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46-576: After completing the GERDA experiment, the GERDA collaboration merged with MAJORANA -collaboration to build a new 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. The experience from GERDA led to the expectation that further background reduction

92-459: A 0νββ 90% CL half-life limit of T 0 ν β β > 2.1 ⋅ 10 25 y r {\displaystyle T_{0\nu \beta \beta }>2.1\cdot 10^{25}yr} . This limit could be combined with previous results, increasing it to 3·10 yr, disfavoring the Heidelberg-Moscow detection claim. A bound on the effective neutrino mass

138-575: 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 counts/(keV·kg·yr) using cleaner materials. This increased the half-life sensitivity to 10 years once 100 kg·yr of data was taken and enabled evaluation of possible ton-scale expansion. Phase I collected data November 2011 to May 2013, with 21.6 kg·yr exposure. No neutrinoless decays were observed, yielding

184-439: A mean lifetime over 10  yr (table below). In a typical double beta decay, two neutrons in the nucleus are converted to protons, and two electrons and two electron antineutrinos are emitted. The process can be thought as two simultaneous beta minus decays . In order for (double) beta decay to be possible, the final nucleus must have a larger binding energy than the original nucleus. For some nuclei, such as germanium-76 ,

230-654: A refined estimate by the same authors stated the half-life was 2.3 × 10  years. This half-life has been excluded at high confidence by other experiments, including in Ge by GERDA . As of 2017, the strongest limits on neutrinoless double beta decay have come from GERDA in Ge, CUORE in Te, and EXO-200 and KamLAND-Zen in Xe. For mass numbers with more than two beta-stable isobars, quadruple beta decay and its inverse, quadruple electron capture, have been proposed as alternatives to double beta decay in

276-561: A ton scale experiment. The experiment will use a mixture of detectors made with natural germanium and enriched germanium, allowing it to confirm or refute the controversial claim for 0νββ observation in Ge by Klapdor-Kleingrothaus et al. (Heidelberg-Moscow experiment). If low enough electronic noise is achieved the Demonstrator may also make a search for WIMPs and axions. The Majorana Demonstrator will proceed in three phases. A prototype cryostat containing 3 strings of non-enriched germanium

322-399: Is a lepton number violating process. In the simplest theoretical treatment, known as light neutrino exchange, a nucleon absorbs the neutrino emitted by another nucleon. The exchanged neutrinos are virtual particles . With only two electrons in the final state, the electrons' total kinetic energy would be approximately the binding energy difference of the initial and final nuclei, with

368-535: Is in commissioning. Two low-background cryostats with enriched detectors are planned, with a total of 40 kg germanium. Electroformed copper and lead bricks protect the cryostats. Polyethylene shields the setup and includes PMTs to act as a veto. Nitrogen flushing removes trace radon. P-type point contact (PPC) germanium detectors are used. This style of detector was chosen for many reasons, but chiefly because PPC detectors allow efficient discrimination of multiply scattering gamma backgrounds. This results from

414-423: Is possible. These theoretical decay branches have not been observed. There are 35 naturally occurring isotopes capable of double beta decay. In practice, the decay can be observed when the single beta decay is forbidden by energy conservation. This happens for elements with an even atomic number and even neutron number , which are more stable due to spin -coupling. When single beta decay or alpha decay also occur,

460-422: Is the lightest observationally stable nuclide whose decay is energetically possible. If the neutrino is a Majorana particle (i.e., the antineutrino and the neutrino are actually the same particle), and at least one type of neutrino has non-zero mass (which has been established by the neutrino oscillation experiments), then it is possible for neutrinoless double beta decay to occur. Neutrinoless double beta decay

506-430: Is the nuclear matrix element, and m ββ is the effective Majorana mass of the electron neutrino. In the context of light Majorana neutrino exchange, m ββ is given by m β β = ∑ i = 1 3 m i U e i 2 , {\displaystyle m_{\beta \beta }=\sum _{i=1}^{3}m_{i}U_{ei}^{2},} where m i are

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552-862: The LEGEND experiment. The enriched germanium detectors operated since June 2015 at the Sanford Underground Research Facility in South Dakota . In March 2021, the enriched germanium detectors were removed from the experiment apparatus and shipped to LNGS in Italy to be used in LEGEND-200 experiment. However, the Majorana Demonstrator experimental apparatus remains operational and is planned to continue operation past March 2021 using germanium detectors made of natural (non-enriched) germanium (i.e. only

