XMASS Refurbishment 2014/6/26
XMASS is an experiment to search for dark matter in the Universe with large mass of liquid xenon. Various astrophysical observations show the existence of dark matter in the Universe, but its nature is not revealed yet. As the interaction between dark matter and ordinary matter is very rare, it is indispensable for a detector to be very clean and very low-background. Considerable efforts were taken at the construction of the XMASS to minimize the background from all the detector materials.
XMASS started commissioning operation from 2010 and we found more background than expected. Detailed studies showed that there were radioactive contaminations (uranium) in aluminum that was used as a sealing between quartz glass windows and bodies of photo multipliers (PMTs).
Therefore, we refurbish the XMASS detector to understand the background more deeply and to obtain much higher sensitivity.
In this refurbishment, we have modified hardware:
（1） We placed Cu rings covering aluminum seals to minimize the background coming from them.
（2） We attached thin Cu plates over the gaps among support structures of PMTs to eliminate the stray light through the gaps.
We also electro-polished the inner Cu surfaces of the detector to peel off the radioactive contaminations. The PMT windows were washed by nitric acid.
Since the dark matter signals are very rare, utmost attentions should be paid for those active contaminations. Dusts in the air contain natural radioactive isotopes and are potential background sources. We reconfigured our experimental area as a clean area (“Class 1”) during the refurbishment works. The air itself contains radioactive gases, among them radon is most serious. We upgraded our radon removal system to keep our working environment better.
The refurbishment has been completed in November 2013 with success. After this modification, we hope we have profound understanding of backgrounds of the detector. We also hope that the low-mass dark matter, which the DAMA/LIBRA group suggested, can be investigated. We note that XMASS is not only sensitive to nuclear recoils but also to electromagnetic events. Accordingly, XMASS is able to search for exotic particles, such as axions with highest sensitivity in the world.
The XMASS collaboration intends to drive forward the search for dark matter, axion, and so on. The experimental results from LHC and theoretical studies, however, indicate that the dark matter may be much heavier and has much smaller interaction cross sections. There is an alternative argument that dark matter may be axion-like particles or something else. Therefore to unveil the secret of dark matter, it is necessary to build a larger detector than the current experiments and that with an ability to detect e/γ events as well as nuclear recoils like XMASS. We have already started to develop and design the next generation detector, XMASS1.5 with 5 tons total mass.
Please keep your eyes on the results from XMASS-Ref and the future XMASS1.5.