|Parts of this article (those related to the current status and planned timeline) are outdated. (November 2013)|
The MAJORANA project is an international effort to search for neutrinoless double-beta (0νββ) decay in 76Ge. The project builds upon the work of previous experiments, notably those performed by the Heidelberg–Moscow and IGEX collaborations, which used high-purity germanium (HPGe) detectors, enriched in 76Ge, to provide the most sensitive limits on neutrinoless double-beta decay half lives to date.
The first stage of the MAJORANA project is the MAJORANA Demonstrator, an experiment designed to demonstrate the feasibility of achieving the background required to justify the construction of a larger tonne-scale experiment. Cryostats housing up to 40 kg of natural and enriched germanium detectors are being deployed in low-background vacuum cryostats, underground at the Sanford Underground Laboratory in Lead, South Dakota. Following the MAJORANA Demonstrator, the MAJORANA collaboration intends to merge with the GERDA collaboration to build a much larger tonne-scale experiment.
The goal of the Majorana project is to search for 0νββ decay in 76Ge using HPGe detectors. The low-energy sensitivity of these detectors allows a secondary goal of searching for low-mass weakly interacting massive particle (WIMP) dark matter and axions. The observation of 0νββ would establish that the neutrino is a Majorana particle and, in so-doing, demonstrate violation of lepton number conservation and permit the seesaw mechanism as the explanation for smallness of the neutrino mass scale. It would also place constraints on the absolute neutrino mass scale.
The principal goal of the Majorana Demonstrator is to demonstrate the feasibility of achieving the background required to justify the construction of a larger tonne-scale experiment. This corresponds to a background count rate of 4 counts per tonne of isotope per year in a 4 keV window around the 0νββ Q value of 2039 keV, which scales to 1 count per tonne of isotope per year in a tonne scale experiment. The experiment will use a mixture of detectors made with natural germanium and germanium enriched in 76Ge, and this will allow it to confirm or refute the controversial existing claim for the observation of 0νββ in 76Ge by Klapdor-Kleingrothaus et al. If low enough electronic noise is achieved the Majorana Demonstrator may also make a sensitive search for WIMP dark matter and axions.
The Majorana Demonstrator will proceed in three phases: a prototype cryostat containing a small number of detectors made from non-enriched germanium, which will be replaced by first one, then two, low-background cryostats containing a mixture enriched and non-enriched detectors. In its most sensitive configuration the experiment will run with up to 40 kg of germanium, up to 30 kg of which will be enriched to 86% 76Ge. The vacuum cryostats and other copper parts will be manufactured from ultra-pure copper electroformed underground to shield from activation by cosmic rays. Each cryostat will contain up to 7 strings of 5 detectors. Each cryostat is mounted on a self-contained monolith that be inserted or removed from the experiment's lead and copper shield.
Point contact detectors
The Majorana Demonstrator will use p-type point contact (PPC) germanium detectors. This style of detector was chosen for many reasons, but chiefly because point contact detectors allow efficient discrimination of multiply scattering gamma backgrounds from single-site 0νββ decays. This results from 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, and thus being able to reject events where there were multiple energy depositions. Other advantages include the low capacitance, which results from the small contact size and offers the potential for low electronic noise and thresholds; and the shielding from surface alpha decays provided by the thick outer n-type contact.
Control of radioactive backgrounds in the materials and environment of the apparatus is one of the experiment's most important aspects. The Majorana Collaboration uses two broad classes of technique to control these backgrounds: minimizing the radioactive contaminants inside the experiment by careful selection and screening of materials, and clean production of components; and using analysis techniques to identify and remove events caused by the residual radioactive contamination.
The Majorana Demonstrator is currently[when?] under construction at the Sanford Underground Laboratory in Lead, South Dakota
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