Radiogenic production of 10Be and 26Al in uranium and thorium ores: Implications for studying terrestrial samples containing low levels of 10Be and 26Al
Radiogenic production of 10Be and 26Al in uranium and thorium ores: Implications for studying terrestrial samples containing low levels of 10Be and 26Al
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铀和钍矿石中 10Be 和 26Al 的放射性生成：对研究含有低含量 10Be 和 26Al 的陆地样品的意义
DOI：
10.1016/0016-7037(89)90013-6
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发表时间：
1989
影响因子：
5
通讯作者：
R. Middleton
中科院分区：
文献类型：
--
作者：
P. Sharma;R. Middleton
10 Be和26 Al是由宇宙射线引起的大气成分的溅射产生的，最近已经表明，也是由暴露于宇宙射线的岩石中的溅射产生的。我们现在提出的实验数据表明，这两种同位素也可以产生的放射性铀和钍衰变链放射性核素包含在主机岩石。我们用加速器质谱法测量了一些屏蔽铀（自发裂变半衰期~ 1026年）和钍（无自发裂变衰变模式）矿石中的~（10）Be和~（26）Al浓度。10 Be和26 Al的浓度范围分别为2.8·106 ~ 29.6·106和1.1 ·107 ~ 1.43·10atoms g−。高浓度的10 Be和26 Al意味着它们由自发裂变产生的高分支比（~ 10− 4到10−1），加上钍矿石中10 Be和26 Al的存在，消除了自发裂变作为这两种同位素的主要来源。在所有样品中发现的大的26 A1到10 Be比率消除了大气产生。最可能的来源是α反应，7 Li（α，p）10 Be和23 Na（α，n）26 Al，以及中子反应，9 Be（n，γ）10 Be，10 B（n，p）10 Be和13 C（n，α）10 Be。我们的工作提供了一个上限的贡献ofsiturradioactivity样品含有低水平的10 Be和26 Al。我们已经计算了10 Be和26 Al的平衡浓度在各种各样的地质材料，包括花岗岩，超镁铁岩，玄武岩，页岩，远洋沉积物，铁锰结核，玻璃陨石和利比亚沙漠玻璃。计算得到的10 Be和26 Al的浓度范围分别为<1 ~ 4100和9 ~ 10.3 ·106 atoms g−1。这些计算的影响进行了讨论。一般来说，地质材料中放射性生成的10 Be的计算量很小，与宇宙生成相比往往可以忽略不计，而26 Al的计算量很大，有时是主要来源。
10Be and26Al are produced by cosmic-ray-induced spallation of atmospheric constituents, and also as has recently been shown, by spallation in the rocks exposed to cosmic rays. We now present experimental data showing that these two isotopes can also be produced by radiation from uranium and thorium decay chain radionuclides contained within the host rocks. We have measured the10Be and26Al concentrations in a number of shielded uranium (spontaneous fission half-life ~1026years) and thorium (no spontaneous-fission-decay mode) ores using accelerator mass spectrometry. The concentrations of10Be and26Al range from 2.8·106to 29.6·106and 1.1 · 107to 1.43·10atoms g−, respectively. The high concentrations of10Be and26Al imply unacceptably-high branching ratios (~ 10−4to 10−1) for their production from spontaneous fission and coupled with the presence of10Be and26A1 in the Th ores, eliminate spontaneous fission as the major source of either isotope. The large26A1 to10Be ratio found in all the samples eliminates atmospheric production. The most likely sources are the alpha reactions,7Li(α,p)10Beand23Na(α,n)26Al, and neutron reactions,9Be(n,γ)10Be,10B(n,p)10Beand13C(n,α)10Be. Our work provides an upper limit on the contribution ofin situradioactivity to samples containing low levels of10Be and26Al. We have calculated the equilibrium concentrations of10Be and26Al in a wide variety of geologic materials including granites, ultramafic rocks, basalts, shales, pelagic sediments, ferromanganese nodules, tektites and Libyan Desert Glass. The calculated concentrations of10Be and26Al range from <1 to 4100 and 9 to 10.3 · 106atoms g−1, respectively. Implications of these calculations are discussed. In general, the calculated amounts of radiogenically produced10Be in geological materials are quite small and frequently negligible compared to cosmogenic production, while that of26Al are significant and sometimes the dominant source.