# Performance of large-scale 6Li-doped pulse-shape discriminating plastic
scintillators

C. Roca<EMAIL_ADDRESS>N. S. Bowden  L. Carman  S. A. Dazeley  S. R.
Durham  O. M. Falana  M. J. Ford  A. M. Glenn  C. Hurlbut  V. A. Li  M. P.
Mendenhall  K. Shipp  F. Sutanto  N. P. Zaitseva

###### Abstract

A 6Li-doped plastic scintillator with pulse-shape discrimination capabilities,
commercially identified as ej-299-50, has been developed and produced at the
kilogram-scale. A total of 44 -scale bars of dimensions 5.5 x 5.5 x 50 of this
material have been characterized. Optical properties like light output and
effective attenuation length have been found to be comparable to 6Li-doped
liquid scintillators. The scintillator ej-299-50 shows good neutron detection
capabilities with an effective efficiency for capture on 6Li of approximately
85%. Stability tests performed on two formulation variations showed no
intrinsic degradation in the material or optical properties during several
months of observations.

###### keywords:

Plastic scintillator , Pulse-shape discrimination , PSD , 6Li-loaded
scintillator , neutron detection , reactor antineutrino detection

††journal: Nuclear Instruments and Methods

[LLNL]organization=Lawrence Livermore National Laboratory, city=Livermore,
state=CA, postcode=94550, country=USA

[Eljen]organization=Eljen Technology, city=Sweetwater, state=TX,
postcode=79556, country=USA

##  1 Introduction

Liquid scintillators have been the standard technology for reactor-
antineutrino detection for decades [1, 2, 3, 4, 5, 6, 7, 8, 9]. Besides
enabling antineutrino interaction through Inverse Beta Decay (ibd), many
liquid scintillators can be used to discriminate between particle ionization
density, e.g. electronic vs nuclear recoil, via Pulse-Shape Discrimination
(psd) [10]. Further particle identification can be provided for thermal-
neutrons by the addition of dopants that have specific neutron capture
reactions. For example, inclusion of 6Li in a detector system can provide a
distinct capture tag via a combination of energy and psd selections [2, 11,
12, 9, 13, 14, 15].

While liquid scintillators are relatively inexpensive, easy to manufacture,
and have demonstrated high performance, they require consideration of
potential safety hazards, handling, transportation, and maintenance of an
appropriate operating environment. Safety, regulatory, practicality, and
mobility requirements can put constraints on the development of novel
detectors, particularly the physical state of the detection medium. Plastic
scintillators alleviate some of these concerns.

It is only recently that such plastics have been developed with psd capability
and 6Li-doping [16, 17, 18, 19, 20, 21]. A key challenge in their production
is achieving a balance between the solubility of the components with the main
monomer and the careful control of curing conditions, all while maintaining
satisfactory light output and optical properties. Introducing 6Li as a
thermal-neutron capture agent to psd plastics, e.g. to achieve similar
capabilities to doped liquid scintillators [22], further complicates the
formulation and production process. Unlike common primary dyes like meta-
terphenyl (mTP) or 2,5-diphenyloxazole (ppo), most organic salts containing
6Li as the cation do not have solubility in monomers like styrene, commonly
used as a base for plastic scintillator production. Recent efforts have
identified 6Li-bearing compounds, scintillator formulations, and curing
techniques that can produce kilogram-scale 6Li-doped psd-capable plastics with
reasonable performance [23, 15]. Eljen Technology has made further significant
advances in performance and scalability for this material, which has been
commercially labelled as ej-299-50.

The demonstration of adequate optical performance and neutron identification
capabilities by ej-299-50 could make it an appropriate candidate for future
safeguards and reactor monitoring experiments using antineutrino detection
that require near-field, readily deployable systems [24, 25]. The ability to
detect fast-neutrons and distinguish them from thermal-neutrons and gamma-rays
also makes ej-299-50 an ideal candidate for neutron spectroscopy and
multiplicity counting applications.

In this work, we characterize the scintillation and neutron detection
performance of ej-299-50. We primarily focus on long bar geometries suitable
for segmented reactor-antineutrino detectors. Sec. 4.2 describes the process
to characterize the effective light output and attenuation length of the
plastic by means of using a collimated gamma-ray source (22Na). In Sec. 4.4,
the response of the plastic when exposed to a mixed neutron/gamma-ray source
(252Cf) is detailed. Quantities such as capture time, psd figure of merit, and
thermal-neutron capture fraction are measured and compared with simulations.
Finally, in Sec. 4.5, we discuss the results of aging tests and the long-term
stability of the plastic.

##  2 Scintillator Description

Plastic scintillators have gained attention as candidate materials for
reactor-antineutrino detection due to the progress made in developing stable
samples with psd capabilities [16, 20, 14, 15]. Desired properties during the
production of such plastics are long-term stability, long attenuation lengths
to allow for large-scale detection elements, and relatively simple processing
requirements. At the same time, maintaining good psd and thermal-neutron
capture capabilities is crucial for antineutrino detection.

With these considerations in mind, 6Li-doped psd plastic scintillator
formulations and casting processes for producing kilogram-scale detection
elements have been developed and tested at LLNL [23]. Eljen Technology have
further advanced this class of material, in the form of ej-299-50, and can now
produce elements at the meter-scale with mass greater than 4 kg. This material
contains about $0.1\%$ 6Li by weight and is based on polyvinyl toluene,
containing ppo as a primary dye. 6Li is introduced in the form of an aliphatic
salt, whose solubility is enhanced by use of a comonomer.

In this work, we report on the performance of 44 bars of ej-299-50 with
dimensions of 5.5 x 5.5 x 50 from a development production run as well as a
set of 2-inch-diameter cylindrical samples. All these were produced by Eljen
Technology between November 2021 and September 2022, during which period minor
variations in formulation and process were implemented to improve the material
properties. A photograph of several of the $50\text{\,}\mathrm{cm}$ long bars
is shown in Fig 1. After the characterization effort described here, 36 out of
44 of these $50\text{\,}\mathrm{cm}$ bars have been installed as active
material of the Reactor Operations Antineutrino Detection Surface Testbed
Rover (ROADSTR) detector [26].

Figure 1: A subset of 10 ej-299-50 bars of 5.5 x 5.5 x 50 dimension produced
by Eljen Technology. Next to them, for scale, a Hamamatsu 2-inch-diameter
H11284-100 photomultiplier tube assembly. Picture taken under UV-light
illumination.

##  3 Experimental Description

Properties of the ej-299-50 psd plastic are initially tested using small
2-inch-diameter cylindrical samples wrapped in a diffuse Teflon reflector and
optically coupled to a 2-inch-diameter r6231-100-sel pmt. The light output and
psd performance of the material were measured using 137Cs and 252Cf sources
respectively. This setup will be referred as the small configuration.

