GERmanium Detector Array – a Search for Neutrinoless Double Beta Decay

X. Liu - MPI für Physik, MünchenSymposium – symmetries and phases in the universe, Kloster Irsee, 23-25 June 2008

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Suppression of background from radioactive decays Background simulation and sensitivity

Schematic view of an array of segmented detectors. The location of radioactive decays are indicated by yellow dots, the segments with energy deposist are shown in green. For the signal process (left) only one segment is hit, whereas for background processes (here: the decay of Co-60) (right) the photons scatter multiple times (red lines) and deposit energy in more than one segment.

Energy spectra for Co-60 for different anti-coincidence requirements: no anti-coincidence (black), between crystals (green) and between segments (red).

Part Background index

[10-4 counts/kg/keV/y]

Detector 11.1

Holder (copper) 1.7

Cabling 1.5

Electronics 3.5

Infrastructure 0.2

Muon & neutron 2.0

SUM 21

Overview of the background contributions. The measured radioimpurities from the material screening is used.

For photons with energy 2MeV and above Compton scattering is the dominant process of energy loss. The range of photons is a few cm‘s and therefore of the size of a crystal segment. The 0v signal has two electrons in the final state with their energy deposited within mm range, much smaller than segment size. By requiring segment anti-coincidence, photon-induced background can be suppressed by a factor of 10-50 depending on the decaying isotope.

The main sources of background are radioactive isotopes with Q-values larger than that of 76Ge, Qββ = 2.039 MeV. If only part of the energy is deposited in the detector with the energy value around the Q-value of 76Ge the observed „event“ can be mis-interpreted as 0νββ. Background events with photons in the final state can be suppressed by anti-coincidence requirements bet-ween segments.

The sensitivity to 0v decay of 76Ge strongly depends on the background index of the experiment. A geant4-based Monte Carlo package, MaGe, is used to simulate the radio-impurities of the surrounding materials and the muon and neutron flux. The background index is calculated to be 2.1·10E-3 counts/(kg keV y) and dominated by radioactive decays from isotopes in the holder structure. Improvements on the material radio-purities and additional analysis techniques using pulse shape infor-mation will help to further decrease the background index. Using a Bayesian analysis on Monte

Carlo data the 90% probability lower limit which can be set on the half-life, in case no observation is made, is calculated. For the envisioned background index of 10-3 counts/(kg keV y) and an exposure of 100 kg years the lower limit on the half-life is T1/2>1.35·1026 years which corres-ponds to a limit on the effective neutrino mass of mee<200 meV/c2.

Neutrinoless double beta decay

Neutrinoless double beta decay (0νββ) is an extremely rare second order weak process which is predicted to occur, if the neutrino is a Majorana particle with non-zero mass. In this process the lepton number is changed by 2. The half-life T1/2 of 0νββ decay is a function of the neutrino masses, their mixing angles and CP-phases. Today‘s best limit on the half-life of the 0νββ-process of 76Ge is measured by the Heidelberg-Moscow collaboration with T1/2 > 1.9·1025 years (90% C.L).

Up: schematic diagram of neutrinoless double beta decay. The nucleus changes its charge by two units. If the neutrino is a Majorana particle, the neutrino emitted by one W boson can be absorbed by the other W boson. Only two electrons remain in the final state.

The GERDA experiment, the detector array and the phase approach

Left: Detector array as simulated. The detectors are placed hexagonally in strings of 3 detectors each. The read-out electronics will be located above the top crystals. Middle: the inner vessel of the cryotank being inserted into the outer vessel (December 2007). RIght:: the cryotank after installed in Hall A of LNGS (March 2008). At present the construction of water tank is almost finished (June 2008).

The detectors will be placed in a hexagonal pattern in strings of three detectors each. The suspension system is designed such that a minimum of high purity material is used.

A phased approach is chosen for GERDA. The phase I detectors are from previous IGEX and Heidelberg-Moscow experiments with a total mass of 17.9 kg. The detectors for phase II will have a total mass of 35 kg. These detectors forsee a true-coaxial geometry with a size of 7 cm in height and 7.5 (1.0) cm outer (inner) diameter. The detectors will be segmented with a 6-fold segmentation in the azimuthal angle φ and a 3-fold segmentation in the height z. All 18 segments for each detector will be read out separately.

The GERDA experiment is currently being constructued in the Hall A of the Gran Sasso National Laboratory, LNGS, in Italy,

The GERmanium Detector Array, GERDA, is a new experiment that will search for neutrinoless double beta decay in the germanium isotope 76Ge. Its main design feature is to submerge and operate high purity germanium detectors, enriched in 76Ge to a level of 86%, directly in liquid argon. The latter serves as coolant and shield from external radiation simultaneously. The cryostat is placed inside a buffer of ultra-pure water which serves as additional shielding and will be instrumented as Cherencov detector in order to veto cosmic muons. With this setup a background index of 10E-3 counts/(kg·keV·y) is expected.

Study of prototype detectors

Effective Majorana neutrino mass

The measurement on half-life of 0νββ decay provides the direct measurement on the so-called effective Majorana neutrion mass, m = where mj is the neutrino mass eigen-value and Uej the neutrino mixing matrix elements. With enough sensitivity on m, the neutrino mass hierarchy as well as the absolute neutrino mass can be probed.

Left:: effective Majorana neutrino mass as a function of the lightest neutrion mass. The green band shows the case for the so-called reversed hierarchy, the red band shows the normal hierarchy.

Search for 0v decay with 76Ge

More than 30 different isotopes decay by 2v and possibly by 0v. For these elements, the single decay is forbidden due to the higher energy level of the daughter elements. 76Ge is among them. As compared to other elements, 76Ge itself can be made into Germanium detector with excellent energy resolution. Thus the signal detection efficiency can reach close to 100% and the energy searching window can be made very small. The detector can be made very pure, further reducing background induced by radioactive elements. These advantages with respect to signal efficiency and low background rate are critical in search for rare 0v decay.

Left:: the gray area illustrates the measured energy spectrum from the standard model allowed 2-neutrino double-beta decay (2v). The 2 neutrinoes in the final state carry away certain amount of energy without being detected. The red line shows the energy peak from the 0v decay with the two electrons in the final state having the full energy equal to the Q-value of 0v decay. The Q-value for 76Ge is 2.039 MeV.

Lightest neutrino mass (eV)

m

(eV

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2ej

jjUm

Performance of GERDA prototype de-tectors have to be tested in cryoliquid enviroument. Phase I p-type detectors have been sucessfully operated in liquid Argon with a resolution (FWHM) of 2.2 keV at 1.3 MeV achieved. The detectors have been cooled down and warmed up many times without deterioration in performance.N-type 18-fold segmented protype detectors for Phase II have also been sucessfully operated in Vacuum. At the moment a prototype detector is being operated in liquid Nitrogen. Further investigation are on going.

Tl208 DEP(signal-like)

Bi212Photon(bg-like)

Left: Phase II prototype 18-fold segmented detector with the cabling scheme. The holder structure is made of copper and Teflon with a total weight of 31 g. Right: results on segment anti-coincidence selection and pulse shape analysis from18-fod segmented detector operated in Vacuum. The so-called double-escape events are barely removed by these selections and events with photon fully absorbed in Germanium detector are strongly suppressed. For more details see: wwwgerda.mppmu.mpg.de publications.

Germanium detector Array

Water / Myon-Veto (Č)

Clean room / lock

Cryo-tank w. Liquid argon

segmentselectrons

photons

0v Signal background (Co60)