Synthesis and Halflives of Heavy Nuclei relevant for the
rp-Process
P. Kienle, T. Faestermann, J. Friese, H.-J. Körner, M. Münch,
R. Schneider, A. Stolz, E. Wefers
Technische Universität München,
85748 Garching, Germany
H. Geissel, G. Münzenberg, C. Schlegel, K. Sümmerer, H. Weick
Gesellschaft für Schwerionenforschung mbH,
64921 Darmstadt, Germany
M. Hellström
Lund University,
22100 Lund, Sweden
P. Thirolf
Ludwig-Maximilians-Universität München
85748 Garching, Germany
November 20, 2000
Abstract
Neutron deficient nuclei up to 100 Sn were produced by fragmentation of a 1A·GeV 112 Sn beam
in a Be target. After isotopic separation by a fragment separator, the radionuclides were implanted
in a stack of position sensitive Si-detectors acting as a microcalorimeter. The halflives and other
decay properties were measured. Here we report halflives of rp-process waiting point nuclei between
80 Zr and 92,93 Pd. In addition the halflifes of the very short lived odd-odd N=Z nuclei 90 Rh, 94 Ag
and 98 In were determined and attributed to superallowed Fermi transitions. Two new isotopes
close to the p-dripline, 76 Y and 78 Zr, were identified for the first time.
1
Introduction
Medium heavy nuclei along the N=Z-line up to 100 Sn are interesting to study for several reasons. Firstly
the rapid proton capture rp-synthesis path is expected to lead along the N=Z line up to 100 Sn [1]. At
high temperatures such as those expected in neutron star x-ray bursters, nuclei between 64 Ge and 100 Sn
may be synthesized by (p, γ) capture reactions until the proton dripline is reached. For a continuation to
heavier masses, nuclei have to decay by the slower β + -decay to a neutron richer daughter nucleus, from
which the (p, γ) reactions can proceed to heavier nuclei. The longer lived β + emitters are called waiting
point nuclei. As all the mass during the rp-process is concentrated in these waiting points, their total
lifetime exclusively determines the flux towards heavier nuclei and the respective isotopic abundances.
The rp-process is not well known, but it is expected to be responsible for the high solar abundances of
nuclei such as 92,94 Mo and 96,98 Ru. From a nuclear structure point of view the waiting point nuclei are
1
also interesting because their decay proceeds mainly by Gamov-Teller transitions, which are quenched,
the mechanism of which is of great actual theoretical interest as pointed out by Arima in this lecture
series [2].
Further interest is attracted by the occurance of superallowed Fermi β-transitions in heavy N=Z
odd-nuclei. An exact determination of their halflives and decay energies would allow sensitive studies of
isospin impurities of the states involved. Precise ft-values for superallowed Fermi transitions are needed
for the determination of the Vud -matrix element of CKM-matrix.
2
Experiment
The experiment was performed at the fragment separator facility FRS of the GSI, Darmstadt. A beam
of 112 Sn was accelerated to an energy of 1 A·GeV in the heavy ion synchrotron SIS after cooling 20
stacks of ions injected from the UNILAC. After slow extraction of 5 × 108 112 Sn ions during 4 s spills
with a repetition rate of 1 in 14 s, the beam was fragmented in a 4 g/cm2 Be target. The fragments were
isotopically separated in the FRS by a combination of magnetic deflection and energy losses caused by
a 1g/cm2 Al degrader inserted after the first 30◦ deflecting magnet and a 5.5 g/cm2 Al degrader after
the second 30◦ magnet. Detector systems placed in front of the third and behind the forth 30◦ magnets
allowed the determination of the fragment trajectories using position sensitive ionisation chambers, the
time of flight between a start and stop plastic scintillator and the energy losses in two special ionisation
chambers placed at the entrance and exit of the third and fourth dipole magnets. With this set up a
mass resolution ∆A = 0.32 (FWHM) and a charge resolution ∆Z = 0.23 (FWHM) was reached, which
allowed a unique isotopic in flight identification of each ion.
3
Isotope Yields
Fig. 1 shows the measured fragment yields in absolute logarithmic scales for each element from Strontium to Indium as function of their corresponding isotopic numbers. The spectra show the previously
unobserved T = -1 nuclei 76 Y (2 events) and 78 Zr (one event) and demonstrate the absence of 81 Nb,
which is probably unstable against p-decay with a halflife shorter than the flight time of 200 ns through
the FRS.
