Isoscalar Spin Response in the Continuum Studied
via the 12Cd d Reaction at 270 MeV
¼
Y. Satou , S. Ishida† , H. Kato , H. Sakai , H. Okamura‡ , N. Sakamoto† ,
T. Uesaka‡ , A. Tamii , T. Wakasa§, T. Ohnishi† , K. Sekiguchi† , K. Yako ,
K. Suda‡ , M. Hatano , Y. Maeda and T. Ichihara†
†
£
Center for Nuclear Study, University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama 351-0198, Japan
££
Department of Physics, University of Tokyo, Bunkyo, Tokyo 113-0033, Japan
‡
Department of Physics, Saitama University, Urawa, Saitama 338-8570, Japan
§
Research Center for Nuclear Physics (RCNP), Ibaraki, Osaka 567-0047, Japan
Abstract. Single and double spin-flip probabilities in inelastic deuteron scattering on 12 C have been
measured at 270 MeV up to 50 MeV in excitation energy using a focal plane deuteron polarimeter
capable of measuring both vector and tensor components of the deuteron polarization. The obtained
S1 values are enhanced relative to the Fermi gas model prediction in highly excited continuum
region. The S 2 values are close to zero over the measured excitation energy range.
INTRODUCTION
Studies of polarization phenomena in nucleon induced inelastic scattering at intermediate energies have been one of the active fields of research in nuclear physics. Not only
giving an insight into the reaction mechanisms, measurements of spin-flip probability,
Snn in particular, provided useful information on spin dependent modes of nuclear excitation. Clear signatures of such specific excitations as the giant Gamow-Teller (GT)
resonance and the spin-flip dipole resonance were obtained, and the relative spin response was extracted in highly excited continuum region [1].
The study of spin-flip processes in inelastic deuteron scattering is an important extension to corresponding studies using the nucleon projectile. A new aspect is the selective
excitation of isoscalar transitions. This will make the reaction an ideal tool for the study
of isoscalar spin-flip modes. Much less is known about this mode due to the lack of
efficient probes as well as to the weakness of the effective interaction in this channel.
Information on the isoscalar spin response should be useful in elucidating “the quenching mechanisms of spin transitions” and “the problem of the enhancement of the relative
spin response in the continuum found in the p p reactions”.
Furthermore, spin-1 nature of the deuteron may offer a unique capability to probe
double spin-flip transitions, such as the proposed double GT state [2]. Up to now
no double GT transition has been identified except for the double beta decay, which
represents the ground state to ground state double GT transition exhausting only a minor
portion of the double GT sum rule [3]. Experimental determination of the double GT
strength distribution should lead to a better understanding of spin-isospin properties of
CP675, Spin 2002: 15th Int'l. Spin Physics Symposium and Workshop on Polarized Electron
Sources and Polarimeters, edited by Y. I. Makdisi, A. U. Luccio, and W. W. MacKay
© 2003 American Institute of Physics 0-7354-0136-5/03/$20.00
696
nuclei. It will also provide an excellent calibration of structure calculations of the double
beta decay nuclear matrix elements, which are necessary ingredients in extracting the
neutrino mass from the double beta decay life time measurements.
In the inelastic deuteron scattering two kinds of spin-flip probabilities S 1 and S2 can
be defined as fractions of deuterons undergoing spin-flip by 1 and 2 units along an axis
normal to the reaction plane. The single spin-flip probability S 1 is expected to be a good
signature of spin excitations, as S nn is in p p , and the double spin-flip probability
S2 a possible probe of double spin-flip excitations. They are given using polarization
observables in the following relations:
S1
S2
1
y¼ y¼
y¼ y¼
4 P
Ayy 2Kyy
9
1
y¼ y¼
y¼
y¼ y¼
4 2P
2Ayy 9Ky Kyy 18
(1)
(2)
where the quantities A, P and K refer to the analyzing power, polarizing power and
polarization transfer coefficient, one and two indices stand for the vector and tensor
polarizations, and lower and upper indices stand for the incident and outgoing beams.
Note that the determination of S1 and S2 requires vector and tensor polarized beams and
a vector and tensor polarimeter.
Recently we have succeeded in extracting S1 and S2 in inelastic deuteron scattering on
12 C at 270 MeV for an excitation energy range between 4 and 24 MeV [4]; the feasibility
of measuring the deuteron spin-flip probabilities over a wide excitation energy range
has been demonstrated. As an continuation to the previous work, we have extended
the measurement up to 50 MeV in excitation energy of 12 C in order to investigate
the isoscalar single and double spin-flip strengths in the continuum. The results of the
experiment are presented.
