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Long duration balloon flight exposure of a Ce:LaBr3 crystal

Cerium-doped lanthanum bromide is an attractive scintillator for use in hard x-ray/low energy gamma ray astronomy instruments. Its high light output and correspondingly excellent energy resolution, fast decay time, and relatively high density offer advantages, but the relatively high internal background due to the presence of the naturally occurring radioactive isotope 138La is a disadvantage. It is desirable to expose LaBr3 to a near-space environment in order to investigate the effects of space exposure on the internal background of the crystal. We have flown an unshielded 1 inch diameter cylindrical LaBr3 and an identical NaI(Tl) scintillator on the ATIC long duration balloon flight from Antarctica for a total of 16 days exposure. We present results from this test, including the background rates at float altitude and changes in the measured spectra over time. No indication is seen of any increase in rate due to activation. The results indicate that LaBr3 appears to be satisfactory as a balloon- or satellite-borne x-ray/gamma ray detector.