Elsevier

Radiation Measurements

Volume 41, Issue 2, February 2006, Pages 235-240
Radiation Measurements

Major parameters affecting the calculation of equilibrium factor using SSNTD-measured track densities

https://doi.org/10.1016/j.radmeas.2005.06.007Get rights and content

Abstract

The equilibrium factor F between radon and its daughters as a function of the track density ratio D/D0 between bare and in can track detectors is solved graphically and gave more accurate solution than that solved mathematically elsewhere. The advantages of the graphical solution come from its simplicity and does not need any tedious mathematical formula or a computer program. The simplicity of this solution makes us study many parameters that affect the equilibrium factor determination such as the detector type, the diffusion chamber dimensions, the membrane specifications and the behavior of α-emitters around the detector. The results show that the equilibrium factor as a function of D/D0 takes different form according to the facility used. The range of this study covers two widely used detectors (CR-39 and LR-115) equipped in two widely used diffusion chambers (small and medium chambers).

Introduction

The hazard of inhaling radon comes from its radioactive decay products which get trapped in the lung depositing their α-particles energy in the tissue. Disintegration, ventilation and deposition (plate-out) are considered as the removal processes which reduce the activity of radon and its daughters in air lower than the equilibrium value. The equilibrium factor F was introduced to simulate this situation and consider a very important parameter to calculate the effective dose from radon and its daughters (Faj and Planinić, 1991).

Solid state nuclear track detectors (SSNTD) become the state-of-the-art nuclear track detector for long-term radon measurements. These detectors can be exposed to radon and/or radon daughters by different configurations (Nikolaev and Ilic`, 1999). The basic configurations are the can-mode and bare-mode configurations. In can-mode configuration, the detector was equipped in the lower part of a cup covered with semi-permeable membrane. The membrane has a role to let radon only entering the cup and prevents thoron, radon daughters and dust. In bare-mode configurations (bare track detector), the detector was exposed freely to radon as well as radon daughters.

The equilibrium factor F was determined by SSNTD based on using can and bare method. In this method, two similar detectors were exposed to radon, one in can-mode configuration (in can detector) and the other in bare-mode configuration (bare detector). F can be found as a function of the track density ratio D/D0 between bare (D) and in can (D0) detector, respectively. The problem of finding this function was solved by Planinic’ et al. (Planinic` and Faj, 1989; Faj and Planinić, 1991) using mathematical solution based on solving the equation of F as a function of the ventilation rate (λv) with D/D0(λv) together using a Cardon's formula and RND computer program to get F as a function of D/D0. This solution was based on the assumptions that the two detectors have the same calibration factor (or response) for radon and registered alpha particles from radon and its daughters by the same efficiency.

Section snippets

Graphical solution of F

The equilibrium factor is strongly dependent on the ventilation rate λv; this dependence was expressed as (Planinic` and Faj, 1990)F(λv)=λ1λ1+λv0.106+0.581λ2λ2+λv+0.38λ2λ3(λ2+λv)(λ3+λv),where, λ1,λ2 and λ3 are the decay constant of 218Po (3.715×10-3s-1), 214Pb (4.311×10-4s-1), 214Bi (5.834×10-4s-1), respectively.

The track density of both bare and in can detector relates the concentration of radon and its daughters as D=K(C0+C1+C4),D0=KC0,where K is the detector response. C0, C1 and C4 are the

Theoretical approaches

The above-mentioned solutions (approximate graphical and Planinic’ solution) have no comment on the used detector type (different detectors are different in response, registration energy window, critical angle, etc.) or the diffusion chamber (the cups are different in dimensions and has permeable membrane). Also, these solutions did not deal with the behavior of alpha emitters around the detectors. The above parameters were discussed in this work.

Results and discussion

The listed parameters in Table 1 were introduced into Eq. (7) and substituting the values of C¯i/C¯i-1 as a function of λv (the ventilation rate) to simply get D/D0 as a function of λv. Substituting λv ranged from 0 to 24h-1 with 0.01 intervals into D/D0 (Eq. (7)) and F (Eq. (1)). Therefore, for each λv one can get F and D/D0 which can be plotted together as shown in Fig. 2 for CR-39 equipped in Terradex cup (curve 2) and in our cup (curve 3) and compared with the approximate solution (curve

Conclusion

The equilibrium factor F as a function of D/D0 was solved graphically and shows a good agreement with the mathematical solution given elsewhere. The simplicity of graphical solution makes us introduce all parameters that affects the equilibrium factor.

This study dealt with two widely different detectors equipped in two widely different diffusion cups. Any diffusion chamber covered with polyethylene membrane has results located within the range of our study. Future work is directed to generalize

References (13)

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