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.
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
In our work, we carried out a simulation of the amplitude spectrum of the proportional
counter. GEANT 4 [
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
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 [
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" [
Energy (keV)
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 largediameter 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 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 5% 1% 100% 90% 80% 70% 60% y iicne 50% c ffE 40% 30% 20% 10% 0%
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. SNM-18 Max.Eff 1000 10 1 0 100 200 300 400 500 600 700 800 Energy (keV)
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.