10 3
10 2
10 1
10 0
103
Strontium
102
78
10
10 1
10 1
10 0
0
10
80
82
84
Ruthenium
10 0
76
2
78
80
Molybdenum
78
10 2
86
Rhodium
10
10 1
10 0
10 0
10 0
90
Silver
102
88
90
92
Cadmium
1
101
10
10 0
100
10 0
92
94
96
94
96
98
86
92
94
98
100
Indium
96
Figure 1: Mass spectra of the observed ions between Sr and In.
2
88
Palladium
90
1
10
Technetium
84
2
10 1
88
82
10 0
84
10 1
86
80
10 1
82
10 2
Zirconium
10 1
100
76
10 2 Niobium
10
10 2
101
74
2
Yttrium
4
The microcalorimeter as implantation detector
After identification of each ion, they were slowed down by a variable degrader placed in the focal plane
of the last magnetic section, such that the ion got stopped in the middle of a stack of position sensitive
Si detectors, which served also as microcalorimeter for the positrons and other charged particles emitted
by the implanted radioactivities. Fig. 2 shows a schematic top and side view of the Si stack detectors.
The implantation zone consists of 4 double sided Si strip detectors with an area of 64 × 25 mm 2 and
0.5 mm thickness each. The strip pitch is 0.5 mm. The dispersive direction is covered by 128 strips,
with single read out, whereas the 50 vertical strips are read out in 16 groups. The implantation zone
detectors are sandwiched between two stacks of 10 Si detectors each with areas of 60×40 mm 2 and 1 mm
thickness. These detectors are 7 fold segmented for effective Compton suppression. The implantation
detectors have a granularity of 8192 pixels, which allows a precise position determination of the stopped
nucleus and an accurate track reconstruction of the decay particles. The correlation of the implantation
point with the decay position leads to an efficient background suppression. Fig. 2 shows as an example
the tracking of the β + - delayed proton decay of a 85 Mo nucleus. The long track of the positron can
be followed with the top and side view of the microcalorimeter starting at the implantation point and
stopping in the second of the surrounding detectors on the right side. The delayed proton is seen as
a very short track with high energy deposition at the implantation point. The implantation depth in
the inner part of the calorimeter could be controlled with an accuracy of about 1mm (two detector
elements) using the variable degrader.
The microcalorimeter was surrounded by a segmented NaI-detector and a Ge-clover detector covering
about 80% of the solid angle. This allowed the first β-γ coincidence decay studies of implanted and
identified nuclei [3].
Figure 2: Schematic top and side view of the Si detector stack. An example of a β-delayed
proton decay event of 85 Mo is shown.
3
5
Halflife measurements
The very low trigger rate per pixel allowed a time correlation of the implantation of an identified nucleus
with its decay chains. Fig. 3 shows as an example the time correlated events of the decay of 77 Y to its
daughter 77 Sr and its decay to 77 Rb on a logarithmic time scale. Note that each first decay from 77 Y is
followed by a second decay from its daughter 77 Sr, exactly from the same pixel.
77
77
Y
77
Sr
(t1/ 2= 57 ms)
Rb
(t1/ 2= 9 s)
1
0
10
-1
0
10
10
decay time [s]
Figure 3: Correlated events of the decay of
on a logarithmic time scale.
77
1
Y to its daughter
10
77
2
Sr and its decay to
77
Rb
The event rates of the decays plotted on a logarithmic time scale, show bell shape distributions
with maxima at the mean life time as has been shown for the first time by H. Bartsch et al. [4] and
K.-H. Schmidt et al. [5]. We used a maximum likelihood method [6], taking into account the decay
of the mother, the daughter and even the granddaughter nuclei during a fixed correlation time. The
reliability of our method is shown in Fig. 4 for the decay of 78 Y into 78 Sr (2.65 min). We find two
halflives for the decay of 78 Y one with 55 ms and the other with 5.7 s in good agreement with recent
measurements 55(12)ms [7] and 5.8(6) s [8] respectively.
10
Figure 4: Decay of 78 Y. The short-living Fermi transition and a long living isomer is shown as
well as a curve drawn for their halflives resulting from our MLH-analysis. In the logarithmic
time scale the exponential decay is transformed into a bell shaped curve with the maximum
at the mean lifetime.
