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, [Formula: see text] and [Formula: see text], 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.

Fig. 1. Decay scheme of ^53mCo.

The scheme is based on previous^,, and present results. Level energies are given in keV and are relative to the ground state of ⁵³Co. The half-life of the isomer is based on the ACTAR-TPC measurement and literature. The absolute proton branching ratio, b_p1, was measured with TASISpec. The relative branching ratio, b_p2/b_p1, was determined with ACTAR TPC.

Fig. 2. Proton branching ratio, b_p1, determination.

a Experimental and simulated energy spectra recorded at the TASISpec implantation detector for ${\beta }^{+}$ particles and protons, the error bars represent the standard error in the number of counts. The simulated spectrum was normalized to the experimental one in the energy range 500 keV–1000 keV. The experimental data are best described when an absolute proton branching ratio of b_p1 = 1.3(1)% is used. The difference between the experimental and simulated spectra can be seen at the bottom. b Result of the Figure-of-Merit (FoM) from a minimization algorithm between experimental and simulated results.

Fig. 3. Experimental proton energy and half-life measurement using ACTAR TPC.

Left: Proton energy spectra associated with the decay of ^53mCo for the two different pressure settings (380 and 292 mbar). Right: Half-life spectra obtained by correlating the proton emission, identified by their characteristic energy-loss profile, with ^53mCo implantation for both pressure regimes. The line represents a least-squares fit of an exponential decay to the data.

Fig. 4. Number of ions per second detected at the microchannel-plate detector behind JYFLTRAP.

The number is displayed as a function of the quadrupolar excitation frequency in the purification trap, the error bars represent the standard error in the number of counts. The relevant frequency region covering the ground and isomeric states of ⁵³Co and ⁵³Fe is shown. The red dotted line indicates one of the frequencies used for selecting the isomer ^53mCo for the TASISpec measurements, the dashed blue line gives the frequency for the ground state of ⁵³Co.