Development of a surface alpha ray detector based on μ-TPC with low background

Radioactive impurities in the detector material have been focused as the serious background in a search for a dark matter. In order to select a pure material which have a low radioactive impurity such as nuclear in uranium-chain and thorium-chain, a precise measurement of radioactivity has been required. A new alpha-ray detector has been developed based on a gaseous μ-TPC. A low-α μ-PIC was installed to the detector for low background measurement. The detector has advantages of a position sensitivity and a ability to take sample and background event on a same time. We performed to calibrate the energy, to evaluate resolution using an alpha-ray source, to inspect a sample as a part of a conventional μ-PIC as a demonstration.