V.R. Brown

Different effects of valence neutrons on the isospin character of transitions to the first 2+ and 3− states of 90,92,94,96Zr

Abstract We have used the 90,92,94,96 Zr( 6 Li, 6 Li) reaction at E = 70 MeV to deduce the ratio of M n M p for transitions to the first 2 + and 3 − excited states. For the first 2 + state, we find M n M p increases from less than N Z to greater than N Z going from 90 Zr to 96 Zr. However, for the 3 − state we find that M n M p is significantly smaller than N Z for all four isotopes, a result which is in sharp disagreement with previous work. We find that our measured values of M n M p are well reproduced by random-phase-approximation calculations including pairing.

Neutron and proton multipole matrix elements in 48Ca and 208Pb

Abstract The ratios of multiple matrix elements M n / M p for the 0 + → 2 1 + transitions in doubly magic nuclei 48 Ca and 208 Pb are compared. For 48 Ca the large value of M n / M p ∼ 2.6 is contrasted with a ratio N / Z for 208 Pb. These results, obtained by comparing hadron scattering and electromagnetic transition rates, are explained by the shell structure and are corroborated by RPA calculations.

Neutron-proton bremsstrahlung experiments

Abstract It is well known that charged particles emit bremsstrahlung radiation when they are accelerated. Classical electron bremsstrahlung occurs when a proton is emitted by an electron accelerated in the field of a nucleus. The bremsstrahlung process also occurs in the scattering of nucleons, for which it is the lowest energy inelastic process that can occur. Like electron bremsstrahlung, nucleon-nucleon bremsstrahlung also requires the exchange of a virtual particle to conserve energy and momentum. In electron bremsstrahlung a virtual photon is exchanged but with two nucleons a meson can be exchanged. Unlike electron bremsstrahlung, in nucleon-nucleon bremsstrahlung the photon can originate from the exchanged meson. This exchange contribution has been shown in calculations to be a significant fraction of bremsstrahlung events. Thus bremsstrahlung serves as a probe of exchange currents in the nucleon-nucleon interaction. Because of a lack of a free neutron target or an intense neutron beam, few measurements of neutron-proton bremsstrahlung exist, each having poor statistical accuracy and poor energy resolution. The white neutron source at the Weapons Neutron Research (WNR) target area at the Los Alamos Meson Physics Facility (LAMPF) produces neutrons with energies from below 50 to above 400 MeV. Using time-of-flight techniques and a liquid hydrogen target, we are measuring the outgoing photons of energies up to 250 MeV at gamma ray angles of around 90° relative to the incident beam. Protons scattered at very forward angles are also detected in coincidence with the gamma rays.

The reversal effect for neutron and proton multipole matrix elements in higher 2+ transitions☆

Abstract Recent experimental evidence is cited showing that there commonly exists a higher prominent 2 + state which has a reversal of the inequality of multiple matrix elements M n and M p from that of the first 2 + state of even nuclei. An explanation in terms of the spin-orbit intruder is presented for nuclei beyond the s-d shell, and examples are given.