The bar castings are tested in batches of four, by means of a support system
that holds them parallel in a 2 x 2 grid configuration. Each bar is read out
by a pair of Hamamatsu 2-inch h11284-100 bi-alkali pmt assemblies [27].
Contact between scintillator and pmt is maintained by inward pressure from a
series of contracted springs attached to the support frame.

Bars are characterized in several configurations. In the wrapped
configuration, the bars are covered with 3M™ df2000ma specular reflector [28],
leaving a small ($\lesssim$ $0.1\text{\,}\mathrm{mm}$) air gap between the bar
and the reflector to allow for efficient optical transport of the fraction of
scintillation photons that meet the conditions for Total Internal Reflection
(tir) at the bar-air interface. pmt assemblies are grease-coupled to bars via
a layer of ej-550 optical grease [29]. This configuration provides a high
light-collection efficiency and, in general terms, corresponds to the optimal
setup for any 2D detector. This configuration has been used to measure the
effective light output of the bars, as described in Sec. 4.3, as well as to
obtain the psd measurement presented in Sec. 4.4.

The possibility of bar-to-bar inconsistency in the wrapping and grease-
coupling process, e.g. flatness of specular reflector strips or grease
coupling uniformity, had the potential to affect the comparison of bar
properties. Thus, for the attenuation length measurements described in Sec.
4.2, the plastic bars are left in the bare configuration, i.e. measured
without coupling grease or specular reflector wrap. Although this choice
results in lower light collection, it allows for a more reliable bar-to-bar
comparison. Additionally, neutron-capture efficiency studies described in Sec.
4.4, benefit from a simple setup that can be easily replicated in simulation.
For this reason, and given that their addition does not bring any particular
benefit to the efficiency measurements, these also used the bare
configuration.

Most of the bar-characterization measurements described here use gamma-ray
sources that are housed in a $5\text{\,}\mathrm{cm}$-wide lead source holder
that collimates gamma emissions through up to four
$0.5\text{\,}\mathrm{cm}$-diameter circular apertures to provide spatially
localized energy depositions. To characterize optical collection as a function
of interaction position, a linear-translation stage is configured to transport
the source holder longitudinally along the main axis of the bar. Both the
stage and the data acquisition are controlled remotely through scripted
routines, providing automated data-collection at specific source positions.

###  3.1 Data acquisition

In the small configuration, pmt signal is recorded using a 14-bit resolution
CompuScope 14200 [30] waveform digitizer with a sampling rate of
$200\text{\,}\mathrm{MHz}$. Readout of the pmts during the bar measurements is
performed using a CAEN V1725S digitizer with $250\text{\,}\mathrm{MHz}$
sampling frequency and 14-bit resolution [31].

Waveform shape depends on the type of event being recorded. Heavily ionizing
particles, like neutron-induced recoil-protons or the products from neutron-
captures in 6Li (6Li$(n,t)\alpha$), generate a population of long-lived
triplet states in the scintillator, leading to slower light emission and
longer pulses than singlet states, induced by electron-like ionization from
gamma-ray events. The psd parameter quantifies this effect:

$\textsc{{psd}}{}=Q_{\text{tail}}/Q_{\text{tot}},$ (1)

where $Q_{\text{tot}}$ is the total integrated charge in a given waveform and
$Q_{\text{tail}}$ is the integrated charge in a specific window corresponding
to the tail of the pulse. The time-integration windows for $Q_{\text{tot}}$
and $Q_{\text{tail}}$ are [-8, 400] ns and [36, 400] ns with respect to the
half-height of the waveform’s leading edge, respectively. Using these
definitions, the psd parameter is calculated for each event using Eq. 1.

These time-integration windows were chosen to maximize the separation Figure
of Merit (fom) between nuclear and electronic-recoil, which can be interpreted
as an indication of the gamma-neutron event discrimination capabilities of the
plastics. It is commonly defined as:

$\textsc{fom}{}=\frac{\mu_{\gamma}-\mu_{n}}{\textsc{fwhm}{}_{\gamma}+\textsc{fwhm}{}_{n}}=0.425\cdot\frac{\mu_{\gamma}-\mu_{n}}{\sigma_{\gamma}+\sigma_{n}}$
(2)

where $\textsc{fwhm}{}_{i}=2.35\sigma_{i}$ corresponds to the Full Width Half
Maximum of the band, approximated as a Gaussian distribution.

The high voltage bias applied to each pmt is chosen to result in matched gain.
Gain curves were obtained from LED pulser calibrations that determined the
analog-digital conversion units (adc) response for single photo-electron (pe)
incidences as a function of applied bias. Further, the chosen gain setting
($40\text{\,}\textsc{adc}\mathrm{/}\textsc{pe}$) results in a linear response
throughout the 18 energy range. A trigger acquisition threshold of
$120\text{\,}\textsc{adc}$ ($3\text{\,}\textsc{pe}$) is also applied.

Figure 2: Example of a pmt waveform sampled in $4\text{\,}\mathrm{ns}$
intervals. Cyan lines delimit the $Q_{tot}$ integration boundaries while
magenta specifies the boundary between rise and tail of the waveform. In
black, crossed by a dashed line, the waveform’s leading edge is depicted.

##  4 Scintillator Performance

In this section, we describe the measurement of the main performance metrics
of the ej-299-50 scintillator.

###  4.1 Small Sample Performance Measurement

As described in the previous section, the first set of measurements of
ej-299-50 are done for a single sample in the small configuration. This
configuration allows us to compare ej-299-50 with other benchmark plastic and
liquid scintillator measurements, also performed in small volumes. The
distribution of reconstructed energy versus psd is presented in Fig. 3. Good
band separation is observed for fast-neutron and gamma-like events, as is for
the neutron-capture feature at $\sim 0.40\text{\,}\mathrm{MeV}_{\mathrm{ee}}$.
The psd fom (Eq. 2) is 2.11 between 0.35;0.50. This result compares favorably
with previous unloaded psd plastic formulations like ej-299-33 [32], with
$\textsc{fom}{}\sim 0.9$ for fast-neutron and gamma bands in the
$0.350.5\text{\,}\mathrm{MeV}$ energy region. ej-299-50 showcases a more
comparable fom to recent unloaded formulations like ej-276[20]. When comparing
plastics to liquids of the same size, like ej-309 in a 2-inch diameter
cylindrical cuvette, a larger discrepancy is observed in favor of the liquid
scintillator, which fom ranges between $3.183.95$ for two different 6Li-doped
variants [33]. Such difference comes mainly from broader psd distributions for
plastics due to less homogeneous energy transfers in the material [20].

