Enriched Germanium Research Articles

Study of cosmogenic activation in 76Ge enriched germanium detectors during fabrication and transportation above ground

Rare event search experiments using germanium detectors are operated in underground laboratories to minimize the background induced by cosmic rays. However, the cosmogenic activation in germanium crystals on the ground during fabrication and transportation generates long half-life radionuclides and contributes a considerable background. We simulated the production rates of cosmogenic radionuclides in germanium and calculated the specific activities of cosmogenic radionuclides according to the scheduled fabrication and transportation processes of 76Ge enriched germanium detectors. The impact of cosmogenic background in germanium crystals for the next generation CDEX experiment was assessed with the scheduled exposure history above ground.

Enriched high purity germanium detectors for the LEGEND-200 experiment: purification and characterization by quadrupole and high resolution inductively coupled plasma mass spectrometry (ICP-MS)

LEGEND-200 (Large Enriched Germanium Experiment for Neutrinoless ββ Decay) is a physics experiment at the Gran Sasso National Laboratories (LNGS) in Italy searching for neutrinoless double beta (0υββ) decay of 76Ge using enriched high purity germanium (HPGe) detectors with a total mass of about 200 kg.During the production of germanium crystals, especially during the crystal cutting, a fraction of the enriched germanium remains as metal residues. To reuse these residual materials again for crystal growing, they must be efficiently purified. A special plant was built to purify and convert Ge metal to GeO2.Quadrupole ICP-MS (Q-ICP-MS) and High Resolution ICP-MS (HR-ICP-MS) were used to characterize the starting materials, reaction and final products. The results of the analyses are presented here.

Search for Spontaneous Radiation from Wave Function Collapse in the Majorana Demonstrator.

The Majorana Demonstrator neutrinoless double-beta decay experiment comprises a 44kg (30kg enriched in ^{76}Ge) array of p-type, point-contact germanium detectors. With its unprecedented energy resolution and ultralow backgrounds, Majorana also searches for rare event signatures from beyond standard model physics in the low energy region below 100keV. In this Letter, we test the continuous spontaneous localization (CSL) model, one of the mathematically well-motivated wave function collapse models aimed at solving the long-standing unresolved quantum mechanical measurement problem. While the CSL predicts the existence of a detectable radiation signature in the x-ray domain, we find no evidence of such radiation in the 19-100keV range in a 37.5kg-y enriched germanium exposure collected between December 31, 2015, and November 27, 2019, with the Demonstrator. We explored both the non-mass-proportional (n-m-p) and the mass-proportional (m-p) versions of the CSL with two different assumptions: that only the quasifree electrons can emit the x-ray radiation and that the nucleus can coherently emit an amplified radiation. In all cases, we set the most stringent upper limit to date for the white CSL model on the collapse rate, λ, providing a factor of 40-100 improvement in sensitivity over comparable searches. Our limit is the most stringent for large parts of the allowed parameter space. If the result is interpreted in terms of the Diòsi-Penrose gravitational wave function collapse model, the lower bound with a 95%confidence level is almost an order of magnitude improvement over the previous best limit.

Material Screening with Mass Spectrometry

Inductively coupled plasma mass spectrometry is a powerful analytical technique. Because of its sensitivity, accuracy, multielement capability, high throughput, rapid analysis times and low detection limits, it is able to determine simultaneously long-lived radionuclides at trace and ultra-trace levels as well as isotope ratios. It has been increasingly applied in the framework of rare events experiments like those investigating the nature of dark matter and neutrinos, where the screening and selection of extremely radiopure materials for the experimental apparatus is crucial. Here, the inductively coupled plasma mass spectrometry (ICP-MS) measurements of the chemical purity of a Cs2HfCl6 crystal scintillator used to study α decay of naturally occurring Hf isotopes and its own raw materials are reported. Moreover, in the framework of the GERDA/LEGEND experiment, an overview of the ICP-MS results to monitor the recycling process of enriched germanium scraps is shown. Significant outcomes, such as low detection limits despite the small amount of sample to analyze and fast ICP-MS results, have been achieved in response to the challenges required by modern low background experiments.

The Future of Neutrinoless Double Beta Decay Searches with Germanium Detectors

The observation of neutrinoless double beta (0vββ) decay would establish the Majorana nature of neutrinos and explicitly show that lepton number conservation is violated. In their search for the rare decay in the isotope 76Ge, the GERDA and MAJORANA DEMONSTRATOR (MJD) experiments have achieved the lowest backgrounds and best energy resolutions in the signal region of interest of any 0vββ decay experiment. Building on the successful results of these experiments, as well as contributions from other groups, the Large Enriched Germanium Experiment for Neutrinoless Double Beta Decay (LEGEND) collaboration aims to develop a phased 0vββ decay experimental program with discovery potential at a half-life beyond 1028 years. To achieve this goal, the enriched germanium detector mass has to be increased up to the tonne-scale and the backgrounds have to be reduced further. The first phase of LEGEND, a 200 kg measurement utilizing the existing GERDA infrastructure at LNGS in Italy, is expected to start in 2021.

