Hafnium-based Superconducting Tunnel Junction Radiation Detectors for a Sterile Neutrino Search

Sterile keV neutrinos are natural extensions to the standard model of particle physics and a possible candidate for warm dark matter. STAR Cryoelectronics has developed X-ray detectors that are currently being used by a collaboration between the Lawrence Livermore National Laboratory (LLNL) and the Colorado School of Mines for the world’s most sensitive sterile neutrino search in the mass range from 100 to 850 keV. We propose to develop next-generation quantum sensors to extend this search to sterile neutrino masses below 100 keV. The same sensors will also be used in our company’s high-resolution X-ray spectrometers for use at synchrotron light sources. The quantum sensors currently used for the sterile neutrino search (and for X-ray spectroscopy) are based on superconducting Ta/Al-AlOx/Ta tunnel junctions (Ta-STJ). The objective of Phase I is to replace tantalum by hafnium and demonstrate the feasibility of fabricating detectors based on Hf/Al-AlOx/Hf junctions, which are expected to have ~5× better energy resolution. We will demonstrate that we can sputter-deposit Hf/Al-AlOx/Hf trilayers with low leakage current and pattern them to fabricate quantum sensors using photolithography. These Hf-STJ quantum sensors will then be tested at LLNL for use in a sterile neutrino search and for high- resolution X-ray spectroscopy. The same type of hafnium sensors can also be used in our company’s high-resolution X-ray spectrometers that we offer for X-ray microscopy and synchrotron science applications. The Hf- STJs will increase instrumental sensitivity for diverse applications such as particle contaminations on microchips, reaction mechanisms of metalloproteins and dopant chemistry in novel materials.