The stratospheric neutron/gamma competition in the beta decay of 83Ga investigated using PARIS at the BEDO decay station

par Elia Nseir

mardi 24 mars 2026, 14:00 → 15:00 Europe/Paris

100/-1-A900 - Auditorium Joliot Curie (IJCLab)

The β decay of the neutron-rich nucleus ⁸³Ga shows a great competition between high-energy γ emission (up to 5–8 MeV) and neutron emission from states located well above the neutron emission threshold [1]. This behavior contradicts the predictions of the traditional statistical model of compound-nucleus decay, established following the work of J. C. Hardy et al. (The Essential Decay of Pandemonium, 1977), in which β-decay strength is assumed to populate highly complex compound states that have lost memory of the entrance channel.

The observed γ/neutron competition above threshold suggests a selective, structure-driven mechanism rather than a purely statistical process [2]. A plausible interpretation is given by the doorway-state concept, introduced by Herman Feshbach and collaborators, where relatively simple configurations couple neutron continuum states to collective excitations—here, notably dipole (E1) modes—thus partially bypassing statistical equilibration.

A dedicated experiment has been performed at the ALTO facility using the high-efficiency γ-ray spectrometer PARIS coupled to the BEDO decay station. The improved resolution and granularity enable precise characterization of high-energy γ spectra and reliable extraction of γ intensities. In addition to validating the γ/neutron competition in ⁸³Ga, a systematic study across the N = 50 shell closure will investigate the β decays of ⁸⁰Ga and ⁸²Ga toward ⁸⁰Ge and ⁸²Ge, respectively, in order to identify structural conditions favoring doorway mechanisms.

The results will be interpreted within the doorway-state framework and compared with fully microscopic shell-model calculations to assess whether β-delayed γ emission above the neutron threshold constitutes a genuine manifestation of doorway dynamics beyond the statistical compound-nucleus description. [3]

[1] Clément Delafosse. "Étude des dérives monopolaires neutron au-delà du ⁷⁸Ni par spectroscopie gamma avec BEDO à ALTO et AGATA au GANIL." PhD thesis, Université Paris-Saclay (2018)

[2] A. Gottardo et al. "Unexpected high-energy γ emission from decaying exotic nuclei." Phys. Lett. B, 772:359–362 (2017)

[3] D. Verney et al. "Pygmy Gamow-Teller resonance in the N = 50 region: New evidence from staggering of β-delayed neutron-emission probabilities." Phys. Rev. C, 95(5):054320 (2017)