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

Byrd, R.C.

DeLucia, S.L.

Goodman, C.

Huang, W.

Luther, B.

Marchlenski, D.G.

Rapaport, J.

Smith, A.

Sugarbaker, E.

Taddeucci, T.

Wang, Y.

English

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WEAK INTERACTION MATRIX ELEMENTS AND (p,n) CROSS SECTIONS 
Y. Wang, C. Goodman, W. Huang and A. Smith 
Indiana University Cyclotron Facility, Bloomington, Indiana 4 74 08 
T. Taddeucci and R. C. Byrd 
Los Alamos National Laboratory, Los Alamos, N e w  Mexico 87545 
J. Rapaport 
Ohio University, Athens,  Ohio 45701 
S. L. DeLucia, B. Luther, D. G. Marchlenski and E. Sugarbaker 
T h e  Ohio S ta te  University, Columbus, Ohio 49210 
The purpose of experiment 320 is to investigate the relationship between the forward 
angle (p,n) cross sections and GT (Gamow-Teller) and F (Fermi) beta decay matrix ele- 
ments and to use such a relationship to determine matrix elements relevant to the capture 
cross section of neutrino detectors. Our initial measurements at 160 MeV reported last 
year suggested that the ratio R2 = eGT/eF for medium-to-heavy odd-mass nuclei had a 
value consistently higher than anticipated from the systematics of previous studies based 
primarily on even-mass nuclei.' The dependence of R on the proton beam energy Ep for 
the latter nuclei has been successfully represented by R = Ep/Eo,  where E, is constant at 
a value of 55 MeV. We have continued these studies with measurements at Ep = 120 MeV 
and a neutron flight path of 121 m using the stripper loop to obtain large pulse separation 
to minimize the background from low-energy "wraparound" neutrons. In addition we have 
measured at 120 MeV the forward-angle cross sections on additional targets related to 
neutrino detection. 
We report extension of experiment 320 to an additional energy and greater precision. 
Targets of 51V, 87Rb, 'l3In, ll8Sn and 14'Pr have again been studied, as they provide 
transitions with known GT strength from beta decay which can be resolved from nearby 
GT transitions within the limits of our energy resolution. We measured (p,n) spectra 
at E, = 120 MeV and at 0°, 3.0" and 6.0". The angular distribution of the differential 
cross sections is used to estimate the size of A L  > 0 contributions at 0'. Such AL > 0 
contributions were found to be small and were neglected. The extracted E, value of 43 f 2 
MeV at Ep = 120 MeV is consistent with that obtained previously at 160 MeV, indicating 
no significant energy-dependence in the small value of E, for these odd-mass nuclei. The 
E, extracted from the ll8Sn data, measured under the same conditions as the odd-mass 
nuclei, is again consistent with the even-mass value of E, = 55 MeV. The experimental 
situation associated with the value of E, for medium-to-heavy odd-mass is presented in 
Fig. 1. The cause of the smaller E, value for odd-mass nuclei is not understood and shall 
be the subject of further investigation. 
By scaling the B(GT) strengths obtained from lower-mass nuclei, Haxton recently 
suggested that a solar neutrino detector using the 1271(p, e)12?Xe reaction might be an 
order of magnitude more efficient than the present 37C1 detector? We measured the OOcross 
section and converted it to a B(GT) spectrum using the techniques described by Taddeucci, 
Figure 1. The E, for medium- to-heavy odd-mass nuclei ( A  2 39). The open circles are 
the E, from Ep = 120 MeV and the diamond symbols from Ep = 160 MeV. The E, for 
Targets of 51V, 87Rb, 'l31n, and l4'Pr are extracted from OO(p,n) spectra. The E, for 39K 
are extracted from DNN measurement reported in Ref. 7. The solid line refers the E, = 55 
MeV and the dot-dashed line indicates the average value of these data points here. The 
length of the thick black bars on 87Rb and '131n data points indicates the uncertainties if 
one does not consider the uncertainties contributed by B (GT). 
e t  aL1 The result for the region below the particle emission threshold (PET) is shown in 
Fig. 2. Using an E, = 50 MeV to normalize the GT strength against the N-Z Fermi 
strength located in the IAS resonance, the integrated GT strength below the PET is 
3.5, with as much as a f 20% uncertainty due to the above mentioned present difficulty 
establishing the appropriate value of E, for this odd-mass nucleus. With a resolution 
of about 300 keV, we were not able to resolve the lowest lying GT transitions recently 
identified.3 However, a fit to the low-excitation region of the B(GT) spectrum constrained 
to these transitions suggests approximate values of 10 and 17 mB(GT) associated with 
the regions centered at 125 and 350 keV, respectively. The observed B(GT) strength 
' 1 " ' 1 ' 1 '  
PET " ' I (~  ,n)'"~e 
Ep=120.14 MeV 
- 
- 
- 
- 
- 
- 
Figure 2. The B(GT) spec- 
trum of 127Xe below the par- 
ticle emission threshold (PET) 
extracted from the 0" (p,n) 
cross sections measured at 120 
MeV. A value of E, = 50 MeV 
was assumed. 
distribution is significantly lower than that estimated by Haxton, suggesting that the 
response of such a detector would not differ much from that of the present 37C1 detector. 
Zero-degree (p,n) cross section measurements were also made for targets of 69Ga and 
97Mo, with comparison spectra obtained for previously studied targets of 71Ga and 98Mo.4,5 
In each case the prior nuclei are significant "contaminants" in current solar neutrino exper- 
iments utilizing the latter nuclei. Knowledge of the B(GT) spectrum for the prior nuclei 
may assist in understanding the contribution of galactic neutrinos to the signal obtained 
in these  detector^.^ 
1. T. N. Taddeucci, e t  al., Nucl. Phys. A496, 125 (1987). 
2. W. C. Haxton, Phys. Rev. Lett. 60, 768 (1988). 
3. A. Garcia, et al., Phys. Rev. C41, 775 (1990). 
4. D. Krofcheck, e t  al., Phys. Rev. Lett. 55, 1051 (1985). 
5 .  J. Rapaport, e t  al., Phys. Rev. Lett. 54, 2325 (1985). 
6. W. C. Haxton and C. W. Johnson, Nature 333, 325 (1988). 
7. W. Huang, Ph.D. dissertation, 1991. 


Weak Interaction Matrix Elements and (p,n) Cross Sections

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Weak Interaction Matrix Elements and (p,n) Cross Sections

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