The processing of enriched germanium for the Majorana Demonstrator and R&D for a next generation double-beta decay experiment

Abstract

The Majorana Demonstrator is an array of point-contact Ge detectors fabricated from Ge isotopically enriched to 88% in 76 Ge to search for neutrinoless double beta decay. The processing of Ge for germanium detectors is a wellknown technology. However, because of the high cost of Ge enriched in 76 Ge special procedures were required to maximize the yield of detector mass and to minimize exposure to cosmic rays. These procedures include careful accounting for the material; shielding it to reduce cosmogenic generation of radioactive isotopes; and development of special reprocessing techniques for contaminated solid germanium, shavings, grindings, acid etchant and cutting fluids from detector fabrication. Processing procedures were developed that resulted in a total yield in detector mass of 70%. However, none of the acid-etch solution and only 50% of the cutting fluids from detector fabrication were reprocessed. Had they been processed, the projections for the recovery yield would be between 80 -85%. Maximizing yield is critical to justify a possible future ton-scale experiment. A process for recovery of germanium from the acid-etch solution was developed with yield of about 90%. All material was shielded or stored underground whenever possible to minimize the formation of 68 Ge by cosmic rays, which contributes background in the double-beta decay region of interest and cannot be removed by zone refinement and crystal growth. Formation of 68 Ge was reduced by a significant factor over that in natural abundance detectors not protected from cosmic rays.

Fabrication and characterization of high-purity germanium detectors with amorphous germanium contacts

“…For example, 3 H, 49 V, 56 Fe, and 65 Zn, produced by cosmogenic activation when the Ge detectors are fabricated on the surface, are main sources of background events in the MAJORANA DEMONSTRATOR and EDELWEISS in the low energy region of interest [5][6]. Similarly, 60 Co and 68 Ge can be the sources of background events in the higher energy region for the detection of neutrinoless double-beta decay [7]. An effective way to reduce the production of cosmogenic isotopes in Ge is to grow Ge crystals and fabricate detectors underground at the site where the experiments will take place.…”

Cosmogenic Activation in Double Beta Decay Experiments

“…The MAJORANA DEMONSTRATOR, built and operated by the MAJORANA Collaboration, consists of an array of ultra-low background HPGe detectors with a total mass of 44 kg (29.7 kg enriched in 76 Ge) installed in the Sanford Underground Research Facility (SURF) in Lead, South Dakota, USA, mainly for DBD investigation. A careful processing procedure to minimize cosmogenic activation of detectors was followed in the fabrication of the point-contact Ge detectors using germanium isotopically enriched to 88% in 76 Ge [79]. The data taking with the entire array of detectors started in 2016 and the measured background level is (11.9 ± 2.0) counts/(FWHM t y) [80].…”

A low energy rare event search with the MAJORANA DEMONSTRATOR

“…Even brief exposures to the cosmic ray flux at high altitudes by transport via airplane can create excessively high tritium levels. The manufacture, processing, and storage of the enr Ge detectors used in the Demonstrator was done to minimize surface exposure time at all points during the manufacturing process [9]. We model the signals of interest from bosonic dark matter and (separately) a 14.4 keV solar axion as a Gaussian peak with a width derived by the ββ(0ν) analysis [4] and taking a constant 30% uncertainty over the energy range of interest.…”