The MAJORANA Neutrinoless Double-beta Decay Experiment

 

The MAJORANA Collaboration is constructing the DEMONSTRATOR, an array of germanium detectors, to search for neutrinoless double-beta decay of germanium-76. The MAJORANA Collaboration consists of over 100 members from 16 institutions in four countries.

 A cross-section of the MAJORANA DEMONSTRATOR, showing installation of a cryostat module .

 

Neutrinos and Neutrinoless Double-beta Decay

The Feynman diagram for neutrinoless double beta decay.

 

MAJORANA is an experiment to search for neutrinoless double-beta decay (0νββ). Neutrinos are fundamental particles that play key roles in the early universe, cosmology and astrophysics, and nuclear and particle physics.

Neutrinoless double-beta decay is a decay mode of an atomic nucleus in which two neutrons convert to two protons and two electrons. This process has not been observed, and it is not known whether it exists. If this decay occurs, the neutrino is its own antiparticle, or a Majorana particle. The observation of neutrinoless double-beta decay would determine whether the neutrino is a Majorana particle and provide information on the absolute scale of neutrino mass. If the neutrino is a Majorana particle, neutrinos could provide a mechanism for the matter/anti-matter imbalance of our universe. The search for 0νββ provides the physics community with the opportunity to build on our successes in understanding the neutrino and further our understanding of the universe.

 

 

The MAJORANA DEMONSTRATOR

The MAJORANA DEMONSTRATOR will contain 40 kg of germanium; up to 30 kg will be enriched to 86% in 76Ge. The DEMONSTRATOR will be deployed deep underground in an ultra-low-background shielded environment in the Sanford Underground Research Facility (SURF) in Lead, SD. The goal of the DEMONSTRATOR is to determine whether a future 1-tonne experiment can achieve a background goal of one count per tonne-year in a 4-keV region of interest around the 76Ge 0νββ Q-value at 2039 keV. MAJORANA plans to collaborate with GERDA for a future tonne-scale 76Ge 0νββ search.

 

Experimental Technology


The MAJORANA Collaboration proposes to search for neutrinoless double-beta decay using an array of germanium crystals enriched in 76Ge. Of the candidate isotopes for 0νββ, 76Ge has some of the most favorable characteristics. Germanium-diode detectors are a well established technology, and in searches for 0νββ of 76Ge, the detectors can work as both source and detecor.

If 0νββ occurs, it is an extremely rare process. Current experimental results indicate that, in 76Ge, the lifetime of this decay is greater than 2 x 1025 years. To achieve sensitivity to such a rare decay, MAJORANA must have a very low background rate.

To achieve this background, the MAJORANA Demonstrator uses materials that are very low in radioactivity, including copper that is electroformed and machined underground. This helps elminate uranium and thorium contamination and reduces activation by high-energy cosmic rays, leading to copper that is up to ten times less radioactive than commercial copper. All of the materials used in the Demonstrator are validated through an extensive radioassay campaign.

The 0νββ background in the MAJORANA Demonstrator is also reduced via the use of P-type Point Contact Detectors. This type of detector gives differently shaped signals for single-site and multi-site events. Since the double-beta decay of 76Ge is always a single-site event, this allows us to cut away multi-site gamma interactions, reducing the background constribution from high-energy radioactivity.