Kavli Affiliate: Tadayuki Takahashi
| First 5 Authors: Hannah Gulick, Hiroki Yoneda, Tadayuki Takahashi, Claire Chen, Kazuhiro Nakazawa
| Summary:
We present measurements of the afterglow signatures in NaI(Tl) and CsI(Tl)
detector modules as part of the Background and Transient Observer (BTO) mission
detector trade-study. BTO is a NASA Student Collaboration Project flying on the
Compton Spectrometer and Imager (COSI) Small Explorer mission in 2027. The
detectors utilized in this study are cylindrical in shape with a height and
diameter of 5.1 cm and were read out by silicon photomultipliers (SiPMs). We
conducted a radiation campaign at the HIMAC accelerator in Japan where the
scintillators were irradiated with a 230 MeV/u helium beam (He beam) and 350
MeV/u carbon beam (C beam). We find that both the CsI and NaI scintillators
exhibit afterglow signatures when irradiated with the C and He beams. The CsI
crystal exhibits a stronger afterglow intensity with afterglow pulses occurring
for an average 2.40 ms for C and 0.9 ms for He after the initial particle
pulse. The duration of afterglow pulses in CsI is 8.6x and 5.6x the afterglow
signal duration in NaI for C and He (0.28 ms and 0.16 ms, respectively).
Although CsI has advantages such as a higher light yield and radiation
hardness, the stronger afterglows in the CsI detector increase the complexity
of the electronics and lead to a ~7x larger dead time per afterglow event or a
~3x higher energy threshold value. We use the measured dead times to predict
the amount of observing time lost to afterglow-inducing events for an
instrument like BTO in low Earth orbit. We simulate the background rates in a
BTO-like orbit and find a total value of 114 counts/s for the full two-detector
system. Based on the particle energies in the HIMAC experiment, we then
determine that an event with sufficient energy to produce an afterglow signal
occurs once every ~70 s and ~1.4 s in NaI and CsI detectors, respectively.
Thus, we conclude that NaI is the better choice for the BTO mission.
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