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The Majorana project is an international effort to search for neutrinoless double-beta (0νββ) decay in 76Ge. Majorana builds upon the work of previous experiments such as the Heidelberg-Moscow and IGEX experiments, which have used 76Ge to provide among the most sensitive limits on 0νββ half lives up to date. Germanium detectors enriched in 76Ge will be used in the Majorana experiment. The first stage of Majorana is known as the Majorana Demonstrator, designed to demonstrate the feasibility of achieving the background required to justify the construction of a larger scale experiment. Cryostats housing 40 kg of natural and enriched high-purity germanium will be deployed underground at the Sanford Underground Laboratory in Lead, South Dakota. The success of this campaign would justify a future tonne-scale experiment to achieve lower background and better granularity. Eventually Majorana intends to merge with the Germanium Detector Array (GERDA) experiment to form an international tonne-scale effort.
Physics Goals[edit]
The primary goal of the Majorana experiment is to observe 0νββ decay in 76Ge using high-purity germanium as both source and detector. Numerous experiments such as the Super Kamiokande experiment, the Sudbury Neutrino Observatory experiment, and the KamLAND experiment have shown that neutrinos have a very tiny nonzero mass. The observation of 0νββ decay would demonstrate that the neutrino is a Majorana particle and provide constraints on the absolute neutrino mass scale. Furthermore, it would also be evidence for lepton number non-conservation.
The aim of the first phase of Majorana, known as the Demonstrator, is to demonstrate the feasibility of achieving a background count of 4 counts per tonne-year in a 4-keV window around 2039 keV, the total energy released in a 0νββ decay of 76Ge. The success of this demonstration would substantiate a grandeur effort for building a tonne-scale experiment with a background rate of 1 count per tonne-year, thereby increasing the half-life sensitivity of the experiment by two orders of magnitude. It is expected that the Majorana Demonstrator should be able to reach a half-life sensitivity of greater than 4 x 1025 years within the first year of operation, allowing scientists to probe for 0νββ in 76Ge.
[Add stuff about low threshold and dark matter]
Experimental Design[edit]
Figure 1 shows a conceptual drawing of the Majorana Demonstrator apparatus. Forty kg of germanium, up to 30 kg of which will be enriched to 86% 76Ge, will be placed in two ultra-pure electroformed copper cryostats. One cryostat is shown in the figure. Each cryostat houses seven strings of five p-type point contact (PPC) detectors, shown in Figure 2.[1][2] The cryostat is mounted in a self-contained module (monolith) that sits on a transporter that can be easily inserted and removed from a lead and electroformed copper door. Other components of the design are also made with extremely pure materials in order to meet the background goals of the experiment.
Point Contact Detectors[edit]
P-type point contact (PPC) detectors were selected as the detector technology of choice for the Majorana Demonstrator project for several reasons. PPC detectors have a strongly peaked weighting potential so that charges drifting in the detector register very little signal until they are in the immediate vicinity of the point contact. Furthermore, the geometry of PPC detectors results in very low electric fields throughout most of the crystal. This means charges drift at a slower velocity and take longer to move about the crystal, allowing the detector to generate sharper pulses in real time to facilitate their identification and discrimination. Figure 3 shows a typical PPC detector with its ultra-high resistance surface and the point contact at the center.
Backgrounds[edit]
Background control is the most significant factor of Majorana. The Majorana Collaboration uses two main techniques to reduce background radiation as much as possible. The first technique is the careful selection, screening, and fabrication of detector components. The second technique is pulse-shape analysis (PSA), an analysis tool that works extremely well with PPC detectors. PSA operates on the principle that ββ−decay events deposit charge locally in a detector (single-site events), as opposed to gamma rays that typically scatter more than once and deposit their energy in several locations (multi-site events). As the charges liberated in these energy depositions drift through the detector, the multiplicity gets recorded in the detector pulses, which can then be subjected to PSA to identify and remove multi-site events from the more important single-site events. Variations of the technique were used in previous experiments with much success including the Heidelberg-Moscow and IGEX experiments.
Status[edit]
The Majorana Demonstrator is currently still under construction.
References[edit]
- ^ Luke P.; et al. (1989). "Low capacitance large volume shaped-field germanium detector". IEEE Transactions on Nuclear Science. 36 (1): 926–930. doi:10.1109/23.34577.
{{cite journal}}: Explicit use of et al. in:|author=(help) - ^ Barbeau P.; et al. (2007). "Large-mass ultralow noise germanium detectors: performance and applications in neutrino and astroparticle physics". Journal of Cosmology and Astroparticle Physics. 2007 (9): 009. doi:10.1088/1475-7516/2007/09/009.
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Bibliography[edit]
- Phillips, David (2011). "The MAJORANA experiment: An ultra-low background search for neutrinoless double-beta decay". Journal of Physics: Conference Series. 381: 012044. arXiv:1111.5578. doi:10.1088/1742-6596/381/1/012044.
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External links[edit]
- The Majorana experiment homepage
- The Majorana group at the University of North Carolina
- The Majorana group at the Lawrence Berkeley National Laboratory