Decay Of 76Ge Research Articles

Searching for 76Ge neutrinoless double beta decay with the CDEX-1B experiment* *Supported by the National Key Research and Development Program of China (2017YFA0402201, 2022YFA1604701, 2022YFA1605000) and the National Natural Science Foundation of China (12322511, 12175112, 12005111, 11725522)

We operated a p-type point contact high purity germanium (PPCGe) detector (CDEX-1B, 1.008 kg) in the China Jinping Underground Laboratory (CJPL) for 500.3 days to search for neutrinoless double beta ( ) decay of 76Ge. A total of 504.3 kg day effective exposure data was accumulated. The anti-coincidence and the multi/single-site event (MSE/SSE) discrimination methods were used to suppress the background in the energy region of interest (ROI, 1989–2089 keV for this work) with a factor of 23. A background level of 0.33 counts/ (keV kg yr) was realized. The lower limit on the half life of 76Ge decay was constrained as , corresponding to the upper limits on the effective Majorana neutrino mass: 3.2–7.5 .

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.

A feasibility study of multi-electrode high-purity germanium detector for 76Ge neutrinoless double beta decay searching

Experiments to search for neutrinoless double-beta (0νββ) decay of 76Ge using a high-purity germanium (HPGe) detector rely heavily on background suppression technologies to enhance their sensitivities. In this work, we proposed a pulse-shape analysis method based on a neural network (NN) and a light gradient boosting machine (lightGBM; LGB) to discriminate single-electron (background) and double-electrons (0νββ signal) events in a multi-electrode HPGe detector. In this paper, we describe a multi-electrode HPGe detector system, a data-processing system, and pulse-shape simulation procedures. We built a fully connected (FC) neural network and an LGB model to classify the single- and double-electron events. The FC network is trained with simulated single- and double-electron-induced pulses and tested in an independent dataset generated by the pulse-shape simulation. The discrimination efficiency of the FC neural network in the test set for the 0νββ double-electron events signal was 77.4%, the precision was 57.7%, and the training time was 430 min. The discrimination efficiency of LGB model was 73.1%, the precision was 64.0%, and the training time was 1.5 min. This study demonstrated that it is feasible to realize single- and double-electron discrimination on multi-electrode HPGe detectors using an FC neural network and LGB model. These results can be used as a reference for future 76Ge 0νββ experiments.

Nuclear transition matrix elements for neutrinoless double-β decay of 76Ge within mechanisms involving sterile neutrinos, Majorons and composite neutrinos

Using HFB wave functions generated with a realistic KUO and an empirical JUNE45 effective two-body interactions, nuclear transition matrix elements (NTMEs) [Formula: see text] for the [Formula: see text] transition of neutrinoless double-[Formula: see text] decay of [Formula: see text]Ge isotope are calculated within mechanisms involving sterile neutrinos, Majorons and compositeness scenario. Uncertainties in nuclear transition matrix elements are estimated by calculating sets of 12 NTMEs with these two sets of wave functions, two alternative forms of finite size effects (FNS) and three different parametrizations of short range correlations (SRC). Uncertainties in NTMEs within mechanisms involving sterile neutrinos, Majorons and composite neutrinos turn out to be about 10%–36% depending on the mass of sterile neutrinos, 10% and 37%, respectively.

Modeling of GERDA Phase II data

The GERmanium Detector Array (Gerda) experiment at the Gran Sasso underground laboratory (LNGS) of INFN is searching for neutrinoless double-beta (0νββ) decay of 76Ge. The technological challenge of Gerda is to operate in a “background-free” regime in the region of interest (ROI) after analysis cuts for the full 100 kg·yr target exposure of the experiment. A careful modeling and decomposition of the full-range energy spectrum is essential to predict the shape and composition of events in the ROI around Qββ for the 0νββ search, to extract a precise measurement of the half-life of the double-beta decay mode with neutrinos (2νββ) and in order to identify the location of residual impurities. The latter will permit future experiments to build strategies in order to further lower the background and achieve even better sensitivities. In this article the background decomposition prior to analysis cuts is presented for Gerda Phase II. The background model fit yields a flat spectrum in the ROI with a background index (BI) of {16.04}_{-0.85}^{+0.78}cdotp {10}^{-3} cts/(keV·kg·yr) for the enriched BEGe data set and {14.68}_{-0.52}^{+0.47}cdotp {10}^{-3} cts/(keV·kg·yr) for the enriched coaxial data set. These values are similar to the one of Phase I despite a much larger number of detectors and hence radioactive hardware components.

