Purification of Germanium Crystals by Zone Refining*

The nature of dark matter (DM) and key properties of neutrinos are two of the most important questions in particle physics. PIRE-GErmanium Materials And Detectors Advancement Research Consortium (PIRE-GEMADARC) is a global partnership created to accelerate the germanium (Ge) material platform used to research these questions and educate the next generation of scientists. In developing high-purity Ge (HPGe) detectors, zone refining is one of the most important techniques. Raw Ge contains many impurities, like Al, B, and P. Our goal is to decrease the distribution density of impurities along a Ge ingot. Zone refining is the simple process of a heating induction coil passing over the ingot and melting a strip that moves as the coil continues its pass. Impurities move within the molten zone; so once this process is done several times, most impurities have been shifted to either end of the ingot and the rest of the rod is purified Ge. The effective isolation of impurities is dependent on many parameters: the travel speed, ratio of ingot length to molten zone width, and number of passes. Refined ingots are grown into crystals that are manufactured into HPGe detectors used in the search for DM and in neutrinoless double beta decay experiments.