利用加速器质谱仪进行快速129I检测的方法研究

摘要

加速器质谱法是129I最灵敏的检测方法。传统方法分析样品时通常需要复杂的化学制样流程，对样品中碘元素进行分离、富集和纯化，测试周期较长，成本高。在核应急分析等类型的应用研究中，需要对大批量样品中的129I含量开展快速检测，针对此类应用需求特点，本研究尝试开发一种快速、流程简便并且成本低廉的129I-AMS检测方法。以三种已知129I水平的国际标准物质作为研究样品，将干燥和研磨均匀化的样品粉末直接与加入过本底碘的电热导体介质（Nb粉末）混合后压入靶座，实现快速制靶，进行加速器质谱测量，再结合制靶时称量的相关质量关系数据就能直接计算得出原始样品中的129I活度。通过对测量数据与已知值进行比较分析验证了方法的可靠性，估算方法的探测下限可达0.15μBq∙ g−1。%Background, aim, and scope Iodine-129 is one of the hard-to-measure radionuclides with a half-lifeT1/2 = 15.7 × 106 years, and Accelerator Mass Spectrometry (AMS) is the most sensitive method for measuring129I. The primary source of129I in the earth have been altered in a signiifcant way by human nuclear actions, and the129I now is an important indicator of the civil and military nuclear activities and nuclear accidents. A drawback of the AMS technique for measuring the129I is the time-consuming chemical separation required to prepare AMS targets from raw samples. This step significantly limits applications requiring rapid analyses and large numbers of samples, for example in monitoring studies associated with nuclear accidents. This work aims to develop a rapid AMS method for rapid determination of129I in environmental samples, especially for the emergency analysis.Materials and methods The only used chemicals is Niobium powder, blank iodine (Woodward iodine, WWI, contains no129I) and the129I standards. Three certiifed reference materials including sediment (IAEA-410), seaweeds (IAEA-446 and NIST-4359) were tested by this rapid method to verify the method. This work was implemented by eliminate the step of chemical separation of iodine from the raw sample materials, the samples were directly packed into target holder after dried and homogenized mixed with Nb powder, and the prepared target is directly used for AMS analysis. All targets are measured on Xi’an 3.0 MV AMS following the same steps as for the measurement of micro-iodine targets method, as described in (Liu, 2015). In this approach, blank iodine (only127I) is added to a matrix of niobium (Nb) powder and mixed with dried raw sample, to improve the measurement of a stable129I /127I ratio.Results The ion source behaved no abnormity on the targets with untreated raw samples materials, except very low stable iodine current output. The comparisons of the spectrometry from the blank targets, standard targets and sample targets showed a good resolution of129I, and the complex component in the targets has no bad effects on the129I spectrometry. The129I /127I ratio measured by this rapid method were found to be in agreement with known values that used a conventional AMS method for the same material, after the subtraction of the background signal. The addition of blank iodine into the targets improved the stable iodine current greatly, but it diluted the129I level of the sample, which leads to the measured129I /127I ratio does not relfect the real129I level, but if corrected by the dilution efifcient, the 129I /127I ratio were also found to be consistent with the well-known values within the errors. Noticed that in the case the dilution approach, the127I signal mostly contributed by the blank iodine added in the Nb powder, we can neglect the iodine content in the raw sample material and directly calculate the129I activity from the 129I /127I ratio and the mass of the raw sample, the amount of Nb, and the iodine added to the Nb. The results are consistent with the established values, within errors.DiscussionThis rapid method were evaluated according to: signal stability and measurement reproducibility. A stable but increased intensity of the127I5+current with iodine concentration was observed for the suite of reference samples, and multiple targets of the same reference material produced consistent127I and129I results. Therefore these criteria are satisifed. Conclusions This work developed a rapid and direct method for sensitive measurement of129I by AMS in 129I-rich environmental samples without wet-chemistry separation of iodine, and it was shown that such a direct AMS measurement method worked reasonably well for the two types of sample materials tested. All the129I /127I ratios and129I activities measured by this method were found to be consistent with the known values within errors. And the time consumption of a batch with 40 unknown samples was shorten to 5 hours, compared to the conventional AMS method. And the detection limit of129I of this method was estimated to be 0.15 μBq ∙ g−1. The ifnal conclusion can be drawn that, it is practicable to directly measure129I /127I ratios and129I activities using AMS without wet-chemistry separation in suitable cases.Recommendations and perspectives Since the seaweeds and sediment were tested as typical materials, it is expected that such a rapid measurement method can be also applied to other environmental solid materials, so long the ion source can function stably. This would be especially useful for the purpose of screening large numbers of samples to determine which ones should be subjected to more time-consuming but precise chemical processing.