Neutrino Ettore Majorana Observatory
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The Neutrino Ettore Majorana Observatory (NEMO experiment) is an international collaboration of scientists searching for neutrinoless double beta decay (0vββ). Observation of 0vββ would indicate neutrinos are Majorana particles and could be used to measure the neutrino mass. It is located in the Modane Underground Laboratory (LSM) in the Fréjus Road Tunnel. Data taking started in January 2003 and ended in January 2011. The NEMO-2 and NEMO-3 detectors produced measurements for double neutrino decays and limits for neutrinoless double-beta decay for a number of elements, such as molybdenum-100 and selenium-82. These double beta decay times are important contributions to understanding the nucleus and are needed inputs for neutrinoless decay studies, which constrain neutrino mass.
The NEMO collaboration remains active[1] and is constructing an improved SuperNEMO detector.
Experiment
The experiment has a cylindrical shape with 20 sectors that contain different isotopes in the form of thin foils with a total surface of about 20 m2. The main isotopes used for the neutrinoless double beta decay search are about 7 kg of enriched molybdenum-100 and about 1 kg of selenium-82. The experiment also contains smaller amounts of cadmium-116, neodymium-150, zirconium-96 and calcium-48 foils. Tellurium and copper foils are used for background measurements.
A tracking detector on each side of the foil detects electrons and positrons from the double beta decay. They are identified by their curvature in a magnetic field and particle energy is measured in a calorimeter. In 0vββ, the sum of the electron and positron energies will be the(Q value) released in double beta decay. For standard double beta decay the neutrinos, which cannot be observed directly, reduce the detected energy.
Results
Neutrinoless double beta decay (0vββ) has not been observed in 5 years of data taking and limits have been set for several isotopes.
NEMO-2 reported 0vββ limits for Majoron models of 100Mo, 116Cd, 82Se and 96Zr.[2]
NEMO-3 reported precision 2νββ half-lives for its 7 isotopes and 0vββ limits for 96Zr, 48Ca, 150Nd at Neutrino08.[3]
NEMO-3 reported 2νββ and more 0vββ limits at SUSY08.[4]
In 2014, NEMO-3 reported a 47 kg⋅y search for 0vββ of molybdenum-100 yielded T1/2 > 1.1×1024 years. This can be translated into an upper limit on the effective neutrino mass: mv < 0.3–0.9 eV, depending on the nuclear model.[5]
NEMO 2νββ Half-life Measurements
| Nuclide | Half-life, years |
|---|---|
| 48Ca | 4.4+0.5 −0.4 ± 0.4 ×1019 |
| 82Se | 9.6 ± 0.3 ± 1.0 ×1019 |
| 96Zr | 2.35 ± 0.14 ± 0.16 ×1019 |
| 116Cd | 2.8 ± 0.1 ± 0.3 ×1019 |
| 130Te | 7.6 ± 1.5(stat) ± 0.8(syst) ×1020 |
| 150Nd | 9.11+0.25 −0.22 ± 0.63 ×1018 |
| 100Mo | 7.11 ± 0.02(stat) ± 0.54(syst) ×1018 |
NEMO Highest 0vββ Decay Lower Limits
| Isotope | T1/2 (yr) | Neutrino mass limit (eV) |
|---|---|---|
| 82Se | 2.1×1023 | |
| 100Mo | 1.1×1024 | 0.9 |
| 116Cd | 1.6×1022 | |
| 96Zr | 8.6×1021 | 20.1 |
| 150Nd | 1.8×1022 | 6.3 |
| 48Ca | 1.3×1022 | 29.7 |
The 96Zr decay is particularly relevant because of its high Q and use in searches for time-dependence of the physical constants. Geochemical measurements of ZrSiO4 allow comparison of its historic and present rates.[6]
SuperNEMO
A next generation experiment, SuperNEMO, is under construction. It is based on technology used by the NEMO-3 experiment, but will be more than a factor of ten bigger.[7] The SuperNEMO detector will consist of 20 modules each containing approximately 5 kg of enriched double beta decay emitting isotope in the form of a thin foil. The installation of a first module (using selenium-82) in the LSM is under way, with data taking expected in the second half of 2015.[8]
References
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- ↑ http://arxiv.org/abs/0810.5497
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