598-604: The Sanford Underground Laboratory in Lead, South Dakota . Following the Demonstrator, the collaboration intends to merge with the GERDA collaboration to build a much larger experiment called LEGEND . The goal of the project is to search for 0νββ decay in Ge using HPGe detectors. Observation of 0νββ would establish that the neutrino is a Majorana particle and demonstrate violation of lepton number conservation , validating

644-1159: The isobar one atomic number higher ( arsenic-76 ) has a smaller binding energy, preventing single beta decay. However, the isobar with atomic number two higher, selenium-76 , has a larger binding energy, so double beta decay is allowed. The emission spectrum of the two electrons can be computed in a similar way to beta emission spectrum using Fermi's golden rule . The differential rate is given by d N ( T 1 , T 2 , cos ⁡ θ ) d T 1 d T 2 d cos ⁡ θ = F ( Z , T 1 ) F ( Z , T 2 ) w 1 p 1 w 2 p 2 ( Q − T 1 − T 2 ) 5 ( 1 − v 1 v 2 cos ⁡ θ ) {\displaystyle {\frac {dN(T_{1},T_{2},\cos \theta )}{dT_{1}dT_{2}d\cos \theta }}=F(Z,T_{1})F(Z,T_{2})w_{1}p_{1}w_{2}p_{2}(Q-T_{1}-T_{2})^{5}(1-v_{1}v_{2}\cos \theta )} where

690-570: The neutrino masses and the U ei are elements of the Pontecorvo–Maki–Nakagawa–Sakata (PMNS) matrix . Therefore, observing neutrinoless double beta decay, in addition to confirming the Majorana neutrino nature, can give information on the absolute neutrino mass scale and Majorana phases in the PMNS matrix, subject to interpretation through theoretical models of the nucleus, which determine

736-410: The seesaw mechanism as the explanation for the neutrino mass scale. It would also place constraints on the absolute neutrino mass. The principal goal of the Majorana Demonstrator is to demonstrate the feasibility of achieving the background required in a ton-scale experiment. This corresponds to 4 counts/tonne/yr in a 4 keV window around the 0νββ Q value of 2039 keV, which scales to 1 count/ton/yr in

782-472: The atom to move closer to the optimal ratio of protons and neutrons. As a result of this transformation, the nucleus emits two detectable beta particles , which are electrons or positrons . The literature distinguishes between two types of double beta decay: ordinary double beta decay and neutrinoless double beta decay. In ordinary double beta decay, which has been observed in several isotopes, two electrons and two electron antineutrinos are emitted from

828-478: The case Gd) nuclides with A ≤ 260 are theoretically capable of double electron capture, where red are isotopes that have a double-electron capture rate measured and black have yet to be measured experimentally: Ar, Ca, Cr, Fe, Ni, Zn, Se, Kr , Sr, Mo, Ru, Pd, Cd, Cd, Sn, Te, Xe , Xe, Ba , Ba, Ce, Ce, Sm, Gd, Gd, Gd, Dy, Dy, Dy, Er, Er, Yb, Hf, W, Os, Pt, Hg, Rn, Rn, Ra, Th, U, Pu, Cm, Fm, and No. In particular, Ar

874-543: The cases Ca, Zr, and Rn ) nuclides with A ≤ 260 are theoretically capable of double beta decay, where red are isotopes that have a double-beta rate measured experimentally and black have yet to be measured experimentally: Ca, Ca , Zn, Ge , Se, Se , Kr, Zr, Zr , Mo, Mo , Ru, Pd, Cd, Cd , Sn, Sn, Te , Te , Xe, Xe , Ce, Nd, Nd, Nd , Sm, Gd, Er, Yb, W, Os, Pt, Hg, Po, Rn, Rn, Ra, Th, U , Pu, Cm, Cf, Cf, and Fm. The following known beta-stable (or almost beta-stable in

920-440: The contacts was explored. No signal was seen, and the project could be a competitive search for low mass WIMPs. The collection of data started June 2015. The construction completed with final configuration taking data since spring 2017. First results were announced October 2017. Data collection continued as of 2018. On 3 March 2021, the Majorana Demonstrator stopped operation with enriched germanium detectors, in preparation for

966-484: 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. MAJORANA The MAJORANA project (styled Majorana ) is an international effort to search for neutrinoless double-beta (0νββ) decay in Ge . The project builds upon

