유효선량 측정시스템(EDMS) 개발 및 복수선량계 기법의 실험적 검증

Abstract

Two dosimeters, positioned on the chest and back, could provide sufficient information for reasonable estimation of effective dose (E) for most exposure situations, excluding the possibility of significant underestimation of effective dose. Use of these two dosimeters with a suitable algorithm could not only solve the underestimation problem of the single-dosimeter approach, but could also alleviate the disadvantages of the multiple-dosimeter approach. However, it has not yet been confirmed experimentally whether the two-dosimeter approach estimates effective dose adequately or merely conservatively. Two-dosimeter algorithms were developed by simplified geometry and Monte Carlo simulations because it was not able to measure the effective dose directly. In this present study, the two-dosimeter approach was experimentally validated with a measurement system called pseudo Effective Dose Measurement System (EDMS) which is developed in the present study to measure effective dose. This system comprises 38 very small isotropic-responding high-sensitivity MOSFET dosimeters in an ATOM adult male phantom supported by 3D image-based Monte Carlo simulation technology to obtain accurate values of organ doses, effective dose and other quantities of interest. The EDMS is portable and very easy to use in the field and it measures the doses on a real-time basis. Because the ATOM adult male phantom has only 4 organs including bone, soft tissue, brain, and lungs, the other organs necessary for calculation of effective dose were defined with reference to the MIRD5 mathematical phantom. The numbers and locations of the MOSFET dosimeters were determined carefully after considering the tissue weighting factors, shapes and volumes of the organs. Each organ dose was determined by the 1-6 point-wise absorbed doses measured with the MOSFET dosimeters. The MOSFET dosimeter had been selected in the present study because it is very small and can measure radiation dose on a real-time basis. The MOSFET dosimeter, however, made mainly of silicon and epoxy, shows some energy dependence for low-energy photons. That is, when used in a phantom, it overestimates absorbed doses due to the existence of low-energy scattered photons. The MOSFET dosimeter shows some degree of angular dependence as well. Therefore, for accurate measurement of organ and tissue doses, the present study determined, by Monte Carlo simulations with the Monte Carlo N-Particle Transport Code System (MCNPX), the relative response (to tissue dose) of the MOSFET dosimeter, and thereby the dose correction factors, at various dosimeter locations in the ATOM adult male phantom. The MOSFET dosimeter is controlled by the MOSFET AutoSenseTM Dose Verification System and all messages generated by the system are recorded in “MsgHistoryOnCOM.txt”. A data process program, which can read the text file and calculate organ doses, E and other dose quantities was developed in C++. The 2007 recommendation of International Commission on Radiological Protection (ICRP 103) uses organ-averaged doses, called equivalent doses, and tissue weighting factors to calculate effective dose. The recommendation also designated two ICRP reference phantoms to be used in calculation of equivalent doses and effective dose. However, the EDMS measures effective dose using an ATOM adult male phantom and 38 MOSFET dosimeters, which results in an approximate measurement of effective dose. Therefore, in the present study, the error of effective dose measurement was determined by Monte Carlo simulations using the ICRP reference phantoms and the ATOM-MIRD hybrid phantoms. The ATOM-MIRD hybrid phantoms are developed in the present study by combining the CT images of the ATOM phantoms (for lungs, bone, brain, and skin) and the MIRD5 mathematical phantom (for the other organs). The effective dose calculated by the ICRP reference phantoms and the ATOM-MIRD hybrid phantoms showed good agreement for high energy photon beams (≥100 keV) for all irradiation geometries. The maximum difference was 35%, which occurs for the 30 keV photon beam in the anterior-posterior (AP) direction. In order to evaluate the performance of the two-dosimeter approach, the EDMS was placed in several non-uniform radiation fields in nuclear power plants (NPPs) and the effective dose measured by the EDMS was compared with the estimated effective dose from the two-dosimeter algorithms. The results were very convincing; that is, the two-dosimeter approach neither significantly overestimates nor seriously underestimates the effective dose. The results confirm the previous Monte Carlo simulation results that the two-dosimeter approach can be used to determine effective dose without significant overestimation or underestimation.