# Evaluation of a Positron-Emission-Tomography-based SiPM readout
for Compact Segmented Neutron Imagers

V. A. Li<EMAIL_ADDRESS>F. Sutanto<EMAIL_ADDRESS>T. M. Classen  S. A.
Dazeley  I. Jovanovic  T. C. Wu  Lawrence Livermore National Laboratory,
Livermore, CA 94550  Department of Nuclear Engineering and Radiological
Sciences, University of Michigan, Ann Arbor, MI 48109

###### Abstract

Gamma-ray emission from special nuclear material (SNM) is relatively easy to
shield from detection using modest amounts of high-Z material. In contrast,
fast-neutrons are much more penetrating and can escape relatively thick high-Z
shielding without losing significant energy. Furthermore, fast neutrons
provide a clear and unambiguous signature of the presence of SNM with few
competing natural background sources. The challenge of detecting fast neutrons
is twofold. First, the neutron flux from SNM are only a fraction of the
corresponding gamma-ray flux. Second, fast neutrons can be difficult to
differentiate from gamma rays. The ability to discriminate gamma rays from
neutrons combined with neutron imaging can yield large benefit to isolate the
localized SNM neutron source from background. With the recent developments of
pulse-shape-sensitive plastic scintillators that offer excellent gamma-
ray/neutron discrimination, and arrays of silicon photomultipliers combined
with highly scalable and fast positron-emission-tomography (PET) multi-channel
readout systems, field-deployable neutron imagers suitable for SNM detection
might now be within reach. In this paper, we present a characterization of the
performance of a recently available commercial PET-scanner readout, including
its sensitivity to pulse-shape differences between fast neutrons and gamma
rays, energy and timing resolution, as well as linearity and dynamic range. We
find that, while the pulse-shape discrimination is achievable with stilbene,
further improvement of the readout is required to achieve it with the best
available plastic scintillators. The time and energy resolution appear to be
adequate for neutron imaging in some circumstances.

###### keywords:

compact neutron imager , pulse-shape discrimination , segmented plastic
scintillator , SiPM arrays , low-power electronics

††journal: Nuclear Instruments and Methods in Physics Research, Part A

##  1 Single-volume neutron-scatter imagers

Worldwide, there are now multiple efforts to develop a “single-volume”
directional neutron detectors [1, 2, 3, 4, 5, 6, 7, 8, 9], motivated primarily
by a desire to reduce size and complexity of neutron imagers. An effective way
to achieve the detection and characterization of multiple neutron scatters is
to use an array of independent detector segments. Better resolution can be
achieved by using smaller segments, since the direction uncertainty depends on
the interaction position uncertainty, and hence it is inversely proportional
to the segment dimensions. The following summarizes some of the key desirable
characteristics of a neutron scatter camera:

  * 1.

Small and mobile

  * 2.

Safe to operate (no liquids or gases under pressure)

  * 3.

Discriminates between gamma rays and neutrons

  * 4.

Uses a scalable, multi-channel, low-power electronic readout

  * 5.

Uses a scalable and inexpensive detection medium

The recent availability of new multi-channel optical sensors in the form of
silicon photomultiplier (SiPM) arrays [10] and pulse-shape discrimination
(PSD)-capable plastic scintillators [11, 12, 13] and organic glass
scintillators [14] are important developments towards the realization of these
detectors. One key missing component however, is a high-channel-density,
lightweight, low-power electronic readout capable of simultaneously measuring
the particle energy and PSD. For example, in our previous setup, like in many
multi-channel systems, we used VME-based full-waveform digitizers mounted in a
crate [15, 16], a solution which was heavy and with high-power consumption.
The important characteristics of an electronic data acquisition system
include:

  * 1.

Good energy resolution over a large dynamic range

  * 2.

Fast sub-nanosecond timing (depending on the detector active volume) to obtain
neutron energy from time of flight, needed for neutron directionality

  * 3.

Fine position resolution on neutron interaction vertices

  * 4.

