A neutron-energy dependent angular distribution was measured for individual $γ$-rays from the 3.2 eV $p$-wave resonance of $^{131}$Xe+$n$, that shows enhanced parity violation owing to a mixing between $s$- and $p$-wave amplitudes. The $γ$-ray transitions from the $p$-wave resonance were identified, and the angular distribution with respect to the neutron momentum was evaluated as a function of the neutron energy for 7132 keV $γ$-rays, which correspond to a transition to the 1807 keV excited state of $^{132}$Xe. The angular distribution is considered to originate from the interference between $s$- and $p$-wave amplitudes, and will provide a basis for a quantitative understanding of the enhancement mechanism of the fundamental parity violation in compound nuclei.

Largely enhanced parity-violating effects observed in compound resonances induced by epithermal neutrons are currently attributed to the mixing of parity-unfavored partial amplitudes in the entrance channel of the compound states. Furthermore, it is proposed that the same mechanism that enhances the parity-violation also enhances the breaking of time-reversal-invariance in the compound nucleus. The entrance-channel mixing induces energy-dependent spin-angular correlations of individual $γ$-rays emitted from the compound nuclear state. For a detailed study of the mixing model, a $γ$-ray yield in the reaction of $^{117}$Sn(n,$γ$)$^{118}$Sn was measured using the pulsed beam of polarized epithermal neutrons and Ge detectors. An angular dependence of asymmetric $γ$-ray yields for the orientation of the neutron polarization was observed.

The neutron energy-dependent angular distribution of $\gamma$ rays from $^{117}{\rm Sn}(n,\gamma)$ reaction was measured with germanium detectors and a pulsed neutron beam. The angular distribution was clearly observed in $\gamma$-ray emissions with an energy of 9327 keV which corresponds to the transition from a neutron resonance of $^{117}{\rm Sn}+n$ to the ground state of $^{118}{\rm Sn}$. The angular distribution causes an angular-dependent asymmetric resonance shape. An asymmetry $A_{\rm LH}$ was defined as $(N_{\rm L}-N_{\rm H})/(N_{\rm L}+N_{\rm H})$, where $N_{\rm L}$ and $N_{\rm H}$ are integrated values for lower- and higher-energy regions of a neutron resonance, respectively. We found that the $A_{\rm LH}$ has the angular dependence of $(A \cos \theta_\gamma +B)$, where $\theta_\gamma$ is the $\gamma$-ray emission angle with respect to the incident neutron momentum, with $A=0.394 \pm 0.073$ and $B = 0.118 \pm 0.029$ in the 1.33 eV p-wave resonance.

Largely enhanced parity-violating effects observed in compound resonances induced by epithermal neutrons are currently attributed to the mixing of parity-unfavored partial amplitudes in the entrance channel of the compound states. Furthermore, it is proposed that the same mechanism that enhances the parity-violation also enhances the breaking of time-reversal-invariance in the compound nucleus. The entrance-channel mixing induces energy-dependent spin-angular correlations of individual $\gamma$-rays emitted from the compound nuclear state. For a detailed study of the mixing model, a $\gamma$-ray yield in the reaction of $^{117}$Sn(n,$\gamma$)$^{118}$Sn was measured using the pulsed beam of polarized epithermal neutrons and Ge detectors. An angular dependence of asymmetric $\gamma$-ray yields for the orientation of the neutron polarization was observed.

Received 25 November 2021DOI:https://doi.org/10.1103/PhysRevC.105.039901©2022 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasNuclear reactionsNuclear tests of fundamental interactionsRadiative captureResonance reactionsProperties90 ≤ A ≤ 149P-symmetryTechniquesRadiation detectorsNuclear Physics

A neutron lifetime measurement conducted at the Japan Proton Accelerator Research Complex (J-PARC) is counting the number of electrons from neutron decays with a time projection chamber (TPC). The gamma rays produced in the TPC cause irreducible background events. To achieve the precise measurement, the inner walls of the TPC consist of Li-6-enriched lithium-fluoride ((LiF)-Li-6) tiles to suppress the amount of gamma rays. In order to estimate the amount of y rays from the (LiF)-Li-6 tile, prompt gamma ray analysis (PGA) measurements were performed using germanium detectors. We reconstructed the measured gamma-ray energy spectrum using a Monte Carlo simulation with the stripping method. Comparing the measured spectrum with a simulated one, the number of gamma rays emitted from the (LiF)-Li-6 tile was (2.3(-0.3)(+0.7)) x 10(-4) per incident neutron. This is 1.4(-0.2)(+0.5) times the value assumed for a mole fraction of the (LiF)-Li-6 tile. We concluded that the amount of gamma rays produced from the (LiF)-Li-6 tile is not more twice the originally assumed value.

