应用氘－氘强流中子发生器开展氩－氩定年

The 40Ar/39Ar dating method utilizes neutron activation of 39K to 39Ar via the 39K(n, p)39Ar reaction as a proxy for the parent isotope 40K. The neutron activation is conventionally achieved by irradiating samples in a 235U fission reactor. The fission spectrum includes neutrons with energies lower than 1MeV to higher than 10MeV, which will result in interference reactions and recoil displacements. After modeling calculation, Renne et al. (2005) proposed that both of these would be significantly reduced by irradiation with 2.45MeV neutrons produced by the deuteron-deuteron (D-D) fusion reaction. However, the neutron fluence in current D-D reactors is about 4 orders lower than 235U fission reactor. While it is generally believed that development of high-flux D-D fusion reactors is the solution, we proposed that improving mass spectrometric 40Ar/39Ar measurements is another solution. To quantify this idea, we calculated 40Ar/39Ar ratio of a series of age standards with different ages and different K content based on current D-D fusion reactor flux. The results indicated that by utilizing high resolution mass spectrometer we could measure 40Ar/39Ar ratio up to 1×104 precisely, which means we could measure samples younger than 30Ma by current D-D fusion reactor. Moreover, by utilizing 58Ni to monitor the neutron fluence of each sample, we could calculate the 40Ar/39Ar age independent to age standard.

传统的235U裂变反应堆释放出的中子能量范围宽，干扰反应多，影响氩－氩定年精度，而且运行时间不定，影响氩－氩年代学工作。氘－氘（D-D）强流中子发生器产生2.45MeV单能中子，位于氩－氩定年最优的能量区间。Renne等（2005）提出用D-D中子源代替反应堆进行氩－氩样品照射，并且计算出该单一能量的中子除了将39K转化为39Ar外，基本不产生其他干扰核反应，大大提高氩－氩定年的准确性和精确性。因此用D-D中子源照射样品是氩－氩年代学的重要发展方向。但D-D中子源中子通量太低导致其至今没有应用到氩－氩定年中。申请人对现有D-D中子源中子通量和一系列氩－氩标样进行了计算，拟通过应用高分辨质谱提高39Ar测量精度克服中子通量低的缺点，建立应用D-D中子源开展氩－氩定年的实验方法，实现对30Ma以来样品的D-D中子源照射和氩－氩定年，而且利用D-D中子源单一中子能量优势可以实现无标样氩－氩定年。

235U裂变反应堆释放出的中子能量范围宽，干扰反应多，影响氩－氩定年精度，而且反应堆运行时间不定，影响氩－氩年代学工作。氘－氘（D-D）强流中子发生器产生2.45MeV单能中子，位于氩－氩定年最优的能量区间。Renne等（2005）提出用D-D中子源代替反应堆进行氩－氩样品照射，并且计算出该单一能量的中子除了将39K转化为39Ar外，基本不产生其他干扰核反应，大大提高氩－氩定年的准确性和精确性。因此用D-D中子源照射样品是氩－氩年代学的重要发展方向。但D-D中子源中子通量太低导致其至今没有应用到氩－氩定年中。我们在中国原子能科学院对氩氩标样透长石SK01进行了持续约90小时的中子活化，并在中国科学院地质与地球物理研究所进行了Ar同位素测量，针对样品中40Ar/39Ar>10000我们优化了测试方法，测得的数据经过本底、干扰峰和大峰拖尾和质量歧视等校正。我们通过Ar同位素结果评估了D-D中子源的中子通量梯度，中子通量是随着距离靶头的距离增大而递减的，Ni片因为其自吸收效应而阻断了25-35%的中子通量，而样品最多阻隔2-5%的中子通量，Ni片在实际的样品测量中是不适合用来检测中子通量的。通过实测的37Ar及其与其他氩同位素比值，并于传统反应堆照射的样品的结果对比表明，D-D中子源照射样品极大程度的降低了干扰反应。