Development of an alpha-particle imaging detector based on a low radioactive micro-time-projection chamber

An important issue for rare-event-search experiments, such as the search for dark matter or neutrinoless double beta decay, is to reduce radioactivity of the detectormaterial and the experimental environment. The selection of materials with low radioactive impurity, such as isotopes in the uranium and thorium chains, requires a precise measurement of surface radioactivity. An alpha- particle detectorhas been developed based on a gaseous micro- time-projection chamber. A low-alpha mu-PIC with reduced alpha-emission background was installed in the detector. The detectoroffers the advantage of position sensitivity, which allows the alpha-particlecontamination of the sample to be imaged and the background to be measured at the same time. The detectorperformance was measured by using an alpha-particlesource. The measurement with a sample was also demonstrated and the sensitivity is discussed.