Preserving pulse-shape information

Figure 1: Rendering of the detector module. The 8$\times$8 array of 6Li-doped
PSD plastic rods (5.4 mm $\times$ 5.4 mm $\times$ 11 cm each) with $\sim$1 mm
spacing in between. Two 8$\times$8-pixel SiPM arrays are coupled to the
scintillator via optical grease. At the back of each SiPM arrays is a splitter
board (shown in red), two carrier boards, each with a 64-channel TOFPET2c
ASIC. Guiding support frames and ribbon cables are not shown for clarity.

The rapid development of specialized technologies for positron emission
tomography (PET), which require fast timing over vast arrays of channels, has
reduced the cost to $\mathcal{O}$($10)/channel with a time resolution of a
few-hundred picoseconds [17, 18]. In the context of neutron imaging, one
important question however, is whether readouts of this type can preserve a
sufficient amount of pulse shape information.

##  2 Technical Approach and Performance

For this study, we chose an application-specific integrated circuit
(ASIC)-based readout system newly developed by the PETsys Electronics
corporation for PET applications. Unlike the full-waveform readouts, the
PETsys readout only outputs the charge and time of each interaction,
colloquially referred to as Qs and Ts. Therefore, a fully digitized waveform
is unavailable for digital pulse processing. This limitation can be addressed
by splitting the signal from each SiPM pixel into two independent readout
channels with different integration times. In this way, some of the
information about the pulse shape can be preserved. The two split channels are
referred to as Head and Total, where Head is integrated over a shorter period
($\sim$100 ns), while Total is integrated over a longer period ($\sim$400 ns).
Particle interaction types are identified on the basis of the ratio of the
Head and the Total signal for each pixel. Unlike waveform digitizers, where
the integration times can be optimized after digitizing the waveforms, the
integration bounds of these PET-based systems must be set at the trigger level
(before data collection).

To test the PETsys readout, we constructed a prototype system referred to in
the following as the intelligent Segmented Autonomous Neutron Directional
Detector (iSANDD). The iSANDD consists of sixty-four 6Li-doped PSD plastic
scintillator rods, each of dimension 5.4 mm $\times$ 5.4 mm $\times$ 110 mm,
coupled to two 8$\times$8-pixel Sensl J-60035 SiPM arrays. The readout of each
SiPM is mounted at the back. A rendering of the primary detection module is
shown in Fig. 1. In the following, we define the axes $x$ and $y$ as being in
the plane of SiPM array face. The $z$-axis runs parallel to the scintillator
rods. The signals from each pixel are split into two (red board) and then
digitized into $T$ and $Q$ values using the green TOFPET2c boards. The $T$ and
$Q$ values are then transmitted digitally via a ribbon cable to a mezzanine
board (not shown). In our data analysis, we require the charge $Q$ and
timestamp $T$ to be available on both ends of a single rod, as illustrated in
Fig. 2. The $z$-coordinate is defined as the distance along the $z$-axis from
the center of each rod. The mezzanine board is shown in Fig. 3 inside the blue
box. The mezzanine board accepts up to 1024 channels from up to 8 ribbon
cables. It incorporates the main clock so that all 1024 channels can be
assigned global timestamps. It also has an FPGA, which enables the user to set
trigger, gain and integration parameters. The SiPM voltage is set via the
PETsys software.

Figure 2: The principle of signal splitting for a single scintillator rod. The
scintillation light propagates along the rod (along the $z$-axis) via total
internal reflection. SiPM pixels on both ends of the rod transform the light
pulse into an electrical signal. A signal from a single SiPM pixel is split
into two independent ASIC channels with different integration times: short
(“Head”) and long (“Total”). Each readout channel reports charge and
timestamp. The $z$ coordinate of where the scintillation light originates is
reconstructed with respect to the center of the rod, either by using the ratio
of Total charges or timestamp differences on opposite sides of the rod. Figure
3: The entire system is placed on a cart. The DAQ computer and power supply
are located at the bottom shelf of the cart. The light-tight temperature-
controlled enclosure and the mezzanine card (blue box) are situated at the top
of the cart.