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1012-531: The decaying nucleus. In neutrinoless double beta decay , a hypothesized process that has never been observed, only electrons would be emitted. The idea of double beta decay was first proposed by Maria Goeppert Mayer in 1935. In 1937, Ettore Majorana demonstrated that all results of beta decay theory remain unchanged if the neutrino were its own antiparticle, now known as a Majorana particle . In 1939, Wendell H. Furry proposed that if neutrinos are Majorana particles, then double beta decay can proceed without

1058-545: The double beta decay half-life of Te was measured by geochemical methods to be 1.4×10  years, reasonably close to the modern value. This involved detecting the concentration in minerals of the xenon produced by the decay. In 1956, after the V − A nature of weak interactions was established, it became clear that the half-life of neutrinoless double beta decay would significantly exceed that of ordinary double beta decay. Despite significant progress in experimental techniques in 1960–1970s, double beta decay

1104-735: The double beta decay rate is generally too low to observe. However, the double beta decay of U (also an alpha emitter) has been measured radiochemically. Two other nuclides in which double beta decay has been observed, Ca and Zr , can also theoretically single beta decay, but this decay is extremely suppressed and has never been observed. Similar suppression of energetically barely possible single beta decay occurs for Gd and Rn, but both these nuclides are rather short-lived alpha emitters. Fourteen isotopes have been experimentally observed undergoing two-neutrino double beta decay (β β ) or double electron capture (εε). The table below contains nuclides with

1150-520: The emission of any neutrinos, via the process now called neutrinoless double beta decay . It is not yet known whether the neutrino is a Majorana particle, and, relatedly, whether neutrinoless double beta decay exists in nature. As parity violation in weak interactions would not be discovered until 1956, earlier calculations showed that neutrinoless double beta decay should be much more likely to occur than ordinary double beta decay, if neutrinos were Majorana particles. The predicted half-lives were on

1196-480: The enriched germanium detectors ceased their operation and were removed from the Majorana Demonstrator in March 2021). Double beta decay#Neutrinoless double beta decay experiments 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

1242-455: The experiment reported lower limit for the 0νββ half-life in Ge-76 of T 0 ν β β > 1.8 ⋅ 10 26 y r {\displaystyle T_{0\nu \beta \beta }>1.8\cdot 10^{26}yr} . The reported final lower limit agreed with the expected value for the sensitivity of the experiment, and was the most stringent value for

1288-665: The isobars with the greatest energy excess. These decays are energetically possible in eight nuclei, though partial half-lives compared to single or double beta decay are predicted to be very long; hence, quadruple beta decay is unlikely to be observed. The seven candidate nuclei for quadruple beta decay include Zr, Xe, and Nd capable of quadruple beta-minus decay, and Xe, Ba, Gd, and Dy capable of quadruple beta-plus decay or electron capture (though Gd and Dy are non-primordial alpha-emitters with geologically short half-lives). In theory, quadruple beta decay may be experimentally observable in three of these nuclei – Zr, Xe, and Nd – with

1334-479: The latest experimentally measured half-lives, as of December 2016, except for Xe (for which double electron capture was first observed in 2019). Where two uncertainties are specified, the first one is statistical uncertainty and the second is systematic. Searches for double beta decay in isotopes that present significantly greater experimental challenges are ongoing. One such isotope is Xe . The following known beta-stable (or almost beta-stable in

1380-406: The most promising candidate being Nd. Triple beta-minus decay is also possible for Ca, Zr, and Nd; triple beta-plus decay or electron capture is also possible for Gd and Dy. Moreover, such a decay mode could also be neutrinoless in physics beyond the standard model. Neutrinoless quadruple beta decay would violate lepton number in 4 units, as opposed to a lepton number breaking of two units in

1426-534: The neutrinoless process, raising the half-life lower bound to approximately 10  years. Geochemical experiments continued through the 1990s, producing positive results for several isotopes. Double beta decay is the rarest known kind of radioactive decay; as of 2019 it has been observed in only 14 isotopes (including double electron capture in Ba observed in 2001, Kr observed in 2013, and Xe observed in 2019), and all have

