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

Byrd, R.C.

Goodman, C.D.

Huang, W.

Kiang, G.C.

Marchlenski, D.S.

Rapaport, J.

Rybarcyk, L.J.

Sugarbaker, E.R.

Taddeucci, T.N.

Wang, Y.

English

IUScholarWorks

CHARGE EXCHANGE 
POLARIZATION TRANSFER MEASUREMENT FOR I9F AND 39K(p,n) 
W. Huang, C.D. Goodman, G.C. Kiang, and Y. Wang 
Indiana University Cyclotron Facility, Bloomington, Indiana 4 7405 
R.C. Byrd, L.J. Rybarcyk, and T.N. Taddeucci 
Los Alamos National Laboratory, Lo3 Alamos, New Mexico 87545 
J. Rapaport 
Ohio University, Athens, Ohio 45701 
E.R. Sugarbaker and D. Marchlenski 
Ohio State Univer~ity, Columbus, Ohio 4321 0 
The absolute transition rate of a nuclear reaction depends on many parameters that 
are usua,lly imprecisely known. Even direct charge-exchange (p,n) reactions depend on 
the effective nucleon-nucleus interaction, the optical potential, and the nuclear structure 
overlap. Thus it is difficult to extract unambiguous information from cross section mea- 
surements alone. Polarization transfer (PT) studies, on the other hand, can provide certain 
information in a way that is insensitive to some parameters. For example, for charge ex- 
change (p,n) reactions at small ~nome~itum transfer, the transverse polarization transfer 
coefficient D,,, based on simple angular momentum arguments alone, is -113 for GT 
( Gamow- Teller) transitions and +I for F (Fermi) transitions. Departures from these sim- 
ple values indicate that other angular momentum couplings axe involved. We have already 
verified that these simple expectations are realized for transitions that are expected to be 
pure F or pure GT (Ref. 1). 
One important application of D,, measurements is to GT strength studies. That GT 
strength can be inferred from (p,n) cross-section measurements was demonstrated a decade 
ago.2 A reliable G T  strength determination, however, depends on calibrating the particular 
spectrum against a known beta-decay transition. In most cases the I A S  (Isobaric Analog 
State) provides the only possible calibration point. But a problem arises if the target 
has non-zero spin, as for odd-mass nuclei. The I A S  peak can contain both GT and F 
components, and, even when the GT content is known, there is a remaining question of 
how accurately the G T I F  interaction strength ratio determined from spin zero targets can 
be applied to spin > O targets. 
An empirically discovered relationship for the ratio of G T I F  strengths3 has been 
extensively used to extract GT strengths relative to an I A S  peak in the spectrum. This 
relationship is based mostly on measurements on even-mass nuclei and may be described as 
R2 E &(GT)/&(F) and R = Ep/EO, where &(a) E a(a)/B(a) (for a = GT or F) and Ep 
is the proton beam energy with Eo a constant 255 MeV. A firm theoretical underpinning 
has not been found so far for this empirical relationship. 
A systematic study of the specific cross sections, 8(GT),  revealed that for some odd- 
mass nuclei 6(GT) appears to be larger than for their even isotopes, and differences of 
as much as 50% have been observed for neighboring A  value^.^ These "enhancements" 
are not understood in the context of DWIA, which predicts a smooth A-dependence for 
8 .  A recent measurement5 suggests tha,t some odd-A nuclei might have different G T I F  
ratio from the systematic value of even-nuclei. It is not clear, therefore, whak part of the 
apparent enhancement is "real" and what part may be a reflection of an inappropriate use 
of the "universal" G T / F  ratio. 
Direct light can be shed on these issues with the a measurement of Dnn(OO) because it 
provides an independent method to separate the GT component in the mixed I A S  peak. 
If the inteference terms from other non-central components can be ignored, the I A S  peak 
is simply an incoherent sum of both GT and F .  So the Dnn(IAS) is simply the weighted 