Figure 3: psd distribution in terms of reconstructed energy for 252Cf data on
the 2-inch-diameter cylindrical elements of ej-299-50.

###  4.2 Light Collection and Attenuation Length in Long-Bars

As shown in the previous section, ej-299-50 shows good performance when cast
as small samples. However, applications like antineutrino detection or neutron
multiplicity counting require larger mass and/or surface area. In such
geometries, light collection may not be spatially uniform. Longer light
propagation paths result in increasing losses through absorption or scattering
in the material and at optical interfaces. Additionally, variation in
travelled path length for different emission angles may cause timing
dispersion that affects the measured scintillation time profile. All of these
can degrade psd performance, making it important to test and characterize the
properties of ej-299-50 in the form of long-bar castings for large detector
applications.

In this work, we estimate an effective attenuation length ($\lambda$) and an
effective light output ($\ell$) for ej-299-50. These estimations are not a
representation of the intrinsic properties of the scintillator but rather a
combined effect of optical properties with light-loss mechanisms. To do these
calculations, a set of measurements with a 22Na source are performed. The
source is placed inside a lead collimator on top of a linear stage, as
described previously in Sec. 3. The stage is configured to take 20 equidistant
measurements across the longitudinal axis of the bar, providing a position-
dependent response. To a reasonable approximation, the collected signal
$S_{0}(S_{1})$ for a given energy deposition $E_{\text{dep}}$, e.g. the
Compton-edge, decreases (increases) exponentially as the source moves away
from (closer to) the readout $\textsc{pmt}{}_{0}$ ($\textsc{pmt}{}_{1}$) on
the bar ends:

$\displaystyle S_{0}$ $\displaystyle=\kappa\cdot\exp{(-x/\lambda)}$ (3)
$\displaystyle S_{1}$ $\displaystyle=\kappa\cdot\exp{((x-L)/\lambda)},$ (4)

where $x$ is the position of the source in the longitudinal axis relative to
the end of the bar, and $L=$50\text{\,}\mathrm{cm}$$ is the length of the bar.
The response scale $\kappa$, measured in adc, is proportional to the effective
light output $\ell$:

$\kappa=\ell\cdot G\cdot E_{\text{dep}},$ (5)

and depends on the pmt gain $G$, measured in adcpe. If both pmts are gain-
matched, their respective $\kappa$ factors are equivalent. Note that, by
definition, $\ell$ in Eq. 5 already includes all unaccounted detection
inefficiencies, and thus is measured in effective pe. Both $\ell$ and
$\lambda$ can then be extracted from Eqs. 3 and 4 through the analysis of the
multi-position 22Na measurements:

  1. 1.

$S_{0}$ and $S_{1}$ can be combined in the following way:

$\ln{\frac{S_{1}}{S_{0}}}=\frac{2x}{\lambda}-\frac{L}{\lambda}.$ (6)

In Eq. 6, the signal ratio $\ln{S_{1}/S_{0}}$ evolves linearly with the source
position $x$ and it is distributed between $\ln{S_{1}/S_{0}}=\pm L/\lambda$
for $x=0,L$ respectively. This variable provides a compact observable that can
be used to calculate $\lambda$.

  2. 2.

Similarly, the geometric mean of the two pmt signals can be computed as
follows:

$\sqrt{S_{0}\cdot S_{1}}=\kappa\cdot\exp{(-L/2\lambda)},$ (7)

where the light collection is position-independent, at least in the regime
where Eqs. 3 and 4 are valid approximations. From Eq. 7 it can also be noted
that light collection degrades as the $L/\lambda$ ratio increases, showcasing
the higher attenuation rate for longer photon paths. Once $\lambda$ is known,
we can extract $\kappa$ by substitution using Eq. 7 for every position. The
effective light output $\ell$ is then calculated by proxy using Eq. 5. It is
useful to scale the segment signal by $E_{\text{dep}}$ for the Compton-edge as
follows:

$\varepsilon=\sqrt{S_{0}\cdot S_{1}}/E_{\text{dep}}=\ell\cdot
G\cdot\exp{(-L/2\lambda)},$ (8)

where the resulting variable $\varepsilon$ can be understood as the energy
scale of the system.

Figure 4: 22Na gamma-ray electron-equivalent energy spectrum. Theoretical
Compton multi-scattering distribution showed in solid black, with individual
scattering contributions showed in dashed black. Solid red represents the
folded detected response, defined by theoretical distribution with Gaussian
smearing. The single bin at $1.22\text{\,}\mathrm{MeV}$ represents the
residual distribution of full-energy deposition events through photoelectric
effect.

However, before analysing the data to extract these properties, there are two
pre-analysis tasks that need to be performed. Firstly, a Compton-edge fit is
designed. Full-energy gamma-ray depositions rarely happen in the relatively
small volume of single bars used in the present study. Furthermore, organic
scintillators are mainly composed by organic chains of C and H (low-Z
elements), that greatly reduce the cross-section for photoelectric effect, in
favour of gamma-electron scatterings. For this reason, the most common
occurrence is that the emitted gamma-ray scatters once in the material before
leaving the active volume. This is represented in the scattered-electron
energy distribution by a continuum with a sharp cut-off. This cut-off is often
referred as Compton-edge [34]. However, there is still a non-negligible
probability that multiple scatterings are observed in one of the bars. Such
Compton multi-scattering distribution can be observed in Fig. 4 as a solid
black line. This distribution has been obtained by numerically integrating the
multi-scattering and photoelectric cross-sections. The individual non-solid
lines represent the decreasing contributions to the total cross-section for an
increasing number of potential scatterings before the gamma-ray either escapes
the volume or is completely absorbed by the photoelectric effect. If we apply
a detection smearing to account for light collection within the bar’s
geometry, we obtain the red curve. This smearing depends on four experimental
parametrizations:

  * 1.

Effective material thickness for gamma-ray scattering, representing the
average number of scatterings per event.

  * 2.

Event rate, working as a normalization factor.

  * 3.

Energy resolution, a measure of the Compton-edge broadening.

  * 4.

Energy scale, defined in Eq. 8. This parameter encapsulates information about
the Compton-edge position.

