The GERmanium Detector Array

The Physics of GERDA


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Papers by subgroups of the GERDA Collaboration 2018 - 2015

Virtual depth by active background suppression: revisiting the cosmic muon induced background of Gerda Phase II
Christoph Wiesinger, Luciano Pandola, and Stefan Schönert
Eur. Phys. J. C (2018) 78:597    
In-situ production of radioisotopes by cosmic muon interactions may generate a non-negligible background for deep underground rare event searches. Previous Monte Carlo studies for the Gerda experiment at LNGS identified the delayed decays of 77Ge and its metastable state 77mGe as dominant cosmogenic background in the search for neutrinoless double beta decay of 76Ge. This might limit the sensitivity of next generation experiments aiming for increased 76Ge mass at background-free conditions and thereby define a minimum depth requirement. A re-evaluation of the 77(m)Ge background for the Gerda experiment has been carried out by a set of Monte Carlo simulations. The obtained 77(m)Ge production rate is (0.21±0.01) nucei/(kg⋅year). After application of state-of-the-art active background suppression techniques and simple delayed coincidence cuts this corresponds to a background contribution of (2.77±0.3)⋅10-6 cts/(keV⋅kg⋅year). The suppression achieved by this strategy equals an effective muon flux reduction of more than one order of magnitude. This virtual depth increase opens the way for next generation rare event searches.
Pulse shape discrimination performance of Inverted Coaxial Ge detectors
A. Domula, M. Hult, Y. Kermaidic, G. Marissens, B. Schwingenheuer, T. Wester, K. Zuber
NIMA 891 (2018) 106-110    
We report on the characterization of two inverted coaxial Ge detectors in the context of being employed in future 76Ge neutrinoless double beta (0νββ) decay experiments. It is an advantage that such detectors can be produced with bigger Ge mass as compared to the planar Broad Energy Ge (BEGe) or p-type Point Contact (PPC) detectors that are currently used in the Gerda and Majorana Demonstrator 0vbb decay experiments respectively. This will result in a lower background for the search of 0νββ decay due to a reduction of detector surface to volume ratio, cables, electronics and holders which are dominating nearby radioactive sources. The measured resolution near the 76Ge Q-value at 2039 keV is 2.3 keV FWHM and their pulse-shape discrimination of background events are similar to BEGe and PPC detectors. It is concluded that this type of Ge-detector is suitable for usage in 76Ge 0νββ decay experiments.
Mitigation of 42Ar/42K background for the GERDA Phase II experiment
A. Lubashevskiy, M. Agostini, D. Budjas, A. Gangapshev, K. Gusev, M. Heisel, A. Klimenko, A. Lazzaro, B. Lehnert, K. Pelczar, S. Schönert, A. Smolnikov, M. Walter, G. Zuzel
Eur. Phys. J. C (2018) 78:15    
Background coming from the 42Ar decay chain is considered to be one of the most relevant for the GERDA experiment, which searches for the neutrinoless double beta decay of 76Ge. The sensitivity strongly relies on the absence of background around the Q-value of the decay. Background coming from 42K, a progeny of 42Ar, can contribute to that background via electrons from the continuous spectrum with an endpoint at 3.5 MeV. Research and development on the suppression methods targeting this source of background were performed at the low-background test facility LArGe . It was demonstrated that by reducing 42K ion collection on the surfaces of the broad energy germanium detectors in combination with pulse shape discrimination techniques and an argon scintillation veto, it is possible to suppress 42K background by three orders of magnitude. This is sufficient for Phase II of the GERDA experiment.
A Compton scattering setup for pulse shape discrimination studies in germanium detectors
K. von Sturm, S. Belogurov, R. Brugnera, A. Garfagnini, I. Lippi, L. Modenese, D. Rosso, M. Turcato
Applied Rad. and Isotopes 125 (2017) 163    
Pulse shape discrimination is an important handle to improve sensitivity in low background experiments. A dedicated setup was built to investigate the response of high-purity germanium detectors to single Compton scattered events. Using properly collimated γ-ray sources, it is possible to select events with known interaction location. The aim is to correlate the position dependent signal shape with geometrical and electrical properties of the detector. We report on design and performance of the setup with a first look on data.
