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Nuclear Inst. and Methods in Physics Research, A 906 (2018) 103–109
Contents lists available at ScienceDirect
Nuclear Inst. and Methods in Physics Research, A
journal homepage: www.elsevier.com/locate/nima
Improved pulse shape discrimination for high pressure 3 He counters
J. Balibrea-Correa a,b ,∗, G.F. Ciani c,d , R. Buompane b,e , F. Cavanna f,g , L. Csedreki d , R. Depalo h ,
F. Ferraro f,g , A. Best a,b ,∗∗
a
Universitá degli Studi di Napoli ‘‘Federico II", Italy
INFN, Sezione di Napoli, 80126 Napoli, Italy
Gran Sasso Science Institute, INFN, Viale F. Crispi 7, 67100 L’Aquila, Italy
d
Laboratori Nazionali del Gran Sasso (LNGS), 67100 Assergi, Italy
e
Universitá degli Studi della Campania, Italy
f
INFN, Sezione di Genova, Via Dodecaneso 33, 16146 Genova, Italy
g
Universitá degli Studi di Genova, Via Dodecaneso 33, 16146 Genova, Italy
h
INFN, Sezione di Padova, Via F. Marzolo 8, 35131 Padova, Italy
b
c
ARTICLE
INFO
Keywords:
Pulse shape discrimination
3 He detectors
Monte Carlo detector response
ABSTRACT
The suppression of the internal detector background in3 He counters through pulse shape discrimination methods
is becoming an area of interest for a growing number of experiments that want to detect low event rate signals.
We discuss two discrimination methods for signals coming from high-pressure (thus high efficiency)3 He counters
read out through charge sensitive preamplifiers. We present a digital filter to convert the integrated signal to
a current pulse. The application of a discrimination method based on the peak shape suppresses the internal
alpha-induced background by > 98.5%. In addition a semi-empirical model of the signal creation in3 He counters
is presented and compared to experimental data.
1. Introduction
In the current era of very low count rate experiments it is of crucial
importance to control and suppress the various backgrounds seen by
the employed experimental setup. Dark matter, double-beta decay and
low-energy nuclear astrophysics experiments can be located in deep
underground environments, where the external background (secondary
cosmic-ray induced muons) is suppressed by orders of magnitude compared to on the surface of the earth [1–3]. In very low background
conditions like these, the intrinsic radioactivity of the detector and other
nearby material becomes a significant, sometimes even the dominant,
source of experimental background.
In this work we focus on the treatment of the intrinsic background
of 3 He counters, which are widely used to detect thermal neutrons with
high efficiency. The counter body, commonly steel or aluminum, contains radioactive impurities that emit alpha particles into the sensitive
area of the detector [4]. In the case of aluminum this can result in
background rates up to 10−4 counts cm−2 s−1 [5]. Steel contains a lower
level of radioactive impurities, nevertheless some background is always
present and needs to be suppressed.
The neutron detection in these counters is based on the capture
reaction 3 He(, )T, which has a very high cross section for thermal
neutron energies. In principle, as two reaction products are emitted
during neutron capture and only one (the alpha particle) in the case of
a background event, pulse shape discrimination (PSD) can be employed
to identify the signal source. Over the years a lot of effort has been spent
on the identification and suppression of the background component.
Two recent examples are rise-time based PSD, where the pulse is read
out through a charge-sensitive (thus integrating) preamplifier [5] and
a method based on the search for multi-peak structures in the current
pulse of the detector [6,7].
A recently commissioned setup at the Laboratory for Underground
Nuclear Astrophysics (LUNA) at the Gran Sasso National Laboratory
employs relatively high-pressure (10 bar) 3 He counters that are read
out through charge-sensitive preamplifiers. As will be shown below, the
rise-time method becomes much less effective for high filling pressures
due to the higher stopping power of the gas minimizing the difference
between background and neutron events. In this work, we describe a
novel methodology that has been developed in order to overcome this
issue and compare it to a Monte Carlo model of the detectors based on
the GEANT4 toolkit [8,9].
