# Neutron spectroscopy with a high-pressure nitrogen-filled spherical
proportional counter

I. Giomataris  S. Green  I. Katsioulas  P. Knights  I. Manthos
<EMAIL_ADDRESS>T. Neep  K. Nikolopoulos  T. Papaevangelou  B. Phoenix
J. Sanders  R. Ward  IRFU, CEA, Universite Paris-Saclay, F-91191 Gif-sur-
Yvette, France  School of Physics and Astronomy, University of Birmingham, B15
2TT, United Kingdom

###### Abstract

The spherical proportional counter is a large volume gaseous detector which
finds application in several fields, including direct Dark Matter searches.
When the detector is filled with nitrogen it becomes an effective neutron
spectrometer thanks to the 14N(n,$\mathrm{\alpha}$)11B and 14N(n,p)14C
reactions. Nitrogen, however, is a challenging operating gas for proportional
counters and requires a high electric field strength to gas pressure ratio.
Benefiting from the latest advances in spherical proportional counter
instrumentation and simulation techniques, we report first neutron
measurements at operating pressures of up to 1.8 bar. This achievement
enhances the prospects of the spherical proportional counter to act as a
neutron spectrometer appropriate for challenging environments, including
underground laboratories, and industrial and medical settings.

###### keywords:

Neutron detectors , Gaseous detectors , Neutron spectroscopy , spherical
proportional counter

††journal: Journal of Nuclear Instruments and Methods in Physics Research A

##  1 Introduction

Neutron spectroscopy is an invaluable tool for a broad range of scientific,
industrial, and medical applications [1]; spanning from direct Dark Matter
(DM) searches [2] and neutrino-less double-beta decay [3] to non-destructive
measurements of materials and medical imaging or cancer treatment. For
example, in underground laboratories, neutron induced backgrounds caused by
cosmic ray muons and natural radioactivity may mimic the DM signal, reducing
experimental sensitivity. A dedicated, precise, and in-situ measurement of
neutron flux would be a valuable tool for characterising and mitigating
neutron background. Amidst several attempts to develop an efficient neutron
spectroscopy system, detection methods are complex, and measurements are
scarce. Furthermore, despite significant effort invested in describing the
neutron background with Monte Carlo simulations [4], there is still
significant uncertainty on the expected neutron background rates.

To date, neutron detection most widely relies on the 3He(n,p)3H reaction,
providing high efficiency for thermal neutrons and very low efficiency for
$\gamma$-rays. However, 3He-based detectors only provide combined slow and
fast neutron flux measurements, while energy measurements for fast neutrons
are plagued by the wall effect, where the recoiling proton escapes the active
volume. Solutions based on gas mixtures with heavy elements and operation at
extremely high pressures are, typically, impractical. Moreover, the world-wide
demand for 3He, which is largely driven by the popularity of such detectors,
is in stark contrast to its supply resulting in a dramatic price increase.
Thus, 3He-based detectors –especially larger ones– require expensive retrieval
systems, leading governments and industries in a search for alternatives.
Existing alternatives [5, 6, 7, 8], have major disadvantages: a) Ga-based
detectors, that exhibit the highest known cross section for thermal neutrons,
present high-cost and long growing time, while require complex experimental
setup for fast neutron spectroscopy; b) The gas in BF3-based proportional
counters is toxic and corrosive; c) Boron-lined tubes exhibit poor efficiency
and high cost; d) Bulk scintillators have complicated response functions,
insufficient $\gamma/$n discrimination, and limited radiation hardness, and;
e) 6Li coated detectors filled with argon exhibit degraded energy resolution.

A novel approach to neutron spectroscopy for thermal and fast neutrons has
been proposed in Ref. [9], relying on the spherical proportional counter [10],
a high-gain and large-volume gaseous detector, operated with nitrogen. It
relies on the 14N(n,$\alpha$)11B and 14N(n,p)14C processes, which have cross-
sections comparable to the 3He(n,p)3H reaction for fast neutrons. Thus, this
approach has the potential to become an inexpensive, robust, and reliable
alternative to 3He-based detectors. Nevertheless, the challenge lies in
achieving sufficient gas gain in nitrogen-filled proportional counters. As a
result, early proof-of-principle demonstrations were limited to low-pressure
measurements up to 0.5 bar, which resulted in non-negligible wall effect.

