New Proton Radioactivity

Abstract

A series of experiments were carried out at Argonne National Laboratory to search for examples of proton emission from ground and low­lying states in odd­Z nuclei at the proton drip­line. Recoils from fusion evaporation reactions were separated from other reaction products and dispersed according to their mass to charge ratio by the Fragment Mass Analyser, before being implanted into a double­sided silicon strip detector system, where their subsequent particle decays (proton or alpha) were measured. Proton emission from 157 Ta was measured for the first time, with a proton energy of 927 +/- 7 keV and a half­life 10.1 +/- 0.4 ms, This was assigned to a 3.4 +/- 1.2 % proton branch from the s 1/2 ground state on the basis of comparisons of measured half­lives with those calculated using barrier penetration codes. Two new proton lines were observed from the s 1/2 ground state and h 11/2 state in 161 Re, with energies of 1192 +/- 6 keV and 1315 +/- 7 keV and half­lives of 0.37 +/- 0.04 ms and 16 +/- 1 ms, respectively. The proton branching ratios from these states were measured at 100 +/- 7 % and 4.8 +/- 0.6 %, respectively. New proton lines were also measured from states in 171 Au and 167 Ir. Proton radioactivity from the ground state of the deformed nucleus 141 Ho was measured with an energy of 1169 +/- 8 keV and a half­life of 4.2 +/- 0.4 ms and was assigned to the 7/2- [532] Nilsson orbital on the basis of proton decay rate calculated using a deformed model of the nucleus. Recent theoretical calculations which incorporate the above results, are presen­ ted and discussed, along with a new model for calculating spectroscopic factors for spherical nuclei in the region 65 < Z < 81. The effect of a large deformation on proton decay rates is discussed with reference to 141 Ho.