4
In table 1 we give a summary of all the results of halflife measurements of neutron deficient nuclei.
In the present context we are most interested in the results for the waiting point nuclei, 80 Zr, 84 Mo,
88,89
Ru and 92,93 Pd.
Table 1: Measured lifetimes for several isotopes from 75 Sr to 99 In.
Isotope Halflife
Isotope Halflife
Isotope Halflife
75
Sr
80+400
−40 ms
83
Mo
6+30
−3 ms
93
Rh
13.9 ± 1.6 s
Mo
92
Pd
1.0+0.3
−0.2 s
93
Pd
1.0 ± 0.2 s
76
Y
> 200 ns
84
77
Y
57+22
−12 ms
86
Tc
3.7+1.0
−0.8 s
59+8
−7 ms
78
Y
55+9
−6
87
Tc
2.2 ± 0.2 s
94
Ag
26+26
−9 ms
78
Yiso
5.7 ± 0.7 s
88
Ru
1.2+0.3
−0.2 s
94
Agiso
0.45 ± 0.2 s
78
Zr
> 200 ns
89
Ru
1.5 ± 0.2 s
98
In
32+32
−11 ms
79
Zr
80+400
−40 ms
90
Rh
12+9
−4 ms
98
Iniso
1.2+1.2
−0.4 s
80
Zr
5.3+1.1
−0.9 s
90
Rhiso
1.0+0.3
−0.2 s
99
In
3.0+0.8
−0.7 s
81
Nb
< 200 ns
91
Rh
1.7 ± 0.2 s
82
Nb
48+8
−6 ms
92
Rh
5.6 ± 0.5 s
ms
A comparison of our results with previous measurements and theoretical expectations [1] are shown
in Fig. 5 upper part. Whereas for 80 Zr our measurement agrees well with a previous result [9] and
a theoretical predictions, there is marked disagreement between our measurements and theoretical
prediction for the heavier nuclei. The discrepancy is such that the measured halflife is for all cases
longer than the predicted ones. This indicates lower Q-values or a stronger quenching for Gamov-Teller
transition than anticipated in the calculations. It leads also to a reduction of the synthesis flux along
the rp-process path.
exp. data
other
theory
Figure 5: Comparison of our results with previous measurements (other) and theoretical
expectations.
Finally we focus on the superallowed Fermi transitions, the halflives of which are plotted on the
lower part of fig. 5. Note that our values for the nuclei 78 Y, 82 Nb and 86 Tc are in good agreement with
recent results from GANIL [7]. We can show that the odd nuclei 90 Rh, 94 Ag and 98 In have low lying
states (presumably the groundstates) which decay by 0+ → 0+ superallowed Fermi transitions to their
even-even daughter nuclei.
5
6
Summary and outlook
We have developed a new method, which allows us to study decays of identified nuclei produced in
fragmentation reactions by stopping them in a 4π-microcalorimeter. First results on halflives of (rp)process waiting point nuclei are given and compared with theoretical expectations, which are in most
cases shorter than the measured values. In the future we will focus also on their decay Q-value measurements. Preliminary results can be obtained from β + -decay energy measurements. More precise results
are expected from Schottky noise mass spectroscopy in the ESR storage ring of GSI.
In addition we have measured halflives of short lived super allowed Fermi transitions of heavy oddodd nuclei. More precise data are needed. Experiments with high intensity 124 Xe available now a GSI
could improve these data appreciably. Also Gamov-Teller transitions could be measured with improved
precision. Of special interest would be the measurement of the Gamov-Teller decay of 100 Sn including
β-γ-coincidence spectroscopy.
References
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
H. Schatz, Phys. Rep. 294 (1998) 167
Arima, this lecture series
E. Wefers et al., GSI Scientific Report 1998, 173
H. Bartsch et al., Nucl. Instr. and Meth. 121 (1974) 185
K.-H. Schmidt et al., Z. Phys. A 316 (1994) 19
R. Schneider, PhD thesis, TU München (1996)
C. Longour et al., Phys. Rev. Lett. 81 (1998) 3337
J. Uusitalo et al., Phys. Rev. C 57 (1998) 2259
J.J. Ressler et al., Phys. Rev. Lett. 84 (2000) 2104
6