Germanium Detector Array: Difference between revisions

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{{Short description|Neutrinoless double beta decay experiment}}

{{refimprovemore citations needed|date=February 2016}}

The '''[[Germanium]] Detector Array''' (or '''GERDA''') experiment iswas searching for [[neutrinoless double beta decay]] (0vββ0νββ) 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 predictspredicted less than one event each year per kilogram of material, appearing as a narrow spike around the 0vββ0νββ Q-value (Q_ββ = 2039 keV) in the observed energy spectrum. This meansmeant b[[Radiation protection|ackgroundbackground shielding]] iswas 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 in 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 usesused high purity enriched [[Germanium|Ge]] crystal [[diodes]] ([[Semiconductor detector#Germanium detectors|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 iswas 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 detectdetected and excludeexcluded background [[muons]]. Pulse-shape discrimination (PSD) iswas 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 will increase the active mass to 38&nbsp;kg using 30 new broad energy germanium (BEGe) detectors. A magnitude reduction in background is planned to 10⁻³ counts/(keV·kg·yr) using cleaner materials. This will increase the half-life sensitivity to 10²⁶ years once 100&nbsp;kg·yr of data is taken and enable evaluation of possible ton-scale expansion.

Phase 2 will increaseincreased the active mass to 38&nbsp;kg using 30 new broad energy germanium (BEGe) detectors. A magnitude reduction in background iswas planned to 10⁻³ counts/(keV·kg·yr) using cleaner materials. This will increaseincreased the half-life sensitivity to 10²⁶ years once 100&nbsp;kg·yr of data iswas taken and enableenabled evaluation of possible ton-scale expansion.

== Results ==

Phase I collected data November 2011 to May 2013, with 21.6&nbsp;kg·yr exposure. No neutrinoless decays were observed, yielding a 0νββ 90% CL half-life limit of <math>T_{0 \nu \beta \beta} > 2.1 \cdot 10^{25} yr </math>. This limit cancould 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: m_vν < 400 meV.

The double beta decay (with two neutrinos) half-life was also measured: T_{2vββ2νββ} = 1.84·10²¹ yr.

Phase I collected data November 2011 to May 2013, with 21.6&nbsp;kg·yr exposure, obtaining a 0vββ 90% CL half-life limit of:

Phase II will havehad additional enriched Ge detectors and reduced background, raising the sensitivity about one order of magnitude.

<math>T_{0 \nu \beta \beta} > 2.1 \cdot 10^{25} yr </math>. This limit can 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: m_v < 400 meV.

Phase II (7 strings, 35.8&nbsp;kg of enriched detectors) was started in Dec 2015.<ref name=G2-07>{{cite conference |title=First results from GERDA Phase II |author=GERDA collaboration |author2=M.Agostini|display-authors=etal<!--|collaboration=GERDA collaboration--> |date=8 July 2016 |url=https://www.mpi-hd.mpg.de/gerda/public/2016/t16_neutrino_gerda_ma.pdf |conference=XXVII International Conference on Neutrino Physics and Astrophysics (Neutrino 2016) |conference-url=http://neutrino2016.iopconfs.org/programme |___location=London}}</ref>{{rp|10}}

The double beta decay half-life was also measured: T_2vββ = 1.84·10²¹ yr.

Preliminary results of Phase II have been published in Nature.<ref>{{citation |author=GERDA collaboration |author2=M.Agostini |display-authors=etal |periodical=Nature |title=Background-free search for neutrinoless double-β decay of ⁷⁶Ge with GERDA |volume=544 |issue=7648 |pages=47–52 |date=2017-04-05 |doi=10.1038/nature21717 |pmid=28382980 |arxiv=1703.00570 |bibcode=2017Natur.544...47A |s2cid=4456764}}</ref> 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&nbsp;kg·yr. Again no neutrinoless decays were observed, bringing the present limit on the half life to T_1/2{{nbsp}}>{{nbsp}}5.3·10²⁵ yr (90% C.L.).

Phase II will have additional enriched Ge detectors and reduced background, raising the sensitivity about one order.

As of 2018, the Phase II data-taking continued.

Phase II (7 strings, 35.8 kg of enriched detectors) was deployed in Dec 2015.<ref name=G2-07>[https://www.mpi-hd.mpg.de/gerda/public/2016/t16_neutrino_gerda_ma.pdf First results from GERDA Phase II. July 2016]</ref>{{rp|10}}

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"/>

Mid 2016 : First results from phase II released.<ref>[https://www.mpi-hd.mpg.de/gerda/public/index.html GERDA publications]</ref> confirming reduced background and (with only 10.8 kg·yr exposure) raising the lower bound on half life.<ref name=G2-07/>{{rp|22-23}}

== References ==

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== Publications ==

* {{cite journal |arxiv=1307.4720 |bibcode=2013PhRvL.111l2503A |doi=10.1103/PhysRevLett.111.122503 |pmid=24093254 |title=Results on Neutrinoless Double-β Decay of ⁷⁶Ge from Phase I of the GERDA Experiment |journal=[[Physical Review Letters]] |volume=111 |issue=12 |pages=122503 |date=19 September 2013 |author=GERDA collaboration, Agostini M. |display-authors=etal|url=http://pubman.mpdl.mpg.de/pubman/item/escidoc:1849853/component/escidoc:1849852/1307.4720.pdf }}

* {{cite journal |arxiv=13071212.47203210 |bibcode=2013JPhG...40c5110T 2013PhRvL.111l2503A|doi=10.11031088/PhysRevLett.111.122503|pmid=0954-3899/40/3/035110 24093254|title=Measurement Resultsof onthe Neutrinolesshalf-life of the Doubletwo-βneutrino Decaydouble beta decay of ⁷⁶Ge from Phase I ofwith the GERDA Experimentexperiment |journal= [[PhysicalJournal of ReviewPhysics LettersG]] |volume=40 111|issue=3 12|pages=035110 122503|yeardate=12 February 2013 |last1author=GERDA collaboration, Agostini|first1= M. |s2cid=119118050 |display-authors= etal}}

* {{cite journal |arxiv=1212.32104067 |bibcode= 2013JPhG2013EPJC...40c5110T73.2330A |doi=10.10881140/epjc/0954s10052-3899/40/3/035110013-2330-0 |doi-access=free |title=The MeasurementGERDA ofexperiment for the half-lifesearch of the two-neutrino double beta0νββ decay ofin ⁷⁶Ge with the GERDA experiment|journal= [[JournalEuropean ofPhysical PhysicsJournal GC]] |volume=73 40|issue= 3 |pages=2330 035110|yeardate=March 2013|last1= Agostini|first1author=GERDA Mcollaboration, Ackermann K.-H. |display-authors= etal}}

* {{cite journal |arxiv=12121703.406700570 |bibcode= 2013EPJC...73.2330A|doi=10.11401038/epjc/s10052-013-2330-0nature21717 |doi-accesspmid=free28382980 |title=Background-free The GERDA experimentsearch for theneutrinoless search of 0νββdouble-β decay inof ⁷⁶Ge with GERDA|journal= [[EuropeanNature Physical Journal C(journal)|Nature]] |volume=544 73|issue= 37648|pages=47–52 2330|yeardate=5 2013|last1=April 2017 Ackermann|first1author=GERDA K.-Hcollaboration, Agostini M. |display-authors= etal|bibcode=2017Natur.544...47A|s2cid=4456764 }}

== External links ==

* [http://www.mpi-hd.mpg.de/gerda/public/index.html GERDA Collaboration]

* [https://inspirehep.net/experiments/1108265 GERDA experiment] record on [[INSPIRE-HEP]]

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