This project is to perform material purification and crystal growth underground for next generation, ultra-low background experiments at the Sanford Underground Laboratory at Homestake and DUSEL. When completed, DUSEL will be the only site in the world where highly pure germanium crystals are grown in a deep underground environment. To accomplish this goal, we must, 1) construct an underground lab for materials purification and crystal growth; 2) implement R&D crystal growth activities on the surface including Monte Carlo simulations for crystal growth; 3) grow germanium crystals for ton-scale experiments; and 4) develop new detectors with the crystals produced on-site. In addition, this project will develop the scientific workforce near the DUSEL site by creating a network of current institutions who are interested in developing new detectors and by training and educating graduate and undergraduate students in areas supportive of DUSEL experiments. The various sub-projects being pursued in this effort are listed below.
The project is to develop an economic way to produce 1-ton enriched 76-Ge for neutrinoless double-beta decay experiments using centrifuges and the existing advantages in zone refining and crystal growth.
This project is to purify the germanium ingots to a level that would yield detector-grade crystals for dark matter and double-beta decay experiments. The zone refining process uses a short, moving molten zone in which the trailing solid is more pure than the adjacent liquid. Therefore, the last liquid to solidify at the ingot's end contains an increased impurity level. Since the impurities concentrate in the molten section, the repeated and sequential melting from one end towards the other concentrates the impurities towards one end of an ingot. This sweeping operation is repeated many times until desired impurity levels are reached in the higher-purity portion. This process will purify germanium ingots to a level of 99.99999999999.
Large single crystals of germanium are grown using the Czochralski technique. A precisely cut seed crystal is dipped into the molten germanium and then withdrawn slowly, while maintaining the temperature of the melt just above the freezing point. The seed is rotated and pulled up while crystal is being formed. The seed and the resulting crystal is typically oriented with a (100) crystal axis parallel to the growth direction. The crystal growth process is conducted in a hydrogen (H) atmosphere inside a quartz envelope. Since the Ge melt "wets" the silica crucible, it will crack the crucible if any Ge melt remains and is allowed to freeze. Therefore, all the Ge in the crucible must be consumed during a crystal growth run-- "grown to dry." Some purification of the Ge also occurs during crystal growth because of impurity segregation, although it is not as effective as the multi-pass zone refining process. The rate of crystal withdrawal and temperature of the melt are adjusted to control the crystal growth.
The grown crystals are characterized to understand carrier concentrations. mobility, resistivity, dislocation density, and impurities. Several techniques can be used to characterize the grown crystals. We will adopt Hall Effect and Photon Thermal Ionization Spectroscopy (PTIS) to measure the net impurity level and dislocations suitable for detector construction.
Grown crystals will be fabricated into detectors for underground experiments. This project will be in collaboration with Lawrence Berkeley National Laboratory and the Majorana collaboration. In addition, we will make devices with the grown crystals for solar energy application.