[论文解读] Copper electroplating for background suppression in the NEWS-G experiment

GSI-FRS ISOMER COLLABORATION, Levente Balogh+48 more|arXiv (Cornell University)|Aug 7, 2020

Dark Matter and Cosmic Phenomena参考文献 30被引用 4

一句话总结

本文提出一种大规模电镀技术，用于在NEWS-G暗物质实验中140厘米直径球形正比计数器的内表面电镀500 µm厚的超低放射性铜。该方法有效抑制了体铜中²¹⁰Pb污染带来的本底，使²³²Th和²³⁸U水平分别低于0.24 µBq/kg和0.11 µBq/kg（95%置信水平），与其它稀有事例探测实验中采用的先进电铸铜相当。

ABSTRACT

New Experiments with Spheres-Gas (NEWS-G) is a dark matter direct detection experiment that will operate at SNOLAB (Canada). Similar to other rare-event searches, the materials used in the detector construction are subject to stringent radiopurity requirements. The detector features a 140-cm diameter proportional counter comprising two hemispheres made from commercially sourced 99.99% pure copper. Such copper is widely used in rare-event searches because it is readily available, there are no long-lived Cu radioisotopes, and levels of non-Cu radiocontaminants are generally low. However, measurements performed with a dedicated 210Po alpha counting method using an XIA detector confirmed a problematic concentration of 210Pb in bulk of the copper. To shield the proportional counter's active volume, a low-background electroforming method was adapted to the hemispherical shape to grow a 500-$μ$m thick layer of ultra-radiopure copper to the detector's inner surface. In this paper the process is described, which was prototyped at Pacific Northwest National Laboratory (PNNL), USA, and then conducted at full scale in the Laboratoire Souterrain de Modane in France. The radiopurity of the electroplated copper was assessed through Inductively Coupled Plasma Mass Spectrometry (ICP-MS). Measurements of samples from the first (second) hemisphere give 68% confidence upper limits of <0.58 $μ$Bq/kg (<0.24 $μ$Bq/kg) and <0.26 $μ$Bq/kg (<0.11 $μ$Bq/kg) on the 232Th and 238U contamination levels, respectively. These results are comparable to previously reported measurements of electroformed copper produced for other rare-event searches, which were also found to have low concentration of 210Pb consistent with the background goals of the NEWS-G experiment.

研究动机与目标

• 抑制在NEWS-G球形正比计数器中使用的体铜材料中²¹⁰Pb污染带来的本底。
• 开发并展示一种可在大型复杂半球形表面实现超低放射性铜电铸的可扩展工艺。
• 实现与其它领先稀有事例实验中所用电铸铜相当的放射纯度水平。
• 通过在加工过程中最小化空气暴露，确保电镀后探测器内表面不被污染。

提出的方法

• 采用低本底电铸工艺，在140厘米直径铜半球的内表面电镀一层500 µm厚的超低放射性铜。
• 该工艺在太平洋西北国家实验室（PNNL）进行原型开发，随后在法国莫尔德地下实验室（LSM）实现规模化。
• 通过优化电解液成分和电镀参数以最小化痕量放射性核素，实现了约1.3 cm/年的沉积速率。
• 通过在氮气氛围下使用酸化过氧化氢溶液进行预蚀刻，减轻表面污染，以去除²¹⁰Pb。
• 从电解液-空气界面附近的电镀层和不锈钢基底处取样，以代表最坏情况的污染。
• 采用ICP-MS分析测量样品中²³²Th和²³⁸U的浓度，结果以68%置信水平报告上限。

实验结果

研究问题

• RQ1 大规模电镀工艺是否能在复杂的大尺寸半球形表面制备出适用于稀有事例实验的超低放射性铜层？
• RQ2 电镀铜的放射纯度水平（以²³²Th和²³⁸U污染计）如何？是否满足NEWS-G实验的本底要求？
• RQ3 该电镀工艺是否能有效抑制体铜中²¹⁰Pb本底，同时不引入新的污染源？
• RQ4 电镀铜的放射纯度与以往在其它暗物质实验及无中微子双贝塔衰变实验中使用的电铸铜相比如何？

主要发现

• 电镀铜层的²³²Th污染水平为<0.24 µBq/kg（95%置信水平），对应第二只半球，样品质量为0.614 g。
• ²³⁸U污染水平测量值为<0.11 µBq/kg（95%置信水平），表明具有极佳的放射纯度。
• 第一只半球的²³²Th污染水平为<0.58 µBq/kg，²³⁸U为<0.26 µBq/kg，与第二只半球性能一致。
• 放射纯度水平与其它领先稀有事例实验（如XMASS）所用电铸铜相当。
• 实现了约1.3 cm/年的沉积速率，证明了未来对整个铜球进行全电铸的可行性。
• 该工艺成功最小化了表面污染，并在蚀刻后保持了无空气环境，确保了探测器内表面长期的放射纯度。