In this work, we implement the latest developments in spherical proportional
counter instrumentation, e.g. the multi-anode resistive sensor [11, 12], to
achieve fast and thermal neutron spectroscopy with a nitrogen-filled spherical
proportional counter at pressures up to 1.8 bar. Results from a dedicated
simulation framework [13] are also presented and compared to the experimental
data.

##  2 Spherical Proportional Counter

(a)

(b)

Figure 1: 1(a) Spherical proportional counter schematic. 1(b) 11-anode multi-
anode ACHINOS sensor.

The spherical proportional counter is presented in Figure 1(a). It is a
gaseous detector where a grounded spherical vessel serves as the cathode and a
small spherical anode is placed at its centre. The operating high voltage is
supplied to the anode through a wire, which is shielded by a grounded metallic
rod that also supports the anode. In the ideal case, the electric field in the
spherical proportional counter is radial and its magnitude can be approximated
as $E(r)\approx\frac{V_{0}}{r^{2}}r_{a}$, where $V_{0}$ is the anode voltage,
$r_{a}$ the anode radius, and $r$ the distance from the detector centre. As a
result the detector volume is separated into a drift and amplification region.

The detector operation is governed by $E/P$, the ratio of the electric field
strength to the gas pressure $P$. For spherical proportional counters
operating with nitrogen, a high $E/P$ is required to achieve sufficient gas
gain. This is achieved with the multi-anode sensor ACHINOS [11, 12], shown in
Figure 1(b). ACHINOS comprises a spherical central resistive electrode with
anodes arranged in a regular pattern. The anodes are placed at a constant
distance from the central electrode, lying on the surface of a virtual sphere.
With ACHINOS, the drift and amplification electric field strengths are
decoupled: the drift region electric field is determined by the collective
field of all the anodes, while the amplification region electric field is
determined by the anode radius. In this way, larger $E/P$ values can be
obtained in the drift region without unacceptably large $E/P$ values in the
amplification region.

##  3 Neutron spectroscopy with nitrogen-filled spherical proportional counter

Neutron detection with a nitrogen-filled spherical proportional counter relies
on two neutron absorption reactions:

${}^{14}\textrm{N}+\textrm{n}\rightarrow{}^{14}\textrm{C}+\textrm{p}+625\,{\textrm{keV}}$
${}^{14}\textrm{N}+\textrm{n}\rightarrow{}^{11}\textrm{B}+\alpha-159\,{\textrm{keV}}\,.$

The first process has significant cross-section for thermal neutrons and fast
neutrons with energies up to 2 MeV, while the latter has large cross-section
for fast neutrons with energies above 2 MeV, which is comparable to that of
the 3He detectors. This is shown in Figure 2(a), where the cross-section of
the two reactions is given as a function of the neutron energy, and it is
compared with 3He(n,p)3H, the process exploited by 3He-based detectors. Thus,
nitrogen-filled spherical proportional counters are highly efficient for fast
neutron detection without the need for moderation. The smaller cross-section
of the 14N(n,p)14C process for thermal neutrons with respect to the 3He-based
detectors can be compensated by the possibility to construct larger volume
detectors operated at high-pressures. Any potential wall effect, which is an
issue for 3He-based detectors, is significantly suppressed due to the higher
atomic number of nitrogen. At the same time, the detector has low sensitivity
to $\gamma$-rays, the gas is inert and harmless and can be readily deployed at
sites with strict safety requirements, while it is abundantly available at an
accessible cost.

During the proof-of-principle demonstration of the method [9], a 1.3 m
diameter spherical proportional counter was equipped with a 3 mm diameter
single-anode sensor and operated with nitrogen up to 500 mbar. An anode
voltage of 6.2 kV was required to achieve sufficient gas gain. Further
increase of the anode voltage was prohibited by the sensor technology at the
time, and thus it was not possible to explore higher-pressure operation. In
this work, the use of the multi-anode sensor ACHINOS with 11 anodes of 1 mm
diameter enables operation with nitrogen at pressures up to 1.8 bar, while
maintaining the required anode voltage below 6 kV.

##  4 Experimental setup and detector characterisation

(a) (b)

Figure 2: 2(a) Neutron interaction cross-section as a function of the neutron
kinetic energy. The inset shows the cross-sections at low energies with the
thermal neutron cross-section of interest shown by the orange point (solid
line). 2(b) Diagram of the experimental setup.

