  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 detector materials and the experimental environment. The selection of
materials with low radioactive impurities, such as isotopes of the uranium and
thorium chains, requires a precise measurement of surface and bulk
radioactivity. Focused on the first one, an alpha-particle detector has 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
detector offers the advantage of position sensitivity, which allows the
alpha-particle contamination of the sample to be imaged and the background to
be measured at the same time. The detector performance was measured by using an
alpha-particle source. The measurement with a sample was also demonstrated and
the sensitivity is discussed.