Read-out Electronics and Signal Processing in GERDA and Future Prospects

The GERDA experiment searches for the neutrinoless double beta decay of 76Ge. The experiment is using 36 kg of high-purity germanium detectors, simultaneously as source and detector, deployed into ultra-pure cryogenic liquid argon. GERDA is one the leading experiment in the field, reporting the highest sensitivity on the half-life of 0νββ decay with 1.1·1026 yr, the lowest background index with 6·10−4 cts/(keV·kg·yr) and an excellent energy resolution of 0.12% (FWHM). The search for the 0νββ decay of the isotope 76Ge will be continued in the next years by the LEGEND-200 experiment, that aims to reach a sensitivity up to 1027 yr using 200 kg of enriched HPGe detectors. The preparation of this experiment already started. The basic concepts of the GERDA read-out electronics, obeying both the severe requirements of ultra high radio-purity and cryogenic operation, are summarized. For LEGEND-200 a new electronics design, including a separation of the preamplifier in two stages, has been already designed and realized: results from tests are presented. Additionally, we will introduce the digital signal processing adopted for the energy reconstruction in GERDA and a new implementation of an optimum digital filter by means of the DPLMS method. This method are discussed and the first application to GERDA data are presented.

Study of the GERDA Phase II background spectrum

The Gerda experiment, located at the Laboratori Nazionali del Gran Sasso (LNGS) of INFN in Italy, searches for the neutrinoless double beta (0νββ) decay of 76Ge. Gerda Phase II is aiming to reach a sensitivity for the 0νββ half life of 1026 yr in ∼ 3 years of physics data taking with 100 kg·yr of exposure and a background index of ∼ 10−3 cts/(keV·kg·yr). After 6 months of acquisition a first data release with 10.8 kg·yr of exposure is performed, showing that the design background is achieved. In this work a study of the Phase II background spectrum, the main spectral structures and the background sources will be presented and discussed.

The GERDA experiment: results and perspectives

The Germanium Detector Array, GERDA, at Laboratori Nazionali del Gran Sasso (Italy), is designed to search for Majorana neutrinos via neutrinoless double beta (0νββ) decay of 76Ge. GERDA completed the Phase I in 2013, after an exposure of 21.6 kg·yr and with a background of about 0.01 cts/(keVkgyr): no signal was found and a limit on the half-life of T0ν1/2 > 2.1 · 1025 yr (90% C.L.) was established. The previous claim of 0νββ observation for 76Ge is strongly disfavoured in a model independent way. The commission for GERDA Phase II is currently ongoing and about 20 kg of additional enriched Ge diodes will be deployed. Pulse- shape analysis, together with the liquid argon instrumentation will allow to reach a background level one order of magnitude lower than in Phase I. In this paper the measurement of the half-life of 0νββ decay from GERDA Phase I and the expected sensitivity for Phase II are discussed.

THE GERDA EXPERIMENT: STATUS AND FUTURE PLANS

The GERDA experiment is searching for the neutrinoless double beta decay of 76Ge. An observation of the neutrinoless double beta decay will not only prove lepton number violation by two units, but also that the neutrino is its own anti-particle, thus of Majorana type. The status of the experiment will be presented.

