Speaker
Description
The discovery of β-delayed neutron emission followed closely that of nuclear fission[1], and the phenomenon was quickly incorporated, into the Bohr-Wheeler compound-nucleus picture[2]. Since that era, β-delayed particle emission has been discussed largely within a two-step statistical framework (the Pandemonium theory [3]).
Over the past decade, several observations have placed this view under severe strain. Among them, unexpectedly strong γ/neutron competition above the neutron separation energy has been reported [4] [5]. An even more striking case is the β decay of the neutron-rich nucleus 83Ga, where high-energy γ emission (up to∼5–8 MeV) competes with neutron emission from states located well above threshold [6]. This extraordinary, totally unexpected behaviour was questioned , calling for a dedicated re-measurement of the high-energy part of the spectrum.
We therefore performed a dedicated experiment at the ALTO facility using the PARIS γ-ray spectrometer coupled to the BEDO decay station. The improved efficiency at high energy and the enhanced segmentation enable a more reliable characterization of the high-energy γ spectra and a more robust extraction of γ-ray intensities. Beyond the 83Ga case, we also carried out a systematic study across the N=50 region (including the β decays of 80Ga and 82Ga) in order to identify structural conditions under which dynamics beyond the purely statistical compound-nucleus description may emerge. The results will be presented and discussed in connection with a microscopic (though phenomenological) structure-driven interpretation that includes the doorway-state concept in its original sense, as introduced by Feshbach and collaborators.
[1] R. B. Roberts, L. R. Hafstad, R. C. Meyer, and P. Wang, The Delayed Neutron Emission which Accompanies Fission of Uranium and Thorium, Phys. Rev. 55, 664, April 1939.
[2] N. Bohr and J. A. Wheeler , The Mechanism of Nuclear Fission, Phys. Rev., 56:426–450, September 1939.
[3] J. C. Hardy et al., The essential decay of pandemonium, Phys. Lett. B, 71(2):307–310, November 1977.
[4] J. L. Tain et al., Enhanced γ-Ray Emission from Neutron Unbound States Populated in β Decay. Phys. Rev. Lett., 115(6), August 2015.
[5] A. Spyrou et al., Strong Neutron-γ Competition above the Neutron Threshold in the Decay of 70Co, Phys. Rev. Lett. 117, 142701, September, 2016
[6] A. Gottardo et al. , Unexpected high-energy γ emission from decaying exotic nuclei, Phys. Lett. B, 772:359–362 (2017)