Secondly, a position cut for 22Na data is defined. During data taking, the
lead collimator is positioned in such a way that the beam of gamma-rays
emitted by the source hits the bars perpendicularly, i.e. at the same position
$x$ as the source across the longitudinal axis. In an ideal scenario, all
events in a single measurement would thus be spatially well-localized around
$x$. However, there are still a non-negligible portion of events with an
interaction point away from the vicinity of $x$, e.g. gamma-rays that scatter
at wide angles within the lead collimator. These events broaden the
distribution of $\ln{S_{1}/S_{0}}$ and make the position reconstruction more
inaccurate. This effect can be observed in Fig. 5(a) for a 22Na measurement at
roughly the middle point ($x=$24\text{\,}\mathrm{cm}$$). Here, the signal
ratio is shown to be correctly centered at $\ln{S_{1}/S_{0}}\sim 0$, specially
visible for events at the Compton-edge $\sqrt{S_{0}\cdot
S_{1}}\approx$18\text{\,}\mathrm{k}\textsc{adc}~{}\mathrm{n}\mathrm{s}$$111The
integrated charge in a waveform includes a multiplicative factor of
$4\text{\,}\mathrm{ns}$ to account for the sample size of the digitizer. This
can prove useful when comparing results from digitizers with different
sampling sizes. This factor is removed later during the analysis of results..
However these events are also widely distributed between $\ln{S_{1}/S_{0}}=\pm
1$. Such position effect provokes a broadening of the Compton-edge, making the
fit of the distribution more challenging. We thus need to impose a position
cut on $\ln{S_{1}/S_{0}}$ to ensure selected events with an interaction point
at $x$ in the longitudinal axis of the bar. The selected region is bounded by
the red lines in Fig. 5(a), which are defined as one standard deviation away
from the mean signal ratio value. An example of the resulting Compton-
scattered electron spectrum for 22Na, after applying the position cut, and the
corresponding fit can be observed in Fig. 5(b).

This process can be performed over each individual pmt signal, as well as for
the geometric mean of the two. The relevant parameter obtained from the
Compton-edge fit for this part of the study is $\varepsilon$. From it, we
calculate the position of the Compton-edge and display its evolution in Fig.
6(a). There it can be observed that individual pmt signals evolve
exponentially with $x$ while $\sqrt{S_{0}\cdot S_{1}}$ is position-
independent, as described in Eqs. 3, 4 and 7. The three lines displayed cross
each other at the middle point ($x=L/2$), which demonstrates that both pmts
are properly gain-matched, i.e $S_{0}=S_{1}=\sqrt{S_{0}\cdot S_{1}}$. Fig.
6(b) shows the evolution of the signal ratio described in Eq. 6, together with
a linear fit of the distribution from which can extract $\lambda$ for each
bar. Finally, using Eq. 8 with extracted $\lambda$ and $\varepsilon$, and
using a pmt gain of $G=$30\text{\,}\textsc{adc}\mathrm{/}\textsc{pe}$$, we
calculate $\ell$. Since this number is position-independent, we calculate the
average for all 20 measurements.

(a) Position cut

(b) Measured Compton-edge

Figure 5: (a) Relationship between signal ratio $\ln{S_{1}/S_{0}}$ and
geometric mean $\sqrt{S_{0}\cdot S_{1}}$ for 22Na data on bar #10 at
$x=$24\text{\,}\mathrm{cm}$$. This position roughly corresponds to the middle
point between both pmts and thus $\ln{S_{1}/S_{0}}\sim 0$ for properly
collimated gamma-rays. Dashed lines correspond to the upper and lower cuts
applied to the distribution, one standard deviation around the main peak in
the $\ln{S_{1}/S_{0}}$ axis (b) Measured spectrum for 22Na using bar #10,
after applying a position cut. Background (red) has been subtracted from data.
The Compton-edge fit is displayed in magenta.

(a) Light-collection evolution

(b) Signal ratio evolution

Figure 6: Measured (a) light-collection and (b) signal-ratio for bar #10 using
22Na source across the longitudinal axis of bar.

(a) Eff. attenuation length $\lambda$ distribution

(b) Eff. light output $\ell$ distribution

Figure 7: Measured effective (a) attenuation length $\lambda$ and (b) light
output $\ell$ distributions for the 44 ej-299-50 scintillator bars tested. Two
subgroups of bars can be distinguished with different average attenuation
$\lambda_{1}$ and $\lambda_{2}$, marked by orange and red shades. These
subgroups cannot be identified when looking at $\ell$ distribution.

Both analyses are thus performed for all ej-299-50 bars:

  1. 1.

The resulting $\lambda$ distribution can be categorized into two subgroups
with average values $\lambda_{1}=$83\pm 12.\text{\,}\mathrm{cm}$$ and
$\lambda_{2}=$110\pm 14.\text{\,}\mathrm{cm}$$, shown in Fig. 7(a). Throughout
the bar production run, improvements to formulation and process were made,
accounting for some of the variation observed. Generally, improved optical
clarity was observed in later castings.

  2. 2.

The distribution of $\ell$ for the 44 bars is shown in Fig. 7(b). No
substantial difference can be observed between production batches. The
relative light output for ej-299-50 in the bare configuration is
$\ell_{\text{bare}}=$141\pm
12.\text{\,}\textsc{pe}\mathrm{/}\mathrm{M}\mathrm{eV}$$. Similar measurement
has been performed with some of the bars with complete wrapping and optical
coupling. This setup, as explained in Sec. 3, gives us an indication of the
typical performance in a detector scenario, where effort is taken to transport
as much scintillation light to the pmts as possible. The results show an
increase of $\sim 200\%$ in terms of light output, bringing the average to
$\ell_{\text{wrapped}}=$421\pm
32.\text{\,}\textsc{pe}\mathrm{/}\mathrm{M}\mathrm{eV}$$. This value is
approximately half of the average effective light output observed for standard
EJ-200 [35] plastic bars of similar geometry in an identical wrapped setup.

###  4.3 Energy Response in Long-Bars

The selection of gamma-ray sources shown in Tab. 1 is used to study the
linearity of the energy response of the ej-299-50 bars. In this study, each
one of the sources is moved to the middle position
($L=$25\text{\,}\mathrm{cm}$$) along the longitudinal axis of the bars. Bars
have been set to the wrapped configuration. The same process as in Sec. 4.2 is
used to ensure localized energy depositions and to fit and extract the
corresponding Compton-edge for each source. Similarly, energy scale is
calculated using Eq. 8. If the material has a linear response, $\varepsilon$
should be the same for all energy depositions listed in Tab. 1, meaning a
linear relationship between $E_{\text{dep}}$ and the detector response
$\sqrt{S_{0}\cdot S_{1}}$.

The observed Compton-edges are plotted versus their nominal energies in Fig. 8
for all sources. From the linear fit we obtain
$\varepsilon=$11.6(0.3)\text{\,}\mathrm{k}\textsc{adc}\mathrm{/}\mathrm{M}\mathrm{eV}$$,
which provides an alternative calculation of the effective light output
$\ell_{\text{wrapped}}=$408\pm
25.\text{\,}\textsc{pe}\mathrm{/}\mathrm{M}\mathrm{eV}$$. This result is
compatible within uncertainties with the valued obtaind in Sec. 4.2. This
measurement demonstrates that the long-bar ej-299-50 system has linear
response at scale energies.

