  The Strontium Iodide Radiation Instrument (SIRI) is a single detector,
gamma-ray spectrometer designed to space-qualify the new scintillation detector
material europium-doped strontium iodide (SrI2:Eu) and new silicon
photomultiplier (SiPM) technology. SIRI covers the energy range from 0.04-8 MeV
and was launched into 600 km sun-synchronous orbit on Dec 3, 2018 onboard
STPSat5 with a one-year mission to investigate the detector's response to
on-orbit background radiation. The detector has an active volume of 11.6 cm3
and a photo fraction efficiency of 50% at 662 keV for gamma-rays parallel to
the long axis of the crystal. Its spectroscopic resolution of 4.3% was measured
by the FWHM of the characteristic Cs-137 gamma-ray line at 662 keV. Measured
background rates external to the trapped particle regions are 40-50 counts per
second for energies greater than 40 keV and are largely the result of short-
and long-term activation products generated by transits of the SAA and the
continual cosmic-ray bombardment. Rate maps determined from energy cuts of the
collected spectral data show the expected contributions from the various
trapped particle regions. Early spectra acquired by the instrument show the
presence of at least 10 characteristic gamma-ray lines and a beta continuum
generated by activation products within the detector and surrounding materials.
As of April 2019, the instrument has acquired over 1000 hours of data and is
expected to continue operations until the space vehicle is decommissioned in
Dec. 2019. Results indicate SrI2:Eu provides a feasible alternative to
traditional sodium iodide and cesium iodide scintillators, especially for
missions where a factor-of-two improvement in energy resolution would represent
a significant difference in scientific return. To the best of our knowledge,
SIRI is the first on-orbit use of SrI2:Eu scintillator with SiPM readouts.
