Dosimeter incorporating radiophotoluminescent detectors for thermal neutrons and γ-rays in n-γ fields

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Abstract

We have developed a dosimeter associating different neutron converters with two radiophotoluminescent detectors to measure thermal neutrons and γ-rays in a mixed n-γ field. Tests show that the H(10) and Hp(10) responses to thermal neutrons and γ-rays are linear with detection limits lower than 0.4 mSv. The angular dependence of the dosimeter response is satisfactory and the influence of a phantom on the results is examined.

Introduction

Compared to other methods of radiation detection (emulsions, track detectors, thermoluminescence, and others), radiophotoluminescent (RPL) detectors offer a unique combination of advantages that include rapid exploitation, reusability, insensitivity to light, temperature and humidity, and stability to fading. RPL glass detects γ, X and β radiations well but it has a low sensitivity for neutrons [1]. A detector readout makes no distinction among the different radiations. In an article by Girod et al. [2] RPL dosimeters were studied for area and criticality monitoring to detect fast neutrons by thermalization and radiative capture in cadmium. Here we propose a compact RPL dosimeter conceived to determine both thermal neutrons and γ-rays in a mixed n-γ field. Thermal neutrons in such fields have been measured by using the 10B(n,α)7Li reaction combined with separate measurements to account for the γ-ray component [3], [4], [5]. However, the (n,α) technique is not transposable to RPL detectors because γ-rays from thermal neutron activation of the Ag contained in the RPL detector cannot be adequately separated from the α-particle signal. In order to measure both the thermal neutron flux and the γ-ray flux in an n-γ field, we have employed side-by-side RPL detectors, each one associated with a different neutron converter. Both detectors are irradiated simultaneously by the same n-γ field. We thus obtain two (different) thermal neutron signals and two (same) γ-ray signals. The detector readings furnish sufficient information to quantify both fields. Each step in the design of the dosimeter was checked by simulations with the Monte Carlo code MCNPX 2.7.0 [6]. For individual dosimetry, it was seen that backscattering from a phantom significantly enhances the magnitude of the dosimeter response. All irradiations were performed with the laboratory’s 241Am-Be calibrator characterized by Amgarou et al. [7]. Thermal neutrons were obtained by placing the source in the center of a 9-inch diameter Bonner sphere.

Section snippets

RPL detectors and reader

The RPL detectors used in this work are rigid plates of Ag-doped phosphate glass (35 × 7 × 1.5 mm3) containing by weight: 48.33% O, 13.24% Na, 6.18% Al, 31.53% P and 0.72% Ag with a 2.6 g/cm3 density. This elemental analysis was made by scanning electron microscopy at the Institute of Physics and Chemistry of Materials of Strasbourg. During an irradiation, luminescence centers are generated in the glass. After exposure to radiation, before reading the detector, the glass must undergo a stabilizing

Principles governing the dosimeter

Fig. 1 shows diagrammatically the structures of the thermal neutron/γ-ray dosimeter.

Results

From Eqs. (1), (2) and the data of Table 1 one finds in the present case thatnCdσCdnBσ478=(2.83×1020×20600)(5.23×1020×3607)=3.09and consequentlyBγ=3.09×R(II)-R(I)2.09where as before RPL(I) and RPL(II) are the detector readings. Bγ registers principally the 4.438 MeV emissions from the 241Am-Be source. The 26.34 and 59.54 keV γ-rays of 241Am are absorbed in the source housing. Knowledge of Bγ enables finding the number of 113Cd(n,γ)114Cd γ-rays detected from Eq. (1).

Conclusions

This study has shown that RPL detectors can be advantageously incorporated in a dosimeter to measure both thermal neutron and ambient γ-ray doses in an n-γ field. The experimental dosimeter response to neutrons in terms of H(10) and Hp(10) is linear with respective detection limits of 0.32 and 0.12 mSv. The dosimeter presents an angular dependence that satisfies ISO performance requirements. The calibration factor obtained for thermalized neutrons of 241Am-Be is (6.7 ± 0.5) × 10−3 mSv.cm2/RPL

Acknowledgment

We thank A. Pape for pertinent comments on the manuscript and A. Sellam and F. Begin for sharing their experience with MCNPX simulations.

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