Observation of Doppler broadening in $β$-delayed proton-$γ$ decay

Background: The Doppler broadening of $γ$-ray peaks due to nuclear recoil from $β$-delayed nucleon emission can be used to measure the energies of the nucleons. This method has never been tested using $β$-delayed proton emission or applied to a recoil heavier than $A=10$. Purpose: To test and apply this Doppler broadening method using $γ$-ray peaks from the $^{26}$P($βpγ$)$^{25}$Al decay sequence. Methods: A fast beam of $^{26}$P was implanted into a planar Ge detector, which was used as a $^{26}$P $β$-decay trigger. The SeGA array of high-purity Ge detectors was used to detect $γ$ rays from the $^{26}$P($βpγ$)$^{25}$Al decay sequence. Results: Radiative Doppler broadening in $β$-delayed proton-$γ$ decay was observed for the first time. The Doppler broadening analysis method was verified using the 1613 keV $γ$-ray line for which the proton energies were previously known. The 1776 keV $γ$ ray de-exciting the 2720 keV $^{25}$Al level was observed in $^{26}$P($βpγ$)$^{25}$Al decay for the first time and used to determine that the center-of-mass energy of the proton emission feeding the 2720-keV level is 5.1 $\pm$ 1.0 (stat.) $\pm$ 0.6 (syst.) MeV, corresponding to a $^{26}$Si excitation energy of 13.3 $\pm$ 1.0 (stat.) $\pm$ 0.6 (syst.) MeV for the proton-emitting level. Conclusions: The Doppler broadening method has been demonstrated to provide practical measurements of the energies for $β$-delayed nucleon emissions populating excited states of nuclear recoils at least as heavy as $A=25$.