Abstract
In searching for the neutrinoless double-beta decay of
76
Ge 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 the operation
experience of the 64 m
3
liquid argon cryostat and its associated
infrastructure. The discussion inludes the challenging safety issues
associated with the operation of a large cryostat in a water
tank.
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.
•ICP-MS analysis of germanium samples.•Purification of germanium crystals.•Use of high resolution ICP-MS.•Online check of purification system.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The radioisotope concentrations in stainless steel for the cryostat of the GERDA experiment at the Laboratori Nazionali del Gran Sasso (LNGS) have been measured. A total of 13 different stainless ...steel batches have been screened by low-level
γ
-ray spectrometry with germanium detectors located at two underground laboratories. The article reports the discovery of commonly available stainless steel with very low concentrations of primordial and cosmogenic radionuclides. The concentrations are in the range of 1
mBq/kg and below. Only the manmade
60Co-isotope is present in all samples with a higher concentration of around 19
mBq/kg. Further, deviations from secular equilibrium in the natural decay chains of U and Th have been observed in some cases.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The GERmanium Detector Array (GERDA) experiment searched for the lepton-number-violating neutrinoless double-β (0νββ) decay of ^{76}Ge, whose discovery would have far-reaching implications in ...cosmology and particle physics. By operating bare germanium diodes, enriched in ^{76}Ge, in an active liquid argon shield, GERDA achieved an unprecedently low background index of 5.2×10^{-4} counts/(keV kg yr) in the signal region and met the design goal to collect an exposure of 100 kg yr in a background-free regime. When combined with the result of Phase I, no signal is observed after 127.2 kg yr of total exposure. A limit on the half-life of 0νββ decay in ^{76}Ge is set at T_{1/2}>1.8×10^{26} yr at 90% C.L., which coincides with the sensitivity assuming no signal.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UL, UM
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 Demonstrator0νββ 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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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
HEROICA (Hades Experimental Research Of Intrinsic Crystal Appliances) is an infrastructure to characterize germanium detectors and has been designed and constructed at the HADES Underground Research ...Laboratory, located in Mol (Belgium). Thanks to the 223 m overburden of clay and sand, the muon flux is lowered by four orders of magnitude. This natural shield minimizes the exposure of radio-pure germanium material to cosmic radiation resulting in a significant suppression of cosmogenic activation in the germanium detectors. The project has been strongly motivated by a special production of germanium detectors for the GERDA experiment. GERDA, currently collecting data at the Laboratori Nazionali del Gran Sasso of INFN, is searching for the neutrinoless double beta decay of super(76)Ge. In the near future, GERDA will increase its mass and sensitivity by adding new Broad Energy Germanium (BEGe) detectors. The production of the BEGe detectors is done at Canberra in Olen (Belgium), located about 30 km from the underground test site. Therefore, HADES is used both for storage of the crystals over night, during diode production, and for the characterization measurements. A full quality control chain has been setup and tested on the first seven prototype detectors delivered by the manufacturer at the beginning of 2012. The screening capabilities demonstrate that the installed setup fulfills a fast and complete set of measurements on the diodes and it can be seen as a general test facility for the fast screening of high purity germanium detectors. The results are of major importance for a future massive production and characterization chain of germanium diodes foreseen for a possible next generation 1-tonne double beta decay experiment with super(76)Ge.
A discovery that neutrinos are Majorana fermions would have profound implications for particle physics and cosmology. The Majorana character of neutrinos would make possible the neutrinoless double-β ...(0νββ) decay, a matter-creating process without the balancing emission of antimatter. The GERDA Collaboration searches for the 0νββ decay of
Ge by operating bare germanium detectors in an active liquid argon shield. With a total exposure of 82.4 kg⋅year, we observe no signal and derive a lower half-life limit of
> 0.9 × 10
years (90% C.L.). Our
sensitivity, assuming no signal, is 1.1 × 10
years. Combining the latter with those from other 0νββ decay searches yields a sensitivity to the effective Majorana neutrino mass of 0.07 to 0.16 electron volts.
Many extensions of the Standard Model of particle physics explain the dominance of matter over antimatter in our Universe by neutrinos being their own antiparticles. This would imply the existence of ...neutrinoless double-β decay, which is an extremely rare lepton-number-violating radioactive decay process whose detection requires the utmost background suppression. Among the programmes that aim to detect this decay, the GERDA Collaboration is searching for neutrinoless double-β decay of
Ge by operating bare detectors, made of germanium with an enriched
Ge fraction, in liquid argon. After having completed Phase I of data taking, we have recently launched Phase II. Here we report that in GERDA Phase II we have achieved a background level of approximately 10
counts keV
kg
yr
. This implies that the experiment is background-free, even when increasing the exposure up to design level. This is achieved by use of an active veto system, superior germanium detector energy resolution and improved background recognition of our new detectors. No signal of neutrinoless double-β decay was found when Phase I and Phase II data were combined, and we deduce a lower-limit half-life of 5.3 × 10
years at the 90 per cent confidence level. Our half-life sensitivity of 4.0 × 10
years is competitive with the best experiments that use a substantially larger isotope mass. The potential of an essentially background-free search for neutrinoless double-β decay will facilitate a larger germanium experiment with sensitivity levels that will bring us closer to clarifying whether neutrinos are their own antiparticles.
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IJS, KISLJ, NUK, SBMB, UL, UM, UPUK
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