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1472-612: The nuclear matrix elements, and models of the decay. The observation of neutrinoless double beta decay would require that at least one neutrino is a Majorana particle , irrespective of whether the process is engendered by neutrino exchange. Numerous experiments have searched for neutrinoless double beta decay. The best-performing experiments have a high mass of the decaying isotope and low backgrounds, with some experiments able to perform particle discrimination and electron tracking. In order to remove backgrounds from cosmic rays, most experiments are located in underground laboratories around

1518-432: The nuclear recoil accounting for the rest. Because of momentum conservation , electrons are generally emitted back-to-back. The decay rate for this process is given by Γ = G | M | 2 | m β β | 2 , {\displaystyle \Gamma =G|M|^{2}|m_{\beta \beta }|^{2},} where G is the two-body phase-space factor, M

1564-434: The order of 10 ~10  years. Efforts to observe the process in laboratory date back to at least 1948 when E.L. Fireman made the first attempt to directly measure the half-life of the Sn isotope with a Geiger counter . Radiometric experiments through about 1960 produced negative results or false positives, not confirmed by later experiments. In 1950, for the first time

1610-473: The process occurs as conversion of two protons to neutrons, emitting two electron neutrinos and absorbing two orbital electrons (double electron capture). If the mass difference between the parent and daughter atoms is more than 1.022 MeV/ c (two electron masses), another decay is accessible, capture of one orbital electron and emission of one positron . When the mass difference is more than 2.044 MeV/ c (four electron masses), emission of two positrons

1656-507: The slightly modified infrastructure of GERDA with the start of data taking planned for 2021. The experiment used high purity enriched Ge crystal diodes ( HPGe ) as a beta decay source and particle detector . 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 cryostat lined with copper and surrounded by an ultra-pure water tank. PMTs in

1702-452: The subscripts refer to each electron, T is kinetic energy, w is total energy, F ( Z , T ) is the Fermi function with Z the charge of the final-state nucleus, p is momentum, v is velocity in units of c , cos ⁡ θ {\displaystyle \cos \theta } is the angle between the electrons, and Q is the Q value of the decay. For some nuclei,

1748-602: The thick outer n-type contact. At Dec 2014 the Majorana Demonstrator was under construction at the Sanford Underground Laboratory in Lead, South Dakota. The first module was expected in early 2015 with full operation expected in late 2015. MALBEK was operated 2011–12 at KURF (Kimballton Underground Research Facility) in Virginia as a WIMP detector to evaluate the broad energy (BEGe) PPCs. The background and behavior of

1794-412: 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 kg germanium detector was used by a Milano group in the first 0νββ decay search with

1840-411: The weighting potential being strongly peaked close to the small electrode, meaning that as charge drifts towards the electrode there is a high probability of seeing distinct signals from each energy deposition, thus being able to reject events these signals. Other advantages include the low capacitance due to the small contacts, reducing electronic noise and thresholds; and shielding surface alpha decays by

1886-639: The work of previous experiments, notably those performed by the Heidelberg–Moscow and IGEX collaborations, which used high-purity germanium (HPGe) detectors , to study neutrinoless double-beta decay. The first stage of the project is the Majorana Demonstrator ( MJD ), designed to demonstrate the technique and evaluate a ton-scale experiment. Cryostats housing up to 40 kg of natural and enriched germanium detectors are being deployed in low-background vacuum cryostats, underground (1,480 m) at

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1932-419: The world. Recent and proposed experiments include: While some experiments have claimed a discovery of neutrinoless double beta decay, modern searches have found no evidence for the decay. Some members of the Heidelberg-Moscow collaboration claimed a detection of neutrinoless beta decay in Ge in 2001. This claim was criticized by outside physicists as well as other members of the collaboration. In 2006,

1978-468: Was 0.7·10 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 T 1/2   >   5.3·10 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

2024-547: Was also reported: m ν < 400 meV. The double beta decay (with two neutrinos) half-life was also measured: T 2νββ = 1.84·10 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

2070-431: 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 28 y r {\displaystyle 10^{28}yr} . In a first phase, it planned to deploy a mass of 200 kg of enriched germanium detectors in

2116-600: Was not observed in a laboratory until the 1980s. Experiments had only been able to establish the lower bound for the half-life – about 10  years. At the same time, geochemical experiments detected the double beta decay of Se and Te . Double beta decay was first observed in a laboratory in 1987 by the group of Michael Moe at UC Irvine in Se . Since then, many experiments have observed ordinary double beta decay in other isotopes. None of those experiments have produced positive results for

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