Status of the GERDA experiment

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Status of the GERDA experiment

J. Janicsk´o-Cs´athy for the GERDA collaboration^a

aMax-Planck-Institut f¨ur Physik, M¨unchen, Germany

The GERDA experiment is being built in the LNGS underground laboratories for the search for neutrinoless double beta decay. Phase I of the experiment GERDA will be able to test the claim of observation of the neutrinoless double beta decay. In a second phase newly developed detectors will be added aiming to a total exposure of 100 kg yr. Status of the experiment and first results from segmented detectors operated in cryogenic liquid are presented.

1. Double beta decay

Double beta decay with the emission of two neutrinos is a second order weak process. It was observed and the half-life measured rather well by the Heidelberg-Moscow (HdMo) collaboration [1].

The neutrinoles double beta decay is allowed only if neutrinos are Majorana particles. So far only lower limit of the half-life was measured. Re- cently a part of the HdMo collaboration claims that it was detected [2] with the half-life ofT_1/2^0ν = 1.2×10²⁵ years at 4σ confidence level.

In order to understand the nature of the neu- trino and verify the results published in [2] a new experiment is needed with higher sensitivity than previous experiments. The most sensitive double beta decay experiments to date used High Purity Germanium (HPGe) detectors. The excellent res- olution of HPGe detectors allows in principle a clear distinction of the neutrinoless double beta peak. One of the advantages of the HPGe detec- tors is that the source and the detector is the same and they can be made of germanium enriched in the double beta decaying⁷⁶Ge isotope.

1.1. The GERDA experiment

The GERmanium Detector Array (GERDA) experiment is being built in the LNGS labora- tory in Italy. In the first phase of the experiment enriched germanium detectors used in HdMo and IGEX experiments will be redeployed. The main difference from the previous experiments is that

in GERDA the HPGe detectors will be oper- ated directly in a cryogenic liquid (liquid ar- gon). The advantage being that the cooling liquid acts as shielding against external radiation. The GERDA cryostat, once filled, will hold around 60 m³ of liquid argon. In addition there is a water tank with 10 m diameter built around the cryo- stat providing additional shielding and acting as Cerenkov veto against muons. An artists view of GERDA can be seen in Fig.1. Part of the actual hardware is already in place, as seen in Fig.2.

In Phase-I of the experiment the expected background rate will be about an order of magni- tude lower than in previous experiments, as low as 10⁻²cts/(keV kg y) in the region of interest.

In Phase I the aim is to reach an exposure of 30 kg y sufficient to test the claim [2].

In Phase-II of the experiment more germanium detectors will be added with the aim of reaching 100 kg y total exposure with a background rate of 10⁻³cts/(keV kg y). The sensitivity of GERDA for the half-life of the neutrinoless double beta decay was evaluated in [3]. In Fig.3 is shown the 90% probability half-life lower limit as a function of exposure for different background conditions.

The construction of the experiment is ap- proaching completion. Many parts of the hard- ware are already installed in hall A of the LNGS laboratory. The heaviest pieces, the cryostat and watertank, are already in place and are being tested right now. The construction of the clean- room and lock system are well advanced. Data taking with the existing germanium detectors is

Nuclear Physics B (Proc. Suppl.) 188 (2009) 68–70

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Figure 1. Artists view of the GERDA experiment.

expected to start in 2009.

2. Phase-II detector production

The development of the Phase-II detectors is done in parallel with building the Phase-I of the experiment and important milestones were al- ready achieved.

In order to produce new crystals for Phase-II of the experiment, 37.5 kg of enriched germanium were bought from the ECP enrichment plant in Russia. The germanium is enriched to 87% in the isotope ⁷⁶Ge from the natural abundance of 7.4%. The material is presently stored under- ground to minimize the exposure to cosmic rays.

One of the important sources of background are the cosmogenically produced ⁶⁰Co and ⁶⁸Ge iso- topes. Special care is being taken to minimize the exposure during transportation, purification and crystal pulling.

