Properties of BGO
Bismuth Germanate (BGO) inorganic scintillator provides a good match to the detector
requirements of BEST. Bismuth (Z=83) has the highest atomic number of any stable
element. This fact makes it better than any other substance for both photoabsorption of
low energy photons and pair production for high energies. Just above lead (Z=82) in the
periodic table, it has all of the absorptive properties of this commonly used material.
Figure XX shows the various components of the absorption cross section for BGO while
Figure YY displays the dectection probability vs. incident photon energy for a 20 keV
deposited energy threshold.
Thallium doped Sodium Iodide (NaI:Tl) is a well known inorganic scintillator so here we
detail the performance of BGO with respect to that detector type. BGO exhibits an
extremely fast risetime at well under 100 ps (ref.) compared to __ of NaI (ref.). Although
several decay modes are present, the dominant one has a decay time of 300 ns compared
to 230 ns for NaI:Tl. BGO’s emission spectrum peaks at 480 nm (higher than NaI:Tl’s
400 nm) and is well matched to an S-11 photocathode. The light yield is about a factor of
5 lower for BGO compared to NaI:Tl, but still about the same as the popular plastic
(organic) scintillators. As a result, BGO’s energy resolution is a factor of 2 worse than
NaI:Tl, (10% FWHM for 1.369 MeV photons –ref.), still easily exceding the
requirements of BEST.
BGO’s density of 7.12 g/cm^3 (about twice NaI:Tl’s) leads to compact packaging for a
given absorption specification. Better than many of its relatives, BGO is insensitive to
water absorption. Its light yield improves with decreasing temperature (50% increase at
-5 C compared to room ambient) but so does its light emission decay constant (a factor of
2 longer for a similar temperature excursion). Inflight calibration (e.g. by cosmic ray
protons and/or He) will be necessary but easily accomodated.