Neutrinoless Double Beta Decay




The process of double beta decay involves two protons transforming into two neutrons at the same time (or viceversa) inside the nucleus of an atom. When this occurs, two beta particles (electrons or positrons) are emitted. So far, this process has been observed to always be accompanied by the emission of two neutrinos or two antineutrinos, but it is hypothesized that the double beta decay can occur without them. This is what is referred to as neutrinoless double beta decay. It has never been observed, but through various experiment it has been found that the lower limit for its half-life is ~1025 years.

In order to find evidence for these decays, very low background is required, as well as a substantial amount of radioactive sources which undergo regular double beta decay. If the elusive process is ever observed, it would make it possible to measure the effective mass ofneutrinos, and it would be the first report of lepton number ever being violated. The violation of lepton number leads to new physics which help explain why there is more matter than antimatter in the universe.




The NEMO collaboration began studying double beta decay in the early 1990s through the development of two prototypes named NEMO-1 and NEMO-2. These prototypes eventually led to the construction of the NEMO-3 detector in the Laboratoire Souterrain de Modane (LSM), which is located inside a tunnel connecting the cities of Modane and Bardonecchia. The success of the NEMO-3 detector ultimately spurred the creation of SuperNEMO, a modern detector that, once commissioned, will continue the search for neutrinoless double beta decay in the LSM.

NEMO-3



NEMO-3 was a cylindrical detector which was radially divided into 20 sections. Each section was identical in size, but not all of them contained the same type of radioactive sources. In fact, NEMO-3 was very unique because it was studying 7 different sources of double beta decay at the same time. These sources were installed inside the sections in the form of thin vertical foils, and they totalled approximately 10 kg of double beta decay isotopes. The detector also had a tracking volume to completely reconstruct the topology of any charged particles passing through it. This was another great advantage of NEMO-3, as it helped significantly to distinguish double beta decays from any other possible events. The energy of the beta particles was measured using scintillator blocks coupled to 8 inch PMTs.




To keep the background as low as possible, the NEMO-3 detector was housed underneath a mountain. This reduced the amount of cosmic rays reaching it by a factor of a million. In addition, every piece of material used to construct NEMO-3 had to be carefully tested to make sure that it had very low radiation. Each test could last more than a month since the radiation emitted by the desired materials needed to be orders of magnitude less than that emitted by humans.




After 8 years of data taking (from 2003 to 2011), the results obtained were very successful. After a lot of analysis, the world’s most accurate measurements of the two-neutrino double beta decay half lives were calculated for each of the sources which were being studied, and the best limits were placed on most of them for the half-life of neutrinoless double beta decay. For more information on these results, see the section “Published Papers”.

SuperNEMO



The success of NEMO-3 motivated an effort to construct a new and better detector called SuperNEMO. It is designed to have larger amounts of double beta decay sources and to be made out of materials which are less radioactive than the ones used to build NEMO-3, further reducing the possible background. SuperNEMO will also be composed of 20 modules. Approximately 5-7 kg of double beta decay isotopes will be installed in each module, allowing it to have at least 10 times more source mass than its predecessor. The materials to build the modules were chosen so that, once built, each module will be less radioactive than 8 bananas.

In contrast to the cylindrical shape of the NEMO-3 detector, the SuperNEMO modules exhibit a rectangular geometry. This allows for a staged approach to the construction of the detector, meaning that a module can be taking data while the rest of the modules are being built. It also makes it possible to build this kind of modules in other underground locations if desired. Lastly, this geometry facilitates the extrapolation of sensitivity and performance from a single module to the whole SuperNEMO detector.

The first module, called the demonstrator, is currently under construction. It will be populated with 82Se as its only source of double beta decays. However, since virtually any double beta decay source may be used, the possibility of enriching and using 150Nd and 48Ca is being investigated. The SuperNEMO experiment is aiming to reach a sensitivity greater than 1026 years for the half life of neutrinoless double beta decay, which translates to an effective neutrino mass of 50-100 meV.

Our group’s involvement with this experiment consists of two calibration systems: The Calibration Source Deployment System, and the Light Injection System. Information about each of them can be found in the following links:

Radioactive Source Deployment System



The radioactive source deployment system consists of six oxygen-free copper plumb bobs suspended from stainless steel wires inside the SuperNEMO source frame. Each wire is wrapped around a wheel on top of the detector (housed inside a stainless steel vessel) which may be rotated by a stepper motor, lowering and raising each plumb bob. At seven fixed positions on each wire above the plumb bobs, 207Bi calibration sources are attached, making it possible to introduce the sources into the detector. At the bottom of the source frame there are six nests with laser light passing through them. Each plumb bob has a hole big enough for the laser beam to pass through. As a plumb bob enters a nest, it first interrupts the laser; this interruption is detected by a computer which slows down the motor. As the plumb bob continues descending, it reaches a position where its hole aligns with the laser beam. The computer is once again alerted by this change, and it stops the motor completely.