One of the complications associated with the use of this system is its
sensitivity to temperature variations. We constructed a dark box covered with
thermally-insulating foam to stabilize the temperature. A Peltier element and
a radiator with a fan are mounted on the top of dark box. The Peltier element
voltage is set at 20 V and the current draw is 2 A. We also equipped each ASIC
with a heat sink and a fan mounted on top of the FEM128, as shwon in Fig. 4.
In our tests, the ASIC temperatures normally settled at about 30 ∘C. An
interlock system was developed to automatically shut down the system if the
temperature on any ASIC rises above 35 ∘C. Additional humidity and temperature
sensors were also mounted within the dark box. Having a humidity sensor is
particularly important to ensure that there is no unwanted condensation inside
the light-tight enclosure due to the Peltier element. Two connections were
provided to the mezzanine board: 12-V power and Ethernet, which provides
connection to a Linux desktop. The nominal power draw of the mezzanine alone
is 12 W; each FEM128 adds 3 W. For our 256-channel system, the power draw was
18 W. For a short mobility test we successfully powered and operated the
electronics readout, including the SiPM bias and cooling fans but not
including the Peltier element, by a 12-V battery pack. The desktop computer
has 3 Ethernet interfaces: internal network, communication with PETsys, and
communication with the power supply. The entire system, including the power
supply and the DAQ computer, is mobile and compact, and was placed on a small
hand-cart, as shown in Fig. 3.

To characterize each interaction, the charge and relative timing of signals
were recorded. Each pixel signal is split into two readout channels, which are
used to record the integration times for Head and Total charge. To determine
the event position within a scintillator rod, the ratio of Total charge
recorded at both ends of the rod is used. Since both the Head and Total are
needed to determine the PSD, the minimum requirement for physics events,
therefore, is a Total and Head trigger on both ends of a single rod. In the
offline analysis, the requirement is therefore four channels (per rod) for an
event to be considered a physics event. There is some tuning of the data
acquisition parameter settings required before physics data can be taken. For
example, the Head and Total integration times need to be set before hand. The
SiPM operating voltage was set to 30 V, and the electronics gain was set to
the highest setting. PETsys uses three different acquisition modes: QDC, TOT,
and mixed [17]. The TOT mode records the time over the threshold. The QDC mode
records an integrated charge for a preset time window. In most tests described
in this study, we used the QDC mode and the same thresholds for all Head and
Total channels. The integration time windows were set to approximately 100 ns
for Head and 400 ns for Total.

Figure 4: iSANDD module incorporating 64 6Li-doped (0.1% wt) plastic PSD
scintillator rods ($110\times 5.4\times 5.4$ mm3) with SiPM arrays and compact
electronics readout. A lead collimator is placed on an motorized translation
stage to calibrate the detector response

SiPM A

SiPM B

Figure 5: SiPM response with scintillator rods coupled. The collimated 137Cs
gamma-ray fan-like beam is oriented at the center of the rods. The pixel
responses are normalized to pixel (3, 3) on SiPM B.

137Cs

54Mn

22Na

Figure 6: Gamma spectra for center-collimated sources: 137Cs, 54Mn, and 22Na.
The response is recorded across multiple rods and is background-subtracted.

The scintillator rods are aligned with the SiPM array using a special support
frame, shown in Fig. 4. Optical grease (EJ-550) is applied to ensure minimal
refractive index mismatch due to an air gap between the scintillator and the
SiPM. The rods are separated from each other by a $\sim$1-mm air gap to ensure
total internal reflection (TIR) within each rod.

###  2.1 Energy characterization

To characterize the energy response and resolution as a function of position
along the scintillator rods, a fan-like beam of gamma rays, was directed
perpendicular to the scintillator rods, as shown in Fig. 4. The figure shows a
motorized translation stage used to automate the detector calibration for any
$z$-position. The translation stage incorporated a 3D-printed holder to
support a lead-brick collimator and a gamma-ray source. With the collimator
set to the center of the scintillator array, the response of each of the
pixels was measured, as shown in Fig. 5. This process is referred to
colloquially as “flat-fielding”, with the ultimate aim of generating a map of
normalization factors for all pixels. The normalization map is used to ensure
consistent response across both SiPM arrays. Spectra generated from 137Cs,
54Mn, and 22Na are shown in Fig. 6, with the collimated beam at the center of
the scintillator array. The mean energy resolution, as measured by a Gaussian
fit of the Compton edge of each of the 64 scintillator rods, was 22% for
137Cs. This was somewhat worse than the resolution of the same detector
($\sim$16%) measured using full-waveform digitizers [15].

