A Monte Carlo based estimate on the background contributions of the detector

1. SignalBackground (Co-60) A Monte Carlo based estimate on the background contributions of the detector support for the Phase II of GERDA I. Abt, M. Altmann, A. Caldwell, D. Kollar, K. Kröninger, X. Liu, B. Majorovits, F. Stelzer MPI für Physik, München Neutrinoless double beta decay The Germanium Detector Array (GERDA) Neutrinoless double beta decay (0νββ) is a second order electro-weak process which is predicted to occour, if the neutrino is a Majorana particle. The halflife of the process is correlated with the mass of the lightest neutrino. The Germanium Detector Array, GERDA [2], is a new double beta experiment. Its main design feature is to operate an array of germani-um detectors directly in a buffer of cryogenic liquid (N2 or Ar). Clean room / lock Water / Myon-Veto (Č) Copper tank / vacuum Liquid nitrogen / argon Germanium detectors Schematic diagram of neutrinoless double beta decay. The nucleus changes its charge by two units. If the neutrino is a Majorana particle, the two neutrinos present can annihilate. Only two electrons remain in the final state. 0νββ The 0νββ-process has only two electrons and no neutrino in the final state. The sum of the kinetic energies of the electrons is therefore equal to the Q-value. For the germanium isotope 76Ge this is Qββ = 2 039 keV. A monoenergetic line is expected in the energy spectrum. With as little as possible amounts of material close to the detectors and the layered shielding approach a background index of 10-3 counts/(kg keV y) is aimed for. Electrons Design of GERDA. The experiment will be located in Hall A of the LNGS, Italy. The array of germanium detectors is submerged in cryoliquid. The cryostat is surrounded by a ultra-pure water buffer. The cleanroom on top of the tank includes a lock through which detectors can be lowered into the cryoliquid. The water is instrumented with photomultiliers in order to function as a Cherenkov detector. Together with a scintillator plate on top of the clean room muon can effectively be vetoed. 2νββ A phased approach is chosen for GERDA. In Phase I detectors will be installed which were previously operated by the IGEX [3] and Heidelberg Moscow [1] experiments. The detectors for Phase II are currently under design. (Te1+Te2 ) / Qββ Spectrum of double beta decay. If the two neutrinos annihilate (0νββ) the electrons carry the full kinetic energy released in the process. If the two neutrinos are emitted (2 νββ) only part of the energy is transfered to the electrons. Today‘s best limits on the halflife of the 0νββ-process in 76Ge is measured by the Heidelberg-Moscow collaboration with T1/2 > 1.9·1025 years (90% C.L) [1]. Design scheme of the fully loaded detector array. The strings are placed hexagonally with each string carrying up to 5 detectors. The electronics is placed about 30 cm above the top detector. MaGe – A Monte Carlo framework for double beta experiments Detectors, suspension and materials The Phase II detectors will be 8x8 cm with a mass of 2.1 kg of germanium. A 3x6-fold segmentation is forseen. A light-weight holder structure made of copper and Teflon will support the crystals. The readout cables will be either be Kapton or Teflon circuits with copper traces. In order to estimate the contributions of the background sources to the energy spectrum a GEANT4 based Monte Carlo framework, MaGe [4], is deve- loped. This is done in collaboration with the Majorana [5] collaboration. Phase II prototype crystal The background contributions are simulated by placing radioactive nuclei in the volume under study and tracking the daughter particles through the detector volume. For the background level envisioned a special selection of radiopure materials is necessay. The radiopurity of the materials is measured down to the μBq/kg level. Infrastructure Single crystal Ideal detector array Radiopurity of materials used in the vicinity of the germanium crystals. For the holder structure specially selected materials have to be chosen so as to minimize possible background contributions from radioactive decays. The values are either measured or estimated. The Phase II detectors will have a mass of 2.1 kg. A 3x6-fold segmen-tation scheme is forseen. The holder structure has a total weight of approximately 37 g. GERDA geometry as simulated in the MaGe package. The suspension consists of the holder, cabling and electronics, located 30 cm above the top crystals. The ideal array consists of 21 segmented detectors which are placed hexagonally into a volume of liquid nitrogen. A plate of scintillator on top of the clean room and photomultipliers in the water tank (both not shown here) provide a muon veto and are also simulated. Background estimate Event selection The signal process has two electrons in the final state. Their energy is deposited on a scale small compared to the size of a crystal segment. Radioactive decays are often accompanied by gamma-radiation. In the energy region around 2 MeV Compton scattering is the dominant process of energy loss for photons. The range of photons is on the order of centimeters and therefore of the size of a crystal segment. In order to further suppress background, an anti-coincidence between segments is required. With this, an additional factor of approximatly 10 in background suppression can be reached [6] The Monte Carlo based estimates on the background contributions help design the segmentation scheme and holder structure. Limits on the permissable radio-purity are calculated and influence the choice of material. An ongoing communication between Monte Carlo simulations and design considerations helps to find the optimal design for the Phase II detector and suspension system. Overview of the background contributions to the spectrum expected in the GERDA experiment using the Monte Carlo package MaGe and the expected level of radioimpurities from the material screening. 2νββ Schematic view of an array of segmented detectors. Local energy deposits are indicated by the yellow dots, the segments with energy deposist are shown in green. For the signal process (left) only one segment is hit, whereas for the decay of Co-60 (right) two photons scatter multiple times (red lines) and deposit energy in six segments. Energy spectra for Co-60 for different anti-coincidence requirements: no anti-coincidence (black), between crystals (green) and between segments (red). For the events with only one segment hat the reduction is larger than order of magnitude than for those with only one crystal hit. Monte Carlo spectrum generated under the assumption of a halflife of 1.6·1025 years, a background index of 10-3 counts/(kg keV y) and an exposure of 50 kg years. The number of signal events in the region on interest (inset) is approximately 14. [1] M. Gunther et al., ”Heidelberg - Moscow beta beta experiment with Ge-76: Full setup with five detectors”, Phys. Rev. D 55 (1997) 54. [2] S. Schönert et al., „The GERmanium Detector Array (GERDA) for the search of neutrinoless beta beta decays of Ge-76 at LNGS“, Nucl. Phys. Proc. Suppl. 145 (2005) 242. [3] D. Gonzalez et al., „Pulse shape discrimination in the IGEX experiment“, Nucl. Instrum. Meth. A 515 (2003) 634 [arXiv:hep-ex/0302018]. [4] To be published [5] R. Gaitskell et al., „White paper on the Majorana zero-neutrino double-beta decay experiment“, arXiv:nucl-ex/0311013. [6] I. Abt et al., „Background suppression in neutrinoless double beta decay experiments using segmented detectors - a Monte Carlo study”, to be published.