基于Geant4的肺部计数器效率校准及其受核素分布影响研究

Study on the calibration of lung counter's efficiency and its effect from radionuclide's distribution

效率校准是人体内照射活体监测中的一个关键环节,是影响内污染活体测量准确性的一个重要因素。利用人体躯干物理模型的CT扫描图片和实验确定的探测器晶体尺寸,分别建立了三维人体数字模型和高纯锗探测器的计算模型,进而通过调用Geant4工具包,编写了模拟肺部计数器的蒙特卡罗程序,用以校准计数器效率。在此基础上,结合ICRP生物动力学模型,模拟计算全身其他器官滞留核素对肺部计数器计数的贡献,研究核素在其他器官的分布对计数器效率的影响。结果表明:肺部计数器效率的蒙特卡罗校准结果与实验测量结果相对偏差为2.47%(对59.5 ke V)和-8.58%(对17.5 ke V);在摄入核素241Am的最初2天内和50天后,人体的实际探测效率要高于只考虑肺部滞留的探测效率,在其他器官分布的核素在肺部计数器的计数占总计数的60%以上,而在摄入中期(第3—50天),二者结果较为一致,AMAD为1μm时,其他器官内核素的计数小于20%,AMAD为5μm时,其他器官内核素的计数在30%以内。

Efficiency calibration is a key link in in vivo internal contamination monitoring of human body,and also is an important factor of the measurement accuracy. According to the CT images of the torso phantom,a three-dimensional voxel phantom is established. The Computational models of the HPGe detector is characterized by means of Monte Carlo transport codes,validated experimentally using point sources to verify the detector response at different energies and to further improve it with the inclusion of dead layers and inactive volumes on the crystals. A computer program is developed by Geant 4 to simulate the lung counter. By comparing on of the results of simulation and experimental results for lung counter detection efficiency of241 Am,the differences are found as 2. 47%( 59. 5 ke V) and- 8. 58%( 17. 5 ke V). Meanwhile a software was developed to solve the predicted values due to intakeradionuclides by means of numerical solution of the ICRP biokinetics models. A case was studied on inhalation of class M241 Am. It was analyzed how the other organs retentions influenced the efficiency calibration of lung counter. The results show that the whole body's efficiency is higher than the lung efficiency in the first two days and 50 days after intake241 Am,and both efficiencies are more consistent in 3—50 days after intake241 Am.