Measurement of the angular distribution of γ-rays after neutron capture by ¹³⁹La for a T-violation search

¹ J-PARC center, Japan Atomic Energy Agency, Tokai, Ibaraki 319–1184, Japan
² Department of Physics, Nagoya University, Furocho, Chikusa, Nagoya, Aichi 464–8601, Japan
³ Kobayashi-Maskawa Institute, Nagoya University, Furocho, Chikusa, Nagoya, Aichi 464–8601, Japan
⁴ National Institute of Standards and Technology, 100 Bureau Dr, Gaithersburg, MD 20899, USA
⁵ Department of Physics, Kyushu University, 744 Motooka, Nishi, Fukuoka, Fukuoka 819–0395, Japan
⁶ Research Center of Advanced Particle Physics, Kyushu University, 744 Motooka, Nishi, Fukuoka, Fukuoka 819–0395, Japan
⁷ Institute of Material Structure Science, High Energy Accelerator Organization (KEK), Oho 1–1, Tsukuba, Ibaraki 305–0801, Japan
⁸ Department of Physics, Tokyo Institute of Technology, 2-12-1 Oh-okayama, Meguro, Tokyo 152–8551, Japan
⁹ Department of Chemistry, UBC Faculty of Science, University of British Columbia, Vancouver, British Columbia V6R1Z1, Canada
¹⁰ Department of Earth and Environmental Sciences, Yamagata University, 1-4-12 Kojirakawa, Yamagata 990–8560, Japan
¹¹ Nuclear Science and Engineering Directorate, Japan Atomic Energy Agency, Japan
¹² Research Reactor Institute, Kyoto University, 2 Asashiro-nishi, Kumatoricho, Senngan-gun, Osaka 590–0494, Japan
¹³ Research Center for Nuclear Physics, Osaka University, 10–1 Mihogaoka, ibaraki, Osaka 567–0047, Japan
¹⁴ RIKEN Center for Advanced Photonics, RIKEN, Hirosawa 2–1, Wako, Saitama 351–0198, Japan

^a e-mail: okudaira@post.j-parc.jp

Published online: 12 December 2019

Abstract

Parity violating effects enhanced by up to 10⁶ times compared to proton-proton scattering have been observed in several neutron capture induced compound nuclei. This enhancement is explained as an interference between an s-wave and a p-wave amplitude (s-p mixing). Theory predicts that this mechanism can also enhance T-violating effects. For estimation of this enhancement in a promising candidate nucleus, we measured the angular distribution of γ-rays in the (n,γ) reaction induced in a ¹³⁹La target. Experiments were performed at J-PARC, using a germanium detector array at an intense neutron beam. Analysis of the observed angular dependence around the 0.74 eV p-wave resonance allowed us to determine the ratio of the partial p-wave neutron width to the total neutron width in the entrance channel of the compound nucleus. We also report our development of a ³He spin filter needed as an epithermal-neutron polarizer for the T-violation search.