Speaker
Description
In this workshop, we present the latest results coming out of the data analysis of the IS633 experiment carried out at the IDS on 2017, related with the rare β-delayed proton branch present in the decay of 8B. This branch is supposed to be enhanced by the proton-halo configuration of the 8B, however, there is a lack of experimental results.
This rare decay proceeds through electron capture (EC) to the 17.6 MeV state of 8Be which, instead of breaking up into two α-particles, emits a low-energy proton of 337 keV, known from reaction studies [1]. Assuming a proton-halo configuration of 8B, an extremely low branching ratio has been estimated considering it as a 7Be core plus an orbiting proton [2]. The EC would occur on the 7Be-core and therefore the transition matrix element can be estimated to be the same as for the decay of 7Be g.s. to 7Li g.s. Scaling by the half-live, an upper limit of the branching ratio is set to 2.3·10-8 [3].
This very low branching ratio in combination with the low energy of the proton oblige us to realize a specific and compact set-up in order to enhance the sensitivity to the proton branch. The main decay of 8B is always accompanied by an α-α coincidence in two opposite detectors, we thus aim for an improved anticoincidence efficiency to exclude all events accompanied by a β- or an α-particle.
To determine the experimental upper limit, an accurate knowledge of the set-up and the electronics used in the experiment has been crucial. By comparing the experimental results with accurate Geant4 simulations of the set-up has allowed us to determine the upper limit of the branching ratio to the 17.6 MeV state, reducing by two orders of magnitude the previous experimental estimation [3] with a confidence level of 99.99% (3).
[1] J.A. Wheeler. The alpha-particle model and the properties of the nucleus Be-8. Physical Review, 59(1):27, 1941.
[2] T. Nilsson et al. Halo-nuclei at ISOLDE. Hyperfine Interactions 129, 67-81, 2000.
[3] M.J.G. Borge et al. Rare βp decays in light nuclei. Journal of physics G: nuclear and particle physics, 40(035109), 2013.
