Speaker
Description
Advancing the detection limits in experiments searching for rare nuclear processes, such as direct dark matter interactions and neutrinoless double beta decay, requires material selection with extremely low radioisotope content. Low-background, large-surface alpha spectrometry is one of the techniques used to measure low surface and bulk specific activities of alpha emitters. One of the most sensitive alpha spectrometers is XIA UltraLo-1800. In this detector several sources contribute to the detector background: cosmic-ray muons, decays of Rn-222 and Rn-220 in the counting gas, as well as alpha emissions originating from the anode and detector walls. To determine the instrumental background, a material producing the lowest possible alpha-particle signal is required to serve as a practical “zero” sample (a sample with virtually no alpha-particle emission). For this purpose, several candidate materials were tested: stainless steel, Teflon, etched oxygen-free copper, and electropolished oxygen-free copper. The lowest signal was observed for electropolished oxygen-free copper in a measurement lasting 76 days, with count rates of (12.6 ± 0.4) cts/d in the 1.5 – 4.5 MeV range and (6.4 ± 0.3) cts/d in the 4.5 – 6 MeV range, corresponding to the regions used for bulk and surface Po-210 analysis, respectively. Currently this material is used as a drawer base. Based on Monte Carlo simulations of detection efficiency for different materials in the full-geometry configuration, the minimum detectable surface activity (MDA) of the XIA UltraLo-1800 for the assumed relative uncertainty of 30% is 0.3 mBq/m^2, while for bulk measurements it varies between 7.4 for lead and 22.6 mBq/kg for Teflon. The corresponding detection limit (minimum activity distinguishable from the background) is 0.07 mBq/m^2 for surface measurements and 1.7 – 5.3 mBq/kg for bulk measurements. Moreover, there is a possibility to improve bulk sensitivity by reducing the detector background related to misidentified muons. Therefore an outer veto can be used. The expected improvement is at the level of 30%. Count rates for various background sources were collected over an 8 years of measurements and their analysis demonstrates the long-term stability of the detector. The XIA UltraLo-1800 is used to investigate materials for various low-background applications, including i.e. dark matter and neutrinoless double beta decay searches. As an example, measurement results for aluminium samples from the European Spallation Source will be presented, with the aim of identifying the sample with the lowest alpha emissivity. The purpose of this research is material selection for construction of a neutron detector. Among the five Al plates investigated, the lowest gross specific alpha activity, C = (30 ± 10) mBq/kg, was obtained for the nickel-coated aluminium. Details of the background studies and chosen results will be discussed.