A correlation term ${{ \sigma}_{n} }\cdot ({ k_{n}\times k_\gamma}) $ in the ${}^{139}{\rm La}(\vec{n},\gamma)$ reaction has been studied utilizing epithermal polarized neutrons and germanium detectors. The transverse asymmetry for single $\gamma$-ray transition was measured to be $0.60\pm0.19$ in the $p$-wave resonance.

In this study, the germanium detector assembly, installed at the Accurate Neutron-Nuclear Reaction measurement Instruments (ANNRI) in the Material and Life Science Facility (MLF) operated by the Japan Proton Accelerator Research Complex (J-PARC), has been characterized for extension to the measurement of the angular distribution of individual gamma-ray transitions from neutron-induced compound states. We have developed a Monte Carlo simulation code using the GEANT4 toolkit, which can reproduce the pulse-height spectra of gamma-rays from radioactive sources and (n,gamma) reactions. The simulation is applicable to the measurement of gamma-rays in the energy region of 0.5-11.0 MeV.

The angular distribution of individual gamma rays, emitted from a neutron-induced compound-nuclear state via radiative capture reaction of La-139(n,gamma) has been studied as a function of incident neutron energy in the epithermal region by using germanium detectors. An asymmetry A(LH) was defined as (N-L - N-H)/(N-L + N-H), where N-L and N-H are integrals of low-and high-energy region of a neutron resonance respectively, and we found that A(LH) has the angular dependence of (Acos theta(gamma) + B), where theta(gamma) is the emitted angle of gamma rays, with A = -0.3881 +/- 0.0236 and B = -0.0747 +/- 0.0105 in 0.74 eV p-wave resonance. This angular distribution was analyzed within the framework of interference between s-and p-wave amplitudes in the entrance channel to the compound-nuclear state, and it is interpreted as the value of the partial p-wave neutron width corresponding to the total angular momentum of the incident neutron combined with the weak matrix element, in the context of the mechanism of enhanced parity-violating effects. Additionally, we use the result to quantify the possible enhancement of the breaking of time-reversal invariance in the vicinity of the p-wave resonance.

The parity (P) violations enhanced by a factor of 10 6 compared to the proton-proton scattering have been observed in the several compound nuclear reactions.The enhancement mechanism is theoretically explained as the interference between a s-wave and a p-wave.In the compound nucleus which has the enhanced P-violation, the violation of time-reversal symmetry (T-violation) could be also enhanced.The experiments to select the candidate nuclei for T-violation search are carried out at the neutron beamline ANNRI in J-PARC MLF.The very high intensity neutron beam and 22 Germanium detectors are available at the beamline.We measured the angular distribution of (n, γ) reaction of 117 Sn, which has the large P-violation.In this paper, the measurement result and analysis status of 117 Sn(n, γ) are reported.

P-violating interactions enhanced by a factor of up to 10 6 have been observed in several compound nuclear reactions.This very large P-violation is produced by parity mixing between an s-wave and a p-wave amplitudes in compound nuclei (s-p mixing).The mechanism of the enhancement of Pviolation can be used to enhance the experimental sensitivity of a T-violation search.Experiments to select the candidate nucleus for a T-violation search are carried out at J-PARC/MFL/BL04.We measured the angular distributions of (n, γ) reactions of several candidate nuclei such as 139 La, 131 Xe, 115 In, 117 Sn and 81 Br, which exhibit large P-violation.The (n, γ) reactions were measured very precisely with the high intensity neutron beam of J-PARC and 22 novel Germanium detectors at BL04.The result of the 139 La(n,γ) measurement is reported.