Hydrogen reduction of enriched germanium dioxide and zone-refining for the LEGEND experiment

The LEGEND experiment, now under construction, will operate a large array of germanium detectors for the search of neutrinoless double-beta decay of 76Ge. In this paper we report on the process development for the hydrogen reduction of germanium dioxide enriched in 76Ge as part of the effort to manufacture detectors for the LEGEND experiment. The process was optimized via a kinetic un-reacted shrinking model and tested with a batch of natural GeO2. We completed the reduction of a batch of 23 kg isotopically enriched germanium 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. Using an intermediate underground storage, an average cosmogenic exposure of 156 h was accumulated. Special care was taken to avoid and recycle losses during the process.

Post-upgrade performance of GERDA Phase II

Excellent spectroscopic performance, event topology information and outstanding radiopurity are unique properties of HPGe detectors that facilitate the search for 0vββ decay of 76Ge. A detection of this process would shed light onto physics beyond the standard model, as it would uncover the Majorana nature of neutrinos. The GERDA experiment employs an array of HPGe detectors, made from isotopically modified material, in an active liquid argon shield at the LNGS underground laboratory. The combination of an ultra-low background environment with active background rejection techniques allows the exploration of 0vββ decay half-lives beyond 1026 yr. No signal has been found in the first 58.9 kg⋅yr of Phase II data, from which the most stringent limit on 0vββ decay of 76Ge has been derived. In the course of an upgrade new HPGe detectors of novel type have been added to the array, increasing the total mass of enriched germanium to 44.2 kg. More than one year of data has been collected in this configuration. The first 24.9 kg⋅yr reveal encouraging performance for the ongoing 0vββ decay search.

GERDA results and the future perspectives for the neutrinoless double beta decay search using 76Ge

The GERmanium Detector Array (GERDA) is a low background experiment at the Laboratori Nazionali del Gran Sasso (LNGS) of INFN designed to search for the rare neutrinoless double beta decay ([Formula: see text]) of [Formula: see text]Ge. In the first phase (Phase I) of the experiment, high purity germanium diodes were operated in a “bare” mode and immersed in liquid argon. The overall background level of [Formula: see text] was a factor of ten better than those of its predecessors. No signal was found and a lower limit was set on the half-life for the [Formula: see text] decay of [Formula: see text]Ge [Formula: see text] yr (90% CL), while the corresponding median sensitivity was [Formula: see text] yr (90% CL). A second phase (Phase II) started at the end of 2015 after a major upgrade. Thanks to the increased detector mass and performance of the enriched germanium diodes and due to the introduction of liquid argon instrumentation techniques, it was possible to reduce the background down to [Formula: see text]. After analyzing 23.2 kg[Formula: see text]⋅[Formula: see text]yr of these new data no signal was seen. Combining these with the data from Phase I a stronger half-life limit of the [Formula: see text]Ge [Formula: see text] decay was obtained: [Formula: see text] yr (90% CL), reaching a sensitivity of [Formula: see text] yr (90% CL). Phase II will continue for the collection of an exposure of 100 kg[Formula: see text]yr. If no signal is found by then the GERDA sensitivity will have reached [Formula: see text] yr for setting a 90% CL. limit. After the end of GERDA Phase II, the flagship experiment for the search of [Formula: see text] decay of [Formula: see text]Ge will be LEGEND. LEGEND experiment is foreseen to deploy up to 1-ton of [Formula: see text]Ge. After ten years of data taking, it will reach a sensitivity beyond 10[Formula: see text] yr, and hence fully cover the inverted hierarchy region.

Searching Neutrinoless Double Beta Decay with Gerda Phase II

An observation of neutrinoless double beta ([Formula: see text]) decay would allow to shed light onto the nature of neutrinos. Gerda (GERmanium Detector Array) aims to discover this process in a background-free search using [Formula: see text]Ge. The experiment is located at the Laboratori Nazionali del Gran Sasso (LNGS) of the Istituto Nazionale di Fisica Nucleare (INFN) in Italy. Bare, isotopically enriched, high purity germanium detectors are operated in liquid argon. Gerda follows a staged approach. In Phase II 35.6 kg of enriched germanium detectors are operated since December 2015. The application of active background rejection methods, such as a liquid argon scintillation light read-out and pulse shape discrimination of germanium detector signals, allows to reduce the background index to the intended level of [Formula: see text] cts/(keV⋅kg⋅yr). No evidence for the [Formula: see text] decay has been found in 23.2 kg⋅yr of Phase II data, and together with data from Phase I the up-to-date most stringent half-life limit for this process in [Formula: see text]Ge has been established, at a median sensitivity of 5.8⋅10[Formula: see text][Formula: see text]yr the 90[Formula: see text]% C.L. lower limit is 8.0⋅10[Formula: see text][Formula: see text]yr.

