GERDA Research Articles

Hypernuclear γ-ray spectroscopy: present status and perspectives

Hypernuclear γ-ray spectroscopy experiments using a germanium detector array, Hyperball, have accumulated precise data on various p-shell Λ hypernuclei. A common parameter set of the spin-dependent effective interaction strengths (Δ, S Λ , S N , and T), which reproduces most of the hypernuclear data, has been determined. In the J-PARC facility, further experiments of hypernuclear γ-ray spectroscopy are planned to be performed.

The GERmanium detector array, GERDA

The GERmanium Detector Array, GERDA, is a new experiment which is currently being built at the INFN LNGS Laboratory in Italy with the aim to search for neutrinoless double beta-decay of 76Ge. Unique features of the experiment are (1) to operate germanium detectors directly inside a bath of liquid argon, and (2) to use segmented germanium detectors. The background level is expected to be two orders of magnitude below that of recent experiments. This results in an estimated sensitivity to the half-life of T1/2 > 13.5 · 1025 y for the envisioned exposure of 100 kg·y, corresponding to an effective Majorana neutrino mass of approximately 130 meV.

Neutron interactions as seen by a segmented germanium detector

The GERmanium Detector Array (GERDA) is designed for the search for “neutrinoless double-beta decay” ( 0ν2β with germanium detectors enriched in 76Ge . An 18-fold-segmented prototype detector for GERDA Phase II was exposed to an AmBe neutron source to improve the understanding of neutron-induced backgrounds. Neutron interactions with the germanium isotopes themselves and in the surrounding materials were studied. Segment information is used to identify neutron-induced peaks in the recorded energy spectra. The Geant4-based simulation package MaGe is used to simulate the experiment. Though many photon peaks from germanium isotopes excited by neutrons are correctly described by Geant4, some physics processes were identified as being incorrectly treated or even missing.

Test of pulse shape analysis using single Compton scattering events

Procedures developed to separate single- and multiple-site events in germanium detector are tested with specially selected event samples provided by an 18-fold segmented prototype germanium detector for phase II of the germanium detector array, GERDA. The single Compton scattering, i.e. single-site, events are tagged by coincidently detecting the scattered photon with a second detector positioned at a defined angle. A neural network is trained to separate such events from events which come from multi-site dominated samples. Identification efficiencies of ≈80% are achieved for both single- and multi-site events.

Gamma-ray spectroscopy of hypernuclei: Recent results and future plans

A series of hypernuclear γ-ray spectroscopy experiments using a germanium detector array, Hyperball, have accumulated precise data on various p-shell Λ hypernuclei. Recently, 12ΛC and 11ΛB were studied at KEK using the (π+,K+γ) reaction, and a transition from a directly-populated spin-flip state, 16ΛO(2−), was observed in BNL E930 data. It is discussed whether the spin-dependent ΛN interaction parameters determined from previous experiments can consistently explain other γ spectroscopy data. At the J-PARC facility, further hypernuclear γ-ray spectroscopy experiments are planned, particularly to measure the g factor of a Λ in a nucleus from a spin-flip B(M1) value and to investigate the ΛN interaction more in detail.

Exciting isomers from the first stopped-beam RISING campaign

First results are reported from a major new initiative of experiments, which focus on nuclear structure studies at extreme isospin values by means of isomer spectroscopy. The experiments represent the first part of the so-called stopped-beam campaign within the Rare ISotope INvestigations at GSI (Rising) project. Time-correlated γ decays from individually identified nuclear species have been measured, allowing the clean identification of isomeric decays in a wide range of exotic nuclei both at the proton drip-line and in heavy, neutron-rich systems. An overview of the experimental technique will be given, together with the performance of the new germanium detector array and future research plans for the collaboration.

Effect of an aerosol deposition pattern in the lung on the counting efficiency of a large area germanium detector array

The Human Monitoring Laboratory has extended the use of sliced lungs containing planar sources to simulate heterogeneous radionuclide deposition patterns. This work examined two deposition patterns and their effect on the counting efficiency of low-energy photons. The results have shown that heterogenous distributions can be difficult to detect in some cases and can still lead to large uncertainties (up to a factor of 2.5) in the activity estimate, especially at low photon energies. At higher energies ( approximately 60 keV), the effect of the heterogeneous distribution is greatly reduced and errors in the activity estimate reduced to approximately 25%. The presence of a heterogenous distribution can be detected by comparing the ratio of the individual detector counts with the expected values obtained from measuring multiple lungs sets that contained a homogeneous distribution. The distributions tested in this paper were detectable (at 2sigma) as heterogeneous by two of the four detectors in the counting array.

