  We present measurements of nonproportionality in the scintillation light
yield of bismuth germanate (BGO) for gamma-rays with energies between 6 keV and
662 keV. The scintillation light was read out by avalanche photodiodes (APDs)
with both the BGO crystals and APDs operated at a temperature of approximately
90 K. Data were obtained using radioisotope sources to illuminate both a single
BGO crystal in a small test cryostat and a 12-element detector in a neutron
radiative beta-decay experiment. In addition one datum was obtained in a 4.6 T
magnetic field based on the bismuth K x-ray escape peak produced by a continuum
of background gamma rays in this apparatus. These measurements and comparison
to prior results were motivated by an experiment to study the radiative decay
mode of the free neutron. The combination of data taken under different
conditions yields a reasonably consistent picture for BGO nonproportionality
that should be useful for researchers employing BGO detectors at low gamma ray
energies.
