4] [5] 739.2(4) α (13/2−) (5/2−) (11/2 −) 652.6(2) 633.1(2) 583.2(1) α α α (5/2−) 505.9(2) α (7/2−) (9/2−) HF 7602(5) (7/2−) α irel / % K. Valli et al. Phys. Rev. 167, 1094 (1968) F.P. Heßberger et al. EPJ A8, 521 (2000) R.B. Firestone et al., Table of Isotopes, Wiley 1996 J.O. Rasmussen, Phys. Rev. 113, 1593 (1959) F.P. Heßberger et al., EPJ A15, 335 (2002) (7/2)− 934.48(14)? (5/2,7/2,9/2)− 5/2−,7/2−,9/2− 794.69(5) 789.13(21) 7/2− (13/2)− 745.78(4) 725.06(2) (11/2 −) 577.10(5) α 7/2− 408.33(3) α 9/2− 7/2− 992.27(4) 13/2− 9/2− 931.88(9) 892.32(4) α 7/2− 742.62(4) α 11/2 − 669.63(4) α 0 α M1 (13/2−) Eα / keV E / keV 395.8(1) 9/2− 0 211 Fr 0 209At α 9/2− 207Bi Figure 1: Level schemes of 213Ac, 209At, 207Bi isotones with excitation energies below 1 MeV [3,5] and proposed level scheme of 211Fr. Observed α-decys populating different levels are indicated by horizontal arrows. - 12 - Decay Properties of the Isomers 210m Ra, 211m Ra and 212m Ra F.P. Heßberger1, S. Hofmann1, I. Kojouharov1, D.Ackermann1,2, and P. Kuusiniemi1 1 Ges. für Schwerionenforschung mbH, Darmstadt, Germany; 2Johannes Gutenberg Universität Mainz, Mainz, Germany In irradiations of 204Pb targets with 12C we observed and identified isomeric states in 210-212Ra, which decay by emission of γ-rays. The measurements were performed in the focal plane of SHIP using delayed coincidences between γ-rays emitted from the evaporation residues which were implanted into a Si-strip detector. The isomers were produced by the reaction 204Pb(12C,xn)216-xRa (x = 4,5,6) at bombarding energies of Elab = (68 – 115) MeV. They were identified by excitation functions, γ-γ - coincidences and life-time measurements. The results are presented in table 1. Three groups of γ-rays were observed . Group I is assigned to the decay of the previously known 8+ isomeric state in 212mRa [1]. 212m Ra ( I ) 211m Ra ( II ) 210m Ra (III) Eγ / keV T1/2 /µs Eγ / keV T1/2 /µs Eγ / keV T1/2 /µs 440.2 628.6 824.3 8.3 ± 0.3 396.1 802.0 3.8 ± 0.3 4.3 ± 0.4 96.7 578.0 602.1 604.5 750.5 774.6 2.24 ± 0.05 8.6 ± 0.6 8.0 ± 0.7 sitions. We obtained I(774.8, 578.9) / I(602.1, 750.5) = 2.6, which rather equals a life-time ratio obtained from Weisskopf estimations of τ(602.1)/τ(774.8) = 3.5 than τ(750.5)/τ(774.8) = 1.34. No transitions 4+ → 4+ were observed at a limit I(4+→4+) / I(4+→2+) ≤ 0.05. Table 1: Measured decay properties of 2.44 ± 0.03 2.44 ± 0.03 2.42 ± 0.08 2.27 ± 0.06 210m-212m Ra. It is evident from Fig. 1b that the excitation functions for -lines of groups (I) and (II) coincide with those for the αdecays of 212,211Ra, which form a not resolved line dublett in our α-spectra and that the maximum for group (II) is shifted to higher values compared to (I). Thus group (II) is attribueted to an isomeric state 211mRa. Group (III) coincides with the low energy side of the excitation function for the α-dublett 210,209Ra and is thus attributed to 210m Ra. Indeed, an isomeric state in 210Ra has been identified at the RITU – separator in Jyväskylä, Finland, several year ago [2], but decay properties have not been reported so far. Our data, however, agree with their unpublished results [3]. The decay schemes for 211mRa and 210mRa are shown in Fig. 2. Similar to the neighbouring odd-even nuclei the isomeric state 211mRa is attributed to a 13/2+ - state decaying via a 9/2 -level into the 5/2 -ground-state [4]. The spin assignments 13/2+ and 9/2 are supported by a conversion coefficient αk = (0.73±0.11) for the 396.1 keV – transition obtained from the intensity ratio of K-x-rayand - –coincidences Σ (K-x-rays(Ra) 802.0) / Σ(396.1 802.0), which is in best agreement with the value expected for an M2 transition [5]. The isomer 210mRa is attributed to an 8+ - state, similar to neighbouring even-even isotopes [4]. As a pecularity here the transitions 6+ → 4+ → 2+ run via two different 4+states. Tentatively the energy ordering of the 4+-levels was done on the basis of the intensity ratio of the -tran- Fig. 1: Excitation functions for 210-212Ra obtained a) from their α-decay, b) from -decay of the isomeric states. Each isomer is represented by one -line. Fig. 2: Decay schemes of 210m,211mRa. The low energy levels of 211Ra are from α-decay studies of 215Th[6]. References: [1] T. Kohno et al. Phys. Rev. C 33, 392 (1986) [2] J. Cocks et al. J. Phys. G 25, 839 (1999) [3] R. Julin priv. comm. 2002 [4] R. Firestone et al. (eds) Table of Isotopes, 1996 [5] R.S. Hager, E.C. Seltzer Nucl. Data A4 (1968) [6] F.P. Heßberger et al. Eur. Phys. J. A8, 521 (2001) - 13 - Gamma-ray Spectroscopy of Spontaneous Ternary Fission of 252 Cf Yu.N. Kopatch1,3 , M. Mutterer2 , P. Jesinger2 , J. von Kalben2 , I. Kojouharov1 , H. Schaffner1 , H.-J. Wollersheim1 , N. Kurz1 , E. Lubkiewicz 1,4 , P. Adrich1,4 , H. Sharma5 , A. Wagner6 , Z. Mezentseva3 , W.H. Trzaska6 , A. Krasznahorkay7 and F. Gönnenwein8 1 Gesellschaft für Schwerionenforschung, Darmstadt, Germany; 2 Institut für Kernphysik, Technische Universität, Darmstadt, Germany; 3 Frank Laboratory of Neutron Physics, JINR, Dubna, Russia; 4 Institute of Physics, Jagiellonian University, Cracow, Poland; 5 Forschungszentrum Rossendorf, Dresden, Germany; 6 Department of Physics, University of Jyväskylä, Jyväskylä, Finland; 7 Institute of Nuclear Research (ATOMKI), Debrecen, Hungary; 8 Physikalisches Institut der Universität Tübingen, Tübingen, Germany Figure 1: Sketch of the new experimental setup for the study of 252 Cf ternary fission. The central part is the FF and LCP detector system CODIS2, contained in a cylindrical vessel filled with 570 torr methane as the counting gas. From the list-mode data registered during several weeks of measurement we will be able to deduce the following sets of parameters and their mutual correlations: masses, kinetic energies and emission angles of the FFs; masses, charges, kinetic energies and emission angles of the LCPs; energies and emission angles of the γ-rays. Our main topics of interest include the following issues: • Gamma-ray spectroscopy of fission fragments. By applying Doppler-shift corrections to the γ-rays emitted from the FFs in flight one can unambiguously assign the γ-ray transitions to particular FFs under given conditions on fragment masses and energies. 25 10 20 dE (arb. units) A new experiment has been performed at GSI, aimed at the investigation of various aspects of γ-ray emission in spontaneous ternary fission of 252 Cf. The experimental setup (see Fig. 1) consists of the fission fragment (FF) and light charged particle (LCP) detector system CODIS2, and two segmented Super Clover Ge detectors facing the 252 Cf source at a distance of 8 cm. CODIS2 is the successor of CODIS (see [1, 2, 3, 4]), having a similar Frischgridded 4π twin ionization chamber (IC) with sectored cathode for measuring FF energies and emission angles, and two rings of LCP detectors with 12 ∆E-E telescopes each. Compared to CODIS, several modifications have been made for the FF IC to accept the higher counting rate (2 × 104 fissions/s) and for the LCP telescopes to improve mass and nuclear charge resolution. Figure 2 demonstrates the high separation power achieved for the LCP registration. The GSI segmented Super Clover Ge detectors used in the experiment are among the largest Ge detectors in the world consisting of 4 Ge crystals, each one 14 cm in length and 6 cm in diameter. 