Identification of photons in double beta-decay experiments using segmented germanium detectors—Studies with a GERDA phase II prototype detector

The sensitivity of experiments searching for neutrinoless double beta-decay of germanium was so far limited by the background induced by external γ -radiation. Segmented germanium detectors can be used to identify photons and thus reduce this background component. The GERmanium Detector Array, GERDA, will use highly segmented germanium detectors in its second phase. The identification of photonic events is investigated using a prototype detector. A focus is placed on the comparison with Monte Carlo data.

γ -ray spectroscopy study of 11 ΛB and 12 ΛC

In this paper, a preliminary result from the latest hypernuclear γ -ray spectroscopy experiment (KEK-E566) is presented together with a short discussion. The experiment was performed at the KEK-PS K6 beam line in 2005. In this experiment, the 12C(π+, K +)12 ΛC reaction was employed to populate 12 ΛC/11 ΛB hypernuclei. A germanium detector array, Hyperball2, was constructed to detect γ -rays emitted from the hypernuclei produced. Three hypernuclear γ -ray peaks were observed and assigned.

Pulse shapes from electron and photon induced events in segmented high-purity germanium detectors

Experiments built to search for neutrinoless double beta-decay are limited in their sensitivity not only by the exposure but also by the amount of background encountered. Radioactive isotopes in the surrounding of the detectors which emit gamma-radiation are expected to be a significant source of background in the GERmanium Detector Array, GERDA. Methods to select electron induced events and discriminate against photon induced events inside a germanium detector are presented in this paper. The methods are based on the analysis of the time structure of the detector response. Data were taken with a segmented GERDA prototype detector. It is shown that the analysis of the time response of the detector can be used to distinguish multiply scattered photons from electrons.

Facilities and Methods: Heavy Element Spectroscopy at JYFL

A central theme throughout the history of experimental nuclear physics has been the pursuit of nuclei at the extremes. These extremes, be they in terms of mass, proton-to-neutron ratio, or spin and excitation energy, provide the most stringent tests for our current nuclear structure theories. The pursuit of these extremes also provides impetus to develop and exploit new techniques and instrumentation. At the Department of Physics of the University of Jyväskylä (JYFL), a major part of the experimental program is devoted to the study of heavy nuclei along the proton dripline and superheavy nuclei in the transfermium region. This program was initiated in the mid-1990s when a small array of TESSA-type germanium detectors known as DORIS (DOdecahedral aRray In Suomi) was coupled to the gas-filled recoil separator RITU [1]. Using the Recoil-Decay Tagging (RDT) technique [2], whereby the delayed charged particle radiation measured at the focal plane of the recoil separator is used to identify the gamma-rays of interest, it was possible to observe gamma-rays from the yrast band of the neutron-deficient nucleus 192Po for the first time [3]. The production cross-section of 192Po was on the order of 10 microbarns. This should be compared to the gamma-ray background from fission that is produced with a cross-section of the order of one barn. Over the decade or so since these initial studies, the instrumentation and range of techniques used at JYFL has evolved steadily such that currently in-beam studies of nuclei produced with cross-sections below 100 nanobarns are possible [4]. An overview of the developments in spectroscopic studies of deformed superheavy nuclei was given in the earlier Nuclear Physics News article of Butler and Leino [5]. This article reports on developments at JYFL since that time.

Early experiences with the Mount Allison University four-detector X-ray fluorescence bone lead measurement system

A four-detector (4×500mm2) low energy Germanium detector array has recently been acquired for use in a 109Cd source K-shell X-ray fluorescence bone lead measurement system. In this paper, system calibration results are presented, along with preliminary measurement results using lead-doped bone phantoms. Measurement uncertainty and reproducibility values are noted. System comparisons are made with our conventional (1×2000mm2) detector system, and future directions outlined.

A cloverleaf detector system for in vivo bone lead measurement

A “cloverleaf” low energy germanium detector array has been assembled for the purpose of in vivo bone lead measurement through x‐ray fluorescence. Using as an exciting source, results are reported from a leg phantom simulating measurement of lead in a human tibia. For high activity and low activity sources, measurement results are reported for both the cloverleaf system and a conventional single detector system of equivalent surface area . The mean uncertainty and reproducibility of measurement were both significantly improved for the cloverleaf system with a high activity source. When using a source activity of , measurement of the phantom resulted in an average bone lead uncertainty of and a reproducibility of . These results represent the highest precision yet reported from a bone lead x‐ray fluorescence system.

