Germanium Detector Array
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Germanium Germanium is a chemical element with the symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid in the carbon group that is chemically similar to its group neighbo ...
Detector Array (or GERDA) experiment was searching for neutrinoless double beta decay (0νββ) in Ge-76 at the underground
Laboratori Nazionali del Gran Sasso Laboratori Nazionali del Gran Sasso (LNGS) is the largest underground research center in the world. Situated below Gran Sasso mountain in Italy, it is well known for particle physics research by the INFN. In addition to a surface portion of th ...
(LNGS).
Neutrinoless beta decay In nuclear physics, double beta decay is a type of radioactive decay in which two neutrons are simultaneously transformed into two protons, or vice versa, inside an atomic nucleus. As in single beta decay, this process allows the atom to move clos ...
is expected to be a very rare process if it occurs. The collaboration predicted less than one event each year per kilogram of material, appearing as a narrow spike around the 0νββ Q-value (Qββ = 2039 keV) in the observed energy spectrum. This meant background shielding was required to detect any rare decays. The LNGS facility has 1400 meters of rock
overburden In mining, overburden (also called waste or spoil) is the material that lies above an area that lends itself to economical exploitation, such as the rock, soil, and ecosystem that lies above a coal seam or ore body. Overburden is distinct from t ...
, equivalent to 3000 meters of water shielding, reducing
cosmic radiation Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our ow ...
background. The GERDA experiment was operated from 2011 onwards at LNGS. After completing the GERDA experiment, the GERDA collaboration merged with
MAJORANA Majorana may refer to: * Majorana (surname), an Italian surname * MAJORANA, a physics search for neutrinoless double-beta decay * Majorana fermion * Majorana Prize, a prize for theoretical and mathematical physics See also * Maiorana, a surna ...
-collaboration to build a new experiment
LEGEND A legend is a genre of folklore that consists of a narrative featuring human actions, believed or perceived, both by teller and listeners, to have taken place in human history. Narratives in this genre may demonstrate human values, and possess ...
. GERDA reported its final results in December 2020 in the
Physical Review Letters ''Physical Review Letters'' (''PRL''), established in 1958, is a peer-reviewed, scientific journal that is published 52 times per year by the American Physical Society. As also confirmed by various measurement standards, which include the '' Jou ...
. The experiment reached all the goals that it set to itself, but no detection of any 0νββ events was made. The experience from GERDA led to the expectation that further background reduction was in reach so that a background-free experiment with an even larger source strength, respectively exposure, became possible. The LEGEND collaboration, continuing GERDA's work, was aiming at increasing the sensitivity to the half-life of 0νββ decay up to 10^ yr. In a first phase, it planned to deploy a mass of 200 kg of enriched germanium detectors in the slightly modified infrastructure of GERDA with the start of data taking planned for 2021.


Design

The experiment used high purity enriched Ge crystal diodes ( HPGe) as a beta decay source and
particle detector In experimental and applied particle physics, nuclear physics, and nuclear engineering, a particle detector, also known as a radiation detector, is a device used to detect, track, and/or identify ionizing particles, such as those produced by ...
. The detectors from the HdM ( Heidelberg-Moscow) and IGEX experiments were reprocessed and used in phase 1. The detector array was suspended in a liquid
argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as a ...
cryostat A cryostat (from ''cryo'' meaning cold and ''stat'' meaning stable) is a device used to maintain low cryogenic temperatures of samples or devices mounted within the cryostat. Low temperatures may be maintained within a cryostat by using various r ...
lined with copper and surrounded by an ultra-pure water tank. PMTs in the water tank and plastic scintillators above detected and excluded background
muons A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As wit ...
. Pulse-shape discrimination (PSD) was applied as a cut to discriminate between particle types. GERDA followed in the footsteps of other 0νββ experiments using germanium; already more than 50 years ago (that is, around 1970), a 0.1 kg germanium detector was used by a Milano group in the first 0νββ decay search with a germanium detector. Since then, the sensitivity had been increased by a factor of one million. Phase 2 increased the active mass to 38 kg using 30 new broad energy germanium (BEGe) detectors. A magnitude reduction in background was planned to 10−3 counts/(keV·kg·yr) using cleaner materials. This increased the half-life sensitivity to 1026 years once 100 kg·yr of data was taken and enabled evaluation of possible ton-scale expansion.


Results

Phase I collected data November 2011 to May 2013, with 21.6 kg·yr exposure. No neutrinoless decays were observed, yielding a 0νββ 90% CL half-life limit of T_ > 2.1 \cdot 10^ yr . This limit could be combined with previous results, increasing it to 3·1025 yr, disfavoring the Heidelberg-Moscow detection claim. A bound on the effective neutrino mass was also reported: mν < 400 meV. The double beta decay (with two neutrinos) half-life was also measured: T2νββ = 1.84·1021 yr. Phase II had additional enriched Ge detectors and reduced background, raising the sensitivity about one order of magnitude. Phase II (7 strings, 35.8 kg of enriched detectors) was started in Dec 2015. Preliminary results of Phase II have been published in Nature. The background index for BEGe detectors was 0.7·10−3 counts/(keV·kg·yr), which translated to less than one count in the signal region after an exposure of 100 kg·yr. Again no neutrinoless decays were observed, bringing the present limit on the half life to T1/2>5.3·1025 yr (90% C.L.). As of 2018, the Phase II data-taking continued. In December 2020, the final results of GERDA were reported. There was no detection of 0νββ, and the experiment reported lower limit for the 0νββ half-life in Ge-76 of T_ > 1.8 \cdot 10^ yr . The reported final lower limit agreed with the expected value for the sensitivity of the experiment, and was the most stringent value for the decay of any 0νββ isotope ever measured. Also the background event rate of GERDA was cutting-edge level in the field. In its final phase GERDA deployed 41 germanium detectors with a total mass of 44.2 kg, with very high germanium-76 enrichment percent.


References


Publications

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External links


GERDA Collaboration

GERDA experiment
record on
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