10 15 10 1 5 0 0 5 10 15 E_rest (MeV) 20 25 30 2 Figure 2: Sample plot for the separation of LCPs with the newly designed LCP telescopes. The plot shows ∆E-Erest patterns from He to C LCPs (bottom to top). Note that for the Li LCPs the three parallel lines for the 7,8,9 Li isotopes are well separated from each other. • Angular anisotropy of γ-rays in binary and ternary fission. Measurements of angular distributions of single γ-ray transitions may provide important information on the fragment spins and their alignment. Of special interest is the comparison between the binary and ternary data. In the latter case the FF spin population and alignment might be affected by the emission of the LCPs [4]. • Emission of LCPs in excited states. In addition to the γ-decay of the 10 Be first excited state observed in the previous measurement [2, 3], γ-rays from other LCPs (e.g., Li-isotopes) should become accessible. • LCP yields. New data on isotopic 252 Cf LCP yields are obtained due to the outstanding resolution of the LCP telescopes. This work was supported by the BMBF under contract 06DA913. References [1] M. Mutterer et al., Proc. Int. Conf. on Dynamical Aspects of Nuclear Fission, DANF96, Častá Papiernička, Slovakia, ed. J. Kliman and B.I. Pustylnik, (JINR, Dubna, 1996), p. 250. [2] Yu.N. Kopatch et al., Phys. Rev. C 65, (2002) 044614. [3] P. Singer et al., Proc. Int. Conf. on Dynamical Aspects of Nuclear Fission, DANF96, Častá Papiernička, Slovakia, ed. J. Kliman and B.I. Pustylnik, (JINR, Dubna, 1996), p. 262; P. Singer, Ph.D. Thesis, TU Darmstadt (1997). [4] Yu.N. Kopatch et al., Phys. Rev. Lett. 82, (1999) 303. - 14 Final result from precision study of deeply bound pionic 1s states of Sn nuclei A. Gillitzer, K. Suzukia , M. Fujitab , H. Geisselc , H. Gilgd , R.S. Hayanoa, S. Hirenzakib , K. Itahashie , M. Iwasaki f , P. Kienled,g , L. Maierd , M. Matosc , G. Münzenbergc, T. Ohtsuboh , M. Sato f , M. Shindoa, T. Suzukia , H. Weickc , M. Winklerc , T. Yamazakii, T. Yoneyama f Within the experimental program on deeply bound pionic states at GSI [1] the most recent beamtime was devoted to a systematic study of the (d,3 He) reaction on Sn isotopes, which was considered to be promising for two reasons: i) The existence of 3s1/2 -neutrons in one of the last bound orbitals dramatically enhances the population of the pionic 1s state, being quasi-substitutional at the recoil-free condition. ii) The long chain of stable isotopes of Sn allows to observe for the first time the isotope effect in deeply bound pionic states and thus to seperately deduce the isoscalar and the isovector part of the pion-nucleus potential. The isovector strength parameter b?1 is of fundamental interest due to its direct relation to the pion decay constant and chiral order parameter fπ . 124 30 p (d,3He) π0 Sn( d,3He) (1s) π- 123Sn 20 10 B [MeV] d 2σ/d ΩdE [ µb/sr/MeV] 0 120 0 1 2 3 4 Sn( d, He) 30 5 3 p (d,3He) π0 (1s) π- 119 Sn 20 ibration point with an accuracy of ±7 keV. The observed 1s binding energies (B1s ) for the Sn isotopes 115, 119 and 123 are 3.906 ± 0.024 MeV, 3.820 ± 0.018 MeV, and 3.744 ± 0.018 MeV, the 1s widths (Γ1s ) are 0.441 ± 0.087 MeV, 0.326 ± 0.080 MeV, and 0.341 ± 0.072 MeV, respectively. The binding energies and widths of pionic 1s states of heavy nuclei are almost exclusively determined by the s-wave part of the pion-nucleus interaction, whereas the contribution of the p-wave part is nearly negligible. Using p-wave parameters fixed from global fits of pionic atom data the measurement of deeply bound pionic 1s states reliably determines the s-wave potential. Its isovector and isoscalar parts can then be determined by a precise measurement of the isotope shift, as indicated above. However, due to the smallness of the isotope shift a more accurate determination of the isovector strength is obtained if the isoscalar part is deduced from the known pionic 1s states of light symmetric nuclei. The values of the isovector effective scattering length b?1 obtained individually in this way for each of the Sn isotopes is shown in Fig.2, together with the b?1 value deduced from the previous 205 Pb data [1]. For the weighted average we obtain b?1 = −0.115 ± 0.005 m−1 π , considerably enhanced as compared to the vacuuum value bfree 1 , resulting in the ra? ? 2 2 tio bfree /b = f (ρ ) / f = 0.78 ± 0.03. Since the pion efeff π π 1 1 fectively probes a density of 0.6 ρ0 this corresponds to a reduction of 37% for the chiral order parameter f π2 at normal nuclear density. 10 b1free/b1* αρ0 NUCLEAR MEDIUM B [MeV] 0 30 116 0 1 2 3 4 Sn( d,3He) 5 -0.090 p (d,3He) π0 average 205 (1s) π- 115 Sn 20 1.0 free value -0.100 -0.110 VACUUM 0.1 0.9 Pb 0.3 0.4 -0.120 115 -0.130 -0.140 B [MeV] 0 0 360 3He Fig. 1: 1 2 3 4 0.2 0.8 b1* 10 0.0 Sn 119 Sn 123 Sn 0.7 0.5 0.6 5 365 Kinetic Energy [Me 370 V] Double differential cross sections of the reactions at measured at incident deuteron energy Td = 503.388 MeV versus the 3 He kinetic energy. The scales of the π− binding energies are also indicated. Fig. 2: Values of the in-medium isovector effective scattering length parameter b?1 (in m−1 π ) deduced from the 1s binding energies B1s in 115,119,123Sn. 124,120,116 Sn(d,3 He) Information on the experimental method and the relevant instrumental parameters is given in Refs. [2, 3]. The data analysis has now been completed and binding energies and widths of the pionic 1s state in 115,119,123Sn were determined [3]. The measured double differential (d,3 He) cross sections d 2 σ/dEdΩ with 124,120,116Sn targets are shown in Fig.1. The thin mylar backing attached on purpose to the targets gives rise to a prominent peak due to the p(d,3 He)π0 reaction, whose higher-energy edge serves as an absolute cal- References: [1] T. Yamazaki et al., Z. Phys. A355 (1996) 219; H. Gilg et al., Phys. Rev. C 62 (2000) 025201; K. Itahashi et al., Phys. Rev. C 62 (2000) 025202; H. Geissel et al., Phys. Rev. Lett. 88 (2002) 122301. [2] Annual Report 2001, KFA-IKP, 2002, p. 91 [3] K. Suzuki et al., submitted to Phys. Rev. Lett, preprint nucl-ex/0211023. a University of Tokyo, b Nara Women’s University, c GSI Darmstadt, d Technische Universität München, e Muon Science Lab., RIKEN f Tokyo Institute of Technology, g IMEP, Vienna, h Niigata University, i RI Beam Science Lab., RIKEN - 15 - Study of High Energy Particle Background in Hypernuclear γ-ray Spectroscopy with π + Induced Reaction on 89Y and 12 C targets T.R. Saitoh1 , A. Banu1 , F. Becker1 , C. Ayerbe2 , P. Doornenbal1 , J. Gerl1 , M. Górska1 , I. Kojouharov1 , Y. Kopach3 , J. Li1 , R. Lozeva1 , S. Mandal1 , J. Pochodzalla2 , N. Saito1 , R.S. Simon1 and H.J. Wollersheim1 1 GSI; 2 Uni. Mainz; 3 Dubna, Russia A π + induced reaction used in hypernuclear experiments is characterized by its large probability to produce a Λ hyperon bound nucleus. At KEK, high resolution γ-ray spectroscopy with the Ge-detector array HYPERBALL has been successfully performed for 7Λ Li in a 7 Li(π + , K + )7Λ Li reaction at 1.05 GeV/c by Tamura et al. [1]. A similar experiment for medium heavy nuclei with A ∼ 90 has been also proposed at GSI to investigate inner shell transitions[2]. Gamma-ray spectroscopy with Ge detectors is difficult because of high energy particle background