Abstract
Proton emission is the radioactive decay mode that is expected to determine the limit of observable proton-rich nuclei for most elements. Considerable progress has been made in the study of proton-emitting nuclei since the first observation of direct proton emission nearly 50 years ago. This has led to improvements in our understanding of this decay process and provided invaluable nuclear structure data far from the valley of beta stability. The rapid fall in half-lives with increasing neutron deficiency when proton emission dominates makes it likely that for some elements, the lightest isotopes whose ground states can be observed in conventional experiments have already been reached. The enhanced stability against proton emission of the recently discovered high-lying isomer in 158 Ta raises the possibility that proton emission from multiparticle isomers could be observed in nuclei beyond the expected boundaries of the nuclear landscape.
Highlights
Isomers are a widespread feature across the chart of nuclides and their study can provide an important testing ground for a range of nuclear theories [1]
As these studies reach the limits of nuclei that are accessible using recoil separators, attention may again switch to proton emission from highlying isomeric states, in nuclei far beyond the proton drip line
Experimental studies of odd-Z nuclei have identified more than 30 proton-emitting nuclei, with examples known for most elements from iodine (Z = 53) to bismuth (Z = 83)
Summary
Isomers are a widespread feature across the chart of nuclides and their study can provide an important testing ground for a range of nuclear theories [1]. The first experimentally observed case of direct proton emission was from a high-spin isomer in 53Co [2, 3]. In this unique case, the proton emitter’s ground state is bound to proton emission, but the excitation energy of the protonunbound isomer is sufficient to overcome the retarding effect of the centrifugal barrier and results in a proton-decay branching ratio of ∼1.5 %. Subsequent studies of proton emission have primarily focused on low-lying states in nuclei beyond the proton drip line. As these studies reach the limits of nuclei that are accessible using recoil separators, attention may again switch to proton emission from highlying isomeric states, in nuclei far beyond the proton drip line
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