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Extending neutron activation analysis to materials with high concentrations of neutron absorbing elements.

机译：将中子活化分析扩展到具有高浓度中子吸收元素的材料。

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The purpose of this study was to investigate epithermal neutron self-shielding for all nuclides used in Neutron Activation Analysis, NAA.;The study started with testing the theory and measuring the nuclear factors characterizing thermal and epithermal self-shielding for 1 mL cylindrical samples containing the halogens Cl, Br and I irradiated in a mixed thermal and epithermal neutron spectrum. For mono-element samples, both thermal and epithermal experimental self-shielding factors were well fitted by sigmoid functions. As a result, to correct thermal neutron self-shielding, the sigmoid uses a single parameter, mth, which can be directly calculated for any element from the sample size, the weighted sum of the thermal absorption cross-sections, sigmaabs, of the elements in the sample and a constant kth characteristic of the irradiation site. However, to correct epithermal self-shielding, the parameter mep, a function of sample geometry and composition, irradiation conditions and nuclear characteristics, needs to be measured for each activated nuclide.;Since the preliminary tests were positive and showed that self-shielding, as high as 30%, could be corrected with an accuracy of about 1%, except in cases with significant epithermal shielding of one element by another, we pursued the study with the verification of two additional aspects. First, the dependency of the self-shielding parameters mth, and mep, on the properties of the irradiation site was evaluated using three different irradiation sites of a SLOWPOKE reactor, and it was concluded that the amount of both thermal and epithermal self-shielding varied by less than 10% from one site to another. Second, the variation of the self-shielding parameters, mth, and mep, with the size of the cylinder, as r( r+h), was tested for h/r ratios from 0.02 to 6.0, and this geometry dependence was confirmed even in slightly non-isotropic neutron fields. These results allowed separating from the mep parameter the amount of chemical element and the sample geometrical factor. Therefore, the remaining nuclear factor, considered as a product of nuclide composite nuclear characteristics and irradiation site characteristics, led to the introduction of a so-called epithermal neutron absorption cross-sections, sigmaabs,ep. This new nuclear parameter will allow the calculation of the epithermal self-shielding for all cylindrical samples activated in all types of irradiation sites.;For the 13 cases studied, the epithermal self-shielding factor, Gep, was obtained from the experimental effective self-shielding factor, Geff, by extracting the thermal neutron self-shielding factor, calculated with the sigmoid formulation. A least-squares fit of the experimental Gep values as a function of the mass of element yielded sigmaabs,ep for each activated nuclide. In addition, for all nuclides commonly used in neutron activation analysis, sigmaabs,ep was calculated with the Martinho, Salgado and Goncalves sigmoid formulation, which uses the total cross-section values at the peaks of the resonances. A comparison of the calculated sigmaabs,ep with the 13 measured values reveals that the calculated values are accurate to about 20%.;Finally, for all 76 nuclides commonly used in NAA, a spreadsheet program was written to use experimental or calculated sigmaabs,ep nuclear parameters to perform iterative self-shielding corrections of concentrations measured by neutron activation analysis. The user provides the parameters f and alpha of the neutron spectrum, the sample mass and dimensions, and the measured concentrations. In a typical case with 10% thermal self-shielding and 30% epithermal self-shielding, the corrected concentrations had uncertainties varying from 2% to 3%.;Keywords. Instrumental Neutron Activation Analysis, epithermal, thermal, self-shielding factors. (Abstract shortened by UMI.)
机译：这项研究的目的是研究NAA中子活化分析中使用的所有核素的超热中子自屏蔽;该研究从测试理论和测量表征1 mL圆柱形样品的热和超热自屏蔽的核因子开始卤素Cl，Br和I在热和中热混合中子光谱中辐照。对于单元素样品，S型函数可以很好地拟合热和超热实验自屏蔽因子。结果，为校正热中子自屏蔽，S型使用单个参数mth，该参数可以从样本大小，元素的热吸收横截面的加权总和sigmaabs直接针对任何元素计算得出样品中的α和恒定的第k个辐照部位的特征。但是，要纠正超热自屏蔽，需要测量每种活化核素的参数mep（样品几何形状和成分，辐照条件和核特性的函数）。由于初步测试是肯定的，并且表明自屏蔽，高达30％的温度可以以大约1％的精度进行校正，除非在一个元素对另一元素的显着超热屏蔽的情况下，我们通过另外两个方面的验证来进行研究。首先，使用SLOWPOKE反应器的三个不同辐照位置评估了自屏蔽参数mth和mep对辐照位置特性的依赖性，并得出结论，热和超热自屏蔽量均发生了变化。从一个站点到另一个站点的比例不到10％。其次，测试了自屏蔽参数mth和mep随圆柱体大小的变化，即r（r + h）的h /​​ r比从0.02到6.0，甚至可以确定这种几何依赖性。在稍微各向同性的中子场中。这些结果允许从mep参数中分离出化学元素的量和样品的几何因子。因此，被认为是核素复合核特征和辐照部位特征的产物的剩余核因子导致引入了所谓的超热中子吸收截面sigmaabs，ep。这个新的核参数将允许计算在所有类型的辐照部位中活化的所有圆柱形样品的超热自屏蔽。对于所研究的13种情况，超热自屏蔽因子Gep是从实验有效的自屏蔽获得的。通过提取用S型公式计算出的热中子自屏蔽因子，可以得到Geff屏蔽因子。实验Gep值与元素质量的函数的最小二乘拟合对于每种活化核素产生sigmaabs，ep。此外，对于中子活化分析中通常使用的所有核素，sigabs，ep是使用Martinho，Salgado和Goncalves乙状结肠配方计算的，其使用了共振峰处的总横截面值。将计算出的sigmaabs，ep与13个测量值进行比较，发现计算出的值准确度约为20％。核参数，以对通过中子活化分析测得的浓度进行迭代自屏蔽校正。用户提供中子光谱的参数f和alpha，样品的质量和尺寸以及所测量的浓度。在具有10％热自屏蔽和30％超热自屏蔽的典型​​情况下，校正后的浓度具有2％至3％的不确定度。仪器中子活化分析，超热，热，自屏蔽因素。 （摘要由UMI缩短。）

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