Elucidating the nature of the proton radioactivity and branching ratio on the first proton emitter discovered 53mCo

Abstract

The observation of a weak proton-emission branch in the decay of the 3174-keV ^53mCo isomeric state marked the discovery of proton radioactivity in atomic nuclei in 1970. Here we show, based on the partial half-lives and the decay energies of the possible proton-emission branches, that the exceptionally high angular momentum barriers, ${\mathcal{l}}_{\mathrm{p}}=9$ and ${\mathcal{l}}_{\mathrm{p}}=7$, play a key role in hindering the proton radioactivity from ^53mCo, making them very challenging to observe and calculate. Indeed, experiments had to wait decades for significant advances in accelerator facilities and multi-faceted state-of-the-art decay stations to gain full access to all observables. Combining data taken with the TASISpec decay station at the Accelerator Laboratory of the University of Jyväskylä, Finland, and the ACTAR TPC device on LISE3 at GANIL, France, we measured their branching ratios as b_p1 = 1.3(1)% and b_p2 = 0.025(4)%. These results were compared to cutting-edge shell-model and barrier penetration calculations. This description reproduces the order of magnitude of the branching ratios and partial half-lives, despite their very small spectroscopic factors.

Since the discovery of proton radioactivity in ^53mCo, there has been no direct measurement of its branching ratio. Here the authors report the first measurement of the weak proton decay in ^53mCo by combining results from advanced detection systems.