Geant4 simulation of multi-sphere spectrometer response function and the detection of 241Am-Be neutron spectrum

NUCLEAR ELECTRONICS AND INSTRUMENTATION

Geant4 simulation of multi-sphere spectrometer response function and the detection of ²⁴¹Am-Be neutron spectrum

Xiao-Fei Jiang，

Jing Cao，

Chun-Yu Jiang，

Ze-Jie Yin

Nuclear Science and Techniques

Vol.28, No.12

Article number 174

Published in print 01 Dec 2017

Available online 16 Nov 2017

DOI：10.1007/s41365-017-0328-2

39101

This paper is aimed at detecting the neutron spectrum of ²⁴¹Am-Be, a widely used neutron source, with the SP9 ³He proportional counter, which is a multi-sphere spectrometer system of eight thermal neutron detectors embedded in eight polyethylene (PE) spheres of varying diameters. The transport processes of a neutron in the multi-sphere spectrometer are simulated using the Geant4 code. Two sets of response functions of the PE spheres are obtained for calculating the ²⁴¹Am-Be neutron spectrum. Response Function 1 utilizes the thermal neutron scattering model G4NeutronHPThermalScattering for neutron energies of ≤4 eV, and Response Function 2 has no thermal treatment. Neutron spectra of an ²⁴¹Am-Be neutron source are measured and compared those calculated by using the response functions. The results show that response function with thermal treatment is more accurate and closer to the real spectrum.

Response functionNeutron energy spectrumMulti-sphereGeant4241Am-BeG4NeutronHPThermalScattering.

1. Introduction

Neutron spectrum spans a vast energy region from 10⁻⁷eV to 10⁹eV. The ²⁴¹Am-Be neutron source is a widely used neutron source [1]. The α particles emitted by ²⁴¹Am react with Be to generate neutrons. An ²⁴¹Am-Be neutron source is featured by its long life, simple protection, stable neutron emission, moderate size, and low γ-ray energy in the decay of ²⁴¹Am. It is of significance to detect the neutron spectrum of a ²⁴¹Am-Be source [2-3], using the Bonner sphere made of polyethylene (PE) with copper or lead inlets [4–7]. Their responses to high-energy neutrons increase with the cross-sections of copper and lead, but low energy neutrons are absorbed after moderation. The pure PE spheres with thermal neutron detectors can detect lower-energy neutrons.

In this paper, we present a multi-sphere spectrometer system to detect the neutron spectrum of ²⁴¹Am-Be. This is the SP9 ³He proportional counter consisting of eight thermal neutron detectors in eight pure PE spheres (PSs) of varying diameters [8-11]. Response functions of the eight PSs, as the key to calculating the ²⁴¹Am-Be neutron spectrum, are calculated by using Geant4 simulations with thermal neutron scattering model G4NeutronHPThermalScattering enabled and disabled. The counting rate is measured, and finally the ²⁴¹Am-Be neutron spectrum is calculated.

2. The simulation

2.1. Theory

Neutrons are slowed down to thermal neutron by elastic collisions with hydrogen atoms of the PSs, as the cross section for hydrogen is larger than those for carbon. The reaction, ³He(n, p)T (Q = +764 keV), has a good cross section to thermal neutrons. ³He proportional counter is usually used for detecting the thermal neutrons because of its total cross sections [12] and low sensitivity to γ-rays. For the eight PS of different diameters, the response function of the i^th PS is R_i(E) and the fluence of the neutron spectrum to be tested under different energy is Φ(E) [13-16].The count value N_i of the i^th PS is

N_{i} = \int Φ (E) R_{i} (E) d E, i = 1, 2, \dots, 8

(1)

So, N_i can be obtained by the experiment, and the response function R_i(E) is obtained by Geant4 simulation, which is the key to calculating the ²⁴¹Am-Be neutron spectrum. The calculated response functions of the eight PSs are only applicable for the multi-sphere spectrometer in this paper.

2.2. Geant4 model

Geant4 [17-19] is a toolkit for simulating the passage of particles through matter. Its areas of application include high-energy, nuclear and accelerator physics, and studies in medical and space science as well. The Geant4 version Geant4.9.6 is used in this paper. The SP9 ³He detector is sensitive to thermal neutrons. The corresponding Geant4 models [20-22] of interaction between neutron and matter are shown in Table 1. The G4NeutronHPThermalScattering model includes a thermal treatment below 4 eV. In all Geant4 versions, the user must first download the high precision neutron data files from the Geant4 web page to a local directory (G4NDL/) when they want to use the High Precision Neutron Models.

Geant4 physics models including various models for high-energy physics processes

Process	Geant4 Model	Energy (GeV)		Cross section data
		Minimum	Maximum
Elastic	G4NeutronHPThermalScattering	0	4 eV	G4ThermalScatteringDataset
	G4NeutronHPElastic	0	0.02	G4HadronElasticDataSet
	G4hElasticCHIPS	0.0195	100000	G4CHIPSElasticXS
Inelastic	G4NeutronHPInelastic	0	0.02	G4NeutronHPInelasticData G4HadronInelasticDataSet
	G4LENeutronInelastic	9.5	25	G4NeutronHPInelasticData G4HadronInelasticDataSet
Capture	G4NeutronHPCapture	0	0.02	G4HadronCaptureDataSet G4NeutronHPCaptureData
	G4LCapture	0.0199	20000	G4HadronCaptureDataSet G4NeutronHPCaptureData
Fission	G4NeutronHPFission	0	0.02	G4HadronFissionDataSet G4NeutronHPFissionData
	G4LFission	0.0199	20000	G4HadronFissionDataSet G4NeutronHPFissionData

2.3. Simulation

The outside diameters of the eight PSs are 4'', 5'', 6'', 7'', 8'', 9'', 10'', and 12''. The SP9 ³He proportional counter is 33 mm in diameter (the spherical part) and 134 mm in total length. It is operated at 800–900V with a neutron sensitivity of 8cps for 3.2 mrem/h. Figure 1 shows the arrangement of PS and the SP9 ³He proportional counter.

