Reviewing the evidence for two-proton emission from the high-spin isomer inAg94

Abstract

The emission of two protons, a highly exotic decay process, has been reported from the high-spin ${21}^{+}$ isomer in $^{94}\mathrm{Ag}$ [I. Mukha et al., Nature (London) 439, 298 (2006)]. Recent experimental results from $\ensuremath{\gamma}$-ray spectroscopy and Penning trap mass measurements, as well as theoretical predictions, are difficult to reconcile with this claim. These anomalies motivate a closer look at the supporting evidence. A detailed review of this evidence suggests that is it unsafe to conclude that the methods used in the two-proton identification experiment demonstrate an ``unambiguous signature'' for two-proton emission. Because the apparatus used could not distinguish protons from positrons and electrons, the identification stands or falls on coincidences with $\ensuremath{\gamma}$ rays in the daughter nucleus $^{92}\mathrm{Rh}$. It is shown that spurious peaks from Compton-scattered $\ensuremath{\gamma}$ rays associated with the dominant background from $^{94}\mathrm{Ag}$ $\ensuremath{\beta}$ decay could have been misidentified as $^{92}\mathrm{Rh}$ $\ensuremath{\gamma}$ rays.