PNNL

Abstract

Executive Summary The next generation of direct detection dark matter experiments will require stringent radiopurity in their target materials, internal instrumentation, and shielding components. A survey of major material assay facilities worldwide and of anticipated assay needs of generation 2 (G2) experiments indicate the current assay capability is marginally adequate in sensitivity and inadequate in throughput. High purity germanium (HPGe) gamma-ray assay has been the workhorse for material screening. It appears hundreds of samples will require screening at the limits of current achievable HPGe sensitivity. Mass spectroscopy methods and neutron activation analysis have demonstrated superior assay sensitivity in some specific cases. However, they are also currently throughput limited. In addition, both alpha/beta screening and radon emanation analysis at or beyond the current achievable sensitivity levels will be needed for dozens of samples. Experiments that follow G2 will have even more stringent material radiopurity specifications, exceeding current assay capabilities in both sensitivity and throughput. These screening needs must precede the commissioning of experiments by 3-5 years to inform design and quality control of components. Achieving the radiopurity goals of the next decade’s experiments will require investment in new techniques and tools to improve material assay sensitivity and throughput. Engineered low-background materials are a significant R&D expenditure, requiring developmental lead-time, and should benefit many end-user experiments. Reserving underground real estate for assay methods affected by cosmic rays (e.g. HPGe) and engineered radiopure materials activated by cosmic rays (e.g. electroformed copper) is judicious forward planning. In many cases, these spaces will require radon-suppressed sample preparation and material storage space. The above-mentioned survey of material assay facilities indicated very little development is taking place to improve sensitivity or throughput capability. In order to create the necessary infrastructure, it may help to form a consortium of low-background assay centers in the U.S that is managed by a scientific board with representatives from the community. This would provide for common use of existing screening facilities and a unified plan for new infrastructure and site development (see the Facilities White Paper). Such a loose organization already exists as AARM (Assay and Acquisition of Radiopure Materials), which is currently populating a community-wide materials database with published assay information. A more formal organization such as a Consortium is being actively investigated with DOE and NSF. Eventually, results of all screening performed in Consortium assay centers could be input directly to the open-access database, reducing duplication and providing for more efficient vendor selection.