For the measurements presented below, a spherical proportional counter
installed, calibrated, and operated at the University of Birmingham (UoB) is
used. The experimental configuration is shown in Figure 2(b). The detector
comprises a 30 cm diameter stainless steel vessel, equipped with an 11-anode
ACHINOS, with 1 mm diameter stainless steel spherical anodes, placed at 1.7 cm
from the central electrode. A two-channel readout is implemented, where the
“Near” and “Far” channels correspond to the 5 anodes closest and the 6 anodes
farthest from the grounded rod, respectively, as shown in Figure 2(b) [11].
The signals are fed to two ORTEC 142AH preamplifiers, through which the anode
voltage (HV) is also provided. The signal digitisation is performed by a
“CALI-box” 16-bit analogue-to-digital converter (ADC) constructed by CEA-
Saclay [14] with a dynamic range of $\pm$1.25 V and a maximum sampling
frequency of 5 MHz. The digitised signal is subsequently sent to the
acquisition PC operating the SAMBA data acquisition (DAQ) software [15].

In the presented work the detector response is evaluated using thermal and
fast neutrons from an 241Am-9Be source [16], with an effective activity of
$2.6\,\textrm{MBq}$. Thermal neutrons were produced by placing the source
inside the UoB graphite stack, shown in Figure 3(a). The set-up was simulated
using the Monte Carlo N-Particle radiation transport code (MCNP 6.1) [17]. In
Figure 3(b), the probability of a moderated neutron of a given energy to reach
the gas volume of the spherical proportional counter is presented.

(a)

(b)

Figure 3: 3(a) The spherical proportional counter at the graphite stack room
at the University of Birmingham. 3(b) Probability for a neutron from the
241Am-9Be source to reach the detector volume as a function of its energy at
the detector, obtained with MCNP simulations [17].

The detector response was characterised using two approaches: a) A 210Po
source and b) A gaseous 222Rn source. In the first case, a 210Po source
emitting 5.30 MeV $\alpha$-particles was placed on the inner surface of the
cathode. For measurements at 1 and 1.5 bar N2 the source was only used for
dedicated calibration runs, while for measurements at 1.8 bar the 210Po source
was continuously in the detector providing a direct calibration during neutron
runs. Two sets of measurements with the spherical proportional counter filled
with 1 and 1.5 bar N2 at different anode voltages, provide the opportunity to
extract the amplitude curve of the detector, fitted with an exponential and
shown in Figure 4(a). An example of two such measurements is shown in Figure
4(b) for 1.5 bar pressure. Such curves inform the choice of the operating
anode voltage, so that the energies from 625 keV (corresponds to the recoil
energy of thermal neutrons) to 20 MeV is within the dynamic range of the
electronics chain.

(a) (b)

Figure 4: 4(a) Gas amplitude curve with 5.30 MeV $\alpha$-particles from 210Po
decays with 1 and 1.5 bar N2. Estimated uncertainties are smaller than the
marker size. 4(b) Pulse amplitude distribution on the response of the
spherical proportional counter to 5.30 MeV $\alpha$-particles from 210Po decay
at different anode voltages.

In the second case the detector was calibrated using the 222Rn emanated by a
commercial filter (Entegris MC700 902-F) used to remove H2O and O2 impurities
from the gas. This is relevant for neutron measurements at 1 and 1.5 bar N2.
The 222Rn decay chain results in three distinct peaks at 5.59 MeV, 6.11 MeV,
and 7.83 MeV energies, produced by the 222Rn, 218Po and 214Po $\alpha$-decay,
respectively. The origin of these peaks was confirmed by a study of the time
evolution of the event rate. The extracted half-life was within 3% of the 3.82
days expected for 222Rn. The gas purification for the measurements at 1.8 bar
was performed with a custom made filter [18], specifically designed to reduce
the 222Rn emanation to low levels. Thus, in these measurements, the 222Rn
$\alpha$-particle rate was negligible compared to the neutron detection rate.

##  5 Results

The nitrogen-filled spherical proportional counter was operated at pressures
up to 1.8 bar with various anode voltages.

Figure 5: A typical experimental pulse of the spherical proportional counter.
The amplitude, width and rise time parameters used in the event selection, are
indicated.

