大质量样品厚度对中子活化定量分析的影响和修正

Influence of mass sample thickness on quantitative analysis of neutron activation and its modification

滑石粉的主要化学成分是Mg3[Si_4010](OH)_2,通过检测面粉中^(24)Mg和^(28)Si原子核的含量,可以计算出面粉中滑石粉的含量.利用中子活化分析方法测量面粉中^(24)Mg和^(28)Si原子核的含量问题时,被测样品内部中子通量和能量随厚度的变化以及γ射线自吸收效应会对测量结果有较大的影响.利用MCNP5 (Monte Carlo N-particle transport code system 5)模拟了中子通量和能量与样品厚度变化的关系,并利用氦3正比计数管测量样品不同厚度处中子通量,结果显示MCNP5模拟结果与实验测量结果基本相符.通过MCNP5模拟和碘化钠探测器测量,研究了γ射线自吸收效应与样品厚度的关系,确定了6.6 cm的样品厚度为最佳实验条件,根据模拟结果给出了特征γ射线计数与样品厚度的关系式,并与实验进行对比,结果符合得较好.

With traditional neutron activation analysis,the increase of sample quality leads to some problems in both irradiation process and measurement process.These problems include the neutron flux gradient in the range of the sample,the decrease of the thermal neutron flux rate around the sample and the influence of self-shielding in the sample in the irradiation process,In the process of measurement,the self-attenuation ofγ-ray in the sample and the geometric effect of the sample lead to the effect of the detector on the measurement of characteristic 7-ray emissivity due to the difference in the detection efficiency of each point of the sample.So the neutron activation analysis of mass sample needs to make some additional modifications.By using the neutron activation analysis technique,the content of 24Mg and 28Si in a large amount of flour can be detected,and the content of talc powder in the flour can be given,so that the quality of flour can be monitored.The flour mainly contains C,H,O,N,Ca,and F element,but the main chemical constituent of talc powder is Mg3[Si4010](OH)2.Therefore,the measured content of Si and Mg element can be used to judge whether the flour contains talcum powder and to determine its exact content.When the content of 24Mg and 28Si in flour are measured by the neutron activation analysis,the variation of neutron flux and energy with thickness in the measured sample and the effect ofγ-ray self-absorption will have great influence on the measurement results.The relationship between the neutron flux and energy and the thickness of the sample is simulated by MCNP5(Monte Carlo N-particle transport code system 5),and the neutron fluxes at different thickness of the sample are measured by a 3 He proportional counter tube.The results show that the simulation results of MCNP5 are basically consistent with the experimental results.Using the simulation by MCNP5 and the measurements by a sodium iodide detector,the relationship between 7-ray self-absorption effect and sample thickness is studied,and the sample thickness is determined to be 6.6.cm that is adopted as an optimal experimental condition.Based on the simulated data,the function relationship between the counting of 1.779 MeV 7-ray and the thickness of the sample is obtained as follows:A=1401+3815 x-720 x2+64 x3-2.8 x4+0.05 x5.The curve trend of the experimental results is basically the same as that of the simulation results.