Measurement of effective analyzing powers for the NTOF polarimeter at LAMPF and D(sub LL)(0(degree)) for Gamow-Teller transitions in p-shell nuclei

摘要

Measurements of neutron polarization from ((rvec p),(rvec n)) reactions can provide valuable clues toward understanding the isovector nucleon-nucleus interaction. A neutron time-of-flight polarimeter has been constructed at the Los Alamos Meson Physics Facility to perform such measurements, but before the polarimeter can be used, its effective analyzing powers must be determined. This is accomplished by using the (sup 14)C((rvec p),(rvec n))(sup 14)N reaction at a bombarding energy of 494 MeV to produce a beam of neutrons with known polarization, illuminating the detector with these neutrons, and measuring the azimuthal asymmetries after scattering from a hydrogenous analyzer fluid within the detector. Secondary measurements are made using the (sup 2)H((rvec p),(rvec n))(sup 2)p reaction with bombarding energies of 318 and 494 MeV to produce a polarized neutron beam. The results from ((rvec n)p) analyzing reactions within the detector agree with values anticipated from free nucleon-nucleon analyzing powers, but the results from ((rvec n)p) analyzing reactions display a more than 33% reduction from the anticipated values. Additionally, measurements are made of the polarization transfer coefficient D(sub LL)(0(degree)) for (rvec p),(rvec n) Gamow-Teller reactions on (sup 2)H, (sup 7)Li(sup 12)C, and (sup 14)C targets. For a purely central interaction, one would expect that D(sub LL)(0(degree)) (approximately) (minus)1/3 in the plane wave limit, but a simple average of the J(pi) = 0(sup +) (yields) 1(sup +) results at a bombardment energy of 494 MeV gives D(sub LL)(0(degree)) = (minus)0.689 (plus minus) 0.044. Thus, the measurements indicate that the nucleon-nucleus interaction -- which is largely central at 200 MeV -- has strong tensor contributions at higher energy.