About XMASS Detector

The XMASS detector is located at the Kamioka Observatory of the Institute for Cosmic Ray Research, the University of Tokyo, Japan. It is in the Kamioka mine 1000 m underneath the top of Mt. Ikenoyama (i.e. 2700 m water equivalent underground).

The XMASS detecotor consists of the liquid xenon detecor about 1m in diameter and the water shield tank 10m in height and diameter. The liquid xenon detector consists of 642 photomultiplier tubes surrounding the 800kg liquid xenon, and the detecor is installed inside the vacuum chamber. The liquid xenon is kept in a temperature of -100 degrees. The liquid xenon emits scintillation lights if a dark matter particle recoils a xenon nucleus. The photomultiplier tubes (PMTs) surrounding the liquid xenon target detect the emitted scintillation photons.

The water shield tank plays a role to reduce the radioactive backgroud.

Overview of XMASS detector
The scylindrical water tank about 10m in height and diameter is used for the radiation shield, and 72 20-inch photomultiplier tubes on the inner surface of the tank are used to reject cosmic rays. The liquid xenon detector is installed in the center of the water tank.
The liquid xenon detector
The detector is equipped with 642 inward-facing photomultiplier tubes (PMTs) in an approximate spherical shape in a copper vessel filled with about 800kg pure liquid xenon. The PMT holder is an approximately spherical shape called a pentakis dodecahedron which consists of 60 isosceles triangles. The inner vessel holds liquid xenon and the PMT holder, and the outer vessel is used for vacuum insulation.