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THE (p,p’) SPIN TRANSFER PROGRAM AT THE INDIANA UNIVERSITY CYCLOTRON FACILITY
A. Opper, S. Wissink, A. Bacher, J. Lisantti, C. Olmer, R. Sawafta, E.
Stephenson, S. Wells
To cite this version:
A. Opper, S. Wissink, A. Bacher, J. Lisantti, C. Olmer, et al.. THE (p,p’) SPIN TRANSFER PRO-
GRAM AT THE INDIANA UNIVERSITY CYCLOTRON FACILITY. Journal de Physique Collo-
ques, 1990, 51 (C6), pp.C6-607-C6-610. �10.1051/jphyscol:1990684�. �jpa-00231099�
COLLOQUE DE PHYSIQUE
Colloque C6, supplbment au n022, Tome 51, 1 5 novembre 1990
THE
(g, G '
) SPIN TRANSFER PROGRAM AT THE INDIANA UNIVERSITY CYCLOTRON FACILITYA.K. OPPER, S.W. WISSINK, A.D. BACHER, J. LISANTTI, C. OLMER, R. SAWAFTA, E.J. STEPHENSON and S.P. WELLS
Indiana University Cyclotron Facility, Bloomington, Indiana 47405, U.S.A.
&sum& - En commissionant le polarimktre de plan du foyer du spectrom6tre de haute rksolution K600 B IUCF, on continue les dtudes observables de transfert de spin pour les transitions discrktes.
Ce programme est consacrk aux transitions qui participent comme filtres B certains termes de 1'a.ction due au nucldon-noyau, ou facteurs de forme nuclhaire de structure. Les buts et les trois expkriences - . du programme seront discutks.
Abstract - With the commissioning of a polarimeter in the focal plane of the high resolution K600 spectrometer at IUCF, we are continuing our study of spin transfer obserwbles for discrete tran- sitions. This program focuses on transitions which act as filters for certain terms of the effective nucleon-nucleus interaction and/or nuclear structure form factors. The experimental program and three specific experiments will be discussed.
l - INTRODUCTION
The determination of polarization transfer observables for (p', p'!) reactions at intermediate energies provides a rich source of information on both the effective nucleon-nucleus interaction and the nuclear structure. At intermediate energies, the impulse approximation provides a valid framework to investigate the relationship between the nudeon-nucleon and nucleon-nudeus interactions. Between 200 and 300 MeV, the experimental sensitivity to the spin-orbit and tensor terms of the effective interaction is enhanced relative to the central term. The
( 6 5 1 )
polarization transfer program at IUCF further enhances this sensitivity by focusing on unnatural-parity transitions.With our beamline polarimeters and a high count rate polarimeter in the focal plane of the high resoiution K600 spectrometer, we are now able to measure all three components of the incident polarization, as re11 as the normal and inplane polarizations of protons exciting discrete transitions. Thus, we are able to measure cross sections, analyzing powers, induced polarizations, and all five polarization transfer coefficients ( U , .41, P and Dip's respectively). We have utilized this system to carry out several detailed investigations of particular discrete states which act as filters to enhance or exploit specific terms of the effective nucleon- nucleus interaction and/or nuclear structure form factors.
2 - D N N , AND P IN INELASTIC PROTON SCATTERING FROM ''C AND 160 AT 200 MeV We have measured the normal--component spin observables DNN*, P, and A, at 200 MeV for the I+, T=O
(12.71 MeV) and T = l (15.11 MeV) states in 12C and for the 4-, T=O (17.79 and 19.80 MeV) and
T=l
(18.98 MeV) states in '$0. Measurements were made at five angles for the I2C transitions, at momentum transfers between 80 and 250 MeV/c, and at three angles for I6O, at momentum transfers of 225 to 400 MeV/c. The angles were chosen to match those at which the in-plane spin transfer observables had been measured / l / ; thus, we now have complete sets of polarization transfer observables for these five transitions. With complete sets, we can form particular linear combinations which, in a direct plane wave impulse approximation, are simple products of isolated interaction amplitudes and nudear form factors. The transitions chosen for this work have fairly well determined nuclear structure and unnatural-parity which maximizes the predicted sensitivity to the spin-dependent terms of the efTective interaction.The statistical quality and angular extent of our measurements is illustrated in Figs. 1 and 2, in which preliminary values for D1 and Dz, linear combinations of observables that are related to the tensor amplitude of the interaction, are shown for the isovector 4- transition in '$0 and (P-Ay) and DNN, are shown for the strong isoscalar l+ state in 12C. The data shown are preliminary and the error bars reflect only statistical uncertainties. It is useful to note that in many cases the discrepancies between the data and the calculations
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990684
C6-608 COLLOQUE DE PHYSIQUE
momentum transfer (MeV/c)
Fig. l
-
Preliminary results for D1 and D2 for0 ZOO 400 600
momentum transfer (MeV/c)
Fig. 2
-
Preliminary results for (P-
AV)the T=l, 4- state at 18.98 MeV in lbO..
-1.0
0 100 200 300
momentum transfer (YeV/c) for the T=O, l+ state at 12.71 MeV in ' * C . are greater than the variations among the different calculations. The effective interactions shown are the free Franey-Love (solid line) and Bonn (dotted line) interactions, the Bonn interaction in which nuclear medium effects have been incorporated using a local density-dependent prescription (dashed line) and a relativistic DREX (dashed-dot). For the 12C l+ state, Cohen-Kurath wave functions have been used, while for the 4- states in "0, the wave functions are taken to be d 5 , 2 P ~ ; 2 stretched codgurations which are predominantly T=O, but contain significant T = l admixtures of opposite sign, as suggested by earlier electron and pion scattering measurements./2/
While more detailed analysis needs to be performed, the statistical quality of the data is readily apparent, as are the general trends of the angular distributions. The data shown for the isovector 4- state in 160
show a correlated discrepancy in that all of the calculations for DI (D2) are above (below) the data. The DNNt measurements for the isoscalar 4- states in "0 /3/ are also noteworthy, in that the data for the two transitions are almost identical, while all calculations predict values that differ significantly between the two states. Preliminary investigation indicates that slight changes to the isoscalar tensor interaction may reduce these discrepancies.
