XMASS is a single phase liquid xenon detector. It is designed for a program of physics targets. The present XMASS-I detector targets direct dark matter detection. Background rejection is based on position information. Due to strong stopping power of xenon, beta and gamma rays from the outside cannot penetrate deep into the liquid xenon. By eliminating event interacting outer region, most background can be eliminated.
By using the data obtained by XMASS-I, dark matter, axion and double electron capture searches were carried out and the results were published.
XMASS-1.5 is next step of XMASS project to perform dark matter search with higher sensitivity. Detector design is now ongoing to improve sensitivity below 10-46cm2 cross section for 100GeV WIMP by increasing total amount of liquid xenon and reducing background level below 10-5 /day/kg/keVee.
PMT exchange for surface background reduction is one of the largest modification from XMASS-I. New PMT R13111 has dome-shaped photocathode which is very effective for identification of surface background. Confirmation of performances including sensitivity of side part of photocathode is already done. Radioactivity reduction of R13111 is ongoing by screening all parts. Up to the present, reductions of ~1/3 for U chain(226Ra) and Th chain, <1/8 for 40K and ~1/6 for 60Co from XMASS-I PMT are achieved.