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45-459: The Enriched Xenon Observatory ( EXO ) is a particle physics experiment searching for neutrinoless double beta decay of xenon -136 at WIPP near Carlsbad, New Mexico, U.S. Neutrinoless double beta decay (0νββ) detection would prove the Majorana nature of neutrinos and impact the neutrino mass values and ordering. These are important open topics in particle physics . EXO currently has

90-498: A 200-kilogram xenon liquid time projection chamber ( EXO-200 ) with R&D efforts on a ton-scale experiment ( nEXO ). Xenon double beta decay was detected and limits have been set for 0νββ. EXO measures the rate of neutrinoless decay events above the expected background of similar signals, to find or limit the double beta decay half-life, which relates to the effective neutrino mass using nuclear matrix elements. A limit on effective neutrino mass below 0.01 eV would determine

135-475: A 5000 kg TPC can improve the background by xenon self-shielding and better electronics. Diameter would be increased to 130 cm and a water tank would be added as shielding and muon veto. This is much larger than the attenuation length for gamma rays. Radiopure copper for nEXO has been completed. It is planned for installation in the SNOLAB "Cryopit". An Oct. 2017 paper details the experiment and discusses

180-435: A cryostat also protect the setup. The neutrinoless decays would appear as narrow spike in the energy spectrum around the xenon Q-value (Q ββ = 2457.8 keV), which is fairly high and above most gamma decays. EXO-200 was designed with a goal of less than 40 events per year within two standard deviations of expected decay energy. This background was achieved by selecting and screening all materials for radiopurity. Originally

225-444: A daughter barium ion, while backgrounds, such as radioactive impurities or neutrons, will not. Requiring a barium ion at the location of an event eliminates all backgrounds. Tagging of a single ion of barium has been demonstrated and progress has been made on a method for extracting ions out of the liquid xenon. A freezing probe method has been demonstrated, and gaseous tagging is also being developed. The 2014 EXO-200 paper indicated

270-439: 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 ,

315-663: 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

360-399: 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

405-477: Is a scintillator , so decay particles produce prompt light which is detected by avalanche photodiodes , providing the event time. A large electric field drives ionization electrons to wires for collection. The time between the light and first collection determines the z coordinate of the event, while a grid of wires determines the radial and angular coordinates. The background from earth radioactivity(Th/U) and Xe contamination led to ≈2×10 counts/(keV·kg·yr) in

450-500: Is a Canadian underground science laboratory specializing in neutrino and dark matter physics. Located 2 km below the surface in Vale 's Creighton nickel mine near Sudbury , Ontario , SNOLAB is an expansion of the existing facilities constructed for the original Sudbury Neutrino Observatory (SNO) solar neutrino experiment. SNOLAB is the world's deepest operational clean room facility. Although accessed through an active mine,

495-576: Is also host to biological experiments in an underground environment. The Sudbury Neutrino Observatory was the world's deepest underground experiment since the Kolar Gold Fields experiments ended with the closing of that mine in 1992. Many research collaborations were, and still are, interested in conducting experiments in the 6000 MWE location. In 2002, funding was approved by the Canada Foundation for Innovation to expand

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540-423: 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,

585-422: 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

630-430: 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

675-1159: 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

720-579: 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

765-494: The June issue of Nature reduced the limits on half-life to 1.1×10 yr, and mass to 450 meV. This was used to confirm the power of the design and validate the proposed expansion. Additional running for two years was taken. EXO-200 has performed two scientific operations, Phase I (2011-2014) and after upgrades, Phase II (2016 - 2018) for a total exposure of 234.1 kg·yr. No evidence of neutrinoless double beta decay has been found in

810-614: The SNO facilities into a general-purpose laboratory, and more funding was received in 2007 and 2008. Construction of the major laboratory space was completed in 2009, with the entire lab entering operation as a 'clean' space in March 2011. SNOLAB is the world's deepest underground laboratory, tied with the China Jinping Underground Laboratory since 2011. Although CJPL has more rock (2.4 km) above it,

