The GERDA, a new experiment to search for the double beta decay of 76 Ge, is being installed at Laboratori Nazionali del Gran Sasso. The potentialities of this experiment as well as the status of the project are reviewed.JRC.D.4-Isotope measurement

BELLOTTI E.

GASPARRO JOEL

HULT MIKAEL

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GERDA: a germanium detector array to search for neutrinoless double beta decay
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2006 J. Phys.: Conf. Ser. 39 338
(http://iopscience.iop.org/1742-6596/39/1/087)
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GERDA: a germanium detector array to search for 
neutrinoless double beta decay 
E Bellotti for the GERDA Collaboration a) 
Univ. of  Milano Bicocca and INFN Milano P.le delle Scienze, 3 - 20100 Milano Italy  
E-mail: bellotti@lngs.infn.it 
Abstract. The GERDA, a new experiment to search for the double beta decay of 76 Ge, is being 
installed at Laboratori Nazionali del Gran Sasso. The potentialities of this experiment as well 
the status of the project are reviewed. 
1.  Introduction 
The physics motivations to search for 0νββ decay have been discussed in many theoretical and 
experimental papers, also at this same Conference. In this paper GERDA [1], a new experiment based 
on the use of a large amount of  76Ge, is presented. Ge diodes are excellent devices to search for 
neutrinoless ββ decay. The germanium  acts as source and detection medium; the transition energy is 
reasonably high (2038.5 keV) and the energy resolution is better than 4 keV at this energy. All 
materials used are carefully selected. In addition,  recently developed techniques - diode segmentation 
and pulse shape discrimination – are applied. All this combined, it is possible to greatly reduce the 
background. 
Two collaborations have recently provided new data on 76 Ge: the IGEX Coll. [2] provided a limit 
on the 0νββ decay half-life of 1.57 1025 y; the Heidelberg-Moscow (HdM) Collaboration at Gran Sasso 
obtained a similar; Klapdor-Kleingrothaus and coll. [3], using the full statistics of the HdM experiment 
(72 kg.y) found a positive signal of 29 counts above a background of 60. This signal corresponds to a 
half-life (1.19+.37
-.23 ) 1025 y; the GERDA experiment is designed to clarify the situation and extend the 
ranges of accessible life-times. 
2.  The baseline design of GERDA 
The baseline design of GERDA is shown in the artistic view of the figure (figure 1). The diodes are 
directly immersed in liquid nitrogen (LN) which has two functions: to cool down and to shield the 
diodes from external radioactivity. The cryostat will be built out of  high purity copper  (an alternative 
design based on the use of steel has also been studied) and it will be immersed in a tank filled with 
high purity water tank to further reduce the n’s and γ’s fluxes. Suspension wires and electronics cable 
will pass through the long neck on top of the cryostat. The water will be instrumented with PMT’s to 
veto the muons entering in the set-up. Special counters will be located on the top of the entire structure 
to complete the muon veto. On the top of the water tank, a building will house a clean room, the 
electronics and the needed infrastructures. GERDA will be located in the Hall A of the underground 
Gran Sasso laboratories of LNGS. 
                                                     =
a)
 The Members of GERDA collaboration are reported in http://www.mpi-hd.mpg.de/GERDA 
Institute of Physics Publishing Journal of Physics: Conference Series 39 (2006) 338–340
doi:10.1088/1742-6596/39/1/087 TAUP 2005:  Proc. Ninth Int. Conf. on Topics in Astroparticle and Underground Physics
338© 2006 IOP Publishing Ltd
  
 
 
 
 