Source |  $E$[] |  $E_{\text{dep}}$ []
---|---|---
137Cs | 0.667 | 0.479
54Mn | 0.834 | 0.638
60Co | 1.15 | 0.963
60Co | 1.33 | 1.118
22Na | 1.22 | 1.061
Table 1: List of gamma-sources used during the test of energy respoonse’s
linearity for the ej-299-50 bars. Second column depicts the nominal energy of
the main gamma emission, while third column shows the energy value of the
single-scattering Compton-edge. Figure 8: Fitted Compton-edge position for
gamma-ray sources listed in Tab. 1 using bar #10. Sources have been placed at
$x=L/2$ (middle point). Linear fit for all Compton-edge values, portraying a
linear energy response.

###  4.4 Neutron Characterization for Long-Bars

The neutron-identification performance of the ej-299-50 plastics can be
quantified through the psd parameter defined in Eq. 1 and the figure of merit
fom between psd regions, defined in Eq. 2. For this purpose, the response of
the bars to a 252Cf gamma-neutron source is used, as described in Sec. 3. The
bars are prepared in the wrapped configuration to optimize psd band
separation. Each measurement is 1-hour long with an additional 1-hour of
background data taken after removing the source. This background is later
subtracted from the foreground. After acquisition, integrated charge is
transformed into reconstructed energy by means of an energy scale database,
created previously for each bar from the multi-position gamma-ray measurements
and the calculation of $\varepsilon$ described in Sec. 4.2.

Figure 9: psd parameter in terms of reconstructed energy. Two main bands can
be observed for nuclear and electron-like recoils respectively. neutron-
captures in 6Li appear prominently in the upper psd band at about
$0.4\text{\,}\mathrm{MeV}_{\mathrm{ee}}$ reconstructed energy and psd between
$0.280.42$.

The distribution of reconstructed energy versus psd is shown in Fig. 9 for the
252Cf data. In this plot, three main regions can be identified: (1) an
electronic-recoil band for psd values below $\sim 0.24$, generated by gamma-
rays interacting with electrons in the material; (2) a nuclear-recoil band for
psd values above $\sim 0.24$, generated by fast-neutrons scattering with
nuclei; and (3) a small region within the nuclear-recoil band at energies
$\sim 0.4\text{\,}\mathrm{MeV}_{\mathrm{ee}}$, generated by thermal-neutron-
captures in 6Li isotopes and colloquially referred as capture island. The
exact position and width of both bands have been calculated by approximating
each band’s projection ($i=\gamma,n$) into the psd axis to a Gaussian
function, parameterized by a mean ($\mu_{i}$) and a standard deviation
($\sigma_{i}$). The capture island is fitted to a two-dimensional Gaussian
function parameterized by $E_{\text{Li}},\textsc{{psd}}{}_{\text{Li}}$ and
$\sigma_{E,\text{Li}},\sigma_{\textsc{{psd}}{},\text{Li}}$, means and standard
deviations in the energy and psd axes respectively.

In our studies two fom values are calculated: one between fast-neutron and
gamma bands in the $12\text{\,}\mathrm{MeV}_{\mathrm{ee}}$ region
($\textsc{fom}{}_{n}$), and another one between the capture island and gamma
band in the [$E_{\text{Li}}-\sigma_{\text{Li}}$,
$E_{\text{Li}}+\sigma_{\text{Li}}$] region ($\textsc{fom}{}_{\text{Li}}$). The
results show similar discriminating power for both regions with
$\textsc{fom}{}_{n}=1.27\pm 0.11$ and $\textsc{fom}{}_{\text{Li}}=1.25\pm
0.18$. These numbers are lower than the ones obtained in Sec. 4.1, which is
expected given the larger volume of the bars, and thus the lower light
collection efficiency.

Efficient ibd detection using 6Li-doped material emphasizes captures on that
agent as opposed to other material constituents. Absolute neutron-capture-
efficiency is hard to measure reliably, since a highly detailed understanding
of the detector setup, its environment, and the event selection are required.
Such a measurement would be beyond the scope of this work, so a comparison of
the effective capture-efficiency in 6Li between data and simulation is
performed. Good agreement would allow us to provide an estimate of the
absolute capture-efficiency using truth information from simulations.

The capture-efficiency in 6Li depends on the remaining potential candidates
for thermal-neutron capture in the material. If we exclude the specific
loading material (i.e. 6Li), the largest capture cross-section in organic
scintillators is for 1H nuclei. This type of captures results in the emission
of a $2.2\text{\,}\mathrm{MeV}$ de-excitation gamma-ray by the capturing
nuclei, a type of event that falls within the electronic-recoil band in Fig.
9. These events strongly compete with background in the psd-energy space and
thus are generally discarded for ibd coincidence search in favor of a more
specific event selection provided by captures in 6Li. Thus, to determine the
neutron-capture-efficiency we compare the amount of 6Li captures versus
captures in 1H. Simultaneously, a Geant4 [36] simulation of the setup and data
acquisition is performed and analysed together with the bench-top
measurements, allowing us to estimate how well neutron dynamics are
represented in our simulation model of these plastics. The single bar used for
this study is configured in bare conditions to facilitate a straightforward
geometry for the simulation model.

Selection of thermal-neutron-captures requires a coincidence search throughout
all the events. A coincident prompt-delayed pair is found when both prompt and
delayed-events fulfill psd, energy and time conditions. The selection process
is:

  1. 1.

Scan all recorded events for potential capture delayed-events on 6Li or 1H
according to Tab.2. This scan prioritizes tagging captures in 6Li before 1H.

  2. 2.

Once a delayed-event candidate is found, all events in the range of
$-2500\text{\,}\mathrm{\SIUnitSymbolMicro s}$ happening prior to it are
scanned for a prompt-event candidate according to Tab.2.

  3. 3.

If a prompt-event is found, then all events in the range of
$-3000-250\text{\,}\mathrm{\SIUnitSymbolMicro s}$ happening prior to the
delayed-event are scanned. Any coincidences found in this range will be
treated as accidental, and will be used for accidental subtraction later.
Since this window is larger than the coincidence event window, the accidental
count is down-scaled prior to the subtraction.

Table 2: psd and energy selection cuts for 6Li and 1H thermal-neutron-capture prompt-delayed coincidences. | psd cut | Energy cut
---|---|---
Prompt - H | [n-band - $1\sigma$, 0.4] | [E${}_{\text{Li}}+1\sigma$, 10]
Delayed - H | [0, gamma-band + $3\sigma$] | [0.1, 1.6]
Prompt - 6Li | [n-band - $1\sigma$, 0.4] | [E${}_{\text{Li}}+1\sigma$, 10]
Delayed - 6Li | psd${}_{\text{Li}}\pm 1\sigma$ | E${}_{\text{Li}}\pm 1\sigma$

The delayed-event selection cuts in Tab. 2 are configured for two different
type of energy depositions following the thermal-neutron-capture in 1H and
6Li:

  1. 1.

