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原子阱,氪-81,撒哈拉的地下水

Atom trap, krypton-81, and the groundwater underneath the Sahara Desert

  • 摘要: 自放射性碳年龄测定法在1949年首次在实验中得以证实以来,长寿命的由宇宙射线产生的放射性核素痕量分析这一领域,无论是在分析技术还是在可利用的核素种类上,都取得了长足的进步.它所产生的影响已经渗透到广泛的科学和技术领域.文章介绍了一种新的痕量分析方法——原子阱痕量分析(atom trap trace analysis,ATTA).这种方法已经用于分析环境样品中的氪-81(半衰期为23万年;同位素含量约为1×10-12)和氪-85(半衰期为10.8年;同位素含量约为2×10-11)核素.氪-81主要由宇宙射线产生于大气层上部.它是理想的测定年代范围从5万年到100万年的水和冰样品的放射性核素.另一方面,氪-85是铀-235和钋-239的核裂变产物.对氪-85的同位素含量测定可以用于监督国际核公约的执行情况.在ATTA中,一个由激光光束和磁场共同产生的原子阱可以有选择地捕陷特定核素的原子.被囚禁的原子可以通过观察其荧光来进行探测.作为ATTA技术的第一次实际应用,一个国际合作小组确定了撒哈拉沙漠地区Nubian含水土层中地下水的寿命.利用ATTA对地下水寿命的成功测定,标志着放射性氪年龄测定法在地球科学中的广泛应用已经具备了可行性.

     

    Abstract: Since radiocarbon dating was first demonstrated in 1949, the field of trace analyses of long-lived cosmogenic isotopes has seen steady growth in both analytical methods and applicable isotopes. The impact of such analyses has reached a wide range of scientific and technological areas. In this review article, we discuss a new method, named atom trap trace analysis (ATTA), which was used to analyze 81Kr (t1/2=2.3×105 years, isotopic abundance~10-12) and 85Kr (t1/2=10.8 years, isotopic abundance 2×10-11) in environmental samples. 81Kr is produced by cosmic rays in the upper atmosphere. It is the ideal tracer for dating ice and groundwater in the age range from 50,000 years to 1 million years. On the other hand, analyses of 85Kr, a fission product of uranium-235 and plutonium-239, can serve as a means to help verify compliance with the Nuclear Non-Proliferation Treaty. In ATTA, individual atoms of the desired isotope are selectively captured into an atom trap that consists of laser beams and magnetic fields and detected by observing the fluorescence of trapped atoms. As the first real-world application of ATTA, the age of the groundwater (200,000 years to 1 million years) in the Nubian Aquifer located underneath the Sahara Desert was determined. With the success of this demonstration, wide spread use of 81Kr-dating in Earth sciences seems feasible.

     

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