地时-中国Ar-Ar定年关键技术：绵阳研究堆辐照方法

Many new Ar-Ar laboratories have been established in recent years. More and more Ar-Ar dating samples are needed for research projects in China. However, only the 49-2 reactor can now provide limited opportunities for Ar-Ar dating, far away from meeting the needs. We got a bat samples irradiated in new CMRR reactor in 2014. The J values were in straight lines with the positions with small errors, and high-precision Ar-Ar ages were obtained. The radioactive intensity of the irradiated samples in CMRR is much higher than those irradiated in the 49-2 reactor. It is necessary to obviously reduce the radioactivity so that the CMRR can widely used to irradiate samples for Ar-Ar dating by joint efforts. In this project, we will investigate the neutron energy spectra in different positions within the CMRR center canal, integral neutrons, shielding and packing materials and the composition of samples, to establish irradiation model. We will calculate the thermohydraulics and security conditions for Ar-Ar irradiation to increase the J values. We will also seek the best shielding and packing materials at the same time and test the security of quartz tube for packing samples during irradiation. The schemes are also further considered to remove the packing tubes in the hot room, because these are good ways to reduce the radiactivity for irradiation samples. In a word, we will seek a best irradiation method to irradiate samples with minimum A/J ratio and to promote the precision of Ar-Ar dating, in order to meet the Earthtime program. The powerful CMRR can greatly increase the J values to obtain high-precision Ar-Ar ages for hydrothermal deposits and oil/gas fields.

近年来新建了许多Ar-Ar实验室，Ar-Ar定年任务快速增长，目前仅有49-2堆提供有限的Ar-Ar样品辐照时间，远远不能满足需求。申请者2014年在绵阳新堆CMRR辐照样品J值线性好，误差小，提高了Ar-Ar定年精度，获得高质量年龄数据。但CMRR辐照样品的放射性强度严重偏高，需要联合攻关，降低辐照样品剂量，才能把CMRR推广应用于Ar-Ar样品辐照。本项目拟通过研究CMRR堆芯各个部位中子能谱、积分注量、屏蔽材料、样品的主微量元素含量和包覆密封材料，进行辐照样品堆芯建模、热工水力学和安全分析，确定最佳辐照条件，提高J值。探寻合适的屏蔽和包覆材料，进行石英管安全性试验，设计热室解体方案，尽量降低剂量，获得最优化辐照条件，进一步提高Ar-Ar定年精度，使我国Ar-Ar研究水平与欧美"地时"先进水平接轨。大功率CMRR能大幅度提高J值，实现热液矿床和油气成藏高精度定年。

中文摘要 (对项目的背景、主要研究内容、重要结果、关键数据及其科学意义等做简单概述，800字以内，含标点符号)：..背景：我国已有10多所高校和研究院所20多台质谱仪从事Ar-Ar定年，仅有49-2堆提供有限的辐照服务，辐照质量不稳定，远不能满足需求，严重阻碍我国Ar-Ar年代学的发展，寻找辐照源迫在眉睫。2012年竣工的绵阳堆CMRR快中子占比高，可用于Ar-Ar快中子辐照，但2014-2016年4批辐照样品试验表明，放射性剂量严重偏高，影响实验操作人员健康。.主要研究内容：降低辐照样品管放射性剂量，寻找最佳辐照部位，提高J值精度，从而提高Ar-Ar定年精度，使我国Ar-Ar年代学研究与国际Earthtime要求接轨，成为本项目的联合攻关目标和主要研究内容。.降低辐照样品管剂量：为确保新建CMRR反应堆的安全，此前4批样品采用硬质铝管封装样品，辐照后1根样品管剂量高达30 μSv，且1 支空铝管冷却1 年后仍然高达8 μSv。通过建模和精密热工计算，确定高纯石英管代替硬质铝管，加之辐照盒植入Cd皮阻挡热中子，大幅降低了辐照样品管的剂量。.提高Ar-Ar定年精度：(1)为了避免空气氩的影响，研制了超高真空封装系统，采用氢氧火焰焊封石英管，提高焊封质量；(2)通过堆芯中子通量分布模式计算，把辐照盒由原来斜坡位置移至孔道最大中子通量位置，获得的J值数据质量明显改善，新位置每支样品管标样J值均匀，轴向上变化很小。J值标准偏差σ的相对误差由7.67%降为0.37%，截距标准误差SE的相对误差由0.48%降为0.23%。若严格以J值截距标准误差引入Ar-Ar年龄误差传递计算，对应的Ar-Ar年龄误差降低了50%。.科学意义：通过本项目工作，确定了石英管为封装材料和封装方法，大幅降低了辐照样品管剂量和可能的辐射损伤；确定了最佳辐照位置为每个孔道最大中子通量部位，J值数据质量明显提高，对应的Ar-Ar年龄误差降低了50%。本项目开辟了最佳辐照位置和技术，极大地推动我国Ar-Ar高精度定年达到国际Earthtime目标。