3
-
0+ --+ 0- EXCITATIONS IN "'0 USING 200 MeV INELASTIC PROTON SCATTERINGInelastic proton scattering from a O+ ground state to a 0- excited state is one of the more selective transi- tions, in that the constraint of zero angular momentum in both the initial and h a l nuclear states leads to simplifications in the nuclear structure
(CT
vanishes) and in the scattering matrix. Simple counting argu- ments reveal that there are only three independent obsembles. Therefore, measurements of the cross section, analydng power, and the in-plane spin rotation parameter completely describe the transition amplitude to within an overall phase factor, and should place severe limits on the acceptability of different theoretical models. Study of these transitions provides information on the poorly understood tensor component of the effective interaction since the spin-orbit component vanishes and the central term is weak. Morevoer, these observables are expected to provide information specifically on the exchange terms, since it is this part of the interaction that yields non-zero values for some of the observables./4/We have performed high resolution studies of the 0- states in "0 at 10.957 MeV (T=O) and 12.797 MeV (T=l). Cross section and analydng power distributions for these two states were measured at 200 MeV for c.m. angles between 7.0' and 45.0' in 4' steps, giving the highest quality data available on any 0+ 4 0- transition. Figure 3 shows the cross section and analyzing power for the isoscalar transition along with a relativistic DREX calculation (dashed) and a Franey-Love DW8l calculation (solid). The DREX (DW81) cross section has an overall normalization factor of 1.2 (0.6). Both calculations explicitly include knock- on exchange and distortions. It was initially assumed that the transition could be described by a pure
llp;j2, 2sl12) single particle transition. However, with that representation of the nuclear structure, neither calculation could adequately reproduce the cross section or the analyzing power of the isoscalar state. It has been suggested /5/ from the beta decay rate of the 0- state in 16N at 0.1201 MeV that the isovector transition may have a Ilp$,1d,,,) component with an amplitude of a = 0.125. The use of this or a similar admixture wavefunction for both the isoscalar and isovector is currently being investigated.
Fig. 3
-
Cross section and analyzing power for the T=O, 0- 10.96 MeV state in I'O.4
-
SPIN OBSEWABLES FOR HIGH SPIN STATES IN 208Pb AT 200 MeVThe doubly-magic nucleus 208Pb has long played an important role in linking experimental results with the theoretical descriptions of both nuclear structure and nuclear reactions. This experiment focused on a comparitive study of stretched states, which for this doubly-closed-shell nucleus offers the unique opportunity
C6-6 10 COLLOQUE DE PHYSIQUE
to observe both proton and neutron particlehole excitations that involve different shell-model orbitals. The nuclear structure of true stretched states is greatly simplified in that they have a maximum-J configuration and thus a unique particle-hole description. Recent high resolution measurements with (p, p!) at IUCF and (e,et) at Bates have examined previously identified, high-spin states in '08Pb at 6.43,6.74 and 7.05 MeV with
the configurations ( ~ j ~ ~ / ~ , v i ~ ~ ~ ~ - ~ ) 12- (neutron), ( ~ j ~ ~ / ~ , ~ i ~ ~ / ~ - ~ ) 14- (neutron) and ( ~ i ~ ~ ~ ~ , a h ~ ~ ~ ~ - ~ ) 12- (proton), respectively. This work was undertaken to investigate the sensitivity of the spin observables
to the neutron and/or proton character of these transitions.
MOMENTUM TRANSFER (MeV/c) MOMENTUM TRANSFER (MsV/c)
Fig. 4
-
Calculations of D N N , for 12- pure neutron and pure proton transitions (left) and free n-p and p-p scattering (right).At IUCF, we have measured a, Ay, DNNt and P for these transitions at 200 MeV for momentum transfers between 1.5 and 3.0 fm-l with an overall resolution of 25 kev. Our analyzing power measurements indicate a sensitivity of Ay to the neutron and/or proton character of these nuclear transitions that is similar to DWIA predictions. Figure 4 shows calculations that predict a considerably more pronounced sensitivity of DNNt to the isospin character of the excitations. Since the predictions of D N N , for these excitations strongly resemble those for free nucleon-nucleon scattering, measurements of D N N , should be a sensitive test of these questions of nuclear structure. Analysis of these data is underway.
5 - CONCLUSIONS
The development of a polarimeter for the the high resolution K600 focal plane provides IUCF with the unique opportunity to measure polarization transfer observables for discrete states. Moreover, we are able to use this system at an energy which is especially sensitive to the spin-dependent terms of the effective interaction.
These studies are providing new insight to the poorly understood tensor and exchange interactions, as well as information on unresolved nuclear structure questions.
REFERENCES
/l/ Olmer, C., in Antinucleon- and Nucleon-Nucleu~ Interactions, ed. G.E. Walker, C.D. Goodman, and C.
Olmer (Plenum Press, New York, 1985) p. 261.
121 Carr, J.A., Petrovich, F., Halderson, D., Holtcamp,
D.B.
and Cottingame, W.B., Phys. Rev. C 22, (1983) 1636./3/ Stephenson, E.J., these proceedings.
/4/ Piekarewics, J., Phys. Rev. C
35,
(1987) 675./5/ van der Werf, S.Y., et al.,Phys. Rev. Lett.
