XMASS detector

Installation of PMTs on the top of the tank

Water filling for installation of PMTs on the top of the tank

Installation of the bottom and wall PMTs was finished.

Installation of the wall PMTs

The outer vacuum chamber was installed. The construction of the xenon detector was completed.

The detector and the xenon tank which is out of water tank were connected with pipes.

Installation of the outer vacuum chamber

Superinsulation was installed.

The inner vacuum chamber was installed.

The inner vacuum chamber was carefully lifted up.

Installation of the inner vacuum chamber was started.

Checking the signal from the PMTs

The cables of PMTs went through the cap of the detector to out of water tank.

The cables of PMTs were extended toward the top of the tank.

The cap of the detector was fixed on the frame.

The vacuum chamber was completed and the construction of the detector was restarted. It was brought into the tank from top of the water tank.

XMASS experiment facilities in the laboratory.

Filler installation has been completed.

The filler is designed to fit to a heat insulating container. The image of the whole detector is here.

The detector will be put in a heat insulating container filled with liquid xenon. Cupper structures are installed in order to fill up the space between the detector and a heat insulating container.

The "radon-free" clean air is blown in the container. We cannot see the detector for a while.

Due to the construction of detector stand, the detector is put in the pentagon container.


All cables were gathered above the detector.

The installation of all PMTs was finished.

PMTs are still to be installed along the connection of the upper and lower half spheres.

Finally the upper hemisphere and the bottom hemisphere are united.

The two parts are gradually coming together.

The separated detector is united. The bottom hemisphere is moved under the upper hemisphere and the upper hemisphere is carefully lowered.


Cables are connected to all PMTs.


The cables for the supply of the high voltage and for the readout of the signal are connected to each PMTs.


The PMT mounting was almost done. Now the second layer holder for filling between the PMTs is installed.


Today's shift member


The PMT mounting work is close to finish.



More than 300 PMTs have been installed.


The hexagonal photo-cathode area of the PMT enables to detect the photons efficiently.


The PMT mounting work has been started.


In order to install the PMTs, the assembled pentakisdodecahedron was divided into the upper part and the bottom part. The 642 PMTs will be installed to the holes of the structure.


The pentakisdodecahedron structure consists of 12 pentagons.


The assembly of the PMT holder has started. The detector parts are made of oxygen-free copper.


The parts of PMT holders were carried into the clean room in the water tank. These 60 triangle parts will be assembled into a pentakisdodecahedron structure.


For the construction of the xenon detector, a clean room was built in the tank. The detector needs extremely clean environment. The radioactive substances from human body cause the noise of the experiment, so the number of people in the tank will be restricted.

2009/Nov./16 Entrance of the tank.

Before entering the tank, people wear the clean suits and dust particles are blew off by the air shower behind the door.

Air purification system.

Radon is a radioactive background source, which is contained in the mine air.

Radon is reduced by using activated charcoal. "Rn-free" air is supplied to the detector.

The purified water in the tank is continuously reprocessed in the cycle purification system. This purified water is supplied not only to XMASS, but also to CANDLES experiment in the next laboratory.

A part of the water purification system.

This was newly set up besides that of Super-Kamiokande.

An Electronics hut on the top of the water tank, in which the high voltage power supply system for PMTs and the electronics are located.





The design and construction of the xenon detector is progressing.

Xenon storage tanks.

One tank can store 1000m3 xenon gas. About 1 ton xenon gas is stored in the two tanks.

The xenon distillation tower.

The Krypton which is one of the background source can be reduced to 10-5 by distillation.


We are now starting construction of the liquid xenon 1 ton detector. The following pictures are images of the detector. The 1 ton liquid xenon is surrounded with 642 PMTs spherically.


 The construction of the water shield tank will be started in autumn 2008. The construction of the whole detector will be finished and the data taking will be started in autumn 2009. The detection sensitivity of the dark matter is expected to be 10-45cm2, which will be the best in the world. The signal of the dark matter is expected.


XMASS検出器全体図 Image of the whole detector: 20-inch PMTs are installed on the wall of the water tank which diameter is 10m and height is 10m. The liquid xenon detector is installed in the center of the water tank.
XMASS検出器 Image of the liquid xenon detector: The liquid xenon is surrounded with 642 PMTs.

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Status of Water Tank Construction

The construction of the water shield tank has been started. The status will be reported sometimes.


Inside of the tank
The work stand is used for the detector construction.

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 XMASS experiment will be a multi-purpose detector which observes not only dark matter, but very low energy solar neutrinos which Super-Kamiokande cannot observe and double beta decay to measure the neutrino mass. A 10 ton liquid xenon detector would observe low energy neutrinos about 10 events per day. Thus, XMASS experiment is expected to expand to a multi-purpose experiment for astroparticle detection.

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