The paper is structured as follows: In Section 2 we describe the
experimental setup and the detectors used in this work; Section 3 is
devoted to the description of the PSD algorithm and its performance and
a comparison with the above mentioned PSD methods [5,7]; In Section
∗ Corresponding author at: Universitá degli Studi di Napoli ‘‘Federico II", Italy
∗∗ Corresponding author.
E-mail addresses: javier.balibrea@na.infn.it (J. Balibrea-Correa), andreas.best@na.infn.it (A. Best).
https://doi.org/10.1016/j.nima.2018.07.086
Received 1 July 2018; Received in revised form 25 July 2018; Accepted 25 July 2018
Available online xxxx
0168-9002/© 2018 Elsevier B.V. All rights reserved.
J. Balibrea-Correa et al.
Nuclear Inst. and Methods in Physics Research, A 906 (2018) 103–109
4 we present the Monte Carlo model of the 3 He detector; Monte Carlo
and experimental 3 He(, ) signatures are compared in Section 5; Finally
in Section 6 our conclusions are shown.
2. Experimental setup
In this work, 18 cylindrical 3 He counters filled with 10 bar of
were used.1 Their diameters were 2.54 cm (1 inch), 12 of them
had an active length of 40 cm, the other six were 25 cm in length.
The steel wall of all counters was 0.5 mm thick. The 3 He tubes were
connected to charge sensitive preamplifiers manufactured by CAEN and
were supplied an operating voltage of 1200 V. The preamplified signals
were digitized using CAEN 1724 cards with a resolution of 14 bit and
100 Ms/s sampling rate. The digitized pulses were directly written to
disk for further analysis.
The measurements were performed underground at the Gran Sasso
National Laboratory. Two different data sets were taken: ‘‘neutron’’ and
‘‘alpha’’. Neutron data were measured using an Am/Be source and alpha
data were collected during a long background run. The counters were
embedded inside a borated polyethylene matrix and surrounded by a
2.54 cm thick 5% borated polyethylene shielding to reduce the detection
of environmental neutrons. It has to be emphasized that neither of these
data sets are pure in the sense that they only consist of neutron or
alpha signals. The source measurement contains a small fraction of alpha
pulses while the background runs are a mixture of neutron and alpha
signals. This is discussed in detail in the following sections.
3 He
Fig. 1. Rise-time amplitude diagram for the background (blue) and Am/Be (red) configurations for the long 3 He counters in the region of interest . (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this
article.)
Table 1
Remaining background and3 He(, ) experimental distributions after the rise-time deposited energy PSD methodology compared to Langford et al. [5].
3. Pulse shape analysis of the experimental data
Langford et al. [5]
This work
The digitized experimental data was analyzed as follows. The high
frequency noise picked-up by the charge sensitive preamplifier was
removed from the analysis by applying a Gaussian filter to the digitized
buffer. The baseline was calculated as the average of the buffer samples
before the rising flank of the preamplifier waveform (presamples). For
each event, two important quantities were determined: the maximum
of the pulse and the rise time. The first is defined as the difference
between the highest buffer value and the baseline. The second is the
time difference between the signal reaching 10% and 50% of the pulse
maximum [5].
The maximum of the pulse was converted into deposited energy
assuming a proportional dependence, adjusted to fit the 3 He(, ) full
absorption energy peak.
The rise-time vs deposited energy scatter diagram for the Am/Be
(red) and background (blue) data in the region of interest, i.e. the
energy range covered by the 3 He(, )T ejecta, is shown in Fig. 1.
Following the analysis carried out by Langford et al. [5], events in
the region above the solid black line are identified as / particle
interactions. Preamplifier discharges and remaining high frequency
noise was rejected through selecting a signal rise time cut of 0.2 μs.
Using the Am/Be and background data, rise time-amplitude cuts were
chosen to select neutron or alpha events. The solid blue line enclose
the region accessible to neutron events, while the majority of -induced
signals lie in the region between the dashed black lines.
The PSD region is then defined as the rise-time vs deposited energy
3 He(, ) region not overlapping with the ‘‘background’’ area. The
remaining neutron and  distributions, defined as the ratio of areas
between the discrimination region and the total distribution, were calculated from the Am/Be and background data sets. Compatible remaining
distributions have been obtained for the long and short detectors. The
results from this work are shown in Table 1.
We achieve a worse discrimination between background and
3 He(, ) events than Langford et al., which can be explained by
differences in filling pressure and gas composition. While we achieve
Rem. Bkg. (%)
Rem.3 He(, ) (%)
<1
∼3
45
∼35
very good background rejection, the rather high suppression of 3 He(, )
events makes this PSD methodology not optimal for the counters used
in this work.
Recently, a new and improved method based on the double peak
structure of 3 He(, ) events has been presented by Zeng et al. [7]. The
3 He(, )T reaction (Q=764 keV) is characterized by the emission of two
charged reaction products, a proton (573 keV) and a triton (191 keV),
in opposite directions (for thermal neutrons). The charge deposition in
the 3 He gas following this reaction can be described by two well defined
Bragg peaks that are connected by a weaker ionization charge cloud. If
the signal is read out through a current sensitive preamplifier, the output
pulses show a double peak waveform [6,7]. Only in a few specific cases
that is not true, i.e if one of the reactions products is not detected (wall
effect), the reaction products are emitted nearly parallel to the anode
wire or the neutron capture takes place close to the amplification region.
-decay events occurring inside the detector’s housing show only
a single charge peak followed by a moderate ionization cloud along
the track. Therefore, the signature of  events is defined by singlepeaked waveforms. A schematic picture of both reactions inside the
active volume of the 3 He detectors is shown in Fig. 2.
Based on Zeng et al., in this work the digitized charge sensitive preamplifier waveforms were converted into current sensitive preamplifier
signals by applying a first order digital high pass filter (CR) described
by the following recurrence formula [10]
 =
1+
1+
 −
−1 + −1
2
2
(1)
where { }, { } are the input and output buffers, respectively. The
parameter  is related to the bandwidth of the filter and its cut frequency,  , through  = −2  . The cut frequency must be determined
according to the bandwidth characteristics of the charge preamplifier
signals and the sampling time,  . For the current experimental setup,
this parameter was set to  =1.85 MHz. An example of the filter
performance is shown in Fig. 3.
1
Manufactured by GE Reuter Stokes, model numbers RS-P4-0816-217 and
RS-P4-0810-249.
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J. Balibrea-Correa et al.
Nuclear Inst. and Methods in Physics Research, A 906 (2018) 103–109
Fig. 4.  — amplitude diagram for background (blue) and Am/Be (red) configurations
for the long 3 He counters . (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
Fig. 2. Schematic picture of charge recollection in the 3 He tubes for 3 He(, ) reactions
and -particle decay from the impurities in the detectors walls . (For interpretation of the
references to color in this figure legend, the reader is referred to the web version of this
article.)
Table 2
Remaining background and3 He(, ) distributions after application of the peak-shape PSD
method.
Zeng et al. [7]
This work (Long)
This work (Short)
We define the PSD parameter  for the converted current sensitive
preamplifier signals as
 (0 , 1 )
 (0 , 1 ) +  (1 , 2 )
Rem.3 He(, ) (%)
<1
<1.5
< 1.5
71
78
88
energy decreases, larger  values are more frequently found, corresponding to wall effect events. The background data, mostly composed
of -particles, is restricted to large values of , i.e. single peaked signals.
The events from the background set that fall into the full 3 He(, ) energy
absorption region correspond to the few environmental neutrons that
were able to pass through the shielding and pollute the background
run.
From a comparison between the ‘‘neutron’’ and ‘‘’’ data sets, a
value for  was chosen to maximize both the neutron detection and
the  rejection efficiencies. It is shown the dashed black line in Fig. 4.
Our results and a comparison with the literature are given in Table 2.
Considering the entire  energy range up to 6 MeV (as in [7]), the
fraction of remaining alphas is less than 1.5%.
A large part of the background can be suppressed while preserving
a relatively large number of 3 He(, ) signals. The differences between
the short and long detectors can be explained by their different positions
in the polyethylene matrix: even underground and shielded, the 3 He
detectors are still immersed in a weak environmental neutron field. The
short detectors were placed deeper inside the polyethylene matrix and
are thus better shielded against environmental neutrons, leading to a
‘‘cleaner’’  spectrum.
The results for both detector geometries are displayed in Fig. 5.
The total (black), the rejected  signals (blue) and accepted 3 He(, )
events (red) are shown. In the long detector spectra (top) a clear neutron
signature is visible. Because of their better background suppression, the
spectrum of the short (inner) detectors is cleaner.
Fig. 3. Digitized input charge preamplifier (black) and filtered current sensitive preamplifier waveforms (red) for a 3 He(, ) event. Also shown are the rise time and the maximum
amplitude of the pulse. The shadowed parts of the filtered waveform correspond to the
‘‘fast’’ and ‘‘slow’’ integral regions . (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
(0 , 1 , 2 ) =
Rem. Bkg. (%)
(2)
 (0 , 1 ) and  (1 , 2 ) are the fast and slow integrals of the current
[
]
sensitive preamplifier pulse over two different intervals, 0 , 1 and
]
[
1 , 2 . Note that the sum of both integrals is the full integral of the
filtered pulse. In the current study, 1 − 0 = 1μs and 2 − 1 = 7μs
were chosen to optimize the neutron/alpha discrimination. Both integral
regions are shown as the gray and blue filled areas in Fig. 3.
The  vs deposited energy scatter plot for the Am/Be (red) and
background (blue) data sets is shown in Fig. 4. Inside the region of the
3 He(, ) full energy peak the Am/Be signals are spread over a wide
range of , covering values from 0.9 down to 0.4. This corresponds
to long current pulses, i.e. double peaked waveforms. As the deposited
4. Monte Carlo simulation
In order to understand the detector response to neutron-induced and
 events, a semi-empirical model of the 3 He detectors has been developed. Despite the rather simple detection principle of 3 He counters, an
accurate simulation of the detector response requires a precise description of the particle interactions with the different detector materials, the
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Nuclear Inst. and Methods in Physics Research, A 906 (2018) 103–109
Fig. 6. Simulated 3 He(, ) event. The input charge arriving at the amplification region is
shown in black, the simulated charge sensitive preamplifier signal in blue, and the current
sensitive preamplifier signal in red . (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
 is proportional to the deposited energy at that position, E . That
is,
 ( ,  , 0 ) =  ( ) (0 )
(3)
where  is the proportional constant between the deposited energy and
the charge and  () is a relative gain function (edge effect gain) that
accounts for the incomplete knowledge of the counter internals and
distortions of the electric field at the edges of the detector. The combined
effect of both is translated into a position sensitive gas multiplication
factor (gain) along the anode wire and plays an important role in the
detector simulation [11].
Since electron mobility is typically 1000 times larger than ion
mobility, we assumed that the waveform registered by the charge
sensitive preamplifier is almost entirely produced by electrons arriving
at the amplification region. Therefore, only electrons are transported
from the ionization position to the amplification region by integrating
the kinematic equation () = ()∕. The electron drift velocity was
modeled as
Fig. 5. Total background spectra (black) for the long (top) and short (bottom) detectors.
The events classified as  and 3 He(, ) are displayed as filled blue and red histograms .
(For interpretation of the references to color in this figure legend, the reader is referred
to the web version of this article.)
() = −
transport of the ionized charge to the amplification region and a realistic
model of the electronics present in the experimental setup.
The Monte Carlo simulation of the 3 He detectors was developed covering the following aspects. The simulation of particle interactions with
different materials was based on the GEANT4 toolkit [8,9], including
the ‘shielding’ class for the accurate description of thermal neutron and
ion interactions. A deterministic model was used for the transport of the
ionization charge to the amplification region. Finally, the simulation of
the charge sensitive preamplifier electronics was performed using digital
filters.
The geometry of the detectors was implemented according to the
manufacturer specifications using a gas mixture of 95% 3 He and 5%
CO2 . A point-like isotropic source of thermal neutrons at one meter
distance from the detector was used for the detector response to
neutron captures. For the -induced signals,  particles were emitted
isotropically from random positions and depths within the steel housing
of the 3 He detector.
For each reaction, the track of the simulated ions within the detector volume was saved, including the relevant physical quantities
{ ,  ,  , 0 } corresponding to interaction position, deposited energy
and time, respectively. In this model it is assumed that the number of
ion–electron pairs q generated by the ionization track at each position
()

(4)
where − is the electron mobility in the gas (assumed to be constant in
this work), () is the electric field, and  is the pressure of the gas [12].
Using the electrical field of a cylindrical geometry, Eq. (4) becomes
() =
− 
 log(∕)
(5)
where  is the voltage between anode and cathode, situated at a and b
respectively from the tube axis. Thus, the time needed for the electron
charge produced at position  to arrive in the amplification region, t , is
obtained by integrating Eq. (5) between the original radial position 
and the edge of the amplification region  (),
( )
]
(∕) [ 2

 = 0 +
 − ( ( ))2
(6)
−
2
In Eq. (6) it is assumed that the amplification radial distance depends
on the axial position of the detector. In the central part of the detector,
where no edge effect occurs,  () is five times the anode radius [12].
Towards the end of the counters it is inversely proportional to the edge
effect gain  ().
{
0 ∕ (),  () < 0.95
 () =
(7)
5, ℎ
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J. Balibrea-Correa et al.
Nuclear Inst. and Methods in Physics Research, A 906 (2018) 103–109
Fig. 7. Adjusted  () (solid) and  () (dashed) functions for the long (red) and short
(blue) 3 He detectors. The z distance for both active volume was normalized to 40 cm
length . (For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
Fig. 8. 3 He amplitude signature for long and short detectors. The experimental data are
given in black, Monte Carlo simulation without edge effect in blue, and Monte Carlo
simulation with edge effect in red . (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
The energy resolution of the 3 He counters was described according
to the following equation [12]

() = √

(8)
where  is the total deposited energy and  is an adjustable parameter to reproduce the experimental resolution at the total absorption
3 He(, ) peak.
The ionization track is then multiplied by an avalanche process in the
amplification region. This charge, as a function of the time (current), is
used as input for a simulated charge sensitive preamplifier with a long
RC time constant. The simulated waveforms are then transformed into
current sensitive preamplifier signals by applying Eq. (1). An example
is shown in Fig. 6, where the charge arriving in the amplification region
is shown in black, the charge sensitive preamplifier output in blue and
the converted current sensitive preamplifier waveform in red.
The simulated pulses were processed in the same way as the experimental data, calculating for every pulse amplitude, rise time and the
PSD parameter .
5. Comparison with experimental data
The free parameters of the model,  (), 0 and − , were adjusted to
reproduce the experimental response of the long and short detectors to
3 He(, ) reactions. The final functions for both active lengths (normalized to 40 cm detector length),  () and  (), are shown in Fig. 7.
For both active lengths, the edge effect is modeled by similar functions:
A fast decrease of the gain at the edge of the detector together with a
fast increase of the amplification radial distance.
After adjusting  (), a good agreement is obtained for the 3 He(, )
amplitude spectra between experimental data (black) and the model
(red) (see Fig. 8). The largest differences are found in the region of the
triton wall effect (5%). To illustrate the strong effect of the edge gain
function, we are showing the results of a simulation that does not take
this effect into account (blue lines).
