Monte Carlo modeling provides accurate calibration factors for radionuclide activity meters

Accurate determination of calibration factors for radionuclide activity meters is crucial for quantitative studies and in the optimization step of radiation protection, as these detectors are widespread in radiopharmacy and nuclear medicine facilities. In this work we developed the Monte Carlo model of a widely used activity meter, using the Geant4 simulation toolkit. More precisely the “PENELOPE” EM physics models were employed. The model was validated by means of several certified sources, traceable to primary activity standards, and other sources locally standardized with spectrometry measurements, plus other experimental tests. Great care was taken in order to accurately reproduce the geometrical details of the gas chamber and the activity sources, each of which is different in shape and enclosed in a unique container. Both relative calibration factors and ionization current obtained with simulations were compared against experimental measurements; further tests were carried out, such as the comparison of the relative response of the chamber for a source placed at different positions.The results showed a satisfactory level of accuracy in the energy range of interest, with the discrepancies lower than 4% for all the tested parameters. This shows that an accurate Monte Carlo modeling of this type of detector is feasible using the low-energy physics models embedded in Geant4. The obtained Monte Carlo model establishes a powerful tool for first instance determination of new calibration factors for non-standard radionuclides, for custom containers, when a reference source is not available. Moreover, the model provides an experimental setup for further research and optimization with regards to materials and geometrical details of the measuring setup, such as the ionization chamber itself or the containers configuration.     

不同标准X射线RQR辐射场刻度眼晶状体当量剂量Hp(3)比较研究

目的通过在不同标准X射线RQR辐射场对Hp(3)进行刻度,并对刻度结果进行比较,探究国内标准X射线RQR辐射场刻度Hp(3)的可行性。方法选择直径20 cm、高20 cm的柱模,分别选取国内外标准X射线RQR辐射场对同一TLD进行Hp(3)的刻度,选择射线包括RQR4(60 kV)、RQR7(90 kV)、RQR9(120 kV),刻度内容包括能量响应、角度响应和线性响应。结果在能量响应方面,TLD对国内外标准X射线RQR辐射场响应均较好,响应值与照射值差异均在10%以内。在角度响应方面,TLD在国外辐射场响应值较好,响应值与照射值差异均在6%以内。而在国内辐射场,TLD在20°响应值偏低,响应值与照射值差异>10%。在线性响应方面,TLD在国内和国外标准X射线RQR辐射场拟合程度均较好。结论本研究的各项检测结果表明,国内标准X射线RQR辐射场可以对TLD进行Hp(3)的刻度。     

Optimization of a measurement facility for radioactive waste free release by Monte Carlo simulation

A novel free release measurement facility (FRMF) was developed within the joint research project “Metrology for Radioactive Waste Management” of the European Metrology Research Programme. Before and during FRMF design and construction, Monte Carlo calculations with MCNPX and PENELOPE codes were used to optimize the thickness of the shielding, the dimensions of the container, and the shape of detector collimators. Validation of the numerical models of the FRMF detectors and the results of the optimization are discussed in the paper.