Captures in 1H are defined by the emission of a de-excitation
$2.2\text{\,}\mathrm{MeV}$ gamma-ray by the resulting deuterium nuclei after
the neutron-capture. The energy deposition for these events is limited to
$1.6\text{\,}\mathrm{MeV}$ since that is the maximum energy that can be
deposited in the material before the gamma-ray Compton-scatters out of the
volume.

  2. 2.

Captures in 6Li are the result of the ionization produced by an
$\alpha$-particle plus tritium nuclei, products from the neutron-capture on
6Li. These ions carry a combined kinetic energy of $4.78\text{\,}\mathrm{MeV}$
that results in a compact mono-energetic deposition through a series of ion-
nucleus inelastic scatterings. The reconstructed energy can be found around
the $0.4\text{\,}\mathrm{MeV}_{\mathrm{ee}}$ mark due to quenching, while the
psd parameter for these events is in the nuclear-recoil band.

(a) prompt-events for coincidences in H (b) delayed-events for coincidences in
H (c) prompt-events for coincidences in 6Li (d) delayed-events for
coincidences in 6Li

Figure 10: psd vs reconstructed energy distribution of events after applying
selection cuts in Tab. 2 on 252Cf data. Fast-neutron-induced nuclear recoils
are selected as prompt-events, while thermal-captures in 6Li and 1H are chosen
as delayed-events.

The selection of prompt-events is done identically for both types of
coincidences, and it is primarily defined by nuclear-recoils produced by an
incoming fast-neutron from the 252Cf source before thermalizing and capturing.
The energy spectrum for these fast-neutrons approximately ranges between
$0.510\text{\,}\mathrm{MeV}_{\mathrm{ee}}$ [37]. To avoid mis-selecting
captures in 6Li as prompts, the low-energy end for the prompt selection range
is set $1\sigma$ away from the capture island center while the high-energy end
stretches until the end of the spectrum. psd for this type of events lies
within the nuclear-recoil band, and thus we use $1\sigma$ below the centroid
as low-psd end of the selection, while opening the high-psd end all the way
until 0.4.

The position of the selection cuts in Tab. 2 substantially impacts the
measured capture-fraction. For this reason, the cuts are defined relative to
the position and width of both recoil bands and the thermal-neutron-capture
island. This way, cuts affect the same topology of events in data and
simulation. The events in data surviving each set of selection cuts are
depicted in Fig. 10.

The same events can be plotted in terms of the coincidence time, i.e the time
difference between the prompt-delayed pair. Such distribution for captures in
1H and 6Li is shown in Fig. 11. The bottom figures showcase the time
difference distribution measured for data (black dots) together with the
estimated accidental contribution (red dots). The subtracted coincidence
distribution is depicted in blue, and can be approximated to an exponential
function:

$N_{corr}=A\exp{(-t/\tau)}+c,$ (9)

with $c\sim 0$ when accidental coincidences are properly subtracted, and
$\tau$ being the mean capture-time in a specific isotope and material. Our
results show an excellent agreement between data and simulation for 6Li with
$\tau_{\text{Li,dat}}=$$15.1(0.6)\text{\,}\mathrm{\SIUnitSymbolMicro s}$ and
$\tau_{\text{Li,sim}}=$ $14.7(0.8)\text{\,}\mathrm{\SIUnitSymbolMicro s}$.
Measurement of the capture-time for 1H proved to be more challenging as rate
of correlated events is much smaller than the accidental rate. Data and
simulation capture-times for 1H are
$\tau_{\text{H,dat}}=$8.4(6.7)\text{\,}\mathrm{\SIUnitSymbolMicro s}$$ and
$\tau_{\text{H,sim}}=$14.6(3.0)\text{\,}\mathrm{\SIUnitSymbolMicro s}$$
respectively. These results are compatible with each other but present large
uncertainties and show that the current composition enhances captures in 6Li
which is the desired outcome. The capture-fraction numbers are also found to
be compatible for data and simulation: $f_{\text{dat}}=16.8\pm 2.4$ and
$f_{\text{sim}}=16.5\pm 2.3$.

As mentioned previously, capture-fraction and capture-time calculations are
heavily dependent on the selection cuts and can vary largely depending the
energy and psd limits. These results should not be understood thus as an
absolute measure of neutron-capture statistics, given that selection and
geometry effects have not been quantified. However, an absolute capture-
efficiency can be extrapolated to data from true information in the
simulation, provided the proven compatibility between both.

To avoid geometry effects from using a single plastic bar, a simulation of a
large and symmetric volume of ej-299-50 has been performed. A plastic
scintillator sphere of $1\text{\,}\mathrm{m}$ radius is simulated together
with a $10\text{\,}\mathrm{eV}$ neutron particle-gun at its center. This
configuration has been chosen to ensure complete capture of all thermal-
neutrons within the active volume. True information about neutron-captures is
recorded and categorized according to the capturing isotope. The results show
an absolute efficiency of captures in 6Li of $85\%$ with a mean thermalization
path of $41.75\text{\,}\mathrm{mm}$ and capture-time of
$\tau_{\text{Li,true}}=~{}$37.2\text{\,}\mathrm{\SIUnitSymbolMicro s}$$. The
rest of captures happen in 1H with a $13\%$ efficiency and
$\tau_{\text{H,true}}=$36.2\text{\,}\mathrm{\SIUnitSymbolMicro s}$$, or in
other isotopes like C and N at a $\sim 2\%$ rate.

(a) Coincidences in MC 6Li (b) Coincidences in MC 1H

(c) Coincidences in data 6Li (d) Coincidences in data 1H

Figure 11: Coincidence time distributions for data and MC datasets after
applying corresponding selection times for captures in both 1H and 6Li.

###  4.5 Material Stability

Previous studies [20] have found that ppo-based psd-capable plastic
scintillators can suffer from gradual degradation of their properties over
time due to the ppo dye included at high concentration to provide psd
capabilities. A recurring problem in these type of scintillators is the
outgassing (leaching) of ppo, which can crystallize on the machined
scintillator surfaces degrading light collection.

This effect has been also observed on the ej-299-50 bars measured in this
study. When the bars are stored in the bare configuration, little to no
leaching is observed, probably because outgassed ppo is mixed with a much
larger air volume or/and leachability being reduced on material surface by
oxidation, as evidenced by yellowing. However, when the bars are left in
wrapped condition for weeks or more, ppo leachate can appear between the
wrapping and the bars, as the outgassing is retained in the air pocket
established for tir light transport. Regardless, the original performance is
recovered if the precipitate is removed, which can be accomplished by a wipe
cleaning with ethanol. The amount of ppo leaving the material is small, and
does not appear to affect the inherent material performance. Methods for
reducing or eliminating this phenomenon are under investigation, including
film surface barrier layers and/or reducing the material ppo fraction.

