B. S. Flanders

Electron capture strength for Ni 60 , 62 and Ni 58 , 60 , 62 , 64 ( p , n ) Cu 58 , 60 , 62 , 64 reactions at 134.3 MeV

Background: The strength of electron capture for medium mass nuclei has a significant effect on the evolution of supernovae. There is insufficient knowledge of these strengths and very little data for important radioactive nuclei. Purpose: Determine whether it is feasible to obtain EC strength from studies of T{sub o}+1 excitations in (p, n) reactions, and whether this might yield information for radioactive nuclei. Methods: Cross sections for the {sup 58,60,62,64}Ni(p, n){sup 58,60,62,64}Cu reactions were measured over the angular range of 0.3 deg. to 11.6 deg. at 134.3 MeV using the IUCF neutron time-of-flight facility. Results: The T{sub o}+1 excitations in {sup 60,62}Ni were identified by comparison with inelastic proton scattering spectra, their B(GT) were extracted, and the corresponding electron capture rates in supernovae were calculated. Data from the TRIUMF (n, p) experiments at 198 MeV were reanalyzed; the electron capture rates for the reanalyzed data are in moderately good agreement with the higher resolution (p, n) results, but differ in detail. The possibility of future measurements with radioactive nuclei was considered. Conclusions: It may be possible to obtain low-lying electron capture strength for radioactive nuclei by studying (p, n) reactions in inverse kinematics.

A comparison of methods for determining neutron detector efficiencies at medium energies

Abstract We compare the “Lithium Activation” (LiA) method and the “Isospin Clebsch-Gordan Ratio” (ICGR) method for determining detection efficiencies of neutrons between 100 and 160 MeV. Each method was used to determine the efficiency by relating a measured neutron yield to a cross section measured in another way: γ-ray activation cross sections for LiA; (p, p′) cross sections from experiments at the Indiana University Cyclotron Facility for ICGR. Efficiencies determined by the two methods disagree substantially. Efficiencies calculated with the Monte Carlo code of Cecil et al. agree with the ICGR results. We conclude that the Lithium Activation method is inconsistent at these energies.

Spin-Isospin Excitations in Nuclei by the (p,n) Reaction

The (p,n) reaction has been successful in delineating the Gamow-Teller strength function in the nuclear excitation spectrum; however, the total strength extracted from experiment is less than that expected from the sum rule if only nucleon spin and isospin degrees of freedom are important in nuclei. Apparently the missing strength is shifted to higher excitation energies primarily through mixing with both 2p-2h and A-h configurations; however, an experimental test is needed to decide the relative importance of these two principal mechanisms.