in the Ge detectors, causing large dead time due to saturation of the preamplifiers. The problem was solved by the HYPERBALL collaboration by using transistor-reset preamplifers [1]. However, the nature of the background has not been well understood. experimental target at 1.131 GeV/c. We have observed significant particle background with the π + beam hitting the 89 Y target at 90◦ and we preliminary conclude that the rate of produced particles in the target can be explained by nucleon resonances. For proton beams as shown in the table, we observed less particles from the target, and we observed almost no particles with the other beam particles. The results of the measurement show that the proposed hypernuclear γ-spectroscopy with two VEGA Ge detectors at 3 cm from the target center [3] could be performed with a necessary π + beam intensity of 5 × 105 per second. Authors would like to thank Prof. Tamura of Tohoku University for fruitful discussions. Authors also would like to thank W. Prokopowicz and H. Schaffner for working on the mechanics and the data acquisition system. At the pion beam facility at GSI, we produced a secondary π + beam from a primary 12 C beams at 2 GeV/u on a Be production target, Secondary beams with 0.929 GeV/c (Bρ = 3.094 T·m) and 1.131 GeV/c (Bρ = 3.766 T·m) of π + momentum were transfered to Cave C. Two different experimental targets, 89 Y with 1.25 cm diameter and 3 cm thickness and 12 C with 2 cm diameter and 6 cm thickness were used at each momentum. Since there is no separator in the beam line, other secondary particles in particular 1 H, 2 H, 3 H and 3 He were also delivered to the experimental target. Time-Of-Flight (TOF) was measured for the beam particles by using two plastic scintillators with 5 mm thickness separated by 2.2 m. Figure 1 shows the separation among the beam particles at Bρ = 3.766 T·m. The beam distribution was measured by a positionsensitive Si strip detector with 0.47 mm strip width to be σx = 7.3 and σy = 8.9 mm. Particles produced in the experimental targets, 89 Y and 12 C, were measured by a BaF2 detector at 90◦ surrounded by 6 NaI detectors. The BaF2 detector is hexagonally shaped and is 14 cm long with an inscribed circle radius of 4.34 cm. The NaI detectors are also hexagonal and are 20 cm long with an inscribed circle radius of 2.94 cm. A plastic scintillator with 9 mm thickness was placed in front of the BaF2 detector. The distance of the BaF2 to the target center was 15 cm. Particle identification was performed by pulse shape analyses with information on the plastic scintillator and NaI detectors. Spectra of π + , protons, high energy γ-rays from π 0 decay, high energy neutrons, electrons, heavy ions, and low energy neutral particles, which are mainly low energy γ-rays and neutrons (E < 30 MeV), from the target were obtained for each kind of projectile by using cuts in TOF and the beam position. Table 1 shows preliminary results for the yield of observed particles by π + and proton beams on the 89 Y target with more than 10 MeV energy deposition in BaF2 normalized to the beam intensity at the References Counts/100ps [1] H. Tamura et al., Phys. Rev. Lett. 84 (2000) 5963. [2] J. Gerl et al., LOI of S234 experiment, GSI [3] See internal notes by T.R. Saitoh et al. 10 10 10 10 10 6 1H σ =0.21ns 5 π+ σ =0.27ns 4 3 2H σ =0.20ns 3He σ =0.16ns 2 3H σ =0.16ns 10 1 96 98 100 102 104 106 108 110 112 TDC time [ns] Figure 1: TOF for the beam particles measured by two plastic TOF scintillators at Bρ = 3.766 T·m. Table 1: Preliminary results of particle measurement for energy deposit > 10 MeV in BaF2 with π + and proton beams at 1.131 GeV/c on the 89 Y target. The numbers are normalized to the total intensity of the beams hitting the target. Particles π+ Proton High energy γ-ray Electron Low energy neutral particles (E < 30 MeV) π + beam 4(3) × 10−4 10(4) × 10−4 4(3) × 10−4 1(1) × 10−4 7(3) × 10−4 Proton beam < 2 × 10−6 8(1) × 10−4 5(3) × 10−5 3(2) × 10−5 16(5) × 10−5 - 16 - Correlation studies of the unstable heavy 5 H systemB,G T. Aumann1,2 , M.J.G. Borge3 , L.V. Chulkov1,4 , Th. W. Elze5 , H. Emling1 , C. Forssén6 , H. Geissel1 , M Hellström1 , B. Jonson6 , J. V. Kratz2 , R. Kulessa7 , Y. Leifels1 , K. Markenroth6 , M. Meister1,6 , G. Münzenberg1 , F. Nickel1 , T. Nilsson8 , G. Nyman6 , V. Pribora4 , A. Richter9 , K. Riisager10 , C. Scheidenberger1 , G. Schrieder9 , H. Simon9 , O. Tengblad3 and M. V. Zhukov6 1 GSI Darmstadt, Germany; 2 Institut für Kernchemie, Universität Mainz, Germany; 3 CSIC Madrid, Spain; Kurchatov Institute Moscow, Russia; 5 Institut für Kernphysik, Universität Frankfurt, Germany; 6 Experimentell Fysik, CTH/GU Göteborg, Sweden; 7 Instytut Fizyki, Universytet Kraków, Poland; 8 CERN/EP-Division, Geneva, Switzerland; 9 Institut für Kernphysik, TU-Darmstadt, Germany; 10 Institut for Fysik og Astronomi, Aarhus Universitet, Denmark 4 n 0.8 n W(ε, ϑ) = ϑ 0.6 t 0.4 0.2 -0.5 cos (ϑ 4p ε(1 − ε) · π + C0000 − 2(2ε − 1)C0200 ) 0.5 0.75 0.50 0.25 E tnn nn/ E = ε Figure 1: Two dimensional plot of the probability distribution W(ε, cos (ϑ)), as given in Eq. (1) with amplitudes adjusted to the experimental data. The inset sketches the used coordinate system. Experimental studies of heavy hydrogen isotopes have recently attracted much interest and some intriguing results [1] have been reported. The structure of heavy hydrogen nuclei is expected to be similar to that of neutron-rich helium isotopes, i.e., an inert core (here triton) surrounded by valence neutrons. The experiment was performed using a 6 He beam with 240 MeV/nucleon from the FRS impinging on a carbon target and the one proton knockout channel has been investigated. Advantageous in this case is that the momentum transfer in the reaction is small ( < 30 ∼ MeV/c) and the data can be analyzed in the framework of the sudden approximation. The kinematically complete measurement was done at the ALADIN-LAND reaction setup; the relative momenta of all particles in the t+n+n final state could be reconstructed. Momentum distributions, relative energy spectra together with partial energy distributions (e.g. ε = Enn /Etnn ) and angular correlations could thus be deduced in this experiment. The weights of different configurations in the t+n+n system were determined from experimental data, using a method proposed in Ref. [2]. It is based on a series expansion of the final state wave function into hyperspherical harmonics and represents a three-body generalization of the expansion in sperical harmonics known from two-body systems. The data are presented in two different coordinate systems in momentum space: (i) the T system shown in the inset in Figure 1 where the lines point along the directions of the relative momenta of all involved particles in the center of mass and (ii) the Y system, where one of the two neutrons is exchanged with the triton. In both coordinate frames 2 8ε(1 − ε) sin2 ϑ C1211 (1) 2 p + 4 ε(1 − ε)C0211 cos ϑ , where W(ε, ϑ) is normalized to unity. The transformation of the complex amplitudes CSKlx ly from T to Y coordinates and vice versa are Rfixed through Raynal-Revai coeffiR cients. The projections W(ε, ϑ)dε and W(ε, ϑ)dcos (ϑ) where fitted to the data in both coordinate systems. As result, the CSKlx ly were determined and the corresponding probability distribution W(ε, cos(ϑ)) is shown in Figure 1. The strongest component in the mixed ground state configuration is related to spin and parity J π = 1/2+ . This observation is in agreement with recent theoretical work [3], and is further supported by direct comparison with the measured energy spectrum of the t+n+n system shown in Figure 2. 