Germanium Detector Array: Difference between revisions

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The '''[[Germanium]] Detector Array''' (or '''GERDA''') experiment was searching for [[neutrinoless double beta decay]] (0νββ) in Ge-76 at the underground [[Laboratori Nazionali del Gran Sasso]] (LNGS). [[Neutrinoless beta decay]] 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 (Q_ββ = 2039 keV) in the observed energy spectrum. This meant [[Radiation protection|background shielding]] was required to detect any rare decays. The [[Laboratori Nazionali del Gran Sasso|LNGS facility]] has 1400 meters of rock [[overburden]], equivalent to 3000 meters of water shielding, reducing [[Cosmic ray|cosmic radiation]] [[Background radiation|background]]. The GERDA experiment was operated from 2011 onwards at LNGS.<ref name="gerdafinal"/>

After completing the GERDA experiment, the GERDA collaboration merged with [[MAJORANA]]-collaboration to build a new experiment [[LEGEND (experiment)|LEGEND]].

GERDA reported its final results onin December 2020 in the [[Physical Review Letters]]. The experiment reached all the goals that it set to itself, but no detection of any 0νββ events was made.<ref name="gerdafinal">{{Cite web|url=https://www.appec.org/news/another-milestone-in-the-search-for-neutrinoless-double-beta-decay-final-results-of-gerda|title = Another milestone in the search for neutrinoless double-beta decay – Final results of GERDA » APPEC}}</ref>

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 <math>10^{28} yr</math>. In a first phase, it planned to deploy a mass of 200 &nbsp;kg of enriched germanium detectors in the slightly modified infrastructure of GERDA with the start of data taking planned for 2021.<ref name="gerdafinal"/>

== Design ==

The experiment used high purity enriched [[Germanium|Ge]] crystal [[diodes]] ([[Semiconductor detector#Germanium detectordetectors|HPGe]]) as a beta decay source and [[particle detector]]. The detectors from the HdM ([[Heidelberg-Moscow experiment|Heidelberg-Moscow]]<ref name="article2021">{{Cite journal|doi = 10.1140/epjc/s10052-021-09403-2|title = Calibration of the Gerda experiment|year = 2021|last1 = Agostini|first1 = M.|last2 = Araujo|first2 = G.|last3 = Bakalyarov|first3 = A. M.|last4 = Balata|first4 = M.|last5 = Barabanov|first5 = I.|last6 = Baudis|first6 = L.|last7 = Bauer|first7 = C.|last8 = Bellotti|first8 = E.|last9 = Belogurov|first9 = S.|last10 = Bettini|first10 = A.|last11 = Bezrukov|first11 = L.|last12 = Biancacci|first12 = V.|last13 = Bossio|first13 = E.|last14 = Bothe|first14 = V.|last15 = Brudanin|first15 = V.|last16 = Brugnera|first16 = R.|last17 = Caldwell|first17 = A.|last18 = Cattadori|first18 = C.|last19 = Chernogorov|first19 = A.|last20 = Comellato|first20 = T.|last21 = d'Andrea|first21 = V.|last22 = Demidova|first22 = E. V.|last23 = Marco|first23 = N. Di|last24 = Doroshkevich|first24 = E.|last25 = Fischer|first25 = F.|last26 = Fomina|first26 = M.|last27 = Gangapshev|first27 = A.|last28 = Garfagnini|first28 = A.|last29 = Gooch|first29 = C.|last30 = Grabmayr|first30 = P.|journal = The European Physical Journal C|volume = 81|issue = 8|page = 682|pmid = 34776783|pmc = 8550656|bibcode = 2021EPJC...81..682A|display-authors = 1}}</ref>) and Igex[[IGEX]]<ref name="article2021"/> experiments were reprocessed and used in phase 1. The detector array was suspended in a liquid [[argon]] [[cryostat]] lined with copper and surrounded by an ultra-pure water tank. [[Photomultiplier|PMTs]] in the water tank and plastic [[scintillators]] above detected and excluded background [[muons]]. 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 &nbsp;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.<ref name="gerdafinal"/>

Phase 2 increased the active mass to 38&nbsp;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&nbsp;kg·yr of data was taken and enabled evaluation of possible ton-scale expansion.

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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 <math>T_{0 \nu \beta \beta} > 1.8 \cdot 10^{26} yr </math>. 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 &nbsp;kg, with very high germanium-76 enrichment percent.<ref name="gerdafinal"/>

== References ==