Left: The six different calibration lines installed on top of the SuperNEMO source frame. Right: A close up of one of the vessels which houses the wheels which raise and lower the plumb bobs and calibration sources.

The system is fully automated. Photodiode amplifiers are used to create electrical signals whenever they receive light from the lasers. These signals are the way the computer knows when the lasers are interrupted and uninterrupted. The computer which controls the whole system, called CompactRIO, communicates with the stepper motor drivers which actuate the stepper motors that deploy the plumb bobs. The CompactRIO has 64 inputs/outputs, and it uses LabVIEW as the language that interprets and analyzes the inputs to decide which outputs need to be sent.

Computer drawings showing how a plumb bob interacts with the light passing through a bottom nest in order to find its lowest position.

The radioactive source deployment system has already been installed and incorporated into the SuperNEMO detector. The mechanics of it (introduction and retrieval of the calibration sources) have already been successfully tested, and it will begin performing calibration runs some time in the year 2021.

Vessels mounted on top of the SuperNEMO detector.

Light Injection and Monitoring System



The Light Injection and Monitoring (LIM) System injects pulsed UV light into each optical module of the SuperNEMO calorimeter via optical fibers to control and monitor their energy response over time. The aim of the LIM system is to guarantee the stability of the calorimetric response to within 1%. The LIM system consists of 20 UV-LEDs illuminating ~1500 optical fibers routed to optical modules. Each LED illuminates a bundle of ~75 fibers. Reference optical modules outside the detector are used to monitor the energy of 207 Bi sources as well as the light levels of some optical fibers connected to each of the 20 UV-LEDs. By comparing the light from the UV-LEDs to the constant energy

By comparing the light from the UV-LEDs to the constant energy of the 207 Bi sources, it is possible to correct any fluctuations on the UV-LEDs. After making those corrections, that light can be used to see if the energy response of the SuperNEMO optical modules is drifting over time. Tests with a top-bench version of this system at UT Austin outperformed the 1% stability goal. The system has been installed and run successfully at the site of the SuperNEMO detector. It will also begin performing calibration runs some time in the year 2021.

Schematic of the Light Injection and Monitoring System together with actual photos of the system as it is currently installed.

NEMO-3



NEMO-3 was a cylindrical detector which was radially divided into 20 sections. Each section was identical in size, but not all of them contained the same type of radioactive sources. In fact, NEMO-3 was very unique because it was studying 7 different sources of double beta decay at the same time. These sources were installed inside the sections in the form of thin vertical foils, and they totalled approximately 10 kg of double beta decay isotopes. The detector also had a tracking volume to completely reconstruct the topology of any charged particles passing through it. This was another great advantage of NEMO-3, as it helped significantly to distinguish double beta decays from any other possible events. The energy of the beta particles was measured using scintillator blocks coupled to 8 inch PMTs.




To keep the background as low as possible, the NEMO-3 detector was housed underneath a mountain. This reduced the amount of cosmic rays reaching it by a factor of a million. In addition, every piece of material used to construct NEMO-3 had to be carefully tested to make sure that it had very low radiation. Each test could last more than a month since the radiation emitted by the desired materials needed to be orders of magnitude less than that emitted by humans.




After 8 years of data taking (from 2003 to 2011), the results obtained were very successful. After a lot of analysis, the world’s most accurate measurements of the two-neutrino double beta decay half lives were calculated for each of the sources which were being studied, and the best limits were placed on most of them for the half-life of neutrinoless double beta decay. For more information on these results, see the section “Published Papers”.

Publications

List of publications related to double beta decay research.