(a) (b) (c)

Figure 6: Thermal neutron detection at different nitrogen pressures: 6(a) 1
bar of nitrogen and anode voltage of 3.6 kV, 6(b) 1.5 bar of nitrogen and bias
voltage of 4.5 kV, and 6(c) 1.8 bar of nitrogen and anode voltage of 5.95 kV.
The peak at 162.1$\pm$6.0, 109.2$\pm$0.2 and 364.1$\pm$0.4 ADU correspondingly
defined by Gaussian fit of the amplitude distribution. The small peak at the
left of each histogram is attributed to residual noise.

To distinguish neutrons detected in the gas volume from events originating
from other sources, e.g. electronic noise or neutrons interacting at the
cathode, standard pulse shape parameters were taken into account.
Specifically, the rise time, defined as the time required from the signal to
increase from 10% to 90% of the pulse amplitude, and the pulse full-width at
half-maximum (FWHM) were used. These are described in Figure 5. The signal
pulse is expected to have a rise time of a few tens of
$\mathrm{\SIUnitSymbolMicro}\textrm{s}$ and provides information about the
energy deposited in the detector volume through pulse size parameters (peak
amplitude, integral of the pulse).

Figure 7: Pulse rise time versus amplitude, showing thermal neutrons events
with 1.5 bar N2 and 4.5 kV at the anode at approximately 110 ADU. Additional
to the thermal neutrons, the 222Rn decay chain is shown, ranging from 800 to
1600 ADU.

As a demonstration of thermal neutron detection, results with the detector
filled with N2 at 1, 1.5 and 1.8 bar and operating at 3.6, 4.5 and 5.95 kV
anode voltage, are presented in Figure 6. The rise time versus the amplitude
for the case of 1.5 bar pressure is presented in Figure 7. The concentration
of events in the leftmost part of the plot is attributed to thermal neutrons,
while the 222Rn decay chain is apparent at higher amplitudes. The relative
positions of the peaks from the decay chain of 222Rn confirms the observation
of the thermal neutron peak.

Figure 8: Amplitude distribution with the spherical proportional counter
operating with 1.8 bar N2 and 5.95 kV anode voltage. The peak with mean value
2176$\pm$41 ADU corresponds to the $\alpha$-particles from the 210Po sample
providing the energy reference. The run with thermal neutrons is presented
with the solid line, while the run with the calibration source alone is shown
with the dashed. Figure 9: Mean amplitude of the thermal neutron peak at 1 bar
as a function of the anode voltage. The error bars correspond to the standard
deviation of the Gauss fit. An exponential fit is shown in red.

(a) (b)

Figure 10: 10(a) Pulse amplitude versus rise time from simulation (coloured
circles/squares) compared with data (histogram). There is good agreement
between the prediction for the 14N(n,p)14C process and the measured data.
Events from neutron capture in the vessel have amplitudes lower than the
trigger threshold used in data taking. 10(b) The percentage of thermal neutron
interactions in which energy is lost to the wall of the SPC. The black points
are obtained using the simulation framework described in the text, with error
bars representing statistical uncertainties. The blue line is the expected
wall effect probability calculated using the CSDA range of a 625 keV proton,
taken from the pstar database [19].  Figure 11: Amplitude distribution of
thermal neutrons in comparison to fast neutrons from the 241Am-9Be source when
operating with 1.5 bar N2 and 4.5 kV anode voltage. The three rightmost peaks
correspond to 222Rn decay chain.

(a) (b)

Figure 12: 12(a) Fast neutrons pulse amplitude distribution from the 241Am-9Be
neutron source in comparison with the simulation results that indicate the
origin of the detected events. 12(b) Pulse rise time distribution for the
events of Figure 12(a) indicated as fast.

The response of the detector for neutron measurements at 1.8 bar and 5.95 kV
anode voltage is presented in Figure 8. The response of the spherical
proportional counter to thermalised neutrons and to the $\alpha$-particles
from the 210Po source is shown in black and the background run with only the
210Po source is presented in red. The $\alpha$-particles peak is fitted with
the sum of two Gaussian, providing the energy reference and validating the
thermal neutron peak shown in Figure 6(c).

A compilation of the thermal neutron detection measurements at 1 bar is shown
in Figure 9. The mean of the Gaussian function that fits the peak amplitude
distribution is presented as a function of the anode voltage. The uncertainty
represents the standard deviation of the Gaussian fit, while the response of
the spherical proportional counter to thermal neutrons follows an exponential
dependence with the applied anode voltage, in agreement with the amplitude
curve found during detector characterisation.