Fig.1

Schematic of PS and SP9 ³He proportional counter Part.

For smaller diameter PSs, low-energy neutrons are slowed down after elastic scattering, but they still have high probabilities to reach the PS center and be detected; while high-energy neutrons still have high energy after moderating and tend to escape the PS. For larger diameter PSs, a large number of low-energy neutrons are absorbed after moderation; while high-energy neutrons are slowed down to thermal energies, which still have high probabilities to reach the PS center and be detected. The degree of neutron moderation depends on the PS diameter. The neutron response of each PS is unique.

A general ²⁴¹Am-Be neutron source is used. The response functions can be obtained in the simulation by using a number of different concrete neutron energies. The response function simulation of Geant4 is shown schematically in Fig. 2. Eight SP9 ³He proportional counters are arranged around the neutron source, with a distance of 40 cm between SP9 ³He proportional counter and neutron source. In the Geant4 simulation, ten million neutrons are simulated for each energy point.

Fig.2

(Color online) Schematic diagram of the Geant4 simulation.

Eight sets of counts of PS at different energies are obtained by simulating the response function of the PSs. The response functions with and without thermal treatment are shown in Fig. 3. The response functions of smaller diameter PSs have a peak in the low energy region, and the peak position moves towards high energy region as the PS diameter increases.

Fig.3

(Color online) Response function without (a) and with (b) thermal neutron scattering model G4NeutronHPThermalScattering.

Also, the response functions with thermal treatment have far higher counts than those without thermal treatment. The use of the G4NeutronHPThermalScattering thermal mode in Geant4 simulation slows down the neutrons in PS more effectively, which is critical for the neutron capture interaction that leads to different count measurements, hence the higher counts with the thermal treatment.

3. The measurement

The simulated response functions are usually validated by measuring an ²⁴¹Am-Be neutron source [23-26]. The ²⁴¹Am-Be neutron source we used was produced by Institute of Atomic Energy of China in 1978, numbered as 0078AB473395, with an activity of 2.0×10⁸ Bq. The neutron spectra specified by International Standard ISO 8529-1 was used (from thermal to 20 MeV) [27-29] for the ²⁴¹Am-Be simulation and measurement. Fig.4 shows the eight PSs surrounding the neutron source.

Fig.4

(Color online)The experimental arrangement.

The SP9 ³He detector had been tested with ¹³⁷Cs or ⁶⁰Co γ-ray sources by amplitude discrimination. No waveforms were viewed on an oscilloscope, indicating that the energy deposition of a γ photon in the SP9 ³He detector was much smaller than that of a neutron. So, we discriminated γ from neutrons only with the amplitude in the main electronics system.

Neutron counts of the eight PSs were measured at 662, 1959, 2367, 1768, 1717, 1161, 1077 and 534 for 12'', 10'', 9'', 8'', 7'', 6'', 5'' and 4'' PSs, respectively.

With the simulated response functions, the ²⁴¹Am-Be neutron source spectra was calculated by Eq. (1). Two spectra calculated by the two sets of response functions are shown in Fig.5 (the insert shows the neutron spectra of 2–11MeV).

Fig.5

(Color online) Neutron source spectra of the ²⁴¹Am-Be evaluated by ISO 8529-1(■), and simulated with (▲) and without (●) 4 eV neutron process. B_E is the spectral source strength and E_n is neutron energy.

The RMS difference is 0.666 between the spectra calculated with thermal treatment and evaluated with ISO8529-1, and the RMS difference is 1.323 between spectra calculated without thermal treatment and evaluated with ISO8529-1. In Fig.5, the spectrum calculated with thermal treatment is more accurate in high energy region. The results indicate that neutron scattering model G4NeutronHPThermalScattering below 4 eV is indispensable for the response function simulation.

4. Conclusion

Two sets of response functions for an eight PE spheres spectrometer to detect neutron spectrum of ²⁴¹Am-Be are simulated, by using the scattering model of G4NeutronHPThermalScattering with and without thermal treatment (<4 eV). From the neutron spectra of ²⁴¹Am-Be measured with the eight PSs, the response function with thermal treatment is closer to the spectrum evaluated by ISO 8529-1. Therefore, the thermal model of G4NeutronHPThermalScattering is more appropriate within the limits of this particular response function in this paper.

References

Y.M. Li, J.X. Chen, G.H. Zhang et al.,

Study of physical characteristics for compressed-mixture type 241AmO2-Be neutron source

. At. Energy Sci. Technol. 47, 1-6 (2013). (in Chinese) doi: 10.7538/yzk.2013.47.01.0001