Periodic Reporting for period 3 - GemX (Towards a ton-scale Ge-76 observatory for neutrinoless double beta decay)
Período documentado: 2021-10-01 hasta 2023-03-31
The observation that matter dominates over anti-matter in the Universe is one of the most fundamental open questions in physics and cosmology. A natural explanation of this asymmetry postulates neutrinos as their own anti-particles, usually referred to as Majorana particles. The only practical way to establish the Majorana character of neutrinos is the experimental search for neutrinoless double-beta decay (NDBD). This decay violates lepton-number conservation and would establish new physics beyond the Standard Model of particle physics. The search for neutrinoless double beta decay is worldwide ranked amongst the top research priorities. The Astroparticle European Consortium (APPEC) has recognized its relevance and included a large scale neutrinoless double beta decay experiment into its road maps. The 2015 US Long Range Plan for Nuclear Science has ranked neutrinoless double beta decay as its top priority recommending the timely development and deployment of a ton-scale experiment, following a careful down select amongst the developed experimental techniques. Similarly, Japan, South-Korea and recently also China have taken strong commitments in neutrinoless double beta decay research.
GemX is a basic research project that aims to expand knowledge and train young researchers. It strives to find a path to understand the evolution of the universe, focusing on why matter exists. As common to fundamental research, its outcome is unknown. A discovery that neutrinos are of Majorana type would have transformative potential for science and thus to society. GemX facilitates the education and training of young students and early career scientists to develop a rigorous scientific approach to problems that spills over into society after they leave academia. Breakthrough technological developments such as high-purity germanium detectors made from enriched germanium material with full topological event reconstruction and ultra-low background applications may trigger spin-offs in the applied sciences. This may range from the analysis of environmental traces for climate studies or non-proliferation to the development of novel devices for quantum computing.
GemX investigates new high-purity germanium detector designs with increased mass and improved pulse shape discrimination to enhance background reduction. It develops a crystal growth process from germanium material enriched in 76Ge for large high-purity Ge crystals with suitable net-impurity concentrations in Europe. Moreover, it establishes the production process of large Ge detectors enriched in 76Ge with minimal activation by cosmic radiation and with full control of surface contaminations from alpha contaminations and will deploy, test and operate the novel detectors in the lowest background environment available world-wide. Overall, GemX carries out cutting-edge research towards LEGEND-1000, a ton-scale NDBD decay experiment based on germanium detectors enriched in 76Ge, and thereby sustain a European leadership also in the next-generation worldwide experimental competition.
GemX is a basic research project that aims to expand knowledge and train young researchers. It strives to find a path to understand the evolution of the universe, focusing on why matter exists. As common to fundamental research, its outcome is unknown. A discovery that neutrinos are of Majorana type would have transformative potential for science and thus to society. GemX facilitates the education and training of young students and early career scientists to develop a rigorous scientific approach to problems that spills over into society after they leave academia. Breakthrough technological developments such as high-purity germanium detectors made from enriched germanium material with full topological event reconstruction and ultra-low background applications may trigger spin-offs in the applied sciences. This may range from the analysis of environmental traces for climate studies or non-proliferation to the development of novel devices for quantum computing.
GemX investigates new high-purity germanium detector designs with increased mass and improved pulse shape discrimination to enhance background reduction. It develops a crystal growth process from germanium material enriched in 76Ge for large high-purity Ge crystals with suitable net-impurity concentrations in Europe. Moreover, it establishes the production process of large Ge detectors enriched in 76Ge with minimal activation by cosmic radiation and with full control of surface contaminations from alpha contaminations and will deploy, test and operate the novel detectors in the lowest background environment available world-wide. Overall, GemX carries out cutting-edge research towards LEGEND-1000, a ton-scale NDBD decay experiment based on germanium detectors enriched in 76Ge, and thereby sustain a European leadership also in the next-generation worldwide experimental competition.
The GemX project started with the procurement of 23 kg of high-purity germanium enriched in the isotope Ge-76 >89% in the chemical form of GeO2. We established a detailed quality assurance protocol with elemental and isotopic analysis carried out by the GemX team throughout the production. During and after the enrichment process, the material was stored underground at the producer’s site to minimize activation through cosmic radiation (i.e. production of interfering elements such as 68Ge or 60Co). After verifying the quality of the raw material by measurements ICPMS carried out by the GemX team, the germanium material was transported from the production site in a 15 ton shielded steel container to Munich, Germany, and stored underground.