The spherical proportional counter response was studied with the use of the
UoB simulation framework [13], which combines Geant4 [20] for the simulation
of the passage of particles through matter, Garfield++ [21] for the
interactions of particles with the gas and the signal formation. The electric
field is described with the use of ANSYS [22] finite element method software.
The response of the spherical proportional counter filled with 1 bar of
nitrogen to 3 million thermal neutrons isotropically incident on the detector
was simulated. These simulated events were used to estimate the detection
efficiency and to derive the expected pulse shape parameters for comparison
with the data. Combining the 2.6 MBq activity of the 241Am-9Be neutron source,
the 5$\times$10-3 probability that a neutron emitted by the source reaches the
detector volume –obtained by integrating the spectrum of Figure 3(b)– and the
detection rate which was estimated to be 5.0$\pm$0.1 Hz for all datasets at 1
bar pressure, an efficiency of approximately 0.04% for thermal neutrons was
obtained. This is found to be in good agreement with the simulation that
provides an efficiency $0.057\pm 0.003\leavevmode\nobreak\
\mathrm{(stat.)}\leavevmode\nobreak\ \%$ for thermal neutron detection. At 1.8
bar, the detection rate was measured to be 9.4 Hz, corresponding to an
efficiency of 0.08%, while the simulation predicts an efficiency of $0.104\pm
0.003\leavevmode\nobreak\ \mathrm{(stat.)}\leavevmode\nobreak\ \%$. An
estimation of the fast neutron efficiency for the dataset at 1.5 bar,
accounting for the source activity, its position relative to the detector, and
the detection rate for neutrons of higher energy than the thermal peak,
provide 0.011% efficiency, while simulation resulted to $0.018\pm
0.001\leavevmode\nobreak\ \mathrm{(stat)}\leavevmode\nobreak\ \%$. This is in
accordance with the expected proportional increase of the detector efficiency
with pressure and the cross section difference between fast and thermal
neutrons, as can be seen in Figure 2(a). The pulse amplitude versus rise time
obtained from the simulation is compared with the measured data in Figure
10(a) and provides insights on the different neutron interactions. Thermal
neutron interactions via the 14N(n,p)14C process within the detector volume
are denoted by blue circles.

Figure 13: The background subtracted data compared to the simulation
prediction for the reconstructed neutron energy of the (n,p) and (n,$\alpha$)
reactions. The rise time value is used to discriminate events attributed to
each process.

Neutron capture in the vessel of the detector can produce $\gamma$-rays that
interact in the gas volume, presented in purple. Nevertheless, these are
rejected in the experimental data by the trigger threshold.

The portion of thermal neutrons that do not deposit all their energy in the
volume of the detector, the aforementioned wall effect, was quantified with
simulation studies at several pressure values. Results, presented in Figure
10(b), show how the impact of the wall effect decreases with the increase of
the nitrogen pressure, with the fraction of events affected by the wall effect
estimated to be 3.3% at 1.8 bar, compared with 5.7% at 1.0 bar and 10.4% at
0.5 bar, implying the reach of a plateau beyond approximately 2.5 bar pressure
for the specific 30 cm diameter SPC.

Fast neutron detection with the 241Am-9Be neutron source and the spherical
proportional counter filled with nitrogen at 1.5 bar and 4.5 kV anode voltage
is demonstrated in Figure 11. In this figure a comparison of the fast and
thermalised neutron pulse amplitude distribution is presented. The excess of
events due to the detection of fast neutron is demonstrated with black solid
line, compared to thermal neutron distribution with red dashed line. The
response of the SPC to fast neutrons was investigated with the use of
simulation, and the origin of the events is attributed to the corresponding
reaction, as shown in Figure 12. At the pulse amplitude distribution of Figure
12(a), the dominance of the (n,$\alpha$) reaction is evident for events with
detected energy over 2 MeV. The amplitude distribution of neutrons thermalised
by the graphite stack (attributed to the (n,p) reaction) is obtained from
simulation, while the corresponding distribution of 222Rn decay chain events
is obtained from a data taking run without the presence of fast neutrons from
the source. Both of these contributions are normalised to the observed data.
The dominant reaction at the first bin -denoted as “Other”- represents
$\gamma$-particles produced by neutron capture at the detector vessel and
elastic scatterings in the volume of the gas. The rise time of the events with
amplitude between the vertical dashed lines, that correspond to the detection
of fast neutrons, is presented in Figure 12(b). In these figures, the (n,p)
contribution is scaled by a factor 1.7, with respect to the cross section
presented in Figure 2(a), to match the observed data. Nevertheless, it is
clear that a threshold on approximately
$50\,$\mathrm{\SIUnitSymbolMicro}\textrm{s}$$ discriminates the (n,p) and
(n,$\alpha$) processes, and enables a correction to be applied for the
$Q$-values of the corresponding reactions. As a result the energy of the
incoming neutron is reconstructed and the observed spectrum is the convolution
of the 241Am-9Be spectrum with the cross section of the (n,p) and (n,$\alpha$)
reactions respectively and the detector resolution. This is shown in Figure 13
for the events denoted as “Fast” in Figure 12(a), where the shadowed area
represents a 50% systematic uncertainty in the rates of the 222Rn and “Other”
subtracted backgrounds. In the future the unfolding of the cross section and
the improved detector resolution will help to obtain the underlying energy
spectrum.