The germanium is received from the ECP plant in the form of GeO₂powder. First it has to be re- duced and purified to at least 6N level. The met- allurgical processing is done at PPM Pure Metals GmbH (Germany). The whole process was tested using depleted germanium (leftover of the enrich- ment). Good quality and high yield (up to 90%) were demonstrated. No change in isotopic com- position was detected after the processing.

Figure 2. Watertank with the cryostat inside at LNGS

HPGe detectors are made of the purest man made material. Their net charge carrier concen- tration is in the order of magnitude of 10¹⁰/cm³. The production of such high purity crystals is not a trivial task. The crystal pulling R&D is done at the Institut f¨ur Kristallz¨uchtung (IKZ), Berlin. A Czochralski puller dedicated for high purity ger- manium crystals was set up. After many modifi- cations the puller operated with success first on April 7, 2008.

Eight test crystals were produced so far and their impurity concentration measured with low temperature Hall-effect measurement and the contaminants were identified with Photo- Thermal Ionization Spectroscopy (PTIS). During the first trials an impurity concentration between 10¹¹ and 10¹³/cm³was achieved. In order to test the whole production chain tests with material purified at PPM Pure Metals are done in paral- lel. The first results are promissing but further improvements are needed to achieve the goal of detector grade crystals.

J. Janicskó-Csáthy / Nuclear Physics B (Proc. Suppl.) 188 (2009) 68–70 69

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years]

⋅ Exposure [kg

0 50 100 150 200

y]25 [10 1/290% prob. lower limit T ⁰

5 10 15 20 25

30 No background

⋅ y)

⋅ keV counts/(kg 10-4

⋅ y)

⋅ keV counts/(kg 10-3

⋅ y)

⋅ keV counts/(kg 10-2 Claim

Figure 3. The expected 90% probability lower limit on the half-life for 0ν2β decay versus the exposure under different background conditions.

The claimed observation [2] is also shown.

3. Phase-II prototype detectors

A segmented GERDA prototype detector was already tested in a conventional (vacuum) cryo- stat. The results were reported earlier [4]. The nominal design for Phase-II foresees the opera- tion of n-type, 18-fold segmented, true coaxial HPGe detectors in cryogenic liquid. To asses the problems related to operation in cryogenic liquid we built a test cryostat at MPI M¨unchen. Fig.4 shows the moment when the HPGe crystal is low- ered in the cryostat later filled with liquid ni- trogen. The detector operated in liquid nitrogen for five months without major problems. Despite longer cables and increased noise level a core reso- lution of about 4 keV at 1332 keV was measured.

The leakage current was stable during the opera- tion. As a consequence our prototype detector is considered suitable for operation in liquid nitro- gen.

4. Summary

The construction of the GERDA experiment is well advanced. Major parts of the hardware are already in place or being prepared to be mounted.

Figure 4. 18 fold segmented n-type prototype de- tector being lowered in the test cryostat

Data taking is expected to start during 2009. For the second phase of the experiment many R&D projects are done simultaneously. The production of new HPGe crystals from enriched germanium is under development. Prototype segmented de- tectors are being tested in similar conditions as they will be deployed in GERDA. Within the col- laboration many other R&D projects, not men- tioned here, are pursued with possible applica- tion in Phase-II and beyond. Among others the so-called point-contact detectors are investigated and scintillation light detection in liquid argon is also developed.

REFERENCES

1. M. G¨unther et al., Phys. Rev. D 55 (1997) 54 2. H.V. Klapdor-Kleingrothaus et al.,

Phys.Lett.B 586 (2004) 198

3. A. Caldwell et al., Phys.Rev. D 74 (2006) 092003

4. I. Abt et al. NIM A 577 (2007) 574

J. Janicskó-Csáthy / Nuclear Physics B (Proc. Suppl.) 188 (2009) 68–70 70