This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Bacher, A.D.

Baghaei, H.

Betker, A.C.

Carr, J.A.

Chang, S.

Franklin, W.A.

Lindgren, R.A.

Liu, J.

Petrovich, F.

Schmitt, W.M.

Stephenson, E.J.

Wissink, S.W.

Yu, C.

English

IUScholarWorks

MEDIUM EFFECTS IN THE NORMAL COMPONENT POLARIZATION 
OBSERVABLES FOR THE 13C(p',p")13C, 112- -+ 9/2+ REACTION AT 200 MeV 
W.M. Schmitt, A.D. Bacher, A.C. Betker, S. Chang, W.A. Franklin, J. Liu, 
E. J. Stephenson, S.W. Wissink and C. Yu 
Indiana Universi ty  Cyclotron Facility, Bloomington,  Indiana 4 74 0 8  
H. Baghaei and R.A. Lindgren 
Universi ty  of  Virginia, Charlottesville,  Virginia 22901 
J.A. Carr and F. Petrovich 
Florida S ta te  Universi ty ,  Tallahassee, Florida 32206 
Free nucleon-nucleon scattering can be described by the following t 
where the five individual amplitudes represent the central spin-independent, central spin- 
dependent, spin-orbit, tensor-direct and tensor-exchange parts of the interaction. Each of 
the amplitudes are also functions of the scattering kinematics as well as the total two-body 
isospin. However, when nucleons interact inside the nuclear medium, this fundamental 
interaction may be modified due to effects of the strong nuclear field environment. This 
would lead to a nucleon-nucleon interaction that would depend on the nuclear density. 
Indeed, it has already been proposed, based on QCD sum rule techniques, that the short 
ranged repulsive core becomes stronger with increasing density.3 Similar results also arise 
in other theoretical  treatment^.^)^ Therefore, detailed knowledge of this interaction as a 
function of the nuclear density is important for our understanding of the properties of 
nuclear matter . 
One way to probe such modifications experimentally is through (6, $) measurements 
from nuclear targets. Within an impulse approximation f r a m e ~ o r k , ~  the three main ingre- 
dients for a calculation of a nucleon-nucleus scattering observable are: 1) the structure of 
the transition under study, 2) distortions, and 3) the effective nucleon-nucleon interaction. 
By using (e,et ), (T, T I ) ,  and proton elastic scattering  measurement^,^-'^ it is possible to 
constrain the transition formfactor and the optical model distortions in a way independent 
of the (6, fl) data. With the uncertainties in structure and distortions at a minimum, de- 
tailed knowledge about the in-medium nucleon-nucleon interaction can be obtained. One 
effect known to modify this interaction is Pauli blocking, which inhibits scattering to states 
below the Fermi momentum. The purpose this experiment, E379, was to obtain informa- 
tion about the isoscalar component of the effective interaction. This information will then 
be included in a global analysis with other IUCF data1'J2 to define the effective interaction 
at 200 MeV. 
The collaboration measured the normal component spin observables, A,, P and Dnnt, 
and cross section, da/dQ, for the reaction l3 C(6, p")13 C to the "stretched" 9/2+ state at 
Eexc = 9.50 MeV in the range 8,, = (20" - 80") at 200 MeV bombarding energy. Cross 
sections and analyzing powers were measured in 3' steps while P and Dnnt were observed 
resolution = 10 chan 
I)" dispersion = 4 keV/chan 1 
3000 4000 5000 6000 
XI [channels]  
Figure 1. An online spectrum of scattered protons from 13C for a proton bombarding en- 
ergy of 200 MeV and a spectrometer angle of 36". The state of interest for this experiment 
is labeled as 13C(9.50) and is shown cleanly separated from the neighboring states with a 
resolution of the order of 40 keV (FWHM) (with a 6 mg/cm2 target thickness). 
only in steps of 9". This measurement was performed on the K600 magnetic spectrometer 
at IUCF and utilized the existing medium-dispersion port focal plane detection system to 
detect the scattered protons and the focal plane polarimeter to determine their polarization. 
The targets were fabricated at IUCF by Bill Lozowski of enriched carbon with isotopic 
purities better than 99%, and ranged in thickness from 3.6 mg/cm2 to 16 mg/cm2. With 
the K600 dispersion-matching system, resolutions ranged from 40 keV (FWHM) at best 
to 80 keV (FWHM) for the thickest targets at large angles. Data were also obtained for 
elastic scattering l3 C(5, 5)13 C, and the analyzing powers extracted online agreed well with 
a previous measurement by H.O. Meyer e t  ~ 1 . ~ '  at IUCF. 
