Liquid xenon: An attractive target for direct dark matter searches

The XMASS experiment aims to directly detect dark matter interacting in its liquid xenon target.

There are three main advantages in using a liquid xenon (LXe) target:
(1)its large scintillation light yield
(2)its large atomic number
(3)its radiopurity

Scintillation light is emitted if a charged particle moves through a scintillating medium – in our case the LXe target. A large scintillation light yield means that a signal can be detected even for small energy deposits in this LXe target, a large atomic number means a large interaction rate for weakly interacting dark matter particles, and high radiopurity means that despite its large atomic number xenon has only one very long lived radioactive isotope (¹³⁶Xe), which in turn means very low background from the target itself.

The high sensitivity ensured by the optimal exploitation of LXe's high scintillation light yield through our detector design naturally extends to dark matter candidates that do not interact weakly with a xenon nucleus, but like for example axions rather interact with the electrons in the LXe.

As we do not yet know the particle nature of dark matter, this optimization for best sensitivity to weak signals irrespective of their nature (nuclear or electron induced scintillation) thus offers additional opportunities that the other LXe experiments are neglecting with their deliberate optimization towards nuclear recoil signals.

Low background: an absolute necessity for dark matter detection

Since we already know that dark matter signals are most certainly very rare and most likely also very weak, background events in the target volume introduced by both ambient and intrinsic radiation sources in both the detector components and the detector's surroundings have to be eliminated as far as possible.

The self shielding properties of a LXe target are another important design concept in our detector. Because of its high mass number and density LXe is itself an excellent shielding material against external gamma radiation. The largest source of such external radiation are the PMTs that are needed at the surface of that volume to detect the scintillation signal.

Together with Hamamatsu Photonics XMASS is developing a new PMT for use in XMASS-1.5 and XMASS-II that will have even lower uranium and thorium contamination than the one we developed with them for our current XMASS detector. Already this contamination is reduced by more than two orders of magnitude relative to ordinary PMTs. And the new PMTs dome-shaped photo-sensitive area addresses a background problem that we identified in our current detector. This new shape will finally allow us to fully exploit the self shielding of the LXe volume in our detector.