Germanium Detector Array: Difference between revisions

Revision as of 22:27, 20 January 2015

Revision as of 00:03, 21 January 2015

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{{Underlinked|date=December 2014}}

The [[Germanium|GERmanium]] Detector Array ('''GERDA''') experiment is searching for [[neutrinoless double beta decay]](0vββ) in Ge-76 at the underground [[Laboratori Nazionali del Gran Sasso]] (LNGS). [[Neutrinoless beta decay]] is expected to be a very rare process if it occurs. The collaboration predicts less than one event each year per kilogram of material, appearing as a narrow spike around the 0vββ Q-value(Q_ββ = 2039 keV) in the observed energy spectrum. This means b[[Radiation_protection|ackground shielding]] is required to detect any rare decays. The LNGS facility has 1400 meters of rock [[overburden]], equivalent to 3000 meters of water shielding, reducing [[Cosmic ray|cosmic radiation]] [[Background radiation|background]].

== Design ==

The experiment uses high purity enriched [[Germanium|Ge]] crystal [[diodes]] (HPGe) as a beta decay source and [[particle detector]]. The detectors from the HdM and Igex experiments were reprocessed and used in phase 1. The detector array is suspended in a liquid [[argon]] [[cryostat]] lined with copper and surrounded by an ultra-pure water tank. [[Photomultiplier|PMTs]] in the water tank and plastic [[scintillators]] above detect and exclude background [[muons]]. Pulse-shape discrimination (PSD) is applied as a cut to discriminate between particle types.

Phase 2 will increase the active mass to 38kg using 30 new HPGebroad energy germanium (BEGe) detectors. A magnitude reduction in background is planned to 10^-3 counts/(keV·kg·yr) using cleaner materials. This will increase the half-life sensitivity to 10²⁶ years once 100kg·yr of data is taken and enable evaluation of possible ton-scale expansion.

== Results ==

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Phase I collected data November 2011 to May 2013, with 21.6 kg·yr exposure, obtaining a 0vββ 90% CL half-life limit of:

<math>T_{0 \nu \beta \beta} > 2.1* \cdot 10^{25} yr </math>. This limit can be combined with previous results, increasing it to 3·10²⁵ yr, disfavoring the Heidelberg-Moscow experiment detection claim. A doublebound betaon decaythe half-lifeeffective neutrino mass was also measuredreported: Tm_2vββv = (1.84^+0.14_−0.10)·10²¹ yr400 meV.

The double beta decay half-life was also measured: T_2vββ = 1.84·10²¹ yr.

Phase II will have additional enriched Ge detectors and reduced background, raising the sensitivity about one order.