average of Dnn(GT) and Dnn(F).  Therefore the fraction of GT content in the IAS is 
given by Fgt 5 o(GT)/o(IAS) = 0.75 (1 - Dnn(IAS)), where we have used the fact that 
Dn,(GT) = -113 and D,,,(F) = $1. 
19F and 39K provide two excellent test cases. 39K is one of the few nuclei with 
a simple shell structure - a single d3/2 hole in the doubly magic *OCa core. A special 
structure symmetry was also de~nonstrated for 19F which has three d 5 p  particles in its 
outmost d shell.6 Both targets have non-zero spins: 3/2+ for 39K, and 5/2+ for lgF.  35% 
enhancement of the specific cross section above the general trend was observed for 39K in 
previous ~ t u d i e s , ~  while no appreciable deviation found for lgF. 
Measurements have been made at the IUCF Swinger Facility using a pulsed proton 
beam of averaged intensity of 50 nA and 75% polarization. Da.ta have been taken at both 
120 MeV and 160 MeV. The target used was a compound KF enriched to 99.96% for 39K. 
The kinetic energy of the projectile neutron was determined by TOF with a 75 m flight 
path. The neutron polarization was measured with a neutron polarimeter consisting of 
two detector planes separated by 1 meter and placed transversely at the end of the flight 
path. The polarirneter was calibrated with 14C(p',,n')14N, 0+ -+ O+ transition which has 
D,,(OO) = 1. Optinlized cuts that maximize the figure-of-merit (FOM), A: x (total counts 
accepted), yielded an effective analyzing power A, = 0.29 f 0.02. (The cuts that maximize 
FOM do not maximize A,.) The energy spectrum for 160 MeV is shown in Fig. 1, where 
the typical energy resolution is about 1 MeV. The corresponding Dnn(OO) spectrum with 
error bars is also displayed in the same figure. The I A S  peaks from the two nuclei, l gF  
and 39K, are separated by 3.3 MeV and cleanly resolved. There is no background problem 
in peak fitting. The region around 145 MeV is where the giant G T  resonance from both 
nuclei should appear. The Dnn in that region very close to -113, demonstrating that the 
transitions are indeed dominated by GT. 
The analyses for the I A S  peaks are listed in the tables. Table I compares the mea- 
surement with DWIA calculations for Dnn(OO). 19F agrees with calculation very well, but 
not 39K which is opposite in sign. Two methods of separating GT from the I A S  peak are 
cornpanred in Table 11, where Fgt is the GT  fraction calculated from the empirical relaption 
R = Ep/E,(= 55MeV), and Fjt is from Dnn measurement. Again, we see tha.t 19F is 
consistent for both methods, but for 39K the Dnn method yields a smaller GT content in 
the I A S  peak. It is interesting to parametrize the D,, results the same way as the ratio 
Figure I .  a) Energy spectrum for target K F  at 0 degrees a.t 160 MeV. The typical energy 
resolution is about 1 MeV. b) Dnn spectrum vs. kinetic energy of the neutrons. 
R. The derived E, values are listed in Table 111. The value 5 4 f  2 MeV for 19F is consistent 
with the systematic value 55 MeV. The Eo is only 43 f 2 MeV for 39K, but this result 
is consistent with recent measurement5 where the preliminary results showed that 51V, 
8 7 ~ b ,  '131n and 14'Pr have typically Eo - 44 f 2 MeV while l18Sn has Eo = 51 f 3 MeV. 
Table I. Comparison of D,,(OO) measurements with DWIA calculation at 160 MeV. 
Target Experiment D WIA l z 1  
Table 11. Comparison of fraction of GT in I A S  deduced from two methods. fgt is the 
result from the emperical relation R2 5 +(GT)/&(F) = (Ep/55.0(MeV))2, 
and fLt is the result from 0.75(1 - D,,(IAS)). 
Target E, B(GT) fgt  fLt f g t  Ifit 
19F 0 1.635zt0.016 0.936zt0.015 0.93k0.02 1.01k0.02 
39K 0 0.274 0.79d10.02 0.70d10.02 1.13A0.03 
Table 111. Parameter Eo yielded from the parametization of D,, into R = Ep/EO for 
Ep = 160 MeV. 
Target EO (MeV) 
Why odd-mass nuclei have different G T / F  ratio is not yet clear. An experiment aiming 
