  Background: The Doppler broadening of $\gamma$-ray peaks due to nuclear
recoil from $\beta$-delayed nucleon emission can be used to measure the
energies of the nucleons. This method has never been tested using
$\beta$-delayed proton emission or applied to a recoil heavier than $A=10$.
  Purpose: To test and apply this Doppler broadening method using $\gamma$-ray
peaks from the $^{26}$P($\beta p\gamma$)$^{25}$Al decay sequence.
  Methods: A fast beam of $^{26}$P was implanted into a planar Ge detector,
which was used as a $^{26}$P $\beta$-decay trigger. The SeGA array of
high-purity Ge detectors was used to detect $\gamma$ rays from the
$^{26}$P($\beta p\gamma$)$^{25}$Al decay sequence.
  Results: Radiative Doppler broadening in $\beta$-delayed proton-$\gamma$
decay was observed for the first time. The Doppler broadening analysis method
was verified using the 1613 keV $\gamma$-ray line for which the proton energies
were previously known. The 1776 keV $\gamma$ ray de-exciting the 2720 keV
$^{25}$Al level was observed in $^{26}$P($\beta p\gamma$)$^{25}$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 $\beta$-delayed nucleon emissions
populating excited states of nuclear recoils at least as heavy as $A=25$.

Nuclear Experiment