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Papers by subgroups of the GERDA Collaboration 2015 - 2024
Cross sections and gamma cascades in 77Ge needed for background reduction
in 0νββ experiments on 76Ge
Peter Grabmayr
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Eur. Phys. J. A (2024) 60:115
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abstract
The compound nucleus model is employed to calculate neutron capture on 76Ge and the
consecutive γ decay. Total, inelastic and capture cross sections are compared to
experiments in order to restrict model parameters. The respective γ cascades are
analyzed for possible background suppression schemes in future 0νββ decay searches
with germanium detectors. Neutron energies in the range from 0.2 keV to 5 MeV are
investigated. The spin dependence of the γ cascades is proposed to be employed for
determining the spin of thermal resonances.
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Gamma cascades in gadolinium isotopes
Peter Grabmayr
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Eur. Phys. J. C (2023) 83:444
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abstract
The compound nucleus model is employed to calculate the γ decay after neutron
capture by the gadolinium isotopes 155Gd and 157Gd. The respective
γ cascades are analyzed for possible use in rare-event searches like
0νββ decay as neutron-veto for neutron energies from 0.1 keV to 10 MeV.
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Design and Performance of the GERDA Low-Background Cryostat for Operation in Water
Karl Tasso Knöpfle and Bernhard Schwingenheuer
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2022 JINST 17 P02038
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abstract
In searching for the neutrinoless double-beta decay of 76Ge the
GERmanium Detector Array (GERDA) experiment at the INFN Laboratori Nazionali del
Gran Sasso has achieved an unprecedented low background of well below
10-3 cts/(keV·kg·yr) in the region of interest.
It has taken advantage of the first realization of a novel shielding concept
based on a large cryostat filled with a liquid noble gas that is immersed in a
water tank. The germanium detectors are operated without encapsulation in liquid
argon. Argon and water shield the environmental background from the laboratory
and the cryostat construction materials to a negligible level. The same approach
has been adopted in the meantime by various experiments. This paper provides an
overview of the design and operating experience of the 64 m3 liquid argon
cryostat and its associated infrastructure. The discussion includes the
challenging safety issues associated with the operation of a large cryostat in a
water tank.
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In-situ measurement of the scintillation light attenuation in liquid argon in the GERDA Experiment
Nuno Barros, Alexander R. Domula, Björn Lehnert, Birgit Zatschler, Kai Zuber
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NIM A953 (2020) 163059
arXiv:1906.11589
50 days free access link
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abstract
The GERDA experiment searches for the neutrinoless double beta (0νββ) decay in 76Ge in order to probe whether the
neutrino is a Majorana particle and to shed light on the neutrino mass ordering. For investigating such a rare decay
it is necessary to minimize the background of the experiment. In Phase II of the GERDA experiment the scintillation
light of liquid argon (LAr) is used as an additional background veto. In order to estimate the efficiency of such a
LAr veto it has to be known how far the scintillation light can travel within the LAr. A dedicated setup was built to
measure the attenuation length of the scintillation light in the LAr in-situ within the cryostat of GERDA. The setup
is composed of a steel tube with a photomultiplier tube (PMT) at one side and a moveable 90Sr source at the other side
to measure the light intensity at different distances between source and PMT. Furthermore, a sophisticated simulation
was developed in order to determine the solid angle correction as well as the background for this measurement, both are
needed for the analysis. Additionally, a set of simulation data was generated to confirm the analysis which is performed
for the measured data afterwards and to find a combination of simulation parameters that matches the measured data.
The analysis results in an absorption length of 15.79±0.70 (stat) +1.4 resp -3.14 (syst) cm under the assumption
of a scattering length of 70 cm at 128 nm. The simulation matching best was produced with an absorption length of 18 cm
and a light yield of 2500 γ/MeV.
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Virtual depth by active background suppression: revisiting the cosmic muon induced background of Gerda Phase II
Christoph Wiesinger, Luciano Pandola, and Stefan Schönert
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Eur. Phys. J. C (2018) 78:597
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abstract
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.
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Pulse shape discrimination performance of Inverted Coaxial Ge detectors
A. Domula, M. Hult, Y. Kermaidic, G. Marissens, B. Schwingenheuer, T. Wester, K. Zuber
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NIMA 891 (2018) 106-110
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abstract
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.
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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
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Eur. Phys. J. C (2018) 78:15
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abstract
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.
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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
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Applied Rad. and Isotopes 125 (2017) 163
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abstract
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.
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Resistor-less charge sensitive amplifier for semiconductor detectors
K. Pelczar, K. Panas, and G. Zuzel
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NIMA 835 (2016) 142
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abstract
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.
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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
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abstract
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.
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Optimized digital filtering techniques for radiation detection with HPGe detectors
Marco Salathe and Thomas Kihm
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NIMA 808 (2016) 150
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abstract
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.
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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
| 2015 JINST 10 P12005 |
abstract
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.
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Results from Phase I of the GERDA Experiment
T. Wester
AIP Conf. Procs. 1686 (2015) 020026
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abstract
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.
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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
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abstract
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.
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GERDA: Results and perspectives
C.M. Cattadori
Nucl. and Particle Physics Procs. 265-266 (2015) 38
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abstract
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
T0ν1/2 > 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
T0ν1/2 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
T2ν1/2 = (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.
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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
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abstract
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.
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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
2015 JINST 10 P09009
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abstract
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.
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Double-beta decay with majoron emission in GERDA Phase I
S. Hemmer for the GERDA Collaboration
Eur. Phys. J. Plus 130 (2015) 139
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abstract
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.
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Phase II upgrade of the GERDA experiment for the search of neutrinoless double beta decay
B. Majorovits for the GERDA collaboration
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Physics Procedia 61 (2015) 254
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abstract
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.
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Improvement of the GERDA Ge Detectors Energy Resolution
by an Optimized Digital Signal Processing
G. Benato, V. D'Andrea, C. Cattadori, S. Riboldi
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Physics Procedia 61 (2015) 673
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abstract
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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