During the characterization of the bars described in Sec. 4.2, one of the four
bars tested at any single time is set to be the same for every measurement. In
particular, bar #3 is chosen as the benchmark bar. After each set of
measurements, the bar is extracted from the setup, cleaned up and reintroduced
together with the new batch of plastics. Eff. attenuation length for this
specific bar is thus tracked over approximately two months of measurements.
This monitoring of bar #3 is displayed in Fig. 12, showing a relatively stable
trend over data-taking time. However, results in Fig. 12 cannot be used to
draw definitive conclusions about the long-term stability of the plastics, as
the time frame is relatively short with respect to the operational
expectations.

Figure 12: Evolution of attenuation length for bar #3 in bare conditions
throughout the measurement campaign. Those 15 measurements correspond to
approximately two months of data taking.

Regardless, a $\chi^{2}$ analysis has been performed over bar #3 data to test
a degradation hypothesis over the null case where data is stable. Both a
linear and constant fit have been performed for the data and shown in Fig. 12.
$\chi^{2}$ values are calculated for both models. The relative
$\Delta\chi^{2}=1.55$ corresponds to only one degree of freedom difference
between hypotheses, which yields a $p$-value of $0.021$. This result showcases
that both constant and linear models are compatible and that any discrepancy
with a constant distribution is not statistically significant. Additionally,
the slope obtained from the linear fit is compatible with 0 within the
uncertainty of the fit, which confirms our conclusions.
The performance of ej-299-50 over time was evaluated through a dedicated long-
term testing of two variants of the plastic: “Original” formulation,
representing the main formulation studied throughout this manuscript; and
“Current” formulation, modified to increase the performance stability of the
scintillators. This study was performed using the small configuration, where
the 2-inch plastic samples were left in different background conditions for
varying amounts of time.

The two metrics used to asses the stability of the plastics are the relative
light output (lo), and the PSD fom, measured at different points in time and
representing different stages of the aging process. lo was determined using
the Compton-edge of 137Cs, and fom using neutron-gamma data from 252Cf, both
in a similar fashion as in Secs. 4.2 and 4.4. The evolution of lo after the
aging tests can be observed in the upper graphs in Fig. 13, where the
“Original” variant shows a decrease in lo following a temperature treatment
($60\text{\,}\mathrm{\SIUnitSymbolCelsius}$ in an air environment). In
contrast, the “Current” variant experiences a milder deterioration in lo. The
fom values in the lower graphs in Fig. 13 follow a consistent pattern,
suggesting that there are no substantial effects on the inherent psd
mechanism. After prolonged exposure to air over a period of 19 weeks, no
significant performance effects can be noted. The uncertainties for the
measurements shown in Fig. 13 have an associated relative systematic
uncertainty of 5%.

Generally, we can conclude from these tests that, under the relatively harsh
environment of the heat treatment in air, the tested variants maintained good
lo and psd performance after 19 weeks (almost a half-year) of observations.

Figure 13: lo and fom observed for both ej-299-50 variants during almost one
year of continuous measurements in an air environment. The blue band
represents the short period of aging tests through heat treatment at
$60\text{\,}\mathrm{\SIUnitSymbolCelsius}$

##  5 Conclusions

In this manuscript the performance of the novel 6Li-doped psd-capable
EJ-299-50 plastic scintillator developed by Eljen Technology has been
characterized in the context of casting appropriate for large detector
systems. The results present a promising technology for, e.g., future reactor-
antineutrino detectors. Light output and attenuation length for this plastic
are similar to liquid counterparts, while solid state permits geometrical
versatility and increased mobility. The material has also proven adequate for
neutron identification, which could be useful for future neutron multiplicity,
directionality, and spectroscopy detectors. Finally, aging tests performed on
variants of the main composition show relative stability over time, crucial
for the future of long-term uses of plastic scintillators.

##  6 Acknowledgements

This work was performed under the auspices of the U.S. Department of Energy by
Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. This
work was supported by the LLNL-LDRD Program under Project No. 20-SI-005 and by
the U.S. Department of Energy Office of Defense Nuclear Nonproliferation
Research and Development. LLNL-JRNL-861520.

