Parity violation in232Thneutron resonances above 250 eV

Abstract

The analysis of parity nonconservation ͑PNC͒ measurements performed on 232 Th by the TRIPLE Collaboration has been extended to include the neutron energy range of 250 to 1900 eV. Below 250 eV all ten statistically significant parity violations have the same sign. However, at higher energies PNC effects of both signs were observed in the transmission of longitudinally polarized neutrons through a thick thorium target. Although the limited experimental energy resolution precluded analysis in terms of the longitudinal asymmetry, parity violations were observed and the cross section differences for positive and negative neutron helicities were obtained. For comparison, a similar analysis was performed on the data below 250 eV, for which longitudinal asymmetries were obtained previously. For energies below 250 eV, the p-wave neutron strength functions for the Jϭ1/2 and Jϭ3/2 states were extracted: S 1/2 1 ϭ(1.68Ϯ0.61)ϫ10 Ϫ4 and S 3/2 1 ϭ(0.75Ϯ0.18) ϫ10 Ϫ4. The data provide constraints on the properties of local doorway states proposed to explain the PNC sign effect in thorium.

Parity and Time Reversal Invariance Violation in Neutron-Nucleus Scattering

“…Moreover, upon probing neutron energies higher that 250 eV, the PV effect in 232 Th has 4 resonances with a negative sign, and 2 with a positive sign, see Refs. [9,100].…”

Publisher's Note: Measurement of resolved resonances of232Th(n,γ) at the n_TOF facility at CERN [Phys. Rev. C85, 064601 (2012)]

“…In addition to Monte Carlo simulations [35], we have a dedicated measurement of the flux performed with a 235 U-loaded parallel-plate fission ionization chamber from the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig [49]. Furthermore, during the capture measurements the relative neutron flux was measured with an in-beam neutron monitor SiMon [50], consisting of a 6 Li deposit on a Mylar foil and four off-beam silicon detectors for the detection of the 6 Li(n, 3 H)α reaction products. Up to 1 keV both methods are in good agreement, but at higher energies the 6 Li(n, α) reaction suffers the insufficient knowledge of the angular distribution of the α and triton particles.…”

“…Another field of interest for neutron-induced resonance reactions concerns the large parity-nonconservation effects which have been observed in neutron p-wave resonances of several isotopes including 232 Th [5,6]. These effects, on the order of 10 −7 in nucleon-nucleon interactions, were found to be up to 10% in polarized neutron transmission experiments on several nuclei including 232 Th [7].…”