PMN07 Blaubeuren Segmented germanium detectors in 0νββ-decay experiments Kevin Kröninger (Max-Planck-Institut für Physik, München) Outline: Segmentation in 0νββ-decay experiments Example 1: photons vs. electrons Example 2: neutron scattering Example 3: 2νββ-decay into excited states Example 4: alpha veto Summary and outlook

Kevin Kröninger PMN07 Blaubeuren, – Segmentation in 0νββ-decay experiments Germanium detectors can be segmented Segmentation works for both n- and p-type detectors State-of-the-art: segmentation schemes with up to 36 segments for a cylindrical geometry (e.g. AGATA) In 0νββ-decay experiments: Information about event topology → Identification of final states → Identification of physics processes → Rejection of background In particular: photons vs. electron identification

Kevin Kröninger PMN07 Blaubeuren, – Example 1: electrons vs. photons I 0νββ-decay has two electrons (only) in the final state Sum of the kinetic energies at Q-value (2 039 keV for 76 Ge) Electrons of O(1) MeV have a range of ~ 1 mm in Ge Single-site events Photons with MeV-energies mostly Compton-scatter Range of photons O(1-5) cm in germanium Multi-site events Aim: Distinguish between single-site (electrons) and multi-site events (photons) Range log(R [mm]) PSASegm. Single crystal

Kevin Kröninger PMN07 Blaubeuren, – Example 1: electrons vs. photons II 18-fold segmented n-type detector Pre-amplifiers and filters 60 l dewar with lN 2 Pre-amplifiers and filters I. Abt et al. NIMA 577 (2007) 574

Kevin Kröninger PMN07 Blaubeuren, – Example 1: electrons vs. photons II 18-fold segmented n-type detector 6-fold segmented in azimuthal angle 3-fold segment in height I. Abt et al. NIMA 577 (2007) 574

Kevin Kröninger PMN07 Blaubeuren, – Example 1: electrons vs. photons III Core electrode spectrum ( 60 Co) Data and MC agree (dev. <5%) Pile-up, CCE, etc. not in MC Substructure due to drift anisotropy of charge carriers Effective model in MC Channel ID I. Abt et al. arxiv:nucl-ex/

Kevin Kröninger PMN07 Blaubeuren, – Example 1: electrons vs. photons IV Suppression factor SF L = N (all) / N (single segment) Data and MC agree (dev. <5%) Add segment energies to study effective segmentation 18-fold segmentation best I. Abt et al. arxiv:nucl-ex/

Kevin Kröninger PMN07 Blaubeuren, – Example 1: electrons vs. photons V PartSourceSF C SF S DetectorCo ± ± 1.0 Ge ± ± 1.4 HolderTl ± ± 0.9 Bi ± ± 1.4 Co ± ± 27 ElectronicsTl ± ± 0.6 GERDA expectation: 21 detectors with 18-fold segmentation Monte Carlo study: segmentation improves background rejection by up to an order of magnitude, depending on source I. Abt et al. NIMA 570 (2007) 479

Study AmBe neutron source with 18-fold prototype detector Segmentation allows to observe recoil spectrum Example: inelastic scattering : Kevin Kröninger PMN07 Blaubeuren, – Example 2: neutron scattering 74 Ge(n, n‘ γ ) n recoiling nucleus γ Event selection: N seg = 2 E any = 596 keV (select photon)

Study AmBe neutron source with 18-fold prototype detector Segmentation allows to observe recoil spectrum Kevin Kröninger PMN07 Blaubeuren, – Example 2: neutron scattering 74 Ge(n, n‘ γ ) require N seg = 2 and E any = 596 keV 208 Tl 214 Bi E seg2 [keV] to be published

Kevin Kröninger PMN07 Blaubeuren, – Example 3: 2νββ into excited states I Double beta-decay of 76 Ge can populate excited states of 76 Se Observation could help check reliability of nuclear models Signature for decay: continous electron spectrum up to 917 keV photon of 559 keV photon of 563 keV Segmentation can be used to identify the two photons and the electrons Background about 3 events / (kg·y) KK, L. Pandola, V. Tretyak arxiv:nucl-ex/

Kevin Kröninger PMN07 Blaubeuren, – Example 3: 2νββ into excited states II GERDA: Monte Carlo simulation of the decay and possible background contributions ( 60 Co, 68 Ge, 2νββ,...) Sensitivity (for 100 kg·y): Two orders of magnitude above current limit of T 1/2 > 6.2·10 21 y Allows testing of predictions with T 1/2 ~ 7.5·10 21 y – 3.1·10 23 y Detector arrayT 1/2 limit (90% prob.) 21 unseg. detectors2.2·10 23 y fold segm. det.5.6·10 23 y KK, L. Pandola, V. Tretyak arxiv:nucl-ex/

Evidence for surface contaminations in previous experiments Alpha decays of 210 Pb daughter can cause energy deposition in the crystal For n-type detectors: mantle surface has a thin dead layer → energy deposit > 2 MeV But: top and bottom dead layer have “critical” thickness Add two thin segments on top and bottom as alpