=Paper=
{{Paper
|id=Vol-2507/408-412-paper-75
|storemode=property
|title=Simulation of Spectra of Cylindrical Neutron Counters Using the GEANT4
|pdfUrl=https://ceur-ws.org/Vol-2507/408-412-paper-75.pdf
|volume=Vol-2507
|authors=Andrey Churakov,Alexei Kurilkin,Juan Saiz Lomas
}}
==Simulation of Spectra of Cylindrical Neutron Counters Using the GEANT4==
https://ceur-ws.org/Vol-2507/408-412-paper-75.pdf
Proceedings of the 27th International Symposium Nuclear Electronics and Computing (NEC’2019)
Budva, Becici, Montenegro, September 30 – October 4, 2019
SIMULATION OF SPECTRA OF CYLINDRICAL NEUTRON
COUNTERS USING THE GEANT-4
A.V. Churakov1,2, A.K. Kurilkin1, J. Saiz3
1
Joint Institute for Nuclear Research, Dubna, Russia
2
Dubna State University, Dubna, Russia
3
University of York, York, England
E-mail: churakov@nf.jinr.ru
It is widely known that the amplitude spectrum of the helium proportional counter produced by the
irradiation of thermal and cold neutrons has a peak of full absorption with energy of 768 keV and two
small “shelves”, caused by boundary effects, i.e. the absorption of charged particles (proton or tritium
nuclei) in the detector wall. The simulation of the amplitude spectra of cylindrical counters with
different gas filling is presented in the paper. The possibility of the third peak, not coinciding with that
of full absorption is shown, whilst the peak position depends on the ratio of the path length towards
the counter diameter. The results obtained may be of interest for the development of low efficiency
neutron detectors and neutron monitors.
Keywords: neutron, detector, proportional counter, simulation, GEANT4
Andrey Churakov, Alexei Kurilkin, Juan Saiz Lomas
Copyright © 2019 for this paper by its authors.
Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0).
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Proceedings of the 27th International Symposium Nuclear Electronics and Computing (NEC’2019)
Budva, Becici, Montenegro, September 30 – October 4, 2019
1. Introduction
The 3He-filled Cylindrical proportional counters are perhaps the most widely used neutron
detectors. They are simple, reliable and commercially available devices with high detection efficiency
of thermal neutrons and very low sensitivity to gamma rays. Proportional counters have a good enough
time resolution which allows their use on diffractometers operated in time-of-flight mode.
An important characteristic of the proportional counters is the amplitude spectrum. It is known
that the amplitude spectrum of the helium proportional counter irradiated by thermal or cold neutrons
has a peak of full absorption with an energy of 768 KeV and two small "steps". These “wall effects”
arise from the hit of primary ionizing charged particles (proton or tritium nuclei) in the detector wall.
(fig. 1). When one of these particles products collides with the wall of the detector, its energy is
dissipated and does not contribute to the full energy peak.
Gamma-rays
Figure 1. Typical proportional counter spectrum
2. Simulation assumption and tools
In our work, we carried out a simulation of the amplitude spectrum of the proportional
counter. GEANT 4 [1] was used as a modeling tool. It was designed to simulate the tracks of
elementary particles passing through matter by the Monte Carlo method. The version of GEANT4
used in our simulations is 10.03.03. In addition, the software packages ROOT [2] on its version
6.14.06, for the storage and visualization of the data, and the program SRIM 2013 [3], for calculations
involving particles of primary ionization, were also used.
The simulated cylindrical counter was irradiated with a uniform flow of thermal neutrons. The
particle generator emulates the thermal neutron beam of energy (0.0253 eV). Neutrons pass through
the geometric model of the detector, taking into account their interaction with the walls of the detector.
The wall thickness was considered to be 0.2 mm, which corresponds to typical counter wall
thicknesses. The rate of neutrons was considered low enough to neglect double and triple events. In
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Proceedings of the 27th International Symposium Nuclear Electronics and Computing (NEC’2019)
Budva, Becici, Montenegro, September 30 – October 4, 2019
the simulation, it was assumed that each interaction between a neutron and a 3He nucleus happening
in the sensitive volume of the detector, will be registered, and the induced charge proportional to the
energy losses of particles in the volume of the counter will be collected at the anode. When the
primary particles are completely absorbed in the volume of the counter, the fluctuation of the charge
induced on the anode has a normal distribution. The same assumption was believed to be true for all
other energy values of primary ionizing particles. The value of the energy resolution of the detector
determines the dispersion of the induced charge distribution. On the other hand, the modeling did not
take into account the part of the spectrum caused by gamma radiation, since the level of the gamma
background is determined mainly by the neutron source [4] and the proportional counters themselves
have a sufficiently low sensitivity to gamma radiation.
3. Results and discussion
We started by modelling the spectrum of a standard proportional counter. In many scientific
facilities, counters of the SNM-18 type and their numerous analogues have been used. For comparison
we chose the “Helium-18-200” counter, produced by JSC "Consensus" [5], which is similar to the
SNM-18. On the left side of figure 2 the measured spectrum for these counters is presented. On the
right hand side, the simulation of the neutron component of the spectrum is shown. The simulation
results are in excellent agreement with the measured spectrum (fig. 2).