Resistor-less charge sensitive amplifier for semiconductor detectors
K. Pelczar, K. Panas, and G. Zuzel
NIMA 835 (2016) 142    
A new concept of a Charge Sensitive Amplifier without a high-value resistor in the feedback loop is presented. Basic spectroscopic parameters of the amplifier coupled to a coaxial High Purity Germanium detector (HPGe) are discussed. The amplifier signal input is realized with an n-channel J-FET transistor. The feedback capacitor is discharged continuously by the second, forward biased n-channel J-FET, driven by an RC low - pass filter. Both the analog - with a standard spectroscopy amplifier and a multi-channel analyzer - and the digital - by applying a Flash Analog to Digital Converter - signal readouts were tested. The achieved resolution in the analog and the digital readouts was 0.17% and 0.21%, respectively, at the Full Width at Half Maximum of the registered 60Co 1332.5 keV gamma line.
The performance of the Muon Veto of the GERDA experiment
K. Freund, R. Falkenstein, P. Grabmayr, A. Hegai, J. Jochum, M. Knapp, B. Lubsandorzhiev, F. Ritter, C. Schmitt, A.-K. Schütz, I. Jitnikov, E. Shevchik, M. Shirchenko, D. Zinatulin

EPJC 76 (2016)298    
Low background experiments need a suppression of cosmogenically induced events. The GERDA experiment located at LNGS is searching for the 0νββ decay of 76Ge. It is equipped with an active muon veto the main part of which is a water Cherenkov veto with 66 PMTs in the water tank surrounding the GERDA cryostat. With this system 806 live days have been recorded, 491 days were combined muon-germanium data. A muon detection efficiency of εμd=(99.935±0.015)% was found in a Monte Carlo simulation for the muons depositing energy in the germanium detectors. By examining coincident muon-germanium events a rejection efficiency of εμr=(99.2+0.3-0.4)% was found. Without veto condition the muons by themselves would cause a background index of BIμ=(3.16±0.85)·10-3 cts/(keV·kg·year) at Qββ.
Optimized digital filtering techniques for radiation detection with HPGe detectors
Marco Salathe and Thomas Kihm
NIMA 808 (2016) 150  
This paper describes state-of-the-art digital filtering techniques that are part of GEANA, an automatic data analysis software used for the GERDA experiment. The discussed filters include a novel, nonlinear correction method for ballistic deficits, which is combined with one of three shaping filters: a pseudo-Gaussian, a modified trapezoidal, or a modified cusp filter. The performance of the filters is demonstrated with a 762 g Broad Energy Germanium (BEGe) detector, produced by Canberra, that measures γ-ray lines from radioactive sources in an energy range between 59.5 and 2614.5 keV. At 1332.5 keV, together with the ballistic deficit correction method, all filters produce a comparable energy resolution of ∼ 1.61 keV FWHM. This value is superior to those measured by the manufacturer and those found in publications with detectors of a similar design and mass. At 59.5 keV, the modified cusp filter without a ballistic deficit correction produced the best result, with an energy resolution of 0.46 keV. It is observed that the loss in resolution by using a constant shaping time over the entire energy range is small when using the ballistic deficit correction method.
Production and characterization of 228Th calibration sources with low neutron emission for GERDA
L. Baudis, G. Benato, P. Carconi, C. Cattadori, P. De Felice, K. Eberhardt, R. Eichler, A. Petrucci, M. Tarka and M. Walter
JINST 10 (2015) P12005  
The GERDA experiment at the Laboratori Nazionali del Gran Sasso (LNGS) searches for the neutrinoless double beta decay of 76Ge. In view of the GERDA Phase II data collection, four new 228Th radioactive sources for the calibration of the germanium detectors enriched in 76Ge have been produced with a new technique, leading to a reduced neutron emission rate from (α, n) reactions. The gamma activities of the sources were determined with a total uncertainty of ∼4% using an ultra-low background HPGe detector operated underground at LNGS. The neutron emission rate was determined using a low background LiI(Eu) detector and a 3He counter at LNGS. In both cases, the measured neutron activity is ∼10-6 n/(s⋅Bq), with a reduction of about one order of magnitude with respect to commercially available 228Th sources. Additionally, a specific leak test with a sensitivity to leaks down to ∼10 mBq was developed to investigate the tightness of the stainless steel capsules housing the sources after their use in cryogenic environment.
