电感耦合等离子体质谱法测定单晶铜中痕量放射性核素钍和铀的含量

Determination of thorium and uranium in copper using inductively coupled plasma mass spectrometry

Panda X-Ⅲ实验位于中国锦屏极深地下实验室,将利用高压136Xe气体时间投影探测器进行无中微子双贝塔衰变事件的寻找。建造高灵敏度的大型探测器需要挑选放射性极低的高纯材料以对本底进行严格的控制。本工作应用电感耦合等离子体质谱(Inductively Coupled Plasma Mass Spectrometry,ICP-MS)法测定纯度极高的单晶铜中痕量钍和铀的含量。在样品消解前加入^(230)Th和^(235)U分别作为标记核素,样品经过8 mol·L-1 HNO_3消解后,依次通过TEVA和UTEVA树脂柱,实现待测核素^(232)Th、^(238)U与基体元素Cu的分离。利用同位素稀释法-ICP-MS测定^(232)Th和^(238)U的含量。通过空白样品实验测量,^(232)Th的检出限是0.036 pg(232Th)·g^(-1)(0.15μBq(232Th)·kg^(-1)),238U的检出限是0.070 pg(238U)·g^(-1)(0.86μBq(238U)·kg^(-1));单晶铜样品中232Th及238U的检出限分别是0.21 pg(^(232)Th)·g^(-1)(0.85μBq(232Th)·kg^(-1))和0.45 pg(238U)·g^(-1)(5.56μBq(238U)·kg^(-1))。该方法可为探测器建造过程中的材料选取工作提供技术支持。

[Background] Panda X-III is designed to search for the neutrinoless double-beta decay（0υββ） with high pressure 136 Xe gas time projection chambers at the China Jinping underground laboratory（CJPL）. This rare decay can be easily covered by background, so ultra-low-background materials should be chosen as the construction for Panda X-III. [Purpose] This work attempts to establish a method to determine Th and U in copper based on inductively coupled plasma mass spectrometry（ICP-MS）. [Methods] The samples were digested by 8 mol·L-1 HNO3.230Th and 235U were spiked in the digestion solution for quantitative analysis. TEVA resin and UTEVA resin were chosen for separation of Th and U from copper. After the Th and U were pre-concentration by the resins columns, ICP-MS was applied to the determination. [Results] Detection limits of 0.036 pg（232Th）·g^-1（0.15 μBq（232Th）·kg^-1） and 0.070 pg（238U）·g^-1（0.87 μBq（238U）·kg^-1） were achieved for blank sample, detection limits of 0.21 pg（232Th）·g^-1（0.85 μBq（232Th）·kg^-1） and 0.45 pg（238U）·g^-1（5.56 μBq（238U）·kg^-1） were achieved for copper sample. [Conclusions] The method based on ICP-MS for the determination of Th and U in copper was presented, and can undertake the task of the radioactive assay of detector components for Panda X-III.