Pulse shape discrimination for Gerda Phase I data

The Gerda experiment located at the Laboratori Nazionali del Gran Sasso of INFN searches for neutrinoless double beta (0νββ) decay of 76Ge using germanium diodes as source and detector. In Phase I of the experiment eight semi-coaxial and five BEGe type detectors have been deployed. The latter type is used in this field of research for the first time. All detectors are made from material with enriched 76Ge fraction. The experimental sensitivity can be improved by analyzing the pulse shape of the detector signals with the aim to reject background events. This paper documents the algorithms developed before the data of Phase I were unblinded. The double escape peak (DEP) and Compton edge events of 2.615 MeV γ rays from 208Tl decays as well as two-neutrino double beta (2νββ) decays of 76Ge are used as proxies for 0νββ decay. For BEGe detectors the chosen selection is based on a single pulse shape parameter. It accepts 0.92±0.02 of signal-like events while about 80 % of the background events at Q ββ =2039 keV are rejected. For semi-coaxial detectors three analyses are developed. The one based on an artificial neural network is used for the search of 0νββ decay. It retains 90 % of DEP events and rejects about half of the events around Q ββ . The 2νββ events have an efficiency of 0.85±0.02 and the one for 0νββ decays is estimated to be $0.90^{+0.05}_{-0.09}$ . A second analysis uses a likelihood approach trained on Compton edge events. The third approach uses two pulse shape parameters. The latter two methods confirm the classification of the neural network since about 90 % of the data events rejected by the neural network are also removed by both of them. In general, the selection efficiency extracted from DEP events agrees well with those determined from Compton edge events or from 2νββ decays.

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.

Prompt γ rays in 77Ge and 75Ge after thermal neutron capture

The reactions 74Ge(n, γ) and 76Ge(n, γ) have been measured with cold neutrons generated by the research reactor FRM II near Munich, Germany. The γ rays have been detected by two Compton-suppressed HPGe systems in single and coincidence mode. The number of identified prompt transitions and levels in 77Ge was increased significantly and the decay scheme could be reconstructed for the first time. The decay scheme was also reconstructed for 68 prompt transitions in 75Ge. Previously reported inconsistencies of the γ-ray intensities in the 77Ge β decay could be resolved. In particular, the region around 2039 keV was investigated carefully, since the background to the neutrinoless double-beta decay of 76Ge must be understood well for the upcoming Gerda and Majorana experiments.

Signal modeling of high-purity Ge detectors with a small read-out electrode and application to neutrinoless double beta decay search in Ge-76

The GERDA experiment searches for the neutrinoless double beta decay of76Ge using high-purity germanium detectors enriched in76Ge. The analysis of the signal time structure provides a powerfultool to identify neutrinoless double beta decay events and to discriminate themfrom gamma-ray induced backgrounds. Enhanced pulse shape discriminationcapabilities of Broad Energy Germanium detectors with a small read-outelectrode have been recently reported. This paper describes the full simulationof the response of such a detector, including the Monte Carlo modeling ofradiation interaction and subsequent signal shape calculation. A pulse shapediscrimination method based on the ratio between the maximum current signalamplitude and the event energy applied to the simulated data shows quantitativeagreement with the experimental data acquired with calibration sources. Thesimulation has been used to study the survival probabilities of the decays whichoccur inside the detector volume and are difficult to assess experimentally.Such internal decay events are produced by the cosmogenic radio-isotopes68Ge and 60Co and the neutrinoless double beta decay of76Ge. Fixing the experimental acceptance of the double escape peakof the 2.614 MeV photon to 90%, the estimated survival probabilitiesat Qββ = 2.039 MeV are (86±3)% for 76Geneutrinoless double beta decays, (4.5±0.3)% for the 68Gedaughter 68Ga, and (0.9+0.4−0.2)% for 60Codecays.

Effects of orbital occupancies and spin–orbit partners II: [formula omitted] decays of 76Ge, 82Se and 136Xe to first excited [formula omitted] states

The neutrinoless double beta ( 0 ν β β ) decays of 76Ge, 82Se and 136Xe to the first excited 0 + states ( 0 1 + ) in 76Se, 82Kr and 136Ba are studied in calculations using different sizes of the single-particle model spaces and different occupancies of the individual orbits. This is a continuation of a similar study [J. Suhonen, O. Civitarese, Effects of orbital occupancies and spin–orbit partners on 0 ν β β -decay rates, Nucl. Phys. A 847 (2010) 207–232] of the 0 ν β β decays of 76Ge, 82Se, 128Te, 130Te and 136Xe to the ground states of 76Se, 82Kr, 128Xe, 130Xe and 136Ba. The nuclear matrix elements and half-lives of the decays to the 0 1 + states are calculated by the use of the multiple-commutator model with effective, G-matrix-derived nuclear forces. Interestingly, the basic features of the ground-state and excited-state decays are found to be quite different. The half-lives of the excited-state transitions are suppressed by at least one order of magnitude relative to the ground-state transitions.

