In this seminar, I will revisit a decade-long program of gamma and beta-delayed neutron spectroscopy measurements launched at the "small" ISOL (Isotope Separation On Line) facility, ALTO, built by the Institut de Physique Nucléaire (IPN) in Orsay during the early 2000s. Originally developed within an R&D context for the SPIRAL2 project, ALTO was later repurposed for scientific exploitation, initially by IPN and currently by IJCLab. Since the commissioning of the first decay station, BEDO (Beta Decay Studies in Orsay), in 2012, measurement setups based on beam collection with removable tape systems (the well-known "tape reels") have expanded. Today, ALTO has evolved into a cohesive system comprising no less than three beamlines with removable collectors, accommodating both semi-permanent devices like COeCO (beta-delayed electron spectrometer), TETRA (long neutron He-3 counter), and mobile setups like PARIS (scintillator array) or MONSTER (time-of-flight neutron spectrometer).
These setups have been employed to investigate the nuclear structure of nuclei around the N=50 shell closure, populated through beta decay from nuclei with very high Q_beta values. These studies have revealed two intriguing phenomena, both incompatible with the dominant "Pandemonium" paradigm — a purely statistical framework (ultimately inspired by Bohr’s compound nucleus model) for beta decay to high-lying levels within the Q_beta window. The first discovery, made with the TETRA device, was a marked "oscillation" in the beta-delayed neutron emission probabilities, Pn, from one isotope to the next (whereas, in a statistical view, Pn should increase consistently with the opening of the Q_beta-n window). The second was the observation of an exceptionally strong competition between gamma and neutron decays from levels located even several MeV above the neutron emission threshold in 83Ge, populated by the beta decay of 83Ga. To confirm these findings and explore the origins of these phenomena, additional measurements using PARIS modules were conducted in 2019, and a first neutron spectrometry campaign with the MONSTER setup is scheduled for February 2025.
All these data point toward a new aspect of radioactivity in nuclei with very high Q_beta values. We have proposed a microscopic interpretation based on the concept of "doorway states," derived from the formalism of neutron scattering and capture. This interpretation, termed "doorway decay," is gradually gaining acceptance among beta-decay researchers, signaling a potential shift away from the half-century dominance of the Pandemonium paradigm in the realm of beta decay.
