  A CubeSat with a prototype scintillating detector with a sensitive volume of
GAGG crystal is being developed with a possible launch date of 2025. Its
purpose is to characterize the background signals that mimic the neutrino
interaction that the nuSOL team is looking for. An important part of the
characterization of the backgrounds and the expected real signal is
understanding the gamma ray detection efficiency of the prototype detector. To
this end we have used and developed three techniques to do an absolute
measurement of gamma ray efficiency. The first is using EC sources that emit an
X-ray before prompt emission of a de-excitation gamma ray, specifically Zn-65
and Mn-54. The second is using a beta + decay source wherein a positron
annihilates on an atomic shell electron producing two back-to-back 511 keV
gammas followed promptly by a de-excitation gamma, specifically Na-22. The
third is using a gamma cascade of two near-simultaneous de-excitation gammas
from the same nucleus, specifically from Co-60 decay.