Results from Phase I of the GERDA Experiment
T. Wester
AIP Conf. Procs. 1686 (2015) 020026  
The GERmanium Detector Array Gerda at the Laboratori Nazionali del Gran Sasso of the INFN in Italy is an experiment dedicated to the search for the neutrinoless double beta (0νββ) decay in 76Ge. The experiment employs high purity germanium detectors enriched in 76Ge inside a 64 m3 cryostat filled with liquid argon. Gerda was planned in two phases of data taking with the goal to reach a half-life sensitivity in the order of 1026 yr. Phase I of Gerda was running from November 2011 until May 2013. With about 18 kg total detector mass, data with an exposure of 21.6 kgyr was collected and a background index of 0.01 cts/(keVkgyr) was achieved in the region of interest. No signal was found for the 0νββ decay and a new limit of T1/2 > 2.1x1025 yr (90% C.L.) was obtained, strongly disfavoring the previous claim of observation. Furthermore, the 2νββ decay half-life of 76Ge was measured with unprecedented precision. Other results include new half-life limits of the order of 1023 yr for Majoron emitting double beta decay modes with spectral indices n = 1; 2; 3; 7 and new limits in the order of 1023 yr for 2νββ decays to the first 3 excited states of 76Se. In Phase II, currently in preparation, the detector mass will be doubled while reducing the background index by a factor of 10.
Signal recognition efficiencies of artificial neural-network pulse-shape discrimination in HPGe 0νββ decay searches
A. Caldwell, F. Cossavella, B. Majorovits, D. Palioselitis, O. Volynets
EPJC 75 (2015) 350  
A pulse-shape discrimination method based on artificial neural networks was applied to pulses simulated for different background, signal and signal-like interactions inside a germanium detector. The simulated pulses were used to investigate variations of efficiencies as a function of used training set. It is verified that neural networks are well-suited to identify background pulses in true-coaxial high-purity ger- manium detectors. The systematic uncertainty on the signal recognition efficiency derived using signal-like evaluation samples from calibration measurements is estimated to be 5 %. This uncertainty is due to differences between signal and calibration samples.
GERDA: Results and perspectives
C.M. Cattadori
Nucl. and Particle Physics Procs. 265-266 (2015) 38  
From November 2011 to May 2013, Gerda searched for 0νββ and 2νββ of 76Ge, operating bare in a liquid argon bath Ge detectors enriched up to ~87% in 76Ge (enrGe), for a total mass of ~18 kg of enrGe. A total exposure of 21.6 kgyr, of enrGe was collected, and the existing claim of 0νββ evidence was scrutinized. Gerda did not observe any peak at Qββ or in its immediate surroundings; the limit of T1/2 > 2.1x1025 yr (90 % C.L.) is derived. When combining the Gerda limit with those of past HdM and Igex experiments, the lower limit of 3.0x1025 yr (90 % C.L.) on T1/2 is achieved. The background index (BI) at Qββ (~2039 keV) is ~2.0x10−2 cts/(keV kg yr) and ∼1.0x10−2 cts/(keV kg yr), prior and after the pulse shape cuts respectively. Thanks to the low background the 2νββ dominates the energy spectrum below 1800 keV: the T1/2 = (1.84+0.14-0.10)x 1021yr was derived on a first data set corresponding to 5.1 kgyr exposure. The ongoing experimental program, to double the exposed mass by adding new enrGe detectors with improved pulse shape discrimination features, and to implement the liquid argon scintillation light readout is outlined.
LArGe: active background suppression using argon scintillation for the Gerda 0νββ-experiment
M. Agostini, M. Barnabe-Heider, D. Budjas, C. Cattadori, A. Gangapshev, K. Gusev, M. Heisel, M. Junker, A. Klimenko, A. Lubashevskiy, K. Pelczar, S. Schönert, A. Smolnikov, G. Zuzel
Eur. Phys. J. C 75 (2015) 506  
LArGe is a GERDA low-background test facility to study novel background suppression methods in a low background environment, for future application in the GERDA experiment. Similar to GERDA, LArGe operates bare germanium detectors submersed into liquid argon (1m3, 1.4 tons), which in addition is instrumented with photomultipliers to detect argon scintillation light. The scintillation signals are used in anti-coincidence with the germanium detectors to effectively suppress background events that deposit energy in the liquid argon. The background suppression efficiency was studied in combination with a pulse shape discrimination (PSD) technique using a BEGe detector for various sources, which represent characteristic backgrounds to GERDA. Suppression factors of a few times 103 have been achieved. First background data of LArGe with a coaxial HPGe detector (without PSD) yield a background index of (0.12-4.6)x10−2 cts/(keV kg yr) (90% C.L.), which is at the level of GERDA Phase I. Furthermore, for the first timewe monitor the natural 42Ar abundance (parallel to GERDA), and have indication for the 2νββ-decay in natural germanium. These results show the effectivity of an active liquid argon veto in an ultra-low background environment. As a consequence, the implementation of a liquid argon veto in GERDA Phase II is pursued.