3.  Ge diodes and read out electronics 
GERDA will proceed by phases: in Phase I the crystals from IGEX and HdM will be installed, after an 
appropriate refurbishing. At present, the HdM detectors have been characterized, while the IGEX 
detectors will arrive at Gran Sasso in November.  
In Phase II, new detectors will be installed. The more significant steps in the production of the new 
enriched detectors are: the procurement of enriched material - 37.5 kg of enriched (86% in 76Ge) 
germanium have been produced by ECP Company (Zelenogorosk – Russian Federation); chemical 
purification- typically this step has a yield of 70% only; R&D on the purification has led to an 
efficiency of more than 85%; transport and storage - special care have been taken in order to reduce 
as possible the cosmogenic production of background inducing isotopes; crystal production - the first 
stages of the crystal production are the purification of Germanium by polyzone and consecutive 
monozone refinement; definition of these steps is under discussions with companies specialized in 
crystal production; segmentation - crystals will be truly coaxial type and segmented 6-fold in Φ and 3-
fold along the core axis. Coincidence among different crystals and segments of the same crystal will 
allow a good background rejection, as shown  by many Monte Carlo simulations.  
Old and new diodes will be mounted in a modular, scalable arrangement of strings. The suspension 
system will be constructed out of Teflon and copper; a prototype is under test. 
Read out and signal processing. For the front end three possibilities have been investigated: i) 
AGATA hybrid preamplifier [4] plus BF862FET, ii) IPA4 [5], an integrated preamplifier originally 
developed for Lar and Kr calorimetry and iii) AMPTEK250 + SK152 [6]; moreover a new CMOS 
ASIC circuit which is being developed by the Collaboration. For the signal processing, two possible 
solutions are MD2S [7] system, developed by the GERDA Padua group in the framework of the 
AGATA project and the commercial VME cards SIS 3301 produced by Struck. 
4.  Background evaluation 
The different sources of background are discussed in details in the GERDA proposal. The goal is to 
achieve a background of 10-3 count/ (kg·y.keV)  at 2 MeV. 
The flux of 2.6 MeV γ from 208Tl has been remeasured in the Hall A of the Laboratory, where 
GERDA will be located. The measured flux is (0.031±0.09) γ/(s·cm2); the thickness of the shield was 
adjusted to this with a safety margin of two. The shield is a hybrid construction with a cryogenic and a 
water layer ( see fig.1). All the materials that will be used in the construction have been screened to 
select those with the lowest content of radionuclides.  
A substantial contribution to the background comes from cosmogenically produced 60Co and 68Ge. 
This background  will be different in the detectors of phase I and II, because the new detectors will be 
constructed minimizing the exposure time to cosmic rays. The relevant background reactions have 
photons in the final state which deposit energy at multiple sites. The signal electrons typically deposit 
energy locally. Therefore, an analysis using anticoincidence strongly reduce the background. A careful 
investigation of the background induced by muons crossing the set-up has been carried out by the 
MAGE group (Majorana-GERDA group). The suppression due to the rejection of events by the muon 
veto and a crystal-based anti-coincidence analysis are taken into account. The predicted background is 
more than one order of magnitude below the goal as stated above. In conclusion, we expect a 
background of 10-2 and of 10-3 counts/(kg·y·keV) for crystals of Phase I and II respectively. 
5.  Potentialities of GERDA 
Assuming the energy resolution and the background index quoted above, it is straightforward to 
compute the expected number of events for different values of the half life. The phase I detectors with 
a mass of approximately 19kg (corresponding to more than 200 moles of 76Ge) will yield about ten 
events per year, if the half-life is 1025 y. The number of background events is expected to be one. A 
similar exposure of phase II detectors is expected to be background free. Consider first the Phase I 
detectors. For a half life of 1025 years, we expect, in one year of data taking, 8.3 events of signal and 
0.7 events of background. The same signal is expected for Phase II detectors, while their background 
is negligible. Assuming two years of data taking with old detectors and one year with new diodes 
339
  
 
 
 
 
(which will be installed later) a signal of 25 events is expected, against 1.5 background events. 
Therefore the central value of the signal observed by Klapdor-Kleingrothaus and coll. will be 
confirmed or disproved with a high statistical significance. Should no signal be observed, a limit of 
1026 y   can be achieved un about three years. 
6.  Status of the project and conclusions 
The project is approved and funded. The needed space is allocated at LNGS. The infrastructure, 
including water tank and cryostat are under final design.  
The works on old and new diodes and the definition of the read-out and DAQ electronics is 
progressing. 
Construction will commence in 2006. 
 
 
  
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 1. Artistic view of the GERDA setup. 
References 
[1] GERDA coll. 2004 GERDA The germanium detector array to search of neutrinoless ββ decays 
of 76Ge at LNGS Proposal to the LNGS P38/04  
[2] Aalseth C E et al IGEX coll. 2002 Phys. Rev. D 65 09007 
[3] Klapdor-Kleingrothaus H V et al 2004 Nucl. Inst. Meth. A 522 371 
[4] Pullia A et al 2002 IEEE Trans. Nucl. Scien. 49 5 
[5] Manfredi P F et al 1996 Nucl. Instr. Meth. A 380 308 
[6] www.amptek.com/a250html 
[7] Isocrate R et al 2004 LNL annual Report 2004 226 and 228 
[8] Bauer M et al 2005 MaGe: a Monte Carlo framework for the Gerda and Majorana double beta 
decay experiments. Presented to TAUP2005 Conference 
Clean room
lock  
Water tank / buffer/ muon veto 
Vacuum insulated 
copper vessel 
Liquid N/Ar 
Ge Array 
340


GERDA: a Germanium Detector Array to Search for Neutrinoless Double Beta Decay

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GERDA: a Germanium Detector Array to Search for Neutrinoless Double Beta Decay

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