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

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

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

The Majorana Demonstrator is an array of point-contact Ge detectors fabricated from Ge isotopically enriched to 88% in 76Ge to search for neutrinoless double beta decay. The processing of Ge for germanium detectors is a well-known technology. However, because of the high cost of Ge enriched in 76Ge 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% and 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 68Ge 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 68Ge was reduced by a significant factor over that in natural abundance detectors not protected from cosmic rays.

MAJORANA Collaboration's Experience with Germanium Detectors

The goal of the Majorana Demonstrator project is to search for 0νββ decay in 76Ge. Of all candidate isotopes for 0νββ, 76Ge has some of the most favorable characteristics. Germanium detectors are a well established technology, and in searches for 0νββ, the high purity germanium crystal acts simultaneously as source and detector. Furthermore, p-type germanium detectors provide excellent energy resolution and a specially designed point contact geometry allows for sensitive pulse shape discrimination. This paper will summarize the experiences the MAJORANA collaboration made with enriched germanium detectors manufactured by ORTEC®®.The process from production, to characterization and integration in MAJORANA mounting structure will be described. A summary of the performance of all enriched germanium detectors will be given.

Isotopically modified Ge detectors for GERDA: from production to operation

The GERDA experiment searches for the neutrinoless double beta (0νββ) decay of 76Ge using high-purity germanium detectors made of material enriched in 76Ge. For Phase II of the experiment a sensitivity for the half life T1/20ν ∼ 2·1026 yr is envisioned. Modified Broad Energy Germanium detectors (BEGe) with thick n+ electrodes provide the capability to efficiently identify and reject background events, while keeping a large acceptance for the 0νββ-decay signal through novel pulse-shape discrimination (PSD) techniques. The viability of producing thick-window BEGe-type detectors for the GERDA experiment is demonstrated by testing all the production steps from the procurement of isotopically modified germanium up to working BEGe detectors. Comprehensive testing of the spectroscopic as well as PSD performance of the GERDA Phase II prototype BEGe detectors proved that the properties of these detectors are identical to those produced previously from natural germanium material following the standard production line of the manufacturer. Furthermore, the production of BEGe detectors from a limited amount of isotopically modified germanium served to optimize the production, in order to maximize the overall detector mass yield. The results of this test campaign provided direct input for the subsequent production of the enriched germanium detectors.

The /spl mu/-Witness Detector: A Ruggedized, Portable, Flux Meter for Cosmogenic Activation Monitoring

This paper presents the design and construction of a portable muon flux monitor based on coincidence counts between two scintillator panels to be used as an indicator for cosmogenic exposure history of detector materials. The Muon Witness detector ( <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$\mu$</tex> </formula> -Witness) has been designed to count for long periods of time (up to 40 days) using battery power and is mounted in a ruggedized case to enable measurements in transport container environments. The purpose of this apparatus is to accurately account for the cosmic-ray exposure of enriched germanium batches that will become the active detector crystals in neutrinoless double-beta decay experiments. The success of such experiments depends on the ability to mitigate and quantify backgrounds, including accurately estimating the radioactive isotopes induced by cosmogenic activation while the detectors are on the Earth's surface, prior to going underground.

Measurement of the cross sections for the production of the isotopes 74As, 68Ge, 65Zn, and 60Co from natural and enriched germanium irradiated with 100-MeV protons

The cross sections for the production of the radioactive isotopes 74As, 68Ge, 65Zn, and 60Co in metallic germanium irradiated with 100-MeV protons were measured, the experiments being performed both with germanium of natural isotopic composition and germanium enriched in the isotope 76Ge. The targets were irradiated with a proton beam at the facility for the production of radionuclides at the accelerator of the Institute for Nuclear Research (INR, Moscow). The data obtained will further be used to calculate the background of radioactive isotopes formed by nuclear cascades of cosmic-ray muons in new-generation experiments devoted to searches for the neutrinoless double-beta decay of 76Ge at underground laboratories.