Shown in Fig. 1 is an online spectrum of scattered protons from a 13C target of 
thickness E 6 mg/cm2measured at a spectrometer angle of 36". The elastic peaks from 
13C and 12C are identified, as well as the 13C peak of interest at Eexc = 9.50 MeV with 
JT = 9/2+. The resolution obtained for the state was of the order of 40 keV (FWHM) and 
its peak was cleanly separated from the peaks of the two nearby states, the l3 C(8.86) 112- 
and 13C(9.90) 312-. For all other angles the 9.50 MeV state was cleanly isolated except 
for spectrometer angles near 72") where the scattering peaks from the 12C(7.65) state and 
13C(9.50) state cross through each other. 
Plotted in Fig. 2 as the open circles are the online differential cross sections for the 
reaction 13C(p,p')13C, 112- + 9/2+ at Eexc = 9.50 MeV. The normalizations for these 
measurements were checked by comparing the 13C elastic measurements with published 
data.'' For all but the two largest angle points, the differences were within expected nor- 
malization uncertainties of 20%. The origin of the large normalization error is not known, 
but could have arisen if part of the beam spot, which was wide to facilitate dispersion 
13 C(p,p') 9.50 MeV 
L ' ~ r ' ' ~ ~ . . r r ~ ~ 8 v ' [ ' ' ' -  
0 E379 online - 
2 0 40 6 0 80 
8,, [degrees] 
Figure 2. Online results for the 112- + 9/2+ differential cross section are plotted as open 
circles. The two points indicated by crosses were corrected for an apparent normalization 
problem by comparing the l3 C elastic scattering measurements with published data.'' 
matching, missed the target. Using the l3  C elastic measurements to normalize these points 
yielded the results indicated by crosses. 
Shown in Fig. 3 are the online results for the normal component spin observables for 
this transition. The top panel plots the analyzing power Ay , the middle panel the induced 
polarization P, and the bottom panel the normal component spin transfer coefficient Dnn/ 
as functions of the center of mass scattering angle O,, . Plotted along with the data are 
curves from a DWBA calculation using structure and distortion effects from published 
(e,el) and (p,pl) data for this t r a n ~ i t i o n . ~ ? ~ ~ ' '  The solid curve is the calculation using the 
free nucleon-nucleon t-matrix derived from the SM93 phase shift solution of Arndt.13 This 
result only qualitatively describes the features present in the data. The dashed curve 
includes a modification of the free interaction due to Pauli blocking in the medium.14 It, 
too, is not able to describe the data quantitatively. We expect these measurements over a 
broad range of momentum transfer will give us a clearer picture of the effective interaction 
at 200 MeV. A full analysis of these data is underway at IUCF. 
1. W.G. Love and M.A. Franey, Phys. Rev. C 24, 1073 (1981). 
2. M.A. Franey and W.G. Love, Phys. Rev. C 31, 488 (1985). 
3. T. Hatsuda and S. Huong Lee, Phys. Rev. C 46, R34 (1992). 
4. G.E. Brown and M. Rho, Phys. Lett. B237, 3 (1990). 
5. G.E. Brown and M. Rho, Phys. Rev. Lett. 66, 2720 (1991). 
6. A.K. Kerman, H. McManus and R.M. Thaler, Ann. Phys. 8, 551 (1959). 
7. R.S. Hicks, et al., Phys. Rev. C 34, 1161 (1986). 
8. R.S. Hicks, et al., Phys. Rev. C 36, 485 (1987). 
9. S.J. Seestroom-Morris, e t  al., Phys. Rev. C 26, 594 (1982). 
Figure 3. Online results for the normal com- 
ponent polarization observables, Ay , P and 
D,,I. The solid curve is a DWBA calcula- 
tion using the free nucleon-nucleon t matrix, 
t f ree ,  for the interaction. The dashed curve 
includes the modification to the free interac- 
t ion due to Pauli blocking. 
13 C(5,p")  9.50 M e V  
v
0 E379 online . 
0 
0 
I . . . . I . . . . I . . . . t . . ~  
I ~ ' ' ' I ' " ' I ' ~ ~ ' I ' ~  
20 40 60 80 
0,, [degrees] 
10. H.O. Meyer, e t  al., Phys. Rev. C 23, 616 (1981). 
11. H. Baghaei, e t  al., Phys. Rev. Lett. 69, 2054 (1992). 
12. S. Chang, e t  al., IUCF Sci. and Tech. Rep., May 1992 - April 1993, p. 16. 
13. R. Arndt , private communication. 
14. K. Nakayama and W.G. Love, Phys. Rev. C 38, 51 (1988). 


Medium Effects in the Normal Component Polarization Observables for the 13-C(p,p')13-C, (1/2)- → (9/2)+ Reaction at 200 MeV

https://scholarworks.iu.edu/dspace/bitstream/2022/500/1/iucf1993-1994_23-26.pdf

Medium Effects in the Normal Component Polarization Observables for the 13-C(p,p')13-C, (1/2)- → (9/2)+ Reaction at 200 MeV

Abstract

Similar works

Full text

Available Versions

IUScholarWorks