120 dσ/dEtnn (a.u.) W( ε, cos(ϑ) ) the explicit expression for the probability distribution, can be written as: 5H 3/2+ 1/2+ 80 5/2+ 40 0 0 1 2 3 4 5 6 7 8 Etnn (MeV) Figure 2: Relative energy spectrum of the t + n + n system. The curves show results of theoretical calculations [3] assuming different spins and parities J π for the 5 H ground state. The current work [4] presents a successful application of hypersperical harmonics to the analysis of few body continuum states. The method can readily be extended to reveal information about the three-body continuum structure of heavier exotic nuclei, e.g., 11 Li and 14 Be. References [1] A.A. Korsheninnikov, et al., Phys. Rev. Lett. 87 (2001) 092501 [2] O.V. Bochkarev, et al., Nucl. Phys. A505 (1989) 215 [3] N.B. Shulgina, et al., Phys. Rev. C62 (2000) 014312 [4] M. Meister, et al., submitted to Nucl. Phys A - 17 - Study of Nuclear Matter Distributions of Neutron-Rich He-Isotopes by Proton Scattering in Inverse Kinematics F. Aksouh1 , A. Bleile1 , O.V. Bochkarev2 , L.V. Chulkov2 , D. Cortina-Gil1,* , A.V. Dobrovolsky1,3 , P. Egelhof1 , H. Geissel1 , M. Hellström1 , N.B. Isaev3 , O.A. Kiselev1,3 , B.G. Komkov3 , M. Mátos1 , F.V. Moroz3 , G. Münzenberg1 , M. Mutterer4 , V.A. Mylnikov3 , S.R. Neumaier1 , V.N. Pribora1,2 , D.M. Seliverstov3 , L.O. Sergueev3 , A. Shrivastava1,** , K. Sümmerer1 , H. Weick1 , M. Winkler1 and V.I. Yatsoura3 1 GSI Darmstadt; 2 Kurchatov Institute, Moscow; 3 PNPI, St. Petersburg; 4 IKP TU Darmstadt; * Present address: Depto. de Fisica de Particulas, Universidade de Santiago de Compostela, Spain; ** Present address: Bhabha Atomic Research Centre, Mumbai, India The study of neutron-rich light nuclei near the drip line has attracted much attention as they exhibit a particular nuclear structure, namely an extended distribution of the valence neutrons surrounding a compact core. Recently the differential cross section for elastic proton scattering from 6,8 He and 8,9,11 Li at 700 MeV/u using the technique of inverse kinematics was successfully measured at GSI [1,2,3]. A high-pressure hydrogen-filled ionization chamber was used as the target and a proton detector. The experiments were performed in the small range 0.002 ≤| t |≤ 0.05 (GeV/c)2 of the four-momentum transfer squared t and have yielded valuable information on the nuclear sizes and radial structure of nuclear matter density distributions. Recently, a novel experimental approach has been accomplished with the aim to deduce the differential p6,8 He cross sections in the t-range 0.05 ≤| t |≤ 0.25 (GeV/c)2 close to the expected first diffraction minimum. The experimental setup allowed to track and identify the projectile nuclei in coincidence with the recoil protons. The major difference with respect to the previous experiments was that instead of the active gaseous target, a 600 mg/cm2 liquid hydrogen target was used and a position sensitive scintillator wall measured the recoil proton energies via time-of-flight [4]. The experimental arrangement allowed for a very lowbackground data taking. At present, only preliminary results for the p6,8 He differential cross sections at the larger t-range have been obtained. An example of the measured p6 He cross section is shown on Fig. 1 together with the fit of the combined data sets at low and high momentum transfer using a double-gaussian parametrisation for modelling the matter distribution in core and halo as input for a Glauber calculation (for details see [2,4]). The density distributions of nuclear matter obtained for 6 He are shown in Fig. 2. Solid curves are the results of averaging of densities deduced from fits to the data using different model parametrisations (for details see [4]). As it was predicted by theoretical calculations [5], the recently measured data together with the data from the previous experiment allow to deduce the size and radial shape of the core in 6,8 He with higher precision. The analysis of the p8 He cross section is currently in progress. Figure 2: Density distribution of the total nuclear matter and the core in 6 He. The shaded areas represent the envelopes of the density variation within the different model parametrizations used. The 4 He density distribution is shown for comparison. All density distributions are normalized to the number of nucleons. References 6 Figure 1: Experimental differential cross section of p He elastic scattering versus momentum transfer squared t. Full symbols are data taken from [1], empty symbols are data from the actual measurement. The insert shows the area of overlap. The solid curve is the result of a fit of the combined data set. [1] [2] [3] [4] S.R. Neumaier et al., Nucl. Phys. A 712, 247 (2002) G.D. Alkhazov et al., Nucl. Phys. A 712, 269 (2002) P. Egelhof et al., Europ. Phys. J. A 15, 27 (2002) F. Aksouh et al., PhD thesis, Paris-Sud University (Orsay), 2002 [5] L.V. Chulkov et al., Nucl. Phys. A 587, 291 (1995) - 18 - Low-lying dipole strength and single-particle structure of oxygen isotopes R. Palita , T. Aumannb , K. Boretzkyc , D. Cortinab , U. Datta Pramanikb , Th.W. Elzea , H. Emlinga , H. Geisselb , A. Grünschloßa , M. Hellströmb , S. Ilievskib , N. Iwasab , K.L. Jonesb , L.H. Khiemc , J.V. Kratzc , R. Kulessad , Y. Leifelsb , A. Leistenschneidera , E. Lubkiewiczd , G. Münzenbergb , C. Nociforoc , P. Reitere , C. Scheidenbergerb , H. Simonf , K. Sümmererb , E. Wajdad , and W. Walusd Univ. Frankfurt; b GSI Darmstadt; c Univ. Mainz; The large spatial extension of the wave function of the loosely bound valence neutron(s) of halo nuclei gives rise to non-resonant dipole transitions to the continuum with large transition probabilities close to the neutron threshold. The fact that this dipole strength is characteristic for the ground-state structure of the projectile was used to extract the single-particle properties, so far only for loosely bound nuclei with neutron separation energies below 1.2 MeV [1, 2, 3]. Here we shall discuss the Coulomb breakup of the odd oxygen isotopes, where the last neutron is relatively well bound, e.g. Sn = 4.1 MeV for 17 O and Sn = 2.3 MeV for 23 O. This study will serve as a testing ground to explore the scope of the Coulomb breakup method as a spectroscopic tool. The unstable oxygen ions were produced by fragmentation of a primary 40 Ar beam, separated by the Fragment Separator FRS and directed onto secondary carbon and lead targets. The dipole-strength function has been extracted from the electromagnetic excitation of the projectile (in the Coulomb field of the lead target) to the continuum followed by neutron decay. The coincident measurement of the charged fragment, neutron, and γ-rays allows to determine the differential cross section exclusively for the different fragment states populated. Contributions from nuclear excitations were estimated by measuring cross sections with a carbon target. The spectroscopic factors associated with the individual