Figure 7: The linearity of the energy response obtained from the summed-rod
Compton-edge positions shown in Fig. 6

The linearity of the detector energy response up to 1274 keV is shown in Fig.
7. The line of best fit of the four gamma-ray points intersects the y-axis at
$-0.0013\pm 0.0155$, consistent with the line through the origin.

The effective attenuation length along each of the rods was measured using a
collimated gamma-ray beam at known $z$-positions and the relative signal
strength as measured by the pixels at either end of each scintillator rod, as
shown in Fig. 8.

Figure 8: Attenuation length measurement on SiPM arrays A and B. The
collimated gamma-ray source was moved along the rods.

The position uncertainty can be measured using relative charge as measured at
both ends of the scintillator rod. This ratio is called the “AB” ratio and is
defined as

$R_{AB}=\frac{Q_{A}}{Q_{A}+Q_{B}},$ (1)

where $Q_{A}$ and $Q_{B}$ are the measured charges at both ends of each
scintillator rod. The AB ratio as a function of position is shown in Fig. 9.
The uncertainties shown represent the 1-sigma width of a Gaussian fitted to
the AB ratio at each position and, hence, the position uncertainty.

Figure 9: Dependence of position of interaction along the rod on the charge
ratio $R_{AB}$.

A peculiarity of the PETsys readout was an observation of seemingly random
baseline and gain shifts on a run-to-run basis. Our preliminary results
indicated that these shifts were aggravated by temperature fluctuations. We
also operated iSANDD outside of the temperature controlled enclosure to
measure the effect of changes in temperature on the energy response. We
noticed that the combination of baseline drift and energy response caused as
much as 40% drop in the 511-keV Compton edge ADC value for a 3-degree
temperature increase. For 1274 keV, the equivalent temperature change caused a
deviation of $\sim$15%.

Figure 10: Timing offsets measured between two pairs of pixels using a laser
and a center-collimated 137Cs source show consistency in the mean values and
the width of the distributions. Figure 11: Laser calibration system. The
picosecond laser beam is fed into the enclosure via a fiber and collimator.
The beam is diffused such that multiple pixels are illuminated to study and
calibrate timing response across the SiPM arrays. A reference 1-inch PMT is
also shown in the top right corner. A neutral-density filter is shown covering
several SiPM pixels to study the timing response to varying relative light
intensity between the pixels.

SiPM A

SiPM B

Figure 12: Laser calibration measurements. Left column: SiPM A; Right column:
SiPM B. Top:  Gaussian widths of the timing variations in laser tests for two
8$\times$8 SiPM arrays. Middle:  Mean charge distribution. The nonuniformity
of response is primarily due to the nonuniformity of the diffused laser beam,
as seen by comparison with Fig. 5. Bottom:  Mean time shift. A lookup table
for correction is constructed based on these measurements. For the “Time
Width” and “Mean Time Shift” plots, the pixel (3,3) on the SiPM array B was
used as a reference.

(a)

(b)

(c)

Figure 13: Timing measurements: (a) time spread of only one pixel [SiPM A
(X,Y) : (3,1)] with respect to the reference pixel [SiPM B (X,Y) : (3,3)] with
laser intensity varied across all pixels at the same time; (b) ratio of time
widths for 127 pairs of signal with respect to the reference pixel of between
dim and bright signals (marked as ’low’ and ’high’ intensities, shown as blue
and purple lines on the subfigure (a) for one pair of the pixels); (c) Timing
between two pixels (one pixel had a varying ND filter, as shown in Fig. 11. In
the subfigures (a) and (c), the $\sim$5-ns artifacts are due to the
electronics readout internal 200-MHz clock.

###  2.2 Characterization of detector timing

Neutron double-scatter imaging requires accurate knowledge of the neutron
energy after the first scatter, which can be obtained from a time-of-flight
measurement between two neutron scatters. For time of flight, we need to
ensure a relative timing calibration for all of the channels, since each
channel can have different timing offsets and/or resolutions.

