Parity violation in partial neutron capture reactions

Abstract

In the last decade, the scattering of polarized neutrons on compound nucleus resonances proved to be a powerful experimental technique for probing nuclear parity violation. Longitudinal analyzing powers in neutron transmission measurements on p-wave resonances in nuclei such as 139 La and 232 Th were found to be as large as 10%. Here we examine the possibilities of carrying out a parallel program to measure asymmetries in the (n, γ) reaction on these same compound nuclear resonances. Symmetry-violating (n, γ) studies can also show asymmetries as large as 10%, and have the advantage over transmission experiments of allowing parity-odd asymmetries in several different gamma-decay branches from the same resonance. Thus, studies of parity violation in the (n, γ) reaction using high efficiency germanium detectors at the Los Alamos Lujan facility, for example, could determine the parity-odd nucleon-nucleon matrix elements in complex nuclei with high accuracy. Additionally, simultaneous studies of the E1 and V P N C matrix elements involved in these decays could be used to help constrain the statistical theory of parity non-conservation in compound nuclei.

Searches for Exotic Interactions Using Neutrons

“…Subsequent progress in the measurement of nucleon-nucleon parity violation in two-and few-nucleon systems [77,78], combined with progress in atomic physics enabling measurements of nuclear anapole moments [79], can soon be used to provide the input information needed to confront the predictions of nuclear statistical spectroscopy [80,81] for parity-odd effects in neutron-nuclear resonances. Searches for parity violation in p-wave resonances in heavy nuclei not yet measured [82] and in the (n, γ) cascades from p-wave neutron capture [83] can also provide important new information.…”

Upper bounds on parity-violating γ-ray asymmetries in compound nuclei from polarized cold neutron capture

“…In this case the observables involve the parity-odd correlation s n · k γ , where s n is the neutron spin and k γ is the momentum of the γ photon [6][7][8]. Just as for neutron scattering, neutron capture on elements with a large number of nucleons produces compound nuclei in highly excited states.…”

“…Parity violation in the gamma decays of nuclei is another example where statistical methods may be employed to estimate observables. In this case the observables involve the parity-odd correlation s n • k γ where s n is the neutron spin and k γ is the momentum of the gamma [6,7,8]. Just as for neutron scattering, neutron capture on elements with a large number of nucleons produces compound nuclei in highly excited states.…”