MCNP 시뮬레이션을 통한 450 kVp 엑스레이 튜브의콘크리트 차폐벽 두께 계산 및반가층 방법을 이용한 계산과의 결과 비교

Abstract

Radiation generating devices must be properly shielded for their safe application. Although institutes such as US National Bureau of Standards and National Council on RadiationProtection and Measurements (NCRP) have provided guidelines for shielding X-ray tube ofvarious purposes, industry people tend to rely on ‘Half Value Layer (HVL) method’ whichrequires relatively simple calculation compared to the case of those guidelines. The method isbased on the fact that the intensity, dose, and air kerma of narrow beam incident on shieldingwall decreases by about half as the beam penetrates the HVL thickness of the wall. One canadjust shielding wall thickness to satisfy outside wall dose or air kerma requirements with thiscalculation. However, this may not always be the case because 1) The strict definition of HVL dealswith only Intensity, 2) The situation is different when the beam is not ‘narrow’; the beam qualityinside the wall is distorted and related changes on outside wall dose or air kerma such as buildupeffect occurs. Therefore, sometimes more careful research should be done in order to verify theeffect of shielding specific radiation generating device. High energy X-ray tubes which is operatedat the voltage above 400 kV that are used for ‘heavy’ nondestructive inspection is an example. People have less experience in running and shielding such device than in the case of widely-usedlow energy X-ray tubes operated at the voltage below 300 kV. In this study, Air Kerma value perweek, outside concrete shielding wall of various thickness surrounding 450 kVp X-ray tube werecalculated using MCNP simulation with the aid of Geometry Splitting method which is a famousVariance Reduction technique. The comparison between simulated result, HVL method result,and NCRP Report 147 safety goal 0.02 mGy wk-1 on Air Kerma for the place where the publicare free to pass showed that concrete wall of thickness 80 cm is needed to achieve the safety goal. Essentially same result was obtained from the application of HVL method except that it suggestthe need of additional 5 cm concrete wall thickness. Therefore, employing the result from HVLmethod calculation as an conservative upper limit of concrete shielding wall thickness was foundto be useful; It would be easy, economic, and reasonable way to set shielding wall thickness.