configurations are obtained from the ratio of the measured cross section to the cross section calculated in a direct-breakup model using the wave function of the concerned configuration as an input. Details of the method are found in [2]. For 17 O (J π = 5/2+) the Coulomb breakup reaction yields the 16 O core mainly in the ground state as expected. The differential cross section dσ/dErel is shown in Fig. 1 as a function of relative energy between 16 O(0+ ) and the neutron. A comparison with similar results obtained for 11 Be [3] shows that the distribution is much broader and the peak cross section is much smaller (by about two orders of magnitude). Obviously, this reflects the fact that the valence neutron of 17 O is well bound in a l = 2 state, while 11 Be has a well pronounced halo structure. It clearly demonstrates the tremendous sensitivity of the Coulomb breakup cross section to a halo-like tail of the wave function. Our preliminary analysis yields a spectroscopic factors of 0.8(1), very close to the expected value of 1 and also to the result of S = 1.04(10) obtained from an electron scattering experiment [4], thus giving confidence that Coulomb breakup can be utilized to extract quantitative nuclear structure information. The results obtained for 19 O and 21 O demonstrate the importance of an exclusive measurement including γ-ray coincidences. In both cases, the main contributions to the d Univ. Kraków; dσ/dErel (mb/MeV) a e LMU Garching; f TU Darmstadt 5 17 4.5 Breakup of O 4 1d5/2ν ⊗ 0 〉 + + S(0 ) = 0.8(1) 3.5 3 2.5 2 1.5 1 0.5 0 0 2 4 6 8 10 12 14 16 18 20 Erel (MeV) Figure 1: Relative energy spectrum of (16 O+n) for Coulomb breakup reactions of 17 O populating 16 O in its ground state. The solid curve shows the result of the direct breakup model for a 1d5/2 neutron coupled to the 16 O ground state with a spectroscopic factor of 0.8. ground state involve excited states. From the γ-γ coincidence measurement, the 4+ to 2+ and 2+ to 0+ cascade in 20 O was observed and the spectroscopic weight for the contribution |1d5/2 ν ⊗ 4+ > in the wave function was obtained. A preliminary analysis results in a value of 2.3(2), which is rather close to the shell-model prediction of Brown of 2.59 [5]. Although the cross sections are much smaller for the non-halo nuclei with comparatively large separation energies, the shape of the cross section as well as the absolute magnitude is well reproduced by the direct-breakup model. The enormous sensitivity of the cross section to the tail of the wave function makes Coulomb breakup one of the most efficient spectroscopic methods to extract quantitative structure information on the ground-state configuration of unstable nuclei even with very low beam intensities. In this context we finally mention a recent experiment studying 23 O. A ground-state spin assignment of J π = 1/2+ could be made and a spectroscopic factor was deduced [6] from the differential Coulomb breakup cross section measured with a 23 O beam intensity of about 1 ion/sec only. References [1] T. Nakamura et al., Phys. Rev. Lett. 83 (1999) 1112. [2] U. D. Pramanik et al., Phys. Lett. B 551 (2003) 63. [3] R. Palit et al., GSI scientific report 2001, ISSN 01740814 (2002) 19. [4] S. Burzynski et al., Nucl. Phys. A 399, (1983) 230. [5] B.A. Brown, Prog. Part. Nucl. Phys. 47 (2001) 517. [6] K.L.Jones et al., in preparation. - 19 - Complex nuclear-structure phenomena revealed from the nuclide production in fragmentation reactions M. V. Ricciardi1, K.-H. Schmidt1, T. Enqvist1, A. V. Ignatyuk2, A. Kelic1, P. Napolitani1, F. Rejmund3 and O. Yordanov1 1 GSI Darmstadt, Germany; 2IPPE Obninsk, Russia; 3IPN Orsay, France Nuclear structure manifests itself in many features, which are widely investigated, e.g. in ground-state properties like binding energy, half-life, radius and deformation. Signatures of nuclear structure arise also in the production yields in specific nuclear reactions at low energies, gradually disappearing and transforming into smooth distributions with increasing excitation energy induced in the reaction. In the latest years signatures of nuclear structures were found in the production yields in deep-inelastic and in fragmentation reactions [1, 2, 3, 4, 5, 6, 7, 8], which can be quite violent and which are expected to introduce a large range of excitation energies in the nucleus. Here we will report on the production yields from the projectile fragmentation of 1 A GeV 238U nuclei in a titanium target, measured at GSI. The residual nuclei were fully identified in mass and atomic number with a high-resolution magnetic spectrometer, the FRS, and their production cross section were deduced. Details of the experimental set-up and of the analysis method can be found in ref. [9]. The data were filtered according to the N-Z number. The production cross sections of the observed fragments, grouped according to this filter, are shown in Figure 1. The data reveal a complex structure. All even-mass nuclei present a visible evenodd effect, which seems particularly strong for N=Z nuclei. Odd-mass nuclei show a “reversed” even-odd effect, with enhanced production of odd-Z nuclei. This enhancement is stronger for nuclei with larger values N-Z. However, for nuclei with N-Z=1 the reversed even-odd effect vanished out at about Z=16, and again an enhanced production of even-Z nuclei can be observed. We tested the hypothesis that these fluctuations are produced at the end of the evaporation cascade due to the influence of nuclear structure on the properties of excited levels. Applying a statistical model, where pairing, modeled as a blocking effect, was introduced both in the masses and in the level densities, the number of bound states, representing the number of possible final states, was determined. For the odd-mass nuclei the statistical model reproduced the observed structural effects in all their complexity. For the even-mass nuclei, the statistical model predicted no structure at all. Analysing the experimental masses and energy levels found in literature, two important aspects emerged: 1) compared to other even-even nuclei, the N=Z nuclei, which are multiples of alpha particles, are exceptionally strongly bound, 2) while blocking effects are expected to destroy the even-odd staggering of the ground-state energies immediately, part of the even-odd staggering survives up to excitation energies in the order of 10 MeV above the ground state. These findings go beyond the blocking effect of pairing and indicate more complex structural phenomena. Recently, possible origins for these structural effects, like mean-field contributions to pairing effects, alpha clustering and neutron-proton pairing, were discussed intensively [10, 11, 12]. Our experimental data suggest that structural effects are restored in the end-products of hot decaying nuclei, whose structure is ruled by the available phase space in the last step. It seems that a systematic investigation of the fine structure in the production yields from highly excited nuclei is a rich source of information on nuclear-structure phenomena in slightly excited nuclei found at the end of their evaporation process. It is a challenge to quantitatively interpret these results with theoretical models in order to better understand the complex