Simulation study of the response of a highly sensitive silver-activation detector for neutron detection using MCNP code

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Highlights

  • Monte-Carlo simulations carried out to optimize activation neutron detectors.

  • Obtained results lead specialties to design the most effective neutron detector.

  • Optimized designs can significantly decrease weight and size of big detectors.

  • Optimized designs can significantly decrease the costs of big neutron detectors.

Abstract

In this paper, simulations were carried out using Monte-Carlo N-Particle MCNP-4C code to determine the energy dependence of the response of a highly sensitive silver-activation detector used for fast-neutron detection after moderated to thermal neutrons. The detector consists of sandwiched alternating thin-sheets of silver and crystal scintillator Anthracene. The simulations also had considered different materials and designs to get the most efficient design for detecting signals from fast-neutrons interactions with the active materials (silver) and the scintillator. The energy and charge depositions of photons and electrons created by fast-neutron interactions with detector within the detection-cell were simulated for different scenarios. Owing on the simulations, a method to maximize the response by modifying the design was identified. The optimal configurations are suitable for a wide range of monitoring and detecting neutron sources especially for big detectors implemented for security purposes, where the costs and weighs can be reduced significantly.

Introduction

Scintillation detector which deployed to reveal fast-neutrons from neutron sources could be activation detector which captures neutrons to produce a radioactive species (Shehada and Golovkov, 2020; Gozani et al., 2011; Jiseok et al., 2019). The decay of these species needs to be distinct and without delay related to the incident fast-neutron intensity. Activation detectors wherein silver have been used for many years to measure neutron fluence from neutron sources (Dennis and William, 1979; Bhadra et al., 2010; Kanani et al., 2020; Wolfertz et al., 2020). These instruments consist of a GM-counter wrapped with a silver foil embedded in a hydrogenated moderator. Fast neutrons are slowed down to thermal energies in the hydrogenous material, then captured within the silver to get Ag110 (24.4 s half-life) and Ag108 (2.43 min half-life). After that, beta decay of the generated isotopes is counted by the embedded GM tube, and the fast neutron fluency is derived through an appropriate calibration in a reference neutron field(Simon et al., 2012).

A scintillator is a material that has scintillation property when excited by ionizing radiation. When irradiated by incoming particles absorb their energy and scintillate and re-emit the absorbed energy in the form of light (Leo, 1994).

Anthracene (C14H10) has the highest light output of all organic scintillators and is therefore chosen as a reference. The light outputs of other scintillators are sometimes expressed as a percentage of anthracene light (Leo, 1994). Anthracene is colorless but emits a blue (400–500 nm peak) fluorescence under ultraviolet radiation (Lindsey et al., 2014). There are wide ranges of applications where scintillators can be used. These detectors will have broad-ranging applications in nuclear non-proliferation, radioactive waste management, nuclear worker monitoring, system reliability, dose assessment, and risk analysis. The improvement of such devices is always crucial (Liakos, 2011).

The use of a scintillator in connection with a photomultiplier tube finds wide use in hand-held survey meters used for detecting and measuring radioactive contamination and monitoring nuclear material. Scintillators generate light in fluorescent tubes, to convert the ultra-violet of the discharge into visible light. Scintillation detectors are also used in the petroleum industry as detectors for Gamma Ray logs (Glenn, 2000).

The detection efficiency for electrons (beta radiation) is essentially 100% for most scintillators. Since electrons can make large angle scattering, they can exit the detector without depositing their full energy in it. Organic scintillators, having a lower Z than inorganic crystals, are therefore best suited for the detection of low-energy (<10 MeV) (Leo, 1994). Electrons are charged particles and thus interact continuously through long range Coulomb force. An electron typically undergoes roughly 104 more collisions for the same energy loss than a neutral particle. For example, an electron slowing down from 0.5 MeV to 0.0625 MeV will undergo on the order of 105 collisions. A photon needs only about 20–30 Compton scatters to reduce its energy from several MeV to 50 keV (Nicholas, 1995). Only a small fraction of the kinetic energy lost by a charged particle in a scintillator is converted into fluorescent light. The remainder is dissipated non radiatively, primarily in the form of lattice vibrations and heat. The fraction of energy that is converted into fluorescence energy (scintillation efficiency) depends on the particle type and its energy.

Section snippets

Method

In this work, simulations were carried out using MCNP-4C code to simulate the response and efficiency of fast-neutron detector indicated in the paper (Dennis and William, 1979). Furthermore, different materials and designs of this detector were simulated to get the most efficient design of detecting signals from fast-neutrons interactions with the active materials (silver and others) and the scintillator Anthracene instead of plastic scintillator NE-110. In principle, it is possible to compare

Results and discussion

The fast-neutron detection efficiency is proportional to the energy and charge deposition at the detector-cell which presented as a detector in the input file of MCNP code. As the high-energy electrons emitted and travelled throw the active material and the scintillator, therefore they will generate a cascade of photons which mostly will be absorbed in the light-emitter window of the photo-multiplier tube (PMT). So, logically, the more energy and charge deposited in the detector-cell, the more

Conclusions

The use of Ag-foils as active material is more efficient than using other elements to obtain more fast-neutron detection efficiency. And it is necessary to use 5 cm thickness of hydrogenate materials covering the detector for the detection of fast-neutron source. It is recommended to minimize the foils-length (from 20 cm to 5 cm in the case of this study) if the source is located in front of and parallel to the detector. But at the same time, that depends on the neutron intensity and neutron

Author statement

Firstly, I would like to reviewers for their precious notes on my manuscript, and I am glad to reply on them. The manuscript have been corrected and revised as can as possible by the author, and I hope now it is suitable to be considered for publication.

Declaration of competing interest

I approve that this manuscript has not been published and is not under consideration for publication elsewhere. We have no conflicts of interest to disclose.

Acknowledgments

The work is supported by the National Research Tomsk Polytechnic University. The author greatly appreciated the valuable help by friend Mr. Igor Pyatkov and supports from supervisor Prof. Krivobokov Valery Pavlovich at the School of Engineering & Nuclear Technologies.

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