4500
4000
3500
3000
2500
Counts
2000
1500
1000
500
0
0 100 200 300 400 500 600 700 800 900 1000
Energy (keV)
Figure 2. Measured (left) and simulated (right) proportional counter spectra
It is necessary to say a few words about the efficiency of cylindrical detectors. Counters of
SNM-18 type are believed to absorb almost all thermal neutrons. However, this is not quite true. The
maximum efficiency of such counters is 86% in a narrow area strictly in the middle of the detector,
and the average efficiency of the detector, because of the cylindrical shape, will be 74 %. In large-
diameter detectors, the bulk of neutrons are counted near the input window, so the effect of the
detector's cylindricity is smaller. However, the amount of gas required to fill the detector increases in
proportion to the square of the diameter, which causes a significant increase in the cost of the
experimental installation. It would be more rational to cover the area of interest with a double layer of
neutron counters, saving on the quite expensive gas.
We simulated changes in amplitude spectra of SNM18 - type counters caused by the gas
mixtures. Helium is a fluid gas, so in some years, depending on the quality of the detectors, the gas
mixture may leak out of the sealed housing, and the efficiency and amplitude spectrum of the counter
will change. The results are shown at (fig. 3), where the pressure is expressed as a percentage of the
initial pressure value, 9 bar. The initial composition of the gas mixture is 8 bar 3He+ 0,95 bar Ar +0,05
bar CO2
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Proceedings of the 27th International Symposium Nuclear Electronics and Computing (NEC’2019)
Budva, Becici, Montenegro, September 30 – October 4, 2019
100%
90%
1000
80%
100%
90% 70%
80%
60%
100 70%
Efficiency
Counts
60% 50%
50% SNM-18
40%
40% Max.Eff
30% 30%
10
20%
10% 20%
5% 10%
1%
1 0%
0 100 200 300 400 500 600 700 800 900 1000 0% 10% 20% 30% 40% 50% 60% 70% 80% 90%100%
Energy (keV) Pressure of gas mix.
Figure 3. Amplitude spectra (left, logarithmic scale) and counter efficiency (right) as a
function of the gas mix pressure. The gas mix pressure is represented as a percentage of the initial
pressure. Blue line – average counter efficiency, red line – maximum counter efficiency (in the middle
of the counter).
At 100% to 30% of the initial pressure, the spectra look like the typical counter spectrum
(fig.1), and the decrease in pressure by half does not lead to any noticeable changes in the amplitude
spectrum. At 20% of the original pressure and below, the spectra are not similar to the classic, familiar
spectrum of the cylindrical counter. On the 20% spectrum we see the peak of full gathering, a strong
peak from the triton and a new peak arises, superimposed to it. On subsequent spectra, this new peak
almost completely suppresses the peak of full absorption.
The explanation for this new peak is the following one; primary ionization energy is spent
mainly in the ionization of the gas mix and losses in the counter walls. The amount of primary
ionization charge collected at the detector anode depends on the number of electron-ion pairs
remaining in the detector volume. When the pressure decreases, the mileage of charged particles
increases. Calculations were carried out using the program SRIM 2013. At a pressure value of about
30% of the initial total, the particle range is of 16.97 mm, which is comparable to the diameter of the
detector. Further pressure reduction leads to an increase in the length of the track, and it no longer fits
completely into the volume of the detector. It follows from the simulations that this is the most likely
value of the track, that depends mainly on the ratio between the diameter of the detector and the
maximum length of the track.
Amplitude spectra for different detector diameters are presented. The gas filling used was the
standard one. Up to about 5 mm, we did not observe the distortion of the spectrum and the appearance
of the third peak, since the mean free path of particles in the gas mixture 8 bar 3He + 0.95 bar Ar
+0.05 bar CO2 is about 5.1 mm, but at smaller diameters, the changes in the spectrum and the third
peak are clearly visible.
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Proceedings of the 27th International Symposium Nuclear Electronics and Computing (NEC’2019)
Budva, Becici, Montenegro, September 30 – October 4, 2019
50
1000 38
32
30
Counts
100 25
18
13
10
10
6,4
5
1
0 100 200 300 400 500 600 700 800 900 1000
4
3
Energy (keV)
Figure 4. Measured (left) and simulated (right) proportional counter spectra
4. Conclusion
Amplitude spectra of 3He-filled proportional counters were simulated (Fig. 4). The appearance
of an additional peak (Landau peak) is shown. This peak is caused by the incomplete stacking of the
tracks of primary ionization particles in the counter volume. The position of the peak depends on the
ratio between the diameter of the counter and the range of the primary ionization particles. The results
of this work can be used in the development of some types of new neutron detectors, such as neutron
detectors with small gas pressure: monitors, low-background cold neutron detectors, neutron
calorimeters («neutron telescopes»). The appearance of the new peak under the low gas pressure must
be considered in the developments of thin position-sensitive proportional counter tubes with resistive
readout and during calibration of slow neutron counters.
References
[1] J. Allison, et all., // Geant4 – simulation toolkit // Nuclear Instruments and Methods in Physics
Research, vol. A 506, pp 250-303, 2003
[2] Rene Brun and Fons Rademakers, ROOT - An Object Oriented Data Analysis Framework,
Proceedings AIHENP'96 Workshop, Lausanne, Sep. 1996, Nucl. Inst. & Meth. in Phys. Res. A 389
(1997) 81-86. See also http://root.cern.ch/
[3] James F. Ziegler, Jochen P. Biersack, Matthias D. Ziegler SRIM. The Stopping and Range of Ions
in Matter, 2015, Lulu Press, NY, USA. See also http://www.srim.org/
[4] D.H. Beddingfield, N.H. Johnson, H.O. Menlove. 3He neutron proportional counter performance
in high gamma-ray dose environments // Nucl. Instr. and Meth. in Phys. Res. A, 2000, V.455
pp. 670-682
[5] htttp://consensus-group.ru (accessed 28.09.2019)
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