nuclear-structure phenomena behind. Figure 1: Formation cross sections of the projectile-like products from the reaction 238U + Ti, 1 A GeV. The data are given along specific values of N-Z. The cross section for 32 Al (Z=13, N=Z+6) is an extrapolated value. The chain N=Z shows the strongest even-odd effect, while the chain NZ=5 show the strongest “reversed” even-odd effect. [1] B. Blank et al., Nucl. Instr. Meth. A 286 (1990) 160. [2] W. R. Webber, J. C. Kish, D. A. Schrier, Phys. Rev. C 41 (1990) 547. [3] Ch. O. Bacri et al., Nucl. Phys. A 555 (1993) 477. [4] C. N. Knott et al., Phys. Rev. C 53 (1996) 347. [5] C. Zeitlin et al., Phys. Rev. C 56 (1997) 388. [6] Sl. Cavallaro et al., Phys. Rev. C 57 (1998) 731. [7] L. B. Yang et al., Phys. Rev. C 60 (1999) 041602 (R). [8] E. M. Winchester et al., Phys. Rev. C 63 (2001) 014601. [9] M. V. Ricciardi, PhD Thesis, GSI, in preparation. [10] J. Dobaczewski et al., Phys. Rev. C 63 (2001) 024308. [11] H. Horiuchi, Eur. Phys. J. A 13 (2002) 39. [12] Yu. V. Palchikov, J. Dobe, R. V. Jolos, Phys. Rev. C 63 (2001) 034320. - 20 - Charge-pickup reactions induced by interactions of 1 A GeV 208Pb with different targets A. Keli a, T. Enqvist a , W. Wlazło b, g, P. Armbruster a, J. Benlliure c, M. Bernas d, A. Boudard b , S. Czajkowski e, R. Legrain e, S. Leray b, B. Mustapha f, M. Pravikoff e, F. Rejmund d, K.-H. Schmidt a, C. Stéphan d, J. Taieb b, L. Tassan-Got d, C. Volant b a GSI, Germany, b DAPNIA/SPhN CEA/Saclay, France, c University of Santiago de Compostela, Spain, d IPN Orsay, France, e CENBG, France, f Argonne National Laboratory, USA, g Jagiellonian University, Institute of Physics, Poland The systematic measurement of residual nuclide cross sections from the interaction of relativistic 1 A GeV 208Pb projectiles with different targets forms part of a comprehensive study of fragment formation in a neutron-generating target for accelerator-driven systems (ADS) [1]. The data published so far [2],[3] do not include charge-pickup reactions, which are of specific importance for estimating the production of polonium isotopes in a lead-bismuth ADS target. From the basic understanding of charge-exchange reactions, data on charge-pickup reactions, being sensitive to the nucleonic aspects of relativistic heavy-ion collisions[4], are also an important test for any microscopic model on nucleonnucleon interactions. However, these studies were limited by lack of experimental data especially those with full isotopic resolution. The measurements with a 1 A GeV 208Pb beam were performed at GSI-Darmstadt using the full advantage of relativistic collisions in inverse kinematics. The experimental method and data-analysis procedure have been described in detail in ref. [2], and here only a short overview will be given. The primary beam of 208Pb impinged on a 87 mg/cm2 thick liquid-hydrogen, 206 mg/cm2 liquid-deuteron target, and on the empty target container corresponding to 36 mg/cm2 titanium target. The fragment separator FRS [5] and the associated detector equipment were used in order to separate and to identify the reaction products. The production cross section of each isotope was determined from the measured velocity distributions. To compare our data with calculations, we used two different intra-nuclear cascade models: ISABEL [6] and INCL4[7], both coupled to the same evaporation-fission model ABLA [8]. In the case of the 208Pb + Ti reaction, calculations were performed only with ISABEL, because in the present version of INCL4 the heaviest target that can be used in calculations is 4He. The results of these comparisons are shown in figure 1. While ISABEL is reproducing quite satisfactory the measured bismuth isotopic distribution for the 208Pb + Ti reaction, discrepancies between model calculations and experiment are apparent for proton and deuteron targets. INCL4 is over-predicting the neutron-rich side, especially for the proton-induced reaction. On the other hand, ISABEL is giving better agreement with this part of the distribution, but in the same time over-predicting the neutron-deficient side. It is difficult to judge which model is more suitable for application. The basic physics contained in both models is the same, and the different predictions are the results of different implementations inside INCL4 and ISABEL. Figure 1. Experimental (full squares, open dots, and open triangles) and calculated (ISABEL + ABLA and INCL4 + ABLA) isotopic distributions of charge-pickup products in the interactions of 1 A GeV 208Pb projectiles with hydrogen, deuteron and titanium. [1] http://www.fynu.ucl.ac.be/collaborations/hindas/ T. Enqvist et al., Nucl. Phys. A 686 (2001) 481. [3] T. Enqvist et al., Nucl. Phys. A 703 (2002) 435. [2] [4] K. Sümmerer et al., Phys. Rev. C52 (1992) 1106. H. Geissel et al., Nucl. Instr. Meth. in Phys. Res. B70 (1992) 286. [6] Y. Yariv and Z. Fraenkel, Phys. Rev. C20 (1979) 2227. [7] A. Boudard et al., Phys. Rev. C 66, 044615 (2002). [8] J.-J. Gaimard and K.-H. Schmidt, Nucl. Phys. A 531 (1991) 709. [5] - 21 - The low-energy cross section of the 7 Be(p,γ)8B reaction determined via Coulomb dissociation of 8B F. Schümanna , F. Hammacheb , S. Typelb,c , F. Uhligd , K. Sümmererb , and the S223 Collaboration Ruhr-Universität Bochum, b GSI Darmstadt, c MSU East Lansing, d TU Darmstadt counts Coulomb dissociation (CD) is an interesting tool to derive low-energy radiative-fusion cross sections of astrophysical interest that are difficult or impossible to measure directly (e.g. when unstable nuclei are involved). The application of this method requires, however, that the multipolarity composition relevant in CD (which is in general different from the one involved in the direct-capture process) is known experimentally. Current interest focusses strongly on the 7 Be(p,γ)8 B fusion reaction due to its relevance for solar-neutrino and elementary-particle physics. It turns out that zeroenergy astrophysical S factors S17 (0) from the most recent 7 Be(p,γ) direct-capture experiments [1,2,3,4] do not all agree within their errors, thus it is desirable to cross check their results by indirect methods such as CD. To solve the problem of E2-contribution in CD experimentally, we have performed an exclusive CD experiment that allows to analyze the distributions of the breakup particles (p and 7 Be) in the moving frame of the excited 8 B nucleus prior to breakup (8 B∗ ). We have performed firstorder perturbation-theory (PT) calculations in the semiclassical approach to interpret such distributions. An example is shown in Fig. 1 where the transverse in-plane momenta of the proton are plotted for three different cuts in the classical Rutherford scattering angle (θ8 ), equivalent of impact parameters of 30 fm, 18.5 fm, and 7 fm, respectively. The full (dashed) theoretical curves in Fig. 1 demonstrate what is expected for pure E1 (E1-plus-E2) multipolarities. It is obvious that pure E1 multipolarity fits our data points much better, therefore we derive the S17 factors for the different Erel bins under this assumption. 3000 a) θ8<0.62 b) θ8<1.0 0 40 ● this work ▼ Baby et al. ❍ Iwasa et al. Typel ✩ Kikuchi et al. 30 ❏ Davids et al. (E2 subtr.) 