Monte Carlo simulation of radon SSNT detectors

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Abstract

A Monte Carlo based software for the computation of the sensitivity of etched radon track detectors was developed. It can be applied to the measurement of radon and radon daughters in free air or inside of a measurement chamber. LR 115 and CR-39 detectors, with or without an attenuator, are specifically addressed. Various etching conditions and observation criteria for counting the track density may be specified. The latent track formation and the etching process are realistically modelled. The dependence of the etch-rate ratio on the energy is taken into account. The plate-out phenomenon is included in the model. An inhomogeneous source distribution in the detector cup can be considered.

Introduction

Solid state nuclear track detectors (SSNTDs) are widely used for measuring α-emitting nuclides in various environmental studies. Among the attractive features of such detectors are the availability in different shapes and dimensions, the possibility to be placed practically in any point, the insensitivity to γ and β fields, the high dynamic range, the true time integrating capability. As they do not require power supply or maintenance during exposure and the costs of the detectors as well as of the processing are rather low, they are suitable for large scale surveys.

One of the main applications of the SSNTDs is represented by radon monitoring. A comprehensive review of the application of the etched track radiometers in this field has been published recently by Nikolaev and Ilić (1999). Frequently, the devices are experimentally calibrated, e.g. by the exposure in a controlled radon atmosphere. The experimental calibration can be usefully supported by theoretical methods, for example, in the cases when the conditions of the actual measurement differ from the conditions of the calibration (e.g. the deposition pattern of the radon daughters, the equilibrium factors, pressure, temperature and humidity differences). Various such theoretical methods, based on more or less accurate models, have been proposed in the past (Nakahara et al., 1980; Akber et al., 1980; Abu-Jarad et al., 1980; Somogyi et al., 1984; Damkjaer, 1986; Urban, 1986; Kleis et al., 1991; Qureshi et al., 1991; Nikezić 1992, Nikezić 1995; Andriamanantena and Enge, 1995; Sima, 1995; Andriamanantena et al., 1997; Djeffal et al., 1997; Nikezić and Yu, 1998).

In this paper, a Monte Carlo based software called ETRADET (Etch Track Radon Detector), useful for the computation of the response of the SSNTDs for radon measurement is presented.

The software combines Monte Carlo computation routines with user friendly interfaces. It can be executed on IBM-compatible personal computers.

Section snippets

The model

Basically, the measurement of radon with an SSNTD is accomplished as follows. The detector is exposed in a specific geometry (e.g. open type, cup type, with or without a filter, with or without an attenuator) for a definite time interval. The α particles emitted by 222Rn and its decay products and in specific circumstances, by 220Rn and its decay products, impinging on the detector, produce latent tracks whose parameters depend on the distribution of the α-emitting nuclides in the vicinity of

The ETRADET software

The model presented above is implemented in the ETRADET software. The computational routines written in FORTRAN 90 are compiled in a DLL (Dynamic Link Library) which is called from the interfaces written in Visual Basic.

The parameters are communicated between the interface and the computing software through specific files (detector file, geometry file, etching file, plate-out file). These files are easily prepared or selected using friendly interfaces. The results are written in output ASCII

Conclusion

The ETRADET software is able to provide a realistic calculation of the sensitivity of various etched track radon monitors. In the calculation, typical observation criteria are included. The variable etch-rate ratio, plate-out phenomenon and non-uniform source distribution are implemented in the program.

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