Enhancement of light yield and stability of radio-pure tetraphenyl-butadiene based coatings for VUV light detection in cryogenic environments
L. Baudis, G. Benato, R. Dressler, F. Piastra, I. Usoltsev and M. Walter
JINST 10 (2015) P09009  
The detection of VUV scintillation light in (liquid) argon (LAr) detectors commonly includes a reflector with a fluorescent coating, converting UV photons to visible light. The light yield of these detectors depends directly on the conversion efficiency. Several coating/reflector combinations were produced using VM2000, a specular reflecting multi-layer polymer, and Tetratex R , a diffuse reflecting PTFE fabric, as reflector foils. The light yield of these coatings was optimised and has been measured in a dedicated liquid argon setup built at the University of Zurich. It employs a small, 1.3 kg LAr cell viewed by a 3-inch, low radioactivity PMT of type R11065-10 from Hamamatsu. The cryogenic stability of these coatings was additionally studied. The optimum reflector/coating combination was found to be Tetratex R dip-coated with Tetraphenyl-butadiene with a thickness of 0.9 mg/cm2, resulting in a 3.6 times higher light yield compared to uncoated VM2000. Its performance was stable in long-term measurements, performed up to 100 days in liquid argon. This coated reflector was also investigated concerning radioactive impurities and found to be suitable for current and upcoming low-background experiments. Therefore it is used for the liquid argon veto in Phase II of the GERDA neutrinoless double beta decay experiment.
Double-beta decay with majoron emission in GERDA Phase I
S. Hemmer for the GERDA Collaboration
Eur. Phys. J. Plus 130 (2015) 139  
Neutrinoless double-beta decay with emission of one or two majorons (0νββχ(χ)) is predicted by several beyond–Standard-Model theories. This article reviews the results of a search for 0νββχ(χ) of 76Ge using data from the Germanium Detector Array (GERDA) experiment, located underground at the INFN Laboratori Nazionali del Gran Sasso (LNGS) in Italy. The analysis comprised data with an exposure of 20.3 kg yr from the first phase of the experiment. No indication of contributions to the observed energy spectra was detected for any of the majoron models. The lower limit on the half-life for the ordinary majoron model (spectral index n = 1) was determined to be T0νχ1/2 > 4.2x1023 yr (90 % quantile). This limit and the limits derived for the other majoron modes constitute the most stringent limits on 0νββχ(χ) decay of 76Ge measured to date.
Phase II upgrade of the GERDA experiment for the search of neutrinoless double beta decay
B. Majorovits for the GERDA collaboration
Physics Procedia 61 (2015) 254  
Observation of neutrinoless double beta decay could answer the question regarding the Majorana or Dirac nature of neutrinos. The GERDA experiment utilizes HPGe detectors enriched with the isotope 76Ge to search for this process. Recently the GERDA collaboration has unblinded data of Phase I of the experiment. In order to further improve the sensitivity of the experiment, additionally to the coaxial detectors used, 30 BEGe detectors made from germanium enriched in 76Ge will be deployed in GERDA Phase II. BEGe detectors have superior PSD capability, thus the background can be further reduced. The liquid argon surrounding the detector array will be instrumented in order to reject background by detecting scintillation light induced in the liquid argon by radiation. After a short introduction the hardware preparations for GERDA Phase II as well as the processing and characterization of the 30 BEGe detectors are discussed.
Improvement of the GERDA Ge Detectors Energy Resolution by an Optimized Digital Signal Processing
G. Benato, V. D'Andrea, C. Cattadori, S. Riboldi
Physics Procedia 61 (2015) 673  
GERDA is a new generation experiment searching for neutrinoless double beta decay of 76Ge, operating at INFN Gran Sasso Laboratories (LNGS) since 2010. Coaxial and Broad Energy Germanium (BEGe) Detectors have been operated in liquid argon (LAr) in GERDA Phase I. In the framework of the second GERDA experimental phase, both the contacting technique, the connection to and the location of the front end readout devices are novel compared to those previously adopted, and several tests have been performed.
In this work, starting from considerations on the energy scale stability of the GERDA Phase I calibrations and physics data sets, an optimized pulse filtering method has been developed and applied to the Phase II pilot tests data sets, and to few GERDA Phase I data sets. In this contribution the detector performances in term of energy resolution and time stability are here presented.
The improvement of the energy resolution, compared to standard Gaussian shaping adopted for Phase I data analysis, is discussed and related to the optimized noise filtering capability. The result is an energy resolution better than 0.1% at 2.6 MeV for the BEGe detectors operated in the Phase II pilot tests and an improvement of the energy resolution in LAr of about 8% achieved on the GERDA Phase I calibration runs, compared to previous analysis algorithms.

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