## References

  * [1] F. Reines, C. L. Cowan, Detection of the free neutrino, Phys. Rev. 92 (1953) 830–831.  doi:10.1103/PhysRev.92.830.
  * [2] Y. Declais, et al., Search for neutrino oscillations at 15-meters, 40-meters, and 95-meters from a nuclear power reactor at Bugey, Nucl. Phys. B 434 (1995) 503–534.  doi:10.1016/0550-3213(94)00513-E.
  * [3] M. Apollonio, et al., Initial results from the CHOOZ long baseline reactor neutrino oscillation experiment, Phys. Lett. B 420 (1998) 397–404.  arXiv:hep-ex/9711002, doi:10.1016/S0370-2693(97)01476-7.
  * [4] F. Boehm, et al., Search for neutrino oscillations at the Palo Verde nuclear reactors, Phys. Rev. Lett. 84 (2000) 3764–3767.  arXiv:hep-ex/9912050, doi:10.1103/PhysRevLett.84.3764.
  * [5] K. Eguchi, et al., First results from KamLAND: Evidence for reactor anti-neutrino disappearance, Phys. Rev. Lett. 90 (2003) 021802.  arXiv:hep-ex/0212021, doi:10.1103/PhysRevLett.90.021802.
  * [6] Y. Abe, et al., Indication of Reactor $\bar{\nu}_{e}$ Disappearance in the Double Chooz Experiment, Phys. Rev. Lett. 108 (2012) 131801.  arXiv:1112.6353, doi:10.1103/PhysRevLett.108.131801.
  * [7] F. P. An, et al., Observation of electron-antineutrino disappearance at Daya Bay, Phys. Rev. Lett. 108 (2012) 171803.  arXiv:1203.1669, doi:10.1103/PhysRevLett.108.171803.
  * [8] J. K. Ahn, et al., Observation of Reactor Electron Antineutrino Disappearance in the RENO Experiment, Phys. Rev. Lett. 108 (2012) 191802.  arXiv:1204.0626, doi:10.1103/PhysRevLett.108.191802.
  * [9] J. Ashenfelter, et al., First search for short-baseline neutrino oscillations at HFIR with PROSPECT, Phys. Rev. Lett. 121 (25) (2018) 251802.  arXiv:1806.02784, doi:10.1103/PhysRevLett.121.251802.
  * [10] J. Birks, The theory and practice of scintillation counting, Pergamon, 1964.  doi:10.1016/C2013-0-01791-4.
  * [11] S. D. Kiff, N. Bowden, J. Lund, D. Reyna, Neutron detection and identification using ZnS:Ag/6LiF in segmented antineutrino detectors, Nucl. Instrum. Meth. A 652 (2011) 412–416.  doi:https://doi.org/10.1016/j.nima.2010.07.082.
  * [12] Y. Abreu, et al., Performance of a full scale prototype detector at the BR2 reactor for the SoLid experiment, JINST 13 (05) (2018) P05005.  arXiv:1802.02884, doi:10.1088/1748-0221/13/05/P05005.
  * [13] A. Haghighat, P. Huber, S. Li, J. M. Link, C. Mariani, J. Park, T. Subedi, Observation of Reactor Antineutrinos with a Rapidly-Deployable Surface-Level Detector, Phys. Rev. Applied 13 (3) (2020) 034028.  arXiv:1812.02163, doi:10.1103/PhysRevApplied.13.034028.
  * [14] R. Dorrill, NuLat: A Compact, Segmented, Mobile Anti-neutrino Detector, journal = Journal of Physics: Conference Series 1216 (1) (2019) 012011.  doi:10.1088/1742-6596/1216/1/012011.
  * [15] F. Sutanto, et al., SANDD: A directional antineutrino detector with segmented 6Li-doped pulse-shape-sensitive plastic scintillator, Nucl. Instrum. Meth. A 1006 (2021) 165409.  doi:https://doi.org/10.1016/j.nima.2021.165409.
  * [16] N. P. Zaitseva, et al., Plastic scintillators with efficient neutron/gamma pulse shape discrimination, Nucl. Instrum. Meth. A 668 (2012) 88–93.  doi:https://doi.org/10.1016/j.nima.2011.11.071.
  * [17] N. P. Zaitseva, et al., Pulse shape discrimination with lithium-containing organic scintillators, Nucl. Instrum. Meth. A 729 (2013) 747–754.  doi:https://doi.org/10.1016/j.nima.2013.08.048.
  * [18] R. Breukers, et al., Transparent lithium loaded plastic scintillators for thermal neutron detection, Nucl. Instrum. Meth. A 701 (2013) 58–61.  doi:https://doi.org/10.1016/j.nima.2012.10.080.
  * [19] N. J. Cherepy, et al., Bismuth- and lithium-loaded plastic scintillators for gamma and neutron detection, Nucl. Instrum. Meth. A 778 (2015) 126–132.  doi:https://doi.org/10.1016/j.nima.2015.01.008.
  * [20] N. P. Zaitseva, et al., Recent developments in plastic scintillators with pulse shape discrimination, Nucl. Instrum. Meth. A 889 (2018) 97–104.  doi:https://doi.org/10.1016/j.nima.2018.01.093.
  * [21] C. Frangville, et al., Large solubility of lithium carboxylates reaching high rates of 6li incorporation in polystyrene-based plastic scintillators for fast/thermal neutron and gamma ray detection, Mater. Chem. Front. 3 (2019) 1626–1631.  doi:10.1039/C9QM00153K.
  * [22] M. Andriamirado, et al., Improved short-baseline neutrino oscillation search and energy spectrum measurement with the PROSPECT experiment at HFIR, Phys. Rev. D 103 (3) (2021) 032001.  arXiv:2006.11210, doi:10.1103/PhysRevD.103.032001.
  * [23] M. J. Ford, et al., Synthesis and processing of lithium-loaded plastic scintillators on the kilogram scale, Nucl. Instrum. Meth. A 1050 (2023) 168093\.  doi:https://doi.org/10.1016/j.nima.2023.168093.
  * [24] R. Carr, et al., Neutrino-based tools for nuclear verification and diplomacy in north korea, Science & Global Security 27 (1) (2019) 15–28.  doi:10.1080/08929882.2019.1603007.
  * [25] A. Bernstein, et al., Colloquium: Neutrino detectors as tools for nuclear security, Rev. Mod. Phys. 92 (2020) 011003.  doi:10.1103/RevModPhys.92.011003.
  * [26] O. A. Akindele, et al., ROADSTR: a Mobile Antineutrino Detector Platform for enabling Multi-Reactor Spectrum, Oscillation, and Application Measurements, in: 2021 Snowmass Letter of Interest, 2021.
  * [27] Hamamatsu 2 inch H11284-100 bi-alkali PMT assemblies, https://www.hamamatsu.com/us/en/product/optical-sensors/pmt.html.
  * [28] 3M Enhanced Specular Reflector Film (ESR), https://www.3m.com/3M/en_US/p/d/b5005047091/.
  * [29] Coupling Grease EJ-550, https://eljentechnology.com/products/accessories/ej-550-ej-552.
  * [30] GaGe Compuscope 14200 2 Channel 14-bit 200 MS/s Digitizer, https://vitrek.com/gage/digitizers/14-bit/pcie/razor-express/.
  * [31] CAEN V1725 16/8 Channel 14-bit 250 MS/s Digitizer, https://www.caen.it/products/v1725/.
  * [32] C. C. Lawrence, M. Febbraro, T. N. Massey, M. Flaska, F. Becchetti, S. A. Pozzi, Neutron response characterization for an ej299-33 plastic scintillation detector, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 759 (2014) 16–22.  doi:https://doi.org/10.1016/j.nima.2014.04.062.
URL https://www.sciencedirect.com/science/article/pii/S0168900214004756

  * [33] N. Zaitseva, et al., 6Li-loaded liquid scintillators produced by direct dissolution of compounds in diisopropylnaphthalene (DIPN), Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 1054 (2023) 168389.  doi:https://doi.org/10.1016/j.nima.2023.168389.
URL https://www.sciencedirect.com/science/article/pii/S0168900223003790

  * [34] G. Knoll, Radiation Detection and Measurement, John Wiley and Sons, Inc., 2000.  doi:10.1016/S0969-806X(00)00323-6.
  * [35] EJ-200 Scintillator, https://eljentechnology.com/products/plastic-scintillators/ej-200-ej-204-ej-208-ej-212.
  * [36] T. Mahmood, A. AhmedZafar, T. Hussain, H. Rashid, GEANT4: Applications in High Energy Physics, AIP Conf. Proc. 888 (2006) 301–304.
URL https://cds.cern.ch/record/1027671

  * [37] R. Capote, K. Zolotarev, V. Pronyaev, A. Trkov, Updating and extending the irdf-2002 dosimetry library, in: Reactor Dosimetry: 14th International Symposium, ASTM International, 2012.  doi:10.1520/JAI104119.