热表面电离源同位素丰度测量的理论模型

根据对热表面电离源同位素丰度测量过程中样品蒸发的微观研究 ,提出如下三条假设 :(1 )未被外层原子覆盖的原子才能蒸发 ;(2 )被外层原子覆盖的原子只能扩散不能蒸发 ;(3 )同位素原子的扩散几率与蒸发几率成正比。根据这三条假设建立了样品原子数的微分方程及边界条件 ,推导出了不同扩散系数的样品原子数的理论公式 ,从理论上讨论了熔融态、固态样品以及蒸发分解同时发生的样品同位素丰度比与样品消耗量间的关系。研究表明 :扩散系数 >1 0 -15m2 / s的样品同位素丰度比与样品消耗量的关系与 H.Kanno得出的结论相同 ,扩散系数 <5× 1 0 -2 1m2 / s的样品同位素丰度比与样品消耗量无关 ,等于实际值 ;蒸发分解同时发生的样品同位素丰度比与分解率蒸发率之比有关

Theory about Determination of Isotope Abundance by Thermal Ionization

Three hypotheses is put forward, based on microexamination about sample evaporation during determaination of isotope abundance by thermal ionization:1. Only atoms uncovered by outer layer atoms can be vapoured; 2. Atoms covered by outer layer atoms can diffuse but cannot vapoure; 3. The probability of diffusion of isotope atoms is in direct proportion to its probability of evaporation. The differential eguations and boundary conditions are established, based on these hypotheses. The theoretical formulary about sample atom number with different diffusion coefficient are reasoned out; the relationship between the isotope abundance ratio and the sample wastage of melting and solid samples and the samples simultaneously at evaporation and decomposition are discussed in theory. The study indicates that the relationship between the sample isotope abundance ratios and the sample wastage with diffusion coefficient >10 -15 m 2/s is accordant to the discuss of H.Kanno; the sample isotope abundance ratio with diffusion coefficient <5×10 -21 m 2/s is independent of sample wastage and is equal with real value; the isotope abundance ratio of the samples simultaneously at evaporation and decomposition is related to the ratio of dissolution and evaporation.