基于MCMC的屏蔽环境放射源定位方法研究

Study on Radioactive Source Localization Method in Shielded Environment Based on MCMC

遗失放射源的快速精准搜寻是航空放射源定位的核心问题。针对屏蔽环境下丢失放射源定位问题,提出了一种基于马尔科夫链蒙特卡罗(MCMC)方法的自适应样本集放射源定位算法。算法通过将屏蔽物衰减系数参数化,建立丢失放射源参数估计的物理模型,通过Metropolis-Hastings采样法获得放射源参数的后验分布,并实时判断马尔科夫链的收敛状态,使之自适应样本集长度,在保证定位精度的同时极大地缩短定位时间。分别采用MCMC方法、衰减系数参数化MCMC方法和增加收敛判断的自适应样本集MCMC方法,基于数值仿真和现场实验数据对放射源进行定位。仿真和实验结果表明,该算法可在未知衰减系数的屏蔽物条件下对放射源进行定位。在60m×60m仿真范围内,定位精度优于1 m;在5.4 m×2.4 m的室内实验范围内,定位精度优于0.1 m,定位精度与无屏蔽环境中的定位精度均处于同一数量级。通过实时判断收敛状态,可在保证定位精度的同时大幅缩短时间。在实验条件下,收敛状态实时判断前后的精度差异不大,但后者所需计算时间约为前者的10%。

Searching of lost radioactive sources rapidly and accurately is the core technology for radioactive source locating by aerial measurement.In order to solve the problem of radioactive source location in shielded condition,an adaptive radioactive source location algorithm based on Markov chain Monte Carlo(MCMC)method was proposed.Firstly,the initial values of radioactive source parameters were randomly given,and the physical model of the lost radioactive source parameter estimation was established by parameterizing the shielding attenuation coefficient.Then,based on the Metropolis-Hastings sampling method,the parameters of the radioactive source were sampled to generate Markov chain,and the appropriate sample set was selected to calculate the posterior distribution of the parameters of the radioactive source.Finally,the convergence state of the Markov chain was judged in real time,and the length of the sample set was adaptive,which can greatly shorten the time while ensuring the localization accuracy.To verify the feasibility and effectiveness of this method,based on numerical simulation and field experiment,the radiation field without shielding and the marble shielding radiation field with unknown attenuation coefficient were designed respectively.The original MCMC method,the MCMC method of attenuation coefficient parameterized and the MCMC method of adaptive sample set with convergence judgment were used to locate the radioactive sources respectively.The simulation and experimental results show that the proposed algorithm can locate radioactive sources in the environment with unknown attenuation coefficient shielding.Within the 60 m×60 m simulation search range,the localization accuracy is better than 1 m.Within the 5.4 m×2.4 m field experimental search range,the localization accuracy is better than 0.1 m,and the accuracy of localization is at the same level of magnitude as that in the environment without shielding.Due to the presence of shielding,the number of unknown parameters is increased,and the running time of the program is increased compared with that without shielding.However,by judging the convergence state in real time,it can greatly shorten the time while ensuring the accuracy of localization.Under experimental conditions,there is little difference in accuracy before and after real-time judgment of convergence state,but the time of the latter is about 10%of the former.In addition,the search environment for real radioactive sources is more complex,and in the actual operation process,the influence of irrelevant factors on measurement should be minimized as much as possible to improve positioning accuracy.