MAJORANA
The MAJORANA project (styled Majorana) 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[1] and IGEX[2] collaborations, which used high-purity germanium (HPGe) detectors, to study neutrinoless double-beta decay.
The first stage of the project is the Majorana Demonstrator, designed to demonstrate the technique and evaluate a ton-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 Demonstrator, the collaboration intends to merge with the GERDA collaboration to build a much larger experiment.
Physics
The goal of the project is to search for 0νββ decay in 76Ge using HPGe detectors. Observation of 0νββ would establish that the neutrino is a Majorana particle and demonstrate violation of lepton number conservation, validating the seesaw mechanism as the explanation for the neutrino mass scale. It would also place constraints on the absolute neutrino mass.
The principal goal of the Majorana Demonstrator is to demonstrate the feasibility of achieving the background required in a ton-scale experiment. This corresponds to 4 counts/tonne/yr in a 4 keV window around the 0νββ Q value of 2039 keV, which scales to 1 count/ton/yr in a ton scale experiment. The experiment will use a mixture of detectors made with natural germanium and enriched germanium, allowing it to confirm or refute the controversial claim for 0νββ observation in 76Ge by Klapdor-Kleingrothaus et al.[3] If low enough electronic noise is achieved the Demonstrator may also make a search for WIMPs and axions.
Design
The Majorana Demonstrator will proceed in three phases. A prototype cryostat containing 3 strings of non-enriched germanium is in commissioning. Two low-background cryostats with enriched detectors are planned, with a total of 40 kg germanium.
Electroformed copper and lead bricks protect the cryostats. Polyethylene shields the setup and includes PMTs to act as a veto. Nitrogen flushing removes trace radon.
Point contact detectors
P-type point contact (PPC) germanium detectors are used.[4][5] This style of detector was chosen for many reasons, but chiefly because PPC detectors allow efficient discrimination of multiply scattering gamma backgrounds. 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, thus being able to reject events these signals. Other advantages include the low capacitance due to the small contacts, reducing electronic noise and thresholds; and shielding surface alpha decays by the thick outer n-type contact.
Status
The Majorana Demonstrator is currently (Dec 2014) under construction at the Sanford Underground Laboratory in Lead, South Dakota. The first module is expected in early 2015 with full operation expected in late 2015.[6]
MALBEK was operated 2011-12 at KURF in Virginia as a WIMP detector to evaluate the broad energy (BEGe) PPCs. The background and behavior of the contacts was explored. No signal was seen, and the project could be a competitive search for low mass WIMPs.[7]
References
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Bibliography
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