To measure timing offsets, we directed a collimated fan-like beam of gamma
rays at the center of the scintillator array so that the relative timing of
the two pixels at either end of each scintillator rod can be measured. An
example of two pairs of pixels (viewing two rods) is shown in Fig. 10. For the
example shown, we selected two rods (two pairs of pixels) with a large
difference in offset.

In practice, it is difficult to compare the timing of pixels unless they are
paired by a single scintillator rod. Therefore, this method cannot be used to
obtain the timing offsets of all the possible pairs of pixels. For this
purpose, we developed a laser setup that can illuminate all the pixels of each
SiPM array simultaneously, as shown in Fig. 11. The laser (Picoquant DL-800D
with a 405-nm laser head LDH-P-C) has a pulse rise time of $\leq$25 ps.
However, the light is propagated via a 1-meter multimode fiber from the laser
head to the light-tight enclosure, which has the potential to smear the rise
time. To illuminate the pixels uniformly, the light was directed from the end
of the fiber through a collimator and then a diffuser consisting of several
layers of PTFE tape.

It was measured that the standard deviations in the timing difference between
Total and Head channels, viewing the same pixel, are $\leq$100 picoseconds,
which means that it is possible to use either Head or Total timestamps in the
analysis; for clarity, we only use timing obtained with Total channels.

The timing uncertainty between any SiPM pixel and a reference pixel (pixel 3/3
on SiPM array B) was found to be between $\sim$500–1000 ps (Fig. 12). One
important test is to ensure that the relative timing offsets as measured by
the laser are the same as those measured by the center-collimated gamma-ray
source, as shown in Fig. 10. It can also be seen that the width of the
distributions is dependent on the brightness of the signal. First, we show an
example of the timing dependence on intensity. In Fig. 13 the time difference
between two pixels is plotted for six different laser intensities, while the
values of mean charges of those pixels are reported in the legend. For
brighter laser settings the timing distributions get narrower; this trend
holds for the majority of pixels. Figure 13b shows the distribution of ratios
for timing widths for 127 pixels relative to a single reference pixel using
two different laser brightness settings.

We also used a set of neutral-density filters to explore a scenario where the
relative light intensity between a pixel and the reference pixel changes; this
is shown in Fig. 11. As the brightness observed by the pixel changes, the
offsets move by a few hundred picoseconds (Fig. 13c).

After we calibrated the detector, it was possible to reconstruct position
using timing as shown in Fig. 14 and using 137Cs data.

Figure 14: Position reconstruction along the rods using timing difference
between pixels on the opposite sides using a collimated 137Cs source

###  2.3 Position resolution

Given the previous measurements of timing and energy resolution, it is worth
examining the position uncertainty for the iSANDD along the $z$-axis. The
scintillator used in this work is 6Li-doped PSD plastic with a light yield of
approximately 9,000 photons per MeV [16]. For the iSANDD, the scintillator
bars were about the same length of the first SANDD prototype [15], which used
undoped PSD plastic. The yield of all three detector configurations were
consistent with commercial plastic (within 10%).

We examined the $z$-position resolution that results from timing and energy
separately. For energy, we can reconstruct $z$-coordinate of the interaction
using charge ratio $R_{AB}$, as shown in Fig. 9. From the figure, the
estimated position resolution measured with a 137Cs source was $\sim$2 cm. The
position uncertainty as a function of energy measured with a collimated 22Na
source is shown in Fig. 15. When comparing these results with the earlier
SANDD prototype [15], which used full-waveform digitization, the position
resolution observed here was $\sim$50% worse over the energy range of 0.2–1.2
MeVee.

The position resolution due to timing was also measured. The position
resolution using the 22Na data is shown in Fig. 15.

Time-based:

Charge-based:

Figure 15: The $z$-position uncertainty calculated as a function of energy —
time-based (top panel) and charge-based (bottom panel), both with using the
22Na data. The charge-based uncertainty is slightly smaller compared to the
time-based one.

###  2.4 Pulse-shape discrimination

As mentioned previously, a pulse-shape parameter (PSP) was defined using the
Total and Head charges from each pixel:

$PSP=\frac{Q_{tail}}{Q_{total}}=\frac{Q_{total}-Q_{head}}{Q_{total}}.$ (2)

Using a 252Cf source, the iSANDD proved insensitive to particle-interaction
type via PSD, unlike earlier measurements taken with full-waveform digitizer
readout [15]. Since the characteristic of the scintillator and the
photodetectors were similar in both cases, the significant deterioration in
PSD sensitivity is attributed to the electronics readout.

