Kavli Affiliate: Tadayuki Takahashi
| First 5 Authors: Shunsaku Nagasawa, Shunsaku Nagasawa, , ,
| Summary:
The energy range from a few hundred keV to a few MeV includes important
probes such as nuclear gamma-rays and the 511 keV annihilation line. However,
compared to X-rays and GeV/TeV gamma-rays, this range suffers from lower
sensitivity by orders of magnitude. The upcoming NASA SMEX satellite mission
Compton Spectrometer and Imager (COSI), scheduled for launch in 2027, is
expected to break through this limitation with its Compton telescope utilizing
a germanium semiconductor detector, covering the 0.2-5 MeV energy range. In
addition to the main instrument, two Background and Transient Observer (BTO)
detectors will be installed on COSI. The detectors are NaI(Tl) scintillators
coupled with SiPMs, and they are being developed as a student collaboration
project. BTO aims to 1) measure background radiation in orbit to maximize
COSI’s sensitivity and 2) detect GRBs and other gamma-ray transients. For this
purpose, it is required to cover the lower-energy range from 30 keV to 2 MeV
with < 20% FWHM energy resolution. In addition, large signals and afterglow
generated by heavy ions penetrating the NaI(Tl) crystal should be appropriately
handled. To address these requirements, we have developed a compact signal
processing and data acquisition system comprised of two main components: an
analog board and a digital board. The analog board amplifies signals from the
SiPM, generates triggers, and performs AD conversion. The digital board
features a Microchip SAMV71 microcontroller, and we developed the software to
control the analog board, read ADC data via SPI interface, add timestamps, and
buffer event data. Through this development, we achieved the required wide
bandpass and an energy resolution of 10% FWHM at 662 keV with a processing time
of 20 us per event. We also implemented veto signal generation for large
signals using a discriminator and an onboard detection algorithm for transient
events.
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