MINOWA Group

みのわ研究室

Direct dark matter search experiment with a conventional scintillator



For the SD(Spin Dependent) interaction of WIMPs, 19F is one of the most favorable nuclei to detect WIMPs because of its large nuclear spin and 100% natural abundance. Furthermore, the spin expectation values of protons and neutrons in 19F have the opposite signs while those values in other nuclei usually have the same signs. Therefore, experiments with a 19F-target can set complementary limits to those with nuclei whose spins have the same sign.

Our group had carried out WIMP search experiments using LiF and NaF bolometers. Although these results set complementary limits to those of NaI(Tl) experiments, the background rates below 20 keV could not be decreased sufficiently. The sources of the main background were thought not to be any radiation but to be intrinsic and instrumental noises in the bolometers.

As an alternative method, we have carried out experiments using CaF2(Eu) scintillators. CaF2(Eu) is known as a useful scintillator for WIMP searches because of its high light yield (19,000 photons/MeV).  In order to reduce the background rates and to improve the energy threshold of our experiments, we have tried to eliminate radioactive sources of γ -rays in the detector.

The detector consisted of a CaF2(Eu) crystal with a mass of 310 g. It was installed in Kamioka Observatory (2700 m.w.e.). To reduce the background, low radioactive PMTs (Hamamatsu R8778) and a low radioactive radiation shield were used. The experimental setup is shown in Fig. 1. The crystal was produced from low radioactive CaF2 and EuF3 powder.  Two PMTs were attached to the crystal through 5 cm-long quartz light guides. The photoelectron yield of the scintillator was about 4 photoelectrons/k.e.e. at 60 k.e.e.

The radiation shield consisted of 5 cm of highly pure copper (99.9999%), 10 cm of OFHC copper, 15 cm of lead, and 20 cm of polyethylene. The highly pure copper was supplied by Mitsubishi materials. The whole setup was separated from the mine air by two layers of EVOH sheet so that radon gas in the air could not come into the detector. The radon free air generated in Kamioka Observatory was sent into the detector as shown in the figure.

みのわ研究室