EXPERIMENT
The experiment was performed at RIKEN accelerator research facility using the 270
MeV polarized deuteron beams from the Ring Cyclotron. The measured quantities are
the differential cross sections and eight polarization observables with respect to the yaxis. The beam polarizations were measured using the beam line polarimeter based on
the d p elastic scattering at 270 MeV, and the obtained polarization magnitudes were
60 to 70% of the ideal values. The scattered deuterons were analyzed in momentum
with the magnetic spectrometer SMART, and their polarizations were determined with
a polarimeter DPOL placed at the focal plane of the spectrometer.
Figure 1 shows the detector arrangement of the polarimeter. It was comprised of three
parts: the multiwire drift chamber for track reconstruction, the secondary CH 2 target,
and the counter hodoscope to detect charged particles. Measurements of both vector
and tensor polarizations of outgoing deuterons are crucial in extracting S 1 and S2 . This
was realized by utilizing, as the analyzing reactions, the d C elastic scattering and the
change exchange d 2p reaction on hydrogen, which show large angular asymmetries
depending, respectively, on the vector and tensor components of the deuteron polariza-
697
Fe
%
FF8GEVQT
K6
d
CM
*
FR6GPUQT
666
%
FF
HOD
Ǿ
ǰ
MWDC SC
p
p
*
FR
d
1m
FIGURE 1. Layout of the polarimeter DPOL.
tion. Shown in the inset of Fig. 1 are missing-mass spectra for the d C elastic scattering
and the change exchange 1 Hd 2p reaction. We see clear peaks due to the desired reactions. In order to eliminate contributions from parasitic components, such as those
arising from the d p and 12 Cd 2p reactions in the CH2 scatterer, cuts were applied
on the missing-mass spectra in off-line analysis.
RESULTS
Figure 2 (a) shows an excitation energy spectrum of the differential cross section integrated over the laboratory scattering angles between 2.5Æ and 7.5Æ . Up to 30 MeV
in excitation energy several discrete levels are excited, which include the spin-flip 1 (12.71 MeV) and 2 (18.3 MeV) states and the non-spin-flip 0 (7.65 MeV) and 3
(9.64 MeV) states. All these are isoscalar states. Above 30 MeV in excitation energy the
spectrum exhibits a continuum structure.
Figure 2 (b) shows an excitation energy spectrum of the spin-flip probability S 1 . The
error bars are only statistical ones. We see that the S1 value in a bin at 20.5 MeV shows
a slight enhancement, similarly to the already established spin-flip states such as the 1 and 2 states. In a d d experiment performed at SATURNE, a spin-flip resonance
state has been reported at 20.5 MeV, which was tentatively assigned as J π 1 [5].
The present result shows consistency with its identification as an isoscalar spin-flip
transition. At higher excitation energy region S 1 takes values between 0.1 and 0.3. These
values are, in fact, much larger than the prediction of the noninteracting Fermi gas model
(dotted curve). Such a large deviation of the S1 values from the Fermi gas values in the
highly excited continuum region has not been identified in previous deuteron scattering
experiments. It would be explained either in terms of “the enhancement of the spin
698
ǭ/G8
5
5
FIGURE 2. Excitation energy spectra of (a) the yield, (b) the spin-flip probability S 1 and (c) the double
spin-flip probability S 2 . The dotted curve in (b) represents the Fermi gas model prediction.
response in the isoscalar channel” or “the quenching of the effective interaction in the
scalar isoscalar channel”. Further studies would need to be done to solve this problem.
The S2 values, shown in Fig. 2 (c), are close to zero over the measured excitation energy
range up to 50 MeV, and the presence of double spin-flip states could not be indicated
in the present experiment.
SUMMARY
We have measured single and double spin-flip probabilities S 1 and S2 in inelastic
deuteron scattering on 12 C at 270 MeV up to 50 MeV in excitation energy. An isoscalar
spin-flip nature of the state at 20.5 MeV in 12 C has been confirmed. The values of S1
much larger than those of the Fermi gas prediction were observed in the continuum region. The origin of such large S1 values has not been understood yet; it would need to
be addressed in forthcoming studies. The values of S 2 were close to zero; no indication
of double spin-flip states could be obtained from the present experiment.
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