Ultra Low Background Time Projection Alpha Particle Detector

The search for rare-events, such as double beta decay or dark matter detections, is primarily a fight to reduce the incidence of background events to sufficiently low levels so that the events of interest can be seen. Background sources include trace radioisotopes found within the materials used to construct, support, and shield these experiments, and any surface contamination caused by exposure to atmospheric radon. Direct detection of alpha emission is necessary to accurately characterize surface contamination on these ultra-pure materials, but available screening techniques do not have sufficient sensitivity at the levels required. The DOE – a major supporter of rare-event search activities – therefore has an active interest in the development of ultra low activity materials. By extension, there is also an active interest in developing instrumentation for characterizing such materials, since it is impossible to improve something that cannot be measured. We propose to develop an instrument to characterize radiopure materials for surface alpha emission, whose detection limit will be over 100 times lower than that of the best commercially available instruments. XIA manufactures and sells the UltraLo-1800 alpha particle counter (UltraLo), the present state-of-the-art in low background alpha particle detection. The proposed approach converts an UltraLo from an ionization chamber into a small-scale time projection chamber (TPC) to extract additional signal information and push achievable background rates towards 1 /day/m2, without shielding, at the earth’s surface. We began by converting our two-channel UltraLo into a 64-channel small scale TPC built around a new electrode design comprised of an 8x8 array of 1.3 cm square pixels, each individually connected to a preamplifier. We developed a new preamplifier design whose noise performance was improved by a factor of 4 and is now limited by the FET input capacitance. Signals generated by an alpha source placed in the TPC were captured and shown to match those predicted by our simulations. Analyzing these data, we demonstrated that noise levels were sufficiently low to allow emission points to be localized, ionization track orientations determined, and charge distributions identified – our Phase I proof of principle goals. Finally, we produced a cost effective design for a 1600 channel full-scale TPC instrument, capable of achieving background rates approaching 1 /day/m2. If successful, this instrument’s direct benefit will be its support of a variety of rare-event experiments seeking to expand our understanding of the universe’s basic operating principles. Moreover, the enhancements made to the capabilities of the UltraLo will provide a significant performance advantage in other areas, including environmental remediation alpha measurements, and the semiconductor industry where controlling alpha activity from packaging materials is critical to increasing device reliability. The US plays a leading role in scientific research seeking to expand our understanding of the universe’s basic operating principles. This work furthers these efforts by providing scientists with a major improvement in instrumentation to support the next generation of sensitive experiments.