20 Descouvemont 10 0 0.25 0.5 0.75 1 1.25 1.5 Erel (MeV) Figure 2: Comparison between S17 values from Coulomb dissociation experiments. The full (open) circles denote the present (previous [5]) GSI Coulomb dissociation experiment. Open stars (squares) are plotted for the results of Ref.[6] (Ref.[7]). Triangles indicate the most recent (p,γ) experiment by Baby et al. [4]. c) θ8<2.5 0 The resulting S17 (Erel ) are shown in Fig. 2. They are in good agreement with those from our first experiment (Ref. 5) and for the lowest Erel bins also with those of Ref. 6. The data points of Davids et al. [7], where an E2 contribution was subtracted, are substantially lower than ours. It is interesting to note that the most recent 7 B(p,γ) experiment [4] agrees very well with our data even up to 1.1 MeV. More high-precision measurements are still needed, however, to decide which theory is the best to extrapolate S17 to solar energies. Our data up to Erel = 1.5 MeV fit best to our PT theory (full line), the resulting zeroenergy S factor amounts to S17 (0) = 18.6 ± 0.5 eV b. But the bulk of the data can be equally well described by the cluster-model prediction of Descouvemont [8] (dashed curve); fitted to our data, we obtain S17 (0) = 20.8 ± 0.6 eV b. S17 (eV b) a 0 2000 References 1000 0 -50 0 50 0 ptin (MeV/c) -50 0 50 Figure 1: In-plane transverse momenta, pin t , of the protons in the frame of 8 B∗ for three different cuts in θ8 . The full lines depict our perturbation-theory calculation with only E1 multipolarity, the dashed curves the same with E1 and E2 multipolarity. [1] [2] [3] [4] [5] [6] [7] [8] F. Hammache et al., Phys.Rev.Lett. 86, 3985 (2001). F. Strieder et al., Nucl. Phys. A696, 219 (2001). A. Junghans et al., Phys.Rev.Lett. 88, 041101 (2001). L.T. Baby et al., Phys.Rev.Lett 90, 022501 (2002). N. Iwasa et al., Phys.Rev.Lett. 83, 2910 (1999). T. Kikuchi et al., Eur.Phys.J. A 3, 213 (1998). B. Davids et al., Phys.Rev.C 63, 065806 (2001). P. Descouvemont et al., Nucl.Phys. A567, 341 (1994). - 22 - LUNA:Laboratory for Underground Nuclear Astrophysics C.Rolfs , M.Junker , F.Schümann , F.Strieder and the LUNA Collaboration Institut für Physik mit Ionenstrahlen, Ruhr-Universität Bochum, Germany 14 1E-04 yield [a.u.] During most of its life, a low mass star burns H in the center via the pp chain. However, when the central H mass fraction reduces down to 0.1, the nuclear energy produced by the H-burning becomes not sufficient and the stellar core must contract to extract some energy from its gravitational field. Then, the central temperature (and the density) increases and the H-burning switches from the pp-chain to the more efficient CNO-burning. Thus, the escape from the main sequence is powered by the onset of the CNO burning, whose bottleneck is the 14 N(p,γ)15 O reaction. A modification of the rate of this reaction alters the turn off luminosity, but leaves almost unchanged the stellar lifetime, which is mainly determined by the rate of the pp reaction. 0E+00 7400 7420 7440 7460 7480 7500 7520 7540 7560 7580 Eγ[keV] 15 At solar energies the cross section of N(p,γ) O is dominated by a subthreshold resonance at -504 keV and at energies higher than 100 keV by the resonance at ER = 278 keV with transitions to the excited states at energies of 5.18 MeV, 6.18 MeV and 6.79 MeV and the groundstate in 15 O. According to Schröder et al. [1] the main contribution to the total S factor at zero energy comes from transitions to the ground state in 15 O and to the subthreshold state at Ex = 6.79 MeV. Recently a reanalysis of the experimental data gave a different picture. The main difference concerns the S(0) factor for capture to the 15 O ground state: a factor of 19 lower S(0) factor than the value Schröder et al. suggested, reducing the total rate by about 50 %. In summary, new measurements of the 14 N(p,γ)15 O cross section at energies E ≤ 200 keV are necessary, in particular measurements of the transition to the ground state in 15 O. The peculiarities of the 400 kV LUNA facility are particularly well suited for this study, where reaction γ-ray lines up to ' 7.5 MeV have to be measured with very low intensities. High beam intensities and high detection resolutions have to be coupled to high target stability and purity, which leads to low beam-induced background; cosmic background is strongly suppressed by the mountain shielding and low intrinsic activity detectors are employed. The excitation function of the 14 N(p, γ)15 O reaction (Q = 7.297 MeV) to four final states in 15 O was measured in the region of the resonance at Ep = 278 keV. From a constrained fit to the observed γ-ray positions in the γspectrum, using as free parameters the 15 O energy levels, i.e. the energy of the secondary peaks of the spectrum, and the Q-value, we obtained: E1 = 5180.3 ± 0.3 keV; E2 = 6172.0 ± 0.3 keV; E3 = 6791.6 ± 0.3 keV and Q = 7297.2 ± 0.3 keV. To determine the precise value of the resonance energy and width, we fitted the measured yield using the machine calibration to obtain the beam energy at each point. The values obtained for the resonance parametrs are: ER cm = 259.1 ± 0.3 keV and Γp = 1.07 ± 0.05 keV. Properly taking into account the summing effects due to the finite geometry the following values for the resonance strength and branching ra- Figure 1: The figure shows the γ-spectrum for the groundstate transition at Ecm = 270 keV. tios were found: ωγ = 13.5 ± 0.5 meV, b0 = 1.6 ± 0.1 %, b1 = 16.8±0.2 %, b2 = 58.2±0.3 % and b3 = 23.4±0.3 %. These values were used in order to obtain the absolute normalization of the cross section at the beam energies below and above the resonance. The yield of the 14 N(p,γ)15 O reaction has been measured using deposited targets of thickness larger than 60 keV. In particular we have covered the energy range from Ep = 147 to 400 keV. In order to determine the absolute cross section from the observed γ-ray spectra, we studied in detail the expected line shape. This shape is determined by the cross section behaviour in the proton energy interval spanned by the incident beam during the slowing-down process in the target. The energy loss of the protons in the thick target gives rise to a drop in the yield at the low energy tail of the capture line. Since the above result depends on the stochiometry of the target, to obtain the absolute value of the cross section it is necessary to normalize the yield to the corresponding infinitive resonance yield measured with the same target. An example of the fit is given in figure 1, where the γ-spectrum for the ground-state transition at Ecm = 270 keV is plotted: the thin solid line is the background, the bold and the thin dotted lines represent the non resonant and the resonant part of the cross section, respectively, and the bold solid line is the sum of these functions. It is worth to note that we obtain such a fit using as free parameters only the non resonant astrophysical S factor and the background parameters. The final analysis of the measured excitation curve is in progress. Financial support from GSI F & E Projekt (Bo-Rol) is gratefully acknowledged. References [1] Schröder et al, Nucl. Phys. A 467(1987)240. - 23 - Astrophysical calculations for explosive nucleosynthesis B. Pfeiffer1 and K.