Cosmogenic activation in germanium and copper for rare event searches

Experiments looking for rare events like the direct detection of dark matter particles or the nuclear Double Beta Decay are operated in deep underground locations, to suppress or very effectively reduce the effect of cosmic rays; but cosmogenic activation produced at sea level in detectors and other materials can become a serious hazard for them. Copper and germanium are very frequently used in this kind of experiments requiring an ultra-low radioactive background and therefore have been chosen as activation targets in this work. First, the excitation functions for relevant induced long-lived radioactive isotopes have been estimated; special care has been taken in using those calculations giving the best agreement with measured production cross-sections and in distinguishing production by neutrons or by protons when relevant and possible. Then, the corresponding rates of production of the nuclides in natural and enriched (86% 76Ge and 14% 74Ge) germanium and copper have been evaluated considering two different cosmic ray spectra. Comparison of the obtained activation yields at surface with all the known previous results (based either on calculations or experiments) has allowed to draw conclusions on the general methodology for evaluating cosmogenic activation.

Measurement of the isotopic composition of germanium by k0-INAA and INAA

In experiments searching for neutrinoless double β-decay high-purity Germanium detectors enriched in 76Ge will be used. Due to the nature of these experiments the isotopic composition of the enriched germanium is an important parameter. In order to determine the 76Ge/ 74Ge isotopic ratio in this new material, but also in natural and depleted samples, the feasibility of using k 0- and relative standardisation neutron activation analysis for the determination of isotopic ratios was investigated. The paper will discuss the accuracy and the estimated uncertainty of both methods in general. It was found that both techniques are useful for the determination of isotopic ratios. The analysis also demonstrated that the k 0-data for Ge should be re-evaluated.

Neutron inelastic scattering processes as a background for double-β decay experiments

We investigate several Pb$(n,{n}^{'}\ensuremath{\gamma}$) and Ge$(n,{n}^{'}\ensuremath{\gamma}$) reactions. We measure \ensuremath{\gamma}-ray production from Pb$(n,{n}^{'}\ensuremath{\gamma}$) reactions that can be a significant background for double-\ensuremath{\beta} decay experiments which use lead as a massive inner shield. Particularly worrisome for Ge-based double-\ensuremath{\beta} decay experiments are the 2041-keV and 3062-keV \ensuremath{\gamma} rays produced via Pb$(n,{n}^{'}\ensuremath{\gamma}$). The former is very close to the $^{76}\mathrm{Ge}$ double-\ensuremath{\beta} decay endpoint energy and the latter has a double escape peak energy near the endpoint. We discuss the implications of these \ensuremath{\gamma} rays on past and future double-\ensuremath{\beta} decay experiments and estimate the cross section to excite the level that produces the 3062-keV \ensuremath{\gamma} ray. Excitation \ensuremath{\gamma}-ray lines from Ge$(n,{n}^{'}\ensuremath{\gamma}$) reactions are also observed. We consider the contribution of such backgrounds and their impact on the sensitivity of next-generation searches for neutrinoless double-\ensuremath{\beta} decay using enriched germanium detectors.

Investigation of double-beta decay at the Institute of Theoretical and Experimental Physics (ITEP, Moscow)

Investigation of neutrinoless double-beta (2{beta}0{nu}) decay is presently being considered as one of the most important problems in particle physics and cosmology Interest in the problem was quickened by the observation of neutrino oscillations. The results of oscillation experiments determine the mass differences between different neutrino flavors, and the observation of neutrinoless decay may fix the absolute scale and the hierarchy of the neutrino masses. Investigation of 2{beta}0{nu} decay is the most efficient method for solving the problem of whether the neutrino is a Dirae or a Majorana particle, Physicists from the Institute of Theoretical and Experimental Physics (ITEP, Moscow) have been participating actively in solving this problem. They initiated and pioneered the application of semiconductor detectors manufactured from enriched germanium to searches for the double-beta decay of {sup 76}Ge. Investigations with {sup 76}Ge provided the most important results. At present, ITEP physicists are taking active part in four very large projects, GERDA. Majorana, EXO, and NEMO, which are capable of recording 2{beta}0{nu} decay at a Majorana neutrino mass of {approx} 10{sup -2} eV.

New requirements on enriched isotopes for experiments studying neutrinoless double β-decay (GERDA experiment)

Work on the production of enriched germanium performed at Elektrokhimicheskii Zavod is described. This germanium is to be used for experiments searching for neutrinoless double β-decay using 76Ge. One of the main requirements for the enriched product is that its activation (formation of long-lived 68Ge and 60Co) by the nuclear-active component of cosmic rays must be decreased. A system of measures which greatly decrease the production of these isotopes to a level which satisfies the requirements of the GERDA experiment has been developed and adopted.