The enriched germanium material was converted from oxide to high-purity metal. For this purpose, an optimized process for hydrogen reduction and zone refinement of germanium dioxide was developed by GemX team in collaboration with crystal growth experts. The process was optimized using a kinetic model for unreacted shrinkage and initially tested with a batch of GeO2 with natural isotopic composition. The reduction process of the isotopically enriched germanium was carried out with an average yield of 99.85%. Subsequently, the germanium was purified to intrinsic purity by zone-refining, and an overall germanium yield of 99.05% was achieved. With the intermediate underground storage and the point-to-point car transportation, an average cosmogenic exposure of only 156 h was accumulated over the entire processing period.
In preparation for the detector production, we studied the performance of the novel high-purity germanium detector’s signals. To this end, we investigated the collective effects in clusters of charge carriers in germanium detectors and the impact of such effects on signal formation, with a particular focus on 0-like signals. We determined that the deformation of the signal due to collective effects is relevant for detectors with long drift paths. Using Monte Carlo and pulse shape simulations of gamma radiation from 208Tl and 0s of 76Ge, we determined that such volume dependence does not significantly impact the pulse shape discrimination performances provided an optimized detector geometry and analysis strategy. We could show that despite these significant effects, the inverted coaxial detector design, which GemX will realize, is high-performance for the search for neutrinoless decay.
The enriched germanium material was converted from oxide to high-purity metal. For this purpose, an optimized process for hydrogen reduction and zone refinement of germanium dioxide was developed by GemX team in collaboration with crystal growth experts. The process was optimized using a kinetic model for unreacted shrinkage and initially tested with a batch of GeO2 with natural isotopic composition. The reduction process of the isotopically enriched germanium was carried out with an average yield of 99.85%. Subsequently, the germanium was purified to intrinsic purity by zone-refining, and an overall germanium yield of 99.05% was achieved. With the intermediate underground storage and the point-to-point car transportation, an average cosmogenic exposure of only 156 h was accumulated over the entire processing period.
In preparation for the detector production, we studied the performance of the novel high-purity germanium detector’s signals. To this end, we investigated the collective effects in clusters of charge carriers in germanium detectors and the impact of such effects on signal formation, with a particular focus on 0-like signals. We determined that the deformation of the signal due to collective effects is relevant for detectors with long drift paths. Using Monte Carlo and pulse shape simulations of gamma radiation from 208Tl and 0s of 76Ge, we determined that such volume dependence does not significantly impact the pulse shape discrimination performances provided an optimized detector geometry and analysis strategy. We could show that despite these significant effects, the inverted coaxial detector design, which GemX will realize, is high-performance for the search for neutrinoless decay.
We procured germanium oxide enriched in the isotope Ge-76, carried out the chemical reduction, and zone-refined the material to 50-ohm cm material with unprecedented exposure to cosmic radiation and mass yield. Furthermore, we investigated critical questions related to the collective effects of charge carrier movements in large volume germanium detectors. We could show that the 0-signal properties are preserved for the proposed large-volume detector design provided that electrical field strengths and detector geometries are carefully designed. We currently study background signals from alpha-decays at the p+ signal junctions to facilitate the development of analytical methods that will recognize this background class at the event-by-event level. Also, here, details of the detector design are critical to meet the performance goals for LEGEND-1000.
The next important step is to manufacture these novel high mass high-purity germanium detectors from the enriched germanium material in Europe to avoid transatlantic shipping and cosmic ray exposure. We shall specify the design criteria to fulfill the low background requirements and optimize the detector geometries based on the material properties of the single crystals ingots grown through the Czchorchalski method. We foresee characterizing these detectors in vacuum cryostats in a dedicated underground facility and subsequently deploying them for background studies in the LEGEND-200 experimental setup. Finally, we shall analyze their performance, and the lessons learned will inform the detector design and production for LEGEND-1000. In addition, the GemX detectors will increase the sensitivity for 0-search in LEGEND-200, contributing to the world-best sensitivity.
The next important step is to manufacture these novel high mass high-purity germanium detectors from the enriched germanium material in Europe to avoid transatlantic shipping and cosmic ray exposure. We shall specify the design criteria to fulfill the low background requirements and optimize the detector geometries based on the material properties of the single crystals ingots grown through the Czchorchalski method. We foresee characterizing these detectors in vacuum cryostats in a dedicated underground facility and subsequently deploying them for background studies in the LEGEND-200 experimental setup. Finally, we shall analyze their performance, and the lessons learned will inform the detector design and production for LEGEND-1000. In addition, the GemX detectors will increase the sensitivity for 0-search in LEGEND-200, contributing to the world-best sensitivity.