-L. Kratz1 1 Institut für Kernchemie, Universität Mainz, Mainz, Germany In general, only elemental abundances can be measured in ultra-metal-poor, neutron-capture-rich halo stars by optical spectroscopy to be compared with the values in the solar system. In case of strong hyperfine splitting in absorption lines, from high-resolution, high signal-to-noise spectra also isotopic abundances can be determined. Very recently, Sneden et al. [1] have measured the isotopic abundances of the rare-earth element Eu in three metal-poor giant stars. Lambert et al. have redetermined the isotopic abundances of Ba in the halo star HD140283, as contradictory results were reported in the past (see [2, 3] and references therein). In the case of Ba, the odd isotopes 135,137 Ba (which have a pronounced hyperfine splitting) are produced mainly in r-process nucleosynthesis, whereas the s-process mainly contributes to the even isotopes. The s/r-isotope mixture is therefore reflected in the absorption line widths. As described in detail in [3, 4], we have calculated the rprocess yields of the 3 nuclides 135,137,138 Ba, which lie just beyond the 2nd r-peak. Fig. 1 shows our model predictions r of the abundance ratio fodd as a function of partial sums of 16 nn -components between nn =1020 [cm−3 ] and 1030 [cm−3 ]. Similar to Eu [4], the solar-system r-process ratio can only be reproduced for nn ≥1022 cm−3 . However, given the large uncertainty of the observation, which overlaps with both r- and s-process values, the quantity fodd alone cannot give an unambiguous answer about the s/r-mixture in HD140283. This is shown in Fig. 2, where the observed [Ba/Eu] value is compared to the solar-system ratio for pure s- and rprocess and to our r-process calculations [Ba/Eu]r,calc as a function of nn -ranges. In these calculations, the solar r-process value can only be obtained for nn ≥1024 [cm−3 ]. These are the conditions under which the “main” r-process is just forming the full A'130 peak and the matter flow starts to overcome this bottle-neck. For nn <1022 [cm−3 ] the predicted r-process [Ba/Eu] ratio may “mimic” different s/r-mixtures, even up to the pure solar-system sprocess value. 75.3 1.1 0.9 10 20 10 22 10 24 10 26 10 28 10 30 nn-ranges 1020 T nn T X [cm-3] Figure 2: Calculated r-process abundance ratios [Ba/Eu]=(Baobs/Ba )/(Euobs /Eu ), compared to the solar s- and r-values, as well as to the observed ratio in HD140283 (Ref.[2]). 0.67 0.41 0.13 0.09 20 10 22 10 10 24 10 26 28 10 30 10 nn-ranges 20 -3 10 T nn T X [cm ] Figure 1: Comparison of calculated r-process ratios fodd = (135 Ba+137 Ba)/(135 Ba+137 Ba+138 Ba) with solar s- and r-values and the observation in HD140283 (Ref.[2]). Therefore, a second observational quantity, in this case the elemental abundance ratio of Ba to Eu normalized to the respective solar values, i.e. [Ba/Eu], has to be considered. And indeed, the value observed in HD140283 [2] clearly excludes an s, -ratio and indicates that an rprocess has been the main contributor to this old star. Therefore, it is extremely important to perform more high-resolution optical spectroscopy of these elements in halo stars. A systematic study of the metallicity dependence of element or isotope abundance ratios as shown above can either determine the onset of a secondary “weak” r-process of so far unknown astrophysical site, and/or of s-process nucleosynthesis in medium-mass AGB stars which evolve slower than the high-mass progenitors of supernovae, the most probable site of the “main” rprocess. References [1] C. Sneden et al., ApJ 526, L25 (2002) [2] D.L. Lambert and C. Allende Prieto, MNRAS 335, 325 (2002) [3] B. Pfeiffer, K.-L. Kratz, Ann. Rep. 2001, Inst. f. Kernchemie, Mainz, A18 [4] K.-L. Kratz and B. Pfeiffer, Proc. CGS11, in print - 24 - Actinides and the source of cosmic rays B. Pfeiffer1 , R.E. Lingenfelter2 , J.C. Higdon3 and K.-L. Kratz1 1 Institut für Kernchemie, Universität Mainz, Mainz, Germany; 2 CASS, Univ. of California San Diego, La Jolla, USA; 3 W.M. Keck Science Center, Claremont Colleges, Claremont, USA Supernova shocks provide the energy for the acceleration of Galactic cosmic rays (ranging up to the actinides), but the source material as well as the acceleration mechanism are open questions. The similarity between ultraheavy cosmic rays (UHCR) and the interstellar medium (ISM) suggested that they may be accelerated out of the well-mixed ISM. But, since most of the heavy elements are ejected into the ISM by supernovae (SN) (which are clustered in space and time), the relative abundance ratios will not differ between these ejecta and the well-mixed ISM. However, the UHCR abundances of the actinide elements, Th, U, Pu and Cm, can provide critical constraints on the major sites of their acceleration and metallicity, as well as on the time scales involved [1]. The most probable source for the UHCR are the about 20 SN explosions which took place over the last 10 MYrs in the close-lying ScorpiusCentaurus OB associations. The shock waves of the first SNs blew an enormous bubble in the ISM, the local superbubble, facilitating the propagation of the ejecta from the later SN [2]. Figure 2: Expected cosmic-ray actinide abundance ratios as a function of superbubble core metallicity. Also indicated are the estimated U/Th and Actinide/Pt-Group (Z = 75 to 79) ratios in the current interstellar medium (ISM) and in the supernova-enriched proto-Solar material. with interstellar clouds are predicted (see, Fig. 2). The current measurements of the actinide/Pt-group ratios [6] and preliminary estimates of the UPuCm/Th ratio in cosmic rays [7] are consistent with the predictions if superbubble cores have metallicities of 3 times solar. Future measurements of the abundance ratios with improved statistics will help to solve these questions. First results of experiments performed on the MIR space station (ECCO [8]) and with ultra-long duration balloon flights (TIGER [9], which serves as an engineering model for the future Heavy Nuclei Explorer mission) are promising. In addition to meteoritic material and optic spectroscopy of stars, a new window to extra-solar matter is opened. Figure 1: Mean actinide abundance ratios from r-process yields in core-collapse supernova ejecta averaged over various time intervals. The typical cosmic-ray acceleration time span in the supernova-active cores of superbubbles of roughly 50 Myr is indicated by the dashed line (SB). The Mainz group has calculated the r-process yields in core-collapse SN within the “waiting-point approximation” [3, 4, 5]. From these yields the actinide abundances in the cosmic rays averaged over time following the SN explosions are predicted (see, Fig. 1). Using standard Galactic chemical evolution methods, the expected actinide abundances in the present day ISM and in the SN-active cores of superbubbles as a function of their ages and mean metallicity resulting from dilution References [1] [2] [3] [4] [5] [6] [7] [8] [9] R.E. Lingenfelter et al., submitted to Ap. J. N. Benitez et al., Phys. Rev. Lett. 88 (2002) 1101 K.-L. Kratz et al., Ap.J. 403 (1993) 216 B. Pfeiffer et al., Z. Phys. A357 (1997) 235 B. Pfeiffer et al., Nucl.Phys. A693 (2001) 282 A.J. Westphal et al., Nature 396 (1998) 50 J. Donnelly et al., 27th ICRC, Hamburg, 2001 A.J. Westphal et al., Adv. Space Res. 27 (2001) 797 J.T. Link et al., 27th ICRC, Hamburg, 2001
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