Copper electroplating for background suppression in the NEWS-G experiment

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