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 P26(βpγ)Al25 decay sequence. Methods: A fast beam of P26 was implanted into a planar Ge detector, which was used as a P26 β-decay trigger. The SeGA array of high-purity Ge detectors was used to detect γ rays from the P26(βpγ)Al25 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 Al25 level was observed in P26(βpγ)Al25 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±1.0 (stat.) ±0.6 (syst.) MeV, corresponding to a Si26 excitation energy of 13.3±1.0 (stat.) ±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.