Projects: Solid Xenon
The solid (crystalline) phase of xenon inherits most of the advantages of using liquid xenon as a detector target material for low energy particles; transparency, self-shielding, absence of intrinsic background, and ionization drift. In the solid phase, even more scintillation light yield (∼60/keV) is reported compared to the liquid phase (∼40/keV). Operation at sub-Kelvin temperature is natural for the solid phase, using superconducting sensors to read out photon, ionization, and phonon signals. There is demonstrated progress from liquid xenon experiment group on xenon purification that substantially reduces the radioactive krypton source down to ppt level. However, in the liquid phase, this extreme level of purity has to be maintained continuously to prevent secondary contamination from outer detector materials. The circulation of liquid xenon for purification is potentially another source of bulk contamination. Solid xenon has no such concern for additional contamination. Therefore, solid phase of xenon is a strong candidate for low background counting applications.
Dark matter search is one of the standard application for low background counting detector. The expected recoil energy spectrum of dark matter in most detectors is exponential with typical energies below 50 keV. This featureless recoil spectrum makes it difficult to discriminate the dark matter signals from the other nuclear recoil backgrounds. Therefore, building a background free detector is crucial in the direct detection of dark matter experiments. The solid xenon detector may be regarded as a fundamental solution to achieve ultimate low background level for the unambiguous discovery of dark matter.
A solar axion search represents a unique physics case for using the crystalline phase of xenon. Solar axions, which may be created in the core region of the Sun, can be detected with a terrestrial X-ray detector. The axion-photon conversion by the strong Coulomb field of atoms in the crystalline X-ray detector may cause coherent Bragg scattering of the photons, which depends on the energy and incident angle of the axions. The solid xenon experiment can cover QCD axion parameter space that is not accessible by other technique.
The other strong physics case for using solid xenon is searching for neutrinoless double beta decay. The possible non-vanishing Majorana mass component of neutrinos indicates the non-zero probability of having self-conjugate states of neutrinos, which means the neutrinos can be their own antiparticles. Therefore, there is a small probability of a neutrino emitted from a nucleon through beta decay that can be absorbed into the other nucleon in the same nucleus; a mono-enegetic beta signal would thus be observed with no neutrinos carrying off momentum. The Xe-136 enriched solid phase volume, if phonon readout is demonstrated, is a superior detector for a neutrinoless double beta decay experiment due to substantially more quanta created through phonon channel, hence better energy resolution, compared to the ionization and scintillation channel. Therefore, once the phonon signal readout is guaranteed, the solid phase is the best strategy of using the xenon for the neutrinoless double decay search.
There are two major R&D issues that need to be addressed in order to make a solid xenon particle detector; the demonstration of the scalability of crystalline xenon and the capability to readout signals from solid xenon. The first R&D phase of the solid xenon project, growing approximately a kilogram of xenon crystals was successfully finished. Based on the success of the first phase R&D, we will carry out the second phase R&D at Fermilab where the focus is on the scintillation light readout from solid xenon.
- Jong-Hee Yoo