  The detection efficiency of phoswich detector starts to decrease when Compton
scattering becomes significant. Events with energy deposit in both
scintillators, if not rejected, are not useful for spectral analysis as the
full energy of the incident photon cannot be reconstructed with conventional
readout. We show that once the system response is carefully calibrated, the
full energy of those double deposit events can be reconstructed using a
waveform digitizer as the readout. Our experiment suggests that the efficiency
of photopeak at 662 keV can be increased by a factor of 2 given our
LaBr$_3$/NaI phoswich detector.
