Detection Techniques

Liquid Xenon is better to detect Dark Matter

 XMASS experiment can detect dark matter directly with a liquid Xenon detector.

  The advantages of a liquid Xenon detector are as follows; (1) a large light yield, (2) a large atomic number and (3) liquid and gas phase are available, which is easy to purify. The scintillation light is emitted depending on the energy which is emitted when dark matter is elastically scattered with a Xenon nucleus. The emitted light is detected by many photo-multiplier-tubes surrounding the liquid Xenon.

 

Low background detector is necessary

 The signal of the dark matter is very rare and weak, so it is important to reduce backgrounds and to make the energy threshold low. The XMASS group develops a very low background PMT for this experiment with Hamamatsu Photonics Company(Fig. 5). This PMT has a high quantum efficiency, which is able to detect the scintillation light from the liquid Xenon efficiently. The radioactive components (U or Th) in the PMT are reduced to 10-2 than ordinary PMTs. Because of the large atomic number of Xenon (Z=54), gamma rays are strongly attenuated in a short distance near the surface of the Xenon volume. This "self-shielding" power is effective to reduce the gamma ray background.

 

50 times better sensitivity

 There are a lot of dark matter search experiments in the world. Fig.6 shows the sensitivities of the dark matter experiments as function of dark matter mass. Blue and green lines show the sensitivities from the existing experiments and yellow circles show the more possible regions. XMASS is expected to achieve 50 times better sensitivity and to detect the dark matter directly.

sensitivities

 

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