855-472: 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

900-478: 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

945-493: The case of neutrinoless double beta decay. Therefore, there is no 'black-box theorem' and neutrinos could be Dirac particles while allowing these type of processes. In particular, if neutrinoless quadruple beta decay is found before neutrinoless double beta decay then the expectation is that neutrinos will be Dirac particles. So far, searches for triple and quadruple beta decay in Nd have remained unsuccessful. SNOLAB SNOLAB

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990-543: 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

1035-482: The combined Phase I and II data, giving the lower bound of 3.5 ⋅ 10 25 {\displaystyle 3.5\cdot 10^{25}} years for the half-life and upper mass of 239 meV. Phase II was the final operation of EXO-200. A ton-scale experiment, nEXO ("next EXO"), must overcome many backgrounds. The EXO collaboration is exploring many possibilities to do so, including barium tagging in liquid xenon. Any double beta decay event will leave behind

1080-531: 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

1125-479: The detector. Energy resolution near Q ββ of 1.53% was achieved. In August 2011, EXO-200 was the first experiment to observe double beta decay of Xe, with a half life of 2.11×10 years. This is the slowest directly observed process. An improved half life of 2.165 ±0.016(stat) ±0.059(sys) × 10 years was published in 2014. EXO set a limit on neutrinoless beta decay of 1.6×10 years in 2012. A revised analysis of run 2 data with 100 kg·yr exposure, reported in

1170-545: 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

1215-735: 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

1260-411: The effective depth for science purposes is determined by the cosmic ray muon flux, and CJPL's mountain location admits more muons from the side than SNOLAB's flat overburden . The measured muon fluxes are 0.27 μ/m²/day ( 3.1 × 10  μ/cm²/s ) at SNOLAB, and 0.305 ± 0.020 μ/m²/day ( (3.53 ± 0.23) × 10  μ/cm²/s ) at CJPL, tied to within the measurement uncertainty. (For comparison,

1305-520: 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

1350-665: 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

1395-558: The laboratory proper is maintained as a class-2000 cleanroom , with very low levels of dust and background radiation . SNOLAB's 2070 m (6800 feet) of overburden rock provides 6010 metre water equivalent (MWE) shielding from cosmic rays, providing a low-background environment for experiments requiring high sensitivities and extremely low counting rates. The combination of great depth and cleanliness that SNOLAB affords allows extremely rare interactions and weak processes to be studied. In addition to neutrino and dark matter physics, SNOLAB

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1440-479: 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

1485-406: 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

1530-421: The neutrino mass order. The effective neutrino mass is dependent on the lightest neutrino mass in such a way that that bound indicates the normal mass hierarchy. The expected rate of 0νββ events is very low, so background radiation is a significant problem. WIPP has 650 metres (2,130 ft) of rock overburden—equivalent to 1,600 metres (5,200 ft) of water—to screen incoming cosmic rays. Lead shielding and

1575-537: 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

1620-612: 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

1665-432: 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

1710-437: 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

1755-473: 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

1800-482: The rate on the surface, at sea level, is about 15 million μ/m²/day.) CJPL does have the advantage of fewer radioisotopes in the surrounding rock. As of November 2019 , SNOLAB hosts the following experiments : Additional planned experiments have requested laboratory space such as the next-generation nEXO , and the LEGEND-1000 searches for neutrinoless double beta decay . There are also plans for

1845-593: The sensitivity and the discovery potential of nEXO for neutrinoless double beta decay. Details on the ionization readout of the TPC have also been published. The pre-Conceptual Design Report (pCDR) for nEXO was published in 2018. The planned location is SNOLAB , Canada. Double 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

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1890-456: 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,

1935-522: The vessel was to be made of Teflon, but the final design of the vessel uses thin, ultra-pure copper. EXO-200 was relocated from Stanford to WIPP in the summer of 2007. Assembly and commissioning continued until the end of 2009 with data taking beginning in May 2011. Calibration was done using Th, Cs, and Co gamma sources. The prototype EXO-200 uses a copper cylindrical time projection chamber filled with 150 kilograms (331 lb) of pure liquid xenon. Xenon

1980-487: 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,

2025-603: 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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