IEEE - Aerospace and Electronic Systems - March 2020 - 61

Hardgrove et al.

Figure 8.
Plot of the PSD ratio versus the energy deposited in equivalent
electrons (gamma ray) energy.

Figure 7.
Photograph of the packaged crystal module. The open quartz window shows the inside of the package and is the spot where the
PMT will optically coupled to the module.

particles, but the energy deposition is smaller due to the
smaller Q value.
The transient information from the light flash is captured by the readout electronics, and a pulse shape discrimination (PSD) metric is used to determine if a particle
is a gamma ray or a neutron. The PSD metric is a ratio of
a short integral that captures the light yield from fast
decay processes (<200 ns) over a longer integral that captures the yield from moderate decay processes (<1 ms).
As the longer integral scales with the energy deposited
(full integral 0 to 6 ms), the ratio of the short to long integrals highlights the contribution of the fast decay processes that produce light in the crystal, particularly when
plotted as the PSD ratio versus the energy deposited
(Figure 8). The associated energy states in the crystals to
these faster decay processes are populated more readily
for gamma rays compared to neutron capture events. The
energy calibrated PSD analysis shows a primary neutron
peak around 3 MeV, which is associated with the 6Li reaction and results in particles with a kinetic energy no less
than 4.8 MeV. Events below this peak cannot be caused
by the 6Li reaction, as they deposit less energy. These are
taken as 35Cl reactions, where the protons produce a light
flash similar to the triton and alpha particles from the
6
Li reaction. Above the neutron peak, higher energy neutrons impart their energy into the triton and a particles
indicating more energy deposition. Based on the kinematics of these reactions, epithermal neutrons are within the
dominate neutron peak around 3 MeV in equivalent electron energy. The background signals from GCR will
be primarily above this equivalent electron energy, and
these high-energy protons will produce a signal similar to
gamma rays. The data captured within this region (events
"Gamma Rays" in Figure 7 that lie above and to the right
of the events labeled "Neutrons") provides a measure of
the GCR flux.
MARCH 2020

In the early phases of development [19], the crystals
were packaged and thermally cycled to characterize the
response of the CLYC crystals to the expected thermal
environment. Thermal gradients over the crystals could be
significant due to its high thermal capacity (0.4 J/g/K) and
low conduction (0.007 W/cm/K at 50 C). Since the gradient is dependent on mass, geometry, and thermal rates,
testing has been conducted to establish the nominal instrument operational conditions. We imposed a rate of 0.5 C/
min, which is within the anticipated rate from thermal
modules during flight. Further measures to prevent light
loss from crystal degradation due to thermal cycling
include using a thicker optical gel pad and a softer foam
backing in the Mini-NS module design.

READOUT ELECTRONICS
The signals from the PMTs are summed to a single analog
to digital converter operated at 250 mega-samples per second. The digitized waveform from the detector is captured
by an FPGA (Xilinx Zynq XQ7Z020). In addition to the
summed digitized waveform, the voltage drop from the
last dynode stage on the PMT is sent into a trigger to generate a logic pulse (module hit identifier). The logic pulse
is used to identify which PMT generated the digitized
waveform. The FPGA logic will generate another trigger
to start the integration of the waveform. The algorithm in
the FPGA will generate four integrals: baseline integral,
short integral, long integral, and full integral. Each of the
last three integrals is corrected using the baseline integral.
The short and long integrals are used to generate the PSD
Ratio and the full integral is used to generate the energy
deposition.
A local time and set of temperatures (module and
boards) are captured in the FPGA (using a counter 26-bit
resolution with $200 ms accuracy). The event-by-event
data product is the most comprehensive product generated
on the instrument, consisting of an event number, module

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