Nuclear Track Detectors

Approximately half of the human exposure to radiation is due to indoor airborne shortlived progeny (decay products) of the radon gas (Rn). To measure the airborne concentrations of radon gas and/or its progeny, Nuclear Track Detectors (NTDs) are usually employed. The NTDs register the alpha particles emitted by radon gas and its progeny in the form of tracks in the NTDs, which will become visible under the optical microscope upon suitable chemical etching of the NTDs. The two most commonly used NTDs are commercially known as CR-39 and LR 115. Calibrations of these NTDs for measurements of concentrations of radon and its progeny can be experimentally performed by exposing the NTDs to known concentrations in exposure chambers. Another approach is to calculate their sensitivities through computer simulations, which enable, among others, better understanding of the involved processes, optimization for the measurements, and determination of calibration coefficients in some circumstances where experimental calibration is not feasible. Section 1 of this chapter gives a review of existing methods for calculations of calibration coefficients, including Monte Carlo simulations as well as other approaches. Section 2 describes the geometrical model of alpha-particle track formation in NTDs, which will be employed in computer simulations in subsequent sections of this chapter. The two operation modes of the NTD, namely, open mode and the closed mode, are described in sections 2.3 and 2.4, respectively. In Section 2.3, computer codes for calculation of calibration coefficients for NTDs employed in the bare mode are described. The possibility to perform long-term measurements of radon progeny concentrations with bare-mode LR 115 NTDs (by using the “proxy equilibrium factor” which emerges from computer simulations) is presented with more details. Section 2.4 deals with the NTDs employed in the closed mode, i.e., enclosed in the socalled diffusion chambers. Here, the effects of radon progeny deposition onto the inner walls of the diffusion chamber are taken into account. Computer codes are described in details for the calculation of detector sensitivity to alpha particles emitted in the chamber volume and on the chamber wall. Section 3 gives a brief conclusion. D. Nikezic and K.N. Yu 120