Figure 14: Pulse rise time versus amplitude following the modified pulse
processing, showing thermal neutrons events with 1.5 bar N2 and 4.5 kV at the
anode at approximately 110 ADU. Additional to the thermal neutrons, the 222Rn
decay chain is shown, ranging from 800 to 1600 ADU.

Following the comparison between data and simulation, an investigation of
pulses revealed a noise component. A modified procedure for the estimation of
the pulse shape parameters was derived, which implemented a low pass filter to
remove high frequency noise and focused in the region around the pulse leading
edge. The result of this approach is shown in Figure 14, with the pulse rise
time being in good agreement with the simulation. In Figure 15(a) the thermal
neutron peak is shown, which exhibit a much improved resolution. Nevertheless,
a second peak appears (with dotted line) which is attributed to the different
response of one of the anodes. This is supported by the integral of the small
peak being approximately the 1/4 of the integral of the main peak, that
embodies the response of the 4 other anodes. This was already apparent from
the tail in the right side of the Gauss distribution in Figure 6(b), which was
obscured by the deteriorated resolution. In Figure 15(b) the 222Rn decay chain
is presented with a fit model derived from the observation in Figure 15(a).
This non uniformity in the response among anodes can be corrected for by
individually reading out each anode and supplying an appropriate anode
voltage. This has been also discussed in a recent publication [11] and will be
our next R&D goal. Nevertheless, in order to avoid obfuscating the main
objective of this publication, that is the detection of neutrons at high
pressure, this was not applied to the data.

##  6 Conclusions and future efforts

The spherical proportional counter operated with nitrogen is a promising
alternative to 3He detectors for neutron spectroscopy, thanks to its
simplicity, robustness and large volume. The measurement principle relies on
the 14N(n,$\alpha$)11B and 14N(n,p)14C reactions. Earlier work has
demonstrated the feasibility of the concept, but was limited to low pressure
operation, due to the challenges in obtaining adequate signal amplification.
In this work, benefiting from the latest instrumentation developments,
including the multi-anode sensor ACHINOS, the operating range for neutron
spectroscopy was extended up to 1.8 bar. Measurements were performed at the
University of Birmingham, using an 241Am-9Be source, and the obtained results
were compared with dedicated simulations, presenting increased detection
efficiency and mitigating the wall effect, capitalising on the increased
target mass compared to 3He detectors. The (n,$\alpha$) - (n,p) discrimination
capability was demonstrated that enables the reconstruction of the energy of
the incoming neutron. The improved energy resolution anticipated with the use
of individual anode read-out, will significantly improve the neutron
spectroscopy performance of the system. This will be best demonstrated with
the planned measurements using mono-energetic neutrons. The presented results
are a leap towards the deployment of the spherical proportional counter as a
neutron spectrometer at a variety of scientific, industrial, and medical
settings.

(a) (b)

Figure 15: 15(a) Thermal neutrons pulse amplitude distribution following the
modified pulse processing at 1.5 bar pressure and 4.5 kV anode voltage.15(b)
222Rn decay chain peaks amplitude distribution. The double peak produced by
thermal neutrons (Figure 15(b)) is scaled to the 222Rn (dashed line), 218Po
(dash-dotted line) and 214Po (dotted line) energies, while the sum of these
distributions (solid line) agrees with the data.

## Acknowledgments

This project has received funding from the European Union’s Horizon 2020
research and innovation programme under the Marie Skłodowska-Curie grant
agreements No 845168 (neutronSphere), No 841261 (DarkSphere) and No 101026519
(GaGARin).

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