γ-ray multipolarimetry between low-spin states ofCe136: Search for the21,ms+one-phonon mixed-symmetry state

An experiment was performed on $^{136}\mathrm{Ce}$ to measure $E2/M1$ multipole mixing ratios using the method of $\ensuremath{\gamma}\ensuremath{\gamma}$ angular correlations. The low-spin states of interest were populated by \ensuremath{\beta} decay from the ${2}^{+}$ ground state of $^{136}\mathrm{Pr}$ that has a half-life of 13.1 min. Emitted \ensuremath{\gamma} rays were detected with the Stony Brook cube germanium detector array. Five $E2/M1$ multipole mixing ratios were measured for the first time and two spin and parity quantum numbers were assigned. Candidates for the ${2}_{1,\mathrm{ms}}^{+}$ state of $^{136}\mathrm{Ce}$ were identified from large $M1$ fractions in the $E2/M1$ multipole mixing ratios. The ${2}_{1,\mathrm{ms}}^{+}$ state is suggested to be fragmented between the closely lying ${2}_{3}^{+}$ and ${2}_{4}^{+}$ states. A two state mixing scenario is employed for deducing an $F$-spin mixing matrix element of 43(5) keV.

New results from MINIBALL

Preliminary results from the study of collectivity and single-particle structure in neutron-rich nuclei are presented from selected experiments. The experiments discussed here employed the MINIBALL Germanium detector array for gamma-ray detection. Coulomb-excitation experiments at REX-ISOLDE focused on the collectivity of Cd and Xe isotopes near 132Sn as well as the shell structure near N = 20 , N = 40 , and N = 50 . Another experiment was performed to probe the single-particle structure of 47Ca, 55Ti, and 73Cu at the GSI Fragmentseparator, using single-nucleon knock-out reactions at relativistic energies.

The Nuclear Compton Telescope (NCT): Scientific goals and expected sensitivity

The Nuclear Compton Telescope (NCT) is a balloon-borne telescope designed to study astrophysical sources of gamma-ray emission with high spectral resolution, moderate angular resolution, and novel sensitivity to gamma-ray polarization. The heart of NCT is an array of cross-strip germanium detectors, each of 15-mm thickness and 5400 mm 2 active area, with full 3D position resolution <2 mm 3. NCT will perform Compton imaging in the 0.2–10 MeV gamma-ray band. We are currently planning a 12-detector long duration balloon flight of the NCT instrument from Australia in December 2008. NCT is the prototype of a future satellite mission, which will have a coded mask on top of a larger detector array to carry out imaging in the 20–100 keV X-ray band. This paper describes the scientific goals, as well as the detector design, simulated performance and current status of the project. NCT is a joint effort of several institutions in the US and in Taiwan.

Large area Germanium detector arrays for lung counting: what is the optimum number of detectors?

Using the Lawrence Livermore National Laboratory (LLNL) torso phantom to calibrate a lung counting system can lead to the conclusion that three large area (i.e. >70 mm diameter) Ge detectors will outperform a four-detector array and provide a lower MDA as a four-detector array of large area Ge detectors covers a significant portion of inactive tissue (i.e. non-lung tissue). The lungs of the LLNL phantom, which are approximately 10 cm too short compared with real lungs, also suggests that a two-detector array could be used under limited circumstances. When tested with modified lungs that are more human-like, it was found that the four-detector array showed the best counting efficiency and the lowest MDA. Fortunately, these findings indicate that, although the LLNL phantom's lungs are too short, there is no adverse impact on the calibration of a lung counter.

Status of the Germanium Detector Array (GERDA) in the search of neutrinoless ββ decays of 76Ge at LNGS

The Germanium Detector Array (GERDA) in the search for neutrinoless ββ decays of 76Ge at LNGS will operate bare germanium diodes enriched in 76Ge in an (optional active) cryogenic fluid shield to investigate neutrinoless ββ decay with a sensitivity of T 1/2 > 2 × 1026 yr after an exposure of 100 kg yr. 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, prototyping of low-mass detector support and contacts, and front-end and DAQ electronics, as well as the preparation for construction of the cryogenic vessel and water tank.

The GERmanium Detector Array read-out: Status and developments

The GERmanium Detector Array, GERDA, is designed to search for neutrinoless double beta decay ( 0 ν β β ) of Ge 76 . In this work the status of the R&D on cryogenics front-end circuits of gamma spectrometry class conducted in the framework of GERDA is reviewed.

Background reduction in neutrinoless double beta decay experiments using segmented detectors—A Monte Carlo study for the GERDA setup

The identification of gamma radiation is essential for a new generation of double beta decay experiments. The GERmanium Detector Array, GERDA, located at the INFN Gran Sasso National Laboratory (LNGS) in Italy, uses germanium, enriched in Ge 76 , as source and detector, and aims at a background level of less than 10 - 3 counts / ( kg keV y ) in the region of the Q β β -value. For the first time highly segmented detectors will be installed in a double beta decay experiment. A detailed GEANT4 Monte Carlo study was performed to evaluate the background reduction achievable by anti-coincidence cuts between crystals and segments. Within the overall geometry of GERDA, the segmentation scheme considered here provides around an order of magnitude of extra background reduction.