at understanding important tensor effects will be underway at IUCF in which non-zero 
degree D,, will be measured for 19F and 39K. 
1. T.N. Taddeucci, T.A. Carey, C. Gaarde, C.D. Goodman, J. Larsen, D.J. Horen, T. 
Masterson, J.  Rapaport, T.P. Welch and E. Sugarbaker, Phys. Rev. Lett. 52, 1960 
(1984). 
2. C.D. Goodman, C.A. Goulding, M.B. Greenfield, J.  Rapaport, D.E. Bainum, C.C. 
Foster, W.G. Love and F. Petrovich, Phys. Rev. Lett. 44, 1755 (1980). 
3. T.N. Taddeucci, J. Rapaport, D.E. Bainum, C.D. Goodman, C.C. Foster, C. Gaarde, 
J. Larsen, C.A. Goulding, D.J. Horen, T.  Masterson, and E. Sugarbaker, Phys. Rev. 
C, 25, 1094 (1982). 
4. T.A. Taddeucci, C.A. Goulding, T.A. Carey, R.C. Byrd, C.D. Goodman, C. Gaarde, 
J .  Larsen, D. Horen, J. Rapaport, and Sugarbaker, Nucl. Phys. A469, 125 (1987). 
5. Y. Wang, C.D. Goodman, W. Huang, G.C. Kiang, T.N. Taddeucci, R.C. Byrd, T.  
Carey, B. Sailor, L.J. Rybarcyk, C.A. Gaarde, D.J. Horen, J .  Rapaport, E. Sugarbaker, 
D. Marchlenski and J .  Wagner, IUCF Scientific and Technical Report, May 1988-April 
1989, p.48. 
6. J. Rapaport, C. Gaarde, J .  Larsen, C. Goulding, C.D. Goodman, C. Foster, D.J. 
Horen, T. Masterson, E. Sugarbaker, and T.N. Taddeucci, Nucl. Phys. A431, 301, 
(1984). 
P-A MEASUREMENTS IN THE 48Ca(p,n)48Sc REACTION AT 135 MeV 
B.D. Anderson, W.-M. Zhang, J.W. Watson_, A.R. Baldwin, 
T. Eden, D.M. Manley, R. Madey, and E. Steinfelds 
Kent  State U n i v e r ~ i t y ,  Ken t ,  Ohio 44242 
C.C. Foster 
Indiana U n i u e r ~ i t y  Cyclotron Facility, Bloomington, Indiana 4 7405 
We measured the polarization (P) and the analyzing-power (Ay)  for the 48 Ca(p,n)48 Sc 
reaction at 135 MeV with the beam-swinger facility at the Indiana University Cyclotron 
Facility (IUCF). The experiment was performed using the improved transmission from the 
polarized ion source. The beam intensity was 400-600 nA of protons with a polarization 
of about 72%. The KSU neutron polarimeterl was located along the 0" swinger line at a 
flight path of 37 m; the energy resolution was about 1 MeV. This polarimeter uses three 
BC-517L liquid scintilla.tor neutron analyzers together with six plastic scintillator "side" 
detectors and provides an analyzing power of about 0.35 for neutrons of about 120 MeV. 
These measurements extend our earlier cross ~ e c t i o n , ~  analyzing-power,3 and spin-transfer4 
measurements for this reaction. 
The target nucleus, 48Ca, is understood to be a relatively good closed-shell nucleus 
and the transitions induced by the (p,n) reaction are to predominantly one particle, one- 
hole ( l p lh )  states. The 1+ state at 2.52 MeV in 48Sc is believed to be predominantly 
(f,/, ,f$) and the I+ state at 16.8 MeV is believed to be predominately (fsI2 ,f$) The 
difference in the dominant l p l h  structures of these two 1+ excitations is expected to affect 
the difference P - Ay strongly. Love and Comfort5 shows that for transitions with a single 
shell, one of the nonvanishing form factors for P = A is specified by the matrix element 
of the operator < i L x  a'>. They show that this matrix element will vanish for j(initia1) = 
j(fina1); thus, P and Ay are expected to be the same for the low-lying 1+ excitation, but we 
expect a significant difference for the state at 16.8 MeV. "Standard" nonrelativistic DWIA 
calculations support these expectations, in fact, the signs of P and Ay are predicted to be 
opposite over the entire angular range for the high-lying 1'- excitation. 
Results for a, aP,  and aA for the 48Ca(p,n)48Sc reaction at 5" are shown in Fig. 1. 
These results are qualitatively in agreement with the arguments presented above. The low- 


Polarization Transfer Measurement for 19-F and 39-K(p,n)

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Polarization Transfer Measurement for 19-F and 39-K(p,n)

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