To further investigate PSD sensitivity with the PETsys readout, a stilbene
crystal was coupled to the SiPM array, as shown in Fig. 16. This orientation
and the stilbene crystal as the scintilator were chosen wiht a goal to improve
the PSD performance.

After switching to stilbene, modest improvement in PSD sensitivity was
observed, as shown in Fig. 17. However, swapping the stilbene crystal for the
PSD plastic scintillator in the same orientation showed only modest PSD
sensitivity.

One problem that may be impacting sensitivity to PSD is that, at present time
there is a lack of flexibility in terms of the range of integration times for
Head and Total. For example, the Head integration time cannot be set below
$\sim$100 ns, and the Total integration time cannot be set greater than
$\sim$400 ns. It is hoped that future iterations of the PETsys ASIC and the
splitter board would allow for greater flexibility in integration time
selection.

Figure 16: A stilbene crystal (6 mm $\times$ 6 mm $\times$ 50 mm) wrapped in
PTFE tape on five sides and optically coupled to 8 pixels of the SiPM array
via EJ-550 silicon grease.

Stilbene (252Cf)

Stilbene (137Cs)

Plastic (252Cf)

Figure 17: Examples of detector response for stilbene and PSD plastic, for the
same pixel. Both the stilbene crystal and the plastic rod were wrapped in
teflon and oriented as shown in Fig. 16. Units of charge are PETsys ADC units.
The gamma-ray band in the stilbene 252Cf plot has a slightly higher PSD value,
which might be due to a $\sim$10 times higher event rate.

(10 neutrons)

(100 neutrons)

(829 neutrons)

Figure 18: Neutron directionality maps based on a simulation of 2.45-MeV
neutrons scattering twice within the detector. The red triangle indicates the
true position of the source (azimuth = 158.2∘, altitude = 32.4∘). Each double-
scatter event represents a ring on the $4\pi$ sky map. As the number of double
scatters increases (from left to right), the true position of the source is
more clearly revealed particularly as the number of double-scatters increases
from 10 to 100. Gaussian smearing is applied. The selection criteria are
described in the text.

###  2.5 Assessing neutron-imaging capabilities

In this section, we predict the imaging capabilities of the PETsys system
using the measured energy, timing, and position resolution, while assuming
that some degree of PSD sensitivity improvement can be realized. This analysis
can help determine whether the energy, timing, and position resolutions are
sufficient for practical neutron imaging.

A Geant4-based [19] simulation was performed, where the detector performance
parameters measured with the PETsys readout were used as input parameters. We
also assumed a quenching factor, such that a proton recoil of 1 MeV is seen as
$\sim$0.16 MeVee, while a 2-MeV recoil — $\sim$0.5 MeVee. Figure 18 shows the
result of this simulation for monoenergetic 2.45-MeV neutrons for a random
selection of varying numbers of double-scattered neutrons. The cone with an
opening angle $\theta$ of possible incoming neutron direction is defined as

$\tan\theta=\sqrt{\frac{E_{p}}{E_{\mathrm{TOF}}}}=\frac{t_{2}-t_{1}}{\left|\mathbf{r}_{2}-\mathbf{r}_{1}\right|}\sqrt{\frac{2E_{p}}{m}},$
(3)

where $m$ is the neutron mass, $E_{p}$ is the proton kinetic energy, and
$(\mathbf{r}_{1},t_{1})$ and $(\mathbf{r}_{2},t_{2})$ are positions and times
of the 1st and 2nd scatters. The timing, energy, and position uncertainties
are propagated to calculate the width of each neutron-scatter “ring”.