  1. SuperNEMO Papers
  2. Development of methods for the preparation of radiopure 82 Se sources for the SuperNEMO neutrinoless double-beta decay experiment, A. V. Rakhimov et al. Radiochimica Acta, 108(2), 87-97 (2019).
  3. Calibration Systems for the SuperNEMO Experiment, Poster, R. Salazar in behalf of the SuperNEMO Collaboration, Zenodo, https://doi.org/10.5281/zenodo.1300646 (2018).
  4. The Search for Periodic Modulations of Nuclear Decay Rates with the NEMO-3 Experiment and Development of the Light Injection Monitoring System for the SuperNEMO Experiment, PhD Thesis, J. Cesar, University of Texas at Austin, 2016, http://hdl.handle.net/2152/44541.
  5. The SuperNEMO light injection and monitoring system, J. Cesar, T. Le Noblet, R. Salazar, and SuperNEMO Collaboration, Journal of Physics Conference Series, Volume 888, 2016.
  6. Search for Neutrinoless Double Beta Decay of 116 Cd and 82 Se and Calorimeter Simulations for the SuperNEMO Experiment, PhD Thesis, Z. Liptak, University of Texas at Austin, 2014.
  7. Spectral modeling of scintillator for the NEMO-3 and SuperNEMO detectors, J. Argyriades et al. Nucl.Inst.Meth. A 625(1).
  8. Probing New Physics Models of Neutrinoless Double Beta Decay with SuperNEMO, R. Arnold et al.Eur.Phys.J.C70:927-943(2010).
  9. Results of the BiPo-1 prototype for radiopurity measurements for the SuperNEMO double beta decay source foils, J. Argyriades et al. Nucl. Inst. Meth. A 622 120-128 (2010).
  10. The SuperNEMO project, F. Piquemal Physics of Atomic Nuclei, Volume 69, Issue 12, pp.2096-2100.
  1. NEMO-3 Papers
  2. Search for Periodic Modulations of the Rate of Double-Beta Decay of 100 Mo in the NEMO-3 Detector, NEMO-3 Collaboration, R. Arnold et al. arXiv:2011.07657v1 [nucl-ex], (2020).
  3. Measurement of the 2νββ decay half-life and search for the 0νββ decay of 116Cd with the NEMO-3 detector, R. Arnold et al. Phys. Rev. D 95, 012007.
  4. Measurement of the 2νββ decay half-life of 150Nd and a search for 0νββ decay processes with the full exposure from the NEMO-3 detector, R. Arnold et al. Phys. Rev. D 94, 072003.
  5. Measurement of the double-beta decay half-life and search for the neutrinoless double-beta decay of 48Ca with the NEMO-3 detector, R. Arnold et al. Phys. Rev. D 93, 112008.
  6. Results of the search for neutrinoless double-β decay in 100Mo with the NEMO-3 experiment, R. Arnold et al. Phys. Rev. D 92, 072011.
  7. Investigation of double beta decay of 100Mo to excited states of 100Ru, R. Arnold et al. Nucl. Phys. A 925 (2014) 25.
  8. Search for neutrinoless double-beta decay of 100Mo with the NEMO-3 detector, R. Arnold et al. Phys. Rev. D 89, 111101(R).
  9. Measurement of the ββ Decay Half-Life of 130Te with the NEMO-3 Detector, R. Arnold et al. Phys. Rev. Lett. 107, 062504.
  10. Spectral modeling of scintillator for the NEMO-3 and SuperNEMO detectors, J. Argyriades et al. Nucl.Inst.Meth. A 625(1).
  11. Measurement of the two neutrino double beta decay half-life of Zr-96 with the NEMO-3 detector, J. Argyriades et al. Nucl.Phys.A847:168-179 (2010).
  12. Measurement of the Double Beta Decay Half-life of 150Nd and Search for Neutrinoless Decay Modes with the NEMO-3 Detector, J. Argyriades et al. Phys. Rev. C 80, 032501(R) (2009).
  13. Measurement of the background in the NEMO 3 double beta decay experiment, J. Argyriades et al. Nucl. Inst. Meth. A 606, Issue 3 (2009) 449-465.
  14. Measurement of double beta decay of 100Mo to excited states in the NEMO-3 experiment, R. Arnold et al.Nucl. Phys. A 781 (2007) 209-226.
  15. Limits on different Majoron decay modes of 100Mo and 82Se for neutrinoless double beta decays in the NEMO-3 experiment, R. Arnold et al. Nucl.Phys. A 765 (2006) 483-494
  16. First Results of the Search for Neutrinoless Double-Beta Decay with the NEMO-3 Detector, R. Arnold et al.Phys. Rev. Let. 95, 182302 (2005).
  17. Technical design and performance of the NEMO-3 detector, R. Arnold et al. Nucl. Inst. Meth. A536 (2005) 79-122.
  18. Possible background reductions in double beta decay experiments, R. Arnold et al. Nucl. Instrum. Meth. A503:649-657
  19. Chemical purification of molybdenum samples for the NEMO-3 experiment, R. Arnold et al. Nucl. Inst. Meth. A 474 (2001) 93