Cross sections for 68Ge production in natural- and enriched-germanium targets irradiated with protons of energy 100 MeV and background in experiments devoted to searches for the 2β0ν decay of 76Ge

The rate of 68Ge production at sea level under the effect of the nuclear component of cosmic rays is calculated. The calculation is based on the experimental values of the cross sections for 68Ge production in natural- and enriched-germanium targets (enrichment in 76Ge) irradiated with high-energy protons. The background from the decays of 68Ge can be a serious problem in new-generation experiments devoted to searches for the 2β0ν decay of 76Ge.

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.

Two-neutrino double-beta decay of 76Ge in an anharmonic vibrator approach

We have calculated the nuclear matrix element of two-neutrino double-beta (2νββ) decay of 76Ge by using the proton–neutron microscopic anharmonic vibrator approach (pnMAVA). In the pnMAVA the wavefunctions of the intermediate 1+ states in 76As have a one-phonon part containing the proton–neutron QRPA (pnQRPA) phonons and a two-phonon part built by coupling the pnQRPA and charge-conserving QRPA phonons. We compare the measured GT− and GT+ Gamow–Teller strength functions with the measured ones. The two-phonon components of the pnMAVA wavefunctions cause the pnQRPA strength to redistribute over a finite energy range. This has only a small effect on the value 2νββ nuclear matrix element. This shows that at the spherical limit the pnQRPA is an appropriate tool to deal with the double-beta transitions in nuclear systems with only slight deviations from spherical symmetry.

Effects of orbital occupancies on the neutrinoless ββ matrix element of 76Ge

In this work we use the recently measured neutron occupancies in the 76Ge and 76Se nuclei as a guideline to define the neutron quasiparticle states in the 1p0f0g shell. We define the proton quasiparticles by inspecting the odd-mass nuclei adjacent to 76Ge and 76Se. We insert the resulting quasiparticles in a proton–neutron quasiparticle random-phase approximation (pnQRPA) calculation of the nuclear matrix element of the neutrinoless double beta (0νββ) decay of 76Ge. A realistic model space and effective microscopic two-nucleon interactions are used. We include the nucleon–nucleon short-range correlations and other relevant corrections at the nucleon level. It is found that the resulting 0νββ matrix element is smaller than in the previous pnQRPA calculations, and closer to the recently reported shell-model results.

Muon capture rates in Se and Cd isotopes

Energy and time spectra of γ rays following nuclear capture of negative muons in natural Se and Cd and isotopically enriched 76Se and 106Cd targets have been measured with HPGe detectors. Total muon lifetimes in Se and Cd isotopes and partial μ capture rates to excited levels of 76As and 106Ag are obtained. These results are necessary for calculation of nuclear matrix elements of the 2β decay of 76Ge and 106Cd respectively.

Status of the Germanium Detector Array ( Gerda) for the search of neutrinoless [formula omitted] decays of 76Ge at LNGS

The Germanium Detector Array ( Gerda) for the search of neutrinoless β β decays of 76Ge at LNGS will operate bare germanium diodes enriched in 76Ge in an (optional active) cryogenic fluid shield to investigate neutrinoless β β decay with a sensitivity of T 1 / 2 > 2 × 10 26 y after an exposure of 100 kg y. Recent progress includes the installation of the first underground infrastructures at Gran Sasso, the completion of the enrichment of 37.5 kg of germanium material for detector construction, the prototyping of the low-mass detector support and contacts, the front-end and DAQ electronics, as well as the preparation for construction of the cryogenic vessel and water tank.