A Cryogenic Phonon Detector to Search for Dark Matter Particles.

This thesis discusses the systematic investigation of the thermal and electrical properties of neutron transmutation doped (NTD) Ge at low temperatures, and the development of a detector to search for weakly interacting massive particles (WIMPs), predicted by some theories to make up to 90% of the matter in the universe. We have fabricated NTD Ge phonon sensors operated on a dilution refrigerator near 20 mK. The thermal and electrical properties of NTD Ge were investigated first. The zero bias resistance was found to be governed by variable range hopping, but the nonlinearity of the current-voltage characteristics indicated significant hot electron effects near 20 mK. A detailed investigation of hot-electron effects in NTD Ge is described. NTD Ge sensors were found to be very sensitive to high energy phonons generated by interactions of Ge with alpha-particles and photons. The mean absorption length of the high energy phonons in NTD Ge was found to be about 500 mum. In order to use these phonon sensors in conjunction with a target crystal, we developed a bonding technique using a Au-Ge eutectic. The bonds were found to be mechanically strong, thermally cyclable, and more transparent to high energy phonons than conventional silver-filled epoxies. The eutectic bonding technique was also found to leave the thermal and electrical properties of NTD Ge unchanged. We describe ionization signals observed in pure Ge target crystals, generated by the interaction of Ge with alpha-particles and photons; these ionization signals were observed simultaneously with phonon signals from the NTD Ge sensors. Such simultaneous signals can be used to reject unwanted background events, which is an important requirement for a WIMP detector. Finally, we describe the design and testing of a 60 g Ge detector to which six phonon sensors were attached. Simultaneous phonon and ionization signals were observed when irradiating the crystal with photons from ^{241 }Am. The energy resolution was found to be about 4 keV for both the phonon and ionization signals, and was limited by microphonic noise. With this prototype detector, we have demonstrated the key aspects necessary for a WIMP detector.