Computational and Experimental Photon Energy Dependence for CaSO 4 : Dy TL Phosphor

The energy dependences of a CaSO4:Dy thermoluminescence(TL) phosphor, which is fabricated in Korea Atomic Energy Research Institute(KAERI) for the purpose of personal dosimetry, were calculated by using MCNP4A code and then compared them to the experimental results. First, Monte Carlo calculations for the energy response of CaSO4:Dy phosphor are performed for 21 monoenergetic photons of energy ranging from 0.01 to 3 MeV, which can be most frequently encountered in radiation protection. The calculation results show a good agreement with those of other authors within ±5% of relative error. Calculations and experiments are also performed to determine the energy dependence of CaSO4:Dy phosphor with the Dy concentrations from 0.01 to 1.5 mol%. The photon beams used were 5 X-ray fields(M30, M60, M100, M150, H150) as described in the ANSI N13.11 standard and Cs gamma rays installed in KAERI. As expected, the energy responses show an increasing trend with the increase of the concentration of Dy in both experimental and computational results. The calculated energy responses show a good agreement with the experiment results within ±20% of relative error except for the concentration lower than 0.1 mol % Dy and the low energy regions of M30(Eave.=20keV) and M60(Eave.=34keV). These differences are caused due to the non-homogeneous distribution of Dy concentration and the effect of large grain size in the phosphors. The calculation method used in this study is useful to determine the optimal Dy contents and the energy dependences of the TL materials, and can be extended to the design of the personal dosimeter. Introduction The ICRU has introduced the personal dose equivalent, Hp(d), as an estimator for the effective dose equivalent, HE, or effective dose, E, approximately avoiding excessive overestimation or underestimation for the purpose of personal monitoring. Recently, many types of TL(Thermoluminescence) material are used to evaluate the personal dose equivalent in various radiation fields. For new TL material, analysis and evaluation of the various dosimetric factors and characteristics for this is essential, in order to use as a most suitable dosimeter for a given dose measurement. Among them, in particular, the evaluation of energy dependence is by far the most important. But the data for the energy dependence of TL materials commercialized or being developed are not enough, because it usually depends on the limited evaluation methods through experiments despite of the many advantages of theoretical calculation method except the evaluations of Bassi[1], Ray[2,3] and a few researchers. Therefore, In this study the energy dependence of CaSO4:Dy phosphor recently fabricated in KAERI was evaluated through the Monte Carlo calculation and experiment firstly. And then, the feasibility of the development of optimal thermoluminescence material and dosimeter design was investigated by studying the coupling of the relationship between two approaches and supplementations. Material and evaluation method TL material and Irradiation geometry The irradiation geometry of the TL material is set to be same with Monte Carlo simulation and real experiment as shown in figure 1. That is, we assumed that TL material with the Dy concentrations in the range of 0.01~1.5 mol % in the body material of CaSO4 is irradiated to the wide and parallel irradiation fields of the Xray(M30, M60, M100,M150, H150) of KAERI and Cs radiation field. The distance between source and TL material is 2 meters. In the Monte Carlo simulation, the density of air is taken to be 1.189 × 10 g/cm, which is the value of dry air at 20C and 750mmHg pressure.