The following cuts were set: segment multiplicity = 2; time difference between
segment hits $>$1 ns; quenched energy deposit in each segment $>$0.3 MeVee;
distance between segment hits $>$3 cm. Energy is quenched based on a LS
quenching profile and smeared for photostatistics based on 20%$/\sqrt{E}$. Hit
times are smeared by 750 ps; Hit positions are given by the $x$ and $y$
centers of the segment hit, and with a 2 cm smearing on the position along the
rod ($z$-coordinate). The simulation generated $5\times 10^{6}$ 2.45-MeV
neutrons in a cone aimed at the detector from one meter away such that the
opening angle had $\cos\theta=0.99$. This corresponds to a $\sim$109 isotropic
neutron source at a 1 m distance.

##  3 Conclusion

We assessed the suitability of one example of an ASIC-based miniaturized data
acquisition electronics readout, developed for PET-imaging, for neutron
imaging applications. The readout was tested with a 64-rod 6Li-doped PSD
plastic scintillator prototype, coupled to two 64-pixel SiPM arrays. The 128
individual SiPM pixels were each split into a short- and long-integration
window, and read out by 256 independent electronics channels. The short- and
long-integration windows were combined to create a PSD metric. Energy and
timing measurements indicated resolutions of $\sim$20% (at the 137Cs Compton
edge) and $\sim$750 ps, respectively. The $z$-position resolution was measured
to be $\sim$2 cm using either timing or the charge ratio $R_{AB}$. The
position resolution for $(x,y,z)$ was measured as $(0.5,0.5,2)$ cm. The
resolutions in $x$ and $y$ are 0.5 cm due to segmentation.

The $z$-position uncertainty using the charge ratio ($R_{AB}$) was somewhat
worse than our previous measurements of a similar prototype with full-waveform
digitization [15]. This reduction in position sensitivity is likely caused by
the worsening in energy resolution. We theorize that this may be partly due to
the splitting of the SiPM signals into two independent electronics channels
used to integrate Head and Total charge.

The $z$-position resolution based on timing was a factor $\sim$1.4 worse than
recently reported measurements using fast full-waveform digitizers and fast
commercial plastic scintillators [20]. Since the plastic scintillator used in
this work is not optimized for speed, it is possible that the scintillator
contributes somewhat to the degraded position resolution. The worsening due to
the electronics readout alone can therefore be constrained to $\lesssim$1.4.

To achieve PSD performance sufficient for reliable gamma-ray background
rejection, further improvements to the electronics readout scheme will be
needed. One possible cause might be the increased noise associated with the
Head and Total charge-splitting mentioned earlier. It is possible that the
splitting of the signal from each pixel is a contributing factor to noise and
therefore inferior PSD. Another possible avenue for improvement is waveform
digitization by ASIC-based readouts.

Another problem we have identified with the PETsys readout is a tendency for
the baseline (the pedestal) to shift as a result of temperature fluctuations.
This problem can impact the apparent energy resolution if the temperature
shifts over the course of a data acquisition run. We did not investigate the
impact of temperature fluctuations on PSD. In a field application of iSANDD,
one would have to implement an algorithm that determines the baseline and gain
for every channel as a function of temperature. A suitable way to monitor
baseline shifts would be to trigger the electronics channels externally in
order to set a pedestal.

To estimate the likely angular sensitivity of a neutron-scatter camera with
energy, timing, and position resolutions as presented here, we performed
simulations of a flux of DD-like fast neutrons incident on the camera. These
simulations indicated adequate directional sensitivity from 100 neutron
double-scatter events. Only limited improvements in angular sensitivity were
gained from $\gtrsim$100 such events.

##  4 Acknowledgements

The scintillator used in this project was synthesized by Andrew Mabe, and is
of the same composition as used in the SANDD [16]. We thank Natalia Zaitseva,
Glenn Jocher, and Melinda Sweaney for useful discussions, Nathaniel Bowden for
the interest in the project, and Dom Porcincula for designing and 3D-printing
various mechanical parts used in this project. We further acknowledge the
discussions with PETsys support staff and with Shiva Abbaszadeh.

This work was supported by the U.S. Department of Energy National Nuclear
Security Administration and Lawrence Livermore National Laboratory [Contract
No. DE-AC52-07NA27344, LDRD tracking number 21-FS-025, release number LLNL-
JRNL-830322] and the Consortium for Monitoring, Technology, and Verification
(DE-NA0003920). The research of T. C. W. was partially supported by the
Department of Energy, Nuclear Energy University Program Fellowship.

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