As can be seen, the edge effect mimics the wall effect, decreasing
the ratio between the full 3 He(, ) deposited energy peak and the rest
of the spectrum affected by the wall effect.
The − and 0 parameters were adjusted to reproduce the experimental rise time-deposited energy signature shown in both panels
of Fig. 9. The obtained values are − =1.6⋅10−5 cm2 atm/(ns⋅V) and
0 =1.0 cm.
The emission angle of the reaction products and the radial distance of
the initial neutron interaction have a large influence on the pulse shapes.
Neutron-induced pulses can have short rise times in the following cases:
Fig. 9. Rise-time vs deposited energy 3 He(, ) signature for experimental (top) and
Monte Carlo (bottom) data sets . (For interpretation of the references to color in this
figure legend, the reader is referred to the web version of this article.)
the neutron is captured near the amplification region, near-parallel (to
the anode) emission of the reaction products or wall-effect events, when
one of the two ejecta strikes the housing and the Bragg peak is not
generated.
In Fig. 10 we show a comparison between experimental and Monte
Carlo data for  vs deposited energy.
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Nuclear Inst. and Methods in Physics Research, A 906 (2018) 103–109
Fig. 10.  vs deposited energy 3 He(, ) signature for experimental (top) and Monte
Carlo (bottom) data sets . (For interpretation of the references to color in this figure legend,
the reader is referred to the web version of this article.)
Fig. 11. Top, Monte Carlo rise-time vs deposited energy scatter diagram for 3 He(, ) and
 particles. Bottom, Monte Carlo  vs deposited energy scatter diagram for 3 He(, ) and
 particles . (For interpretation of the references to color in this figure legend, the reader
is referred to the web version of this article.)
The response of the 3 He tubes to 5 MeV  particles was calculated
in the same way as for 3 He(, ) reactions. The top panel of Fig. 11
shows the Monte Carlo rise-time vs deposited energy for thermal neutron
captures. Alpha particles (blue) are characterized by short rise times
while neutrons (red) are characterized by longer tracks and longer rise
times (except for the cases mentioned above). Taking this into account,
we select the region where almost no  events are found (<0.1%) as the
region delimited by the black line. Thus, according to this selection, the
remaining 3 He(, ) distribution is 58% of the initial number.
The  vs deposited energy scatter plot is shown in the bottom
panel of Fig. 11. In this case,  events (blue) are constraint to large 
values, corresponding to single-peaked signals as already pointed out in
Section 2. Thus, selecting the region where  particles are excluded, the
remaining 3 He(, ) distribution amounts to ∼70%. By performing cuts
on the various event types (distance to the central wire, emission angle,
energy deposition) we identified that the most critical parameter is the
interaction radius, where about 60% of all true events are rejected. The
false negative rate in all of the other cases is maximally ∼ 25%.
The Monte Carlo results indicate that the signals from  and 3 He(, )
can be distinguished by the rise-time and  methodologies with a
reasonably large remaining 3 He(, ) distribution. Like seen with the
experimental data, the  vs. deposited energy method performs better
than the discrimination by rise time.
and remaining 3 He(, ) distribution (∼78/88%) values are similar to
literature values for low-pressure 3 He counters and PSD with charge
pulses.
Additionally we developed a semi-empirical model of the detector
response, which can be used to further improve PSD methods. The
Monte Carlo results agree within 5% with the experimental amplitude
data presented in this study. The Monte Carlo pulse shape discrimination
results show that signals from  particles and from neutron captures can
be distinguished by the rise-time and  methodologies with a reasonably large remaining neutron distribution. The  vs deposited energy
method is found to performs better than the rise-time vs deposited
energy method.
Acknowledgments
This research has been carried out in the framework of the STAR
project ‘‘UCAN’’, which is funded by the University of Naples ‘‘Federico
II’’ and the Compagnia di San Paolo.
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