High-Radiopurity Structural Components Made by Chemical Vapor Deposition

Statement of the problem or situation that is being addressed: High-radiopurity structural materials are needed for a variety of high-energy physics experiments, particularly those involved in the search for dark matter in which background radiation from trace impurities can hide the desired signal. The structural material with the highest radiopurity available is electroformed copper, but its yield strength is only 12 ksi, which limits its utility as a structural material. Statement of how this problem or situation is being addressed: Chemical vapor deposition (CVD) converts gaseous reactants to solid products, but only when it is thermodynamically favorable to do so. Thermodynamic calculations have been performed to identify CVD process conditions where desirable refractory metals (e.g. tungsten, which has a reported yield strength as high as 109 ksi) will be deposited but radioactive species (e.g. K40, Th232, U238, and the progeny in their decay chains) will not. What is to be done in Phase I? Additional thermodynamic calculations will be performed to identify deposition conditions for other desirable refractory metals (e.g. tantalum, niobium, molybdenum) such that the simultaneous deposition of undesirable elements is thermodynamically prohibited. Deposition runs will be performed using the specified conditions, and the resulting deposited materials (along with samples of the reactants) will be analyzed via inductively coupled plasma mass spectrometry (ICP-MS) to quantify the concentrations of undesirable radionuclides and the extent to which their concentrations were reduced by the CVD process. Commercial applications and other benefits: Detectors used in high-energy physics experiments, particularly those looking for dark matter, will require materials with extremely high radiopurity. The techniques used to assess the purity of these materials can also be used to quantify the concentrations of heavy metals in consumer plastics, which is an area receiving much greater attention in many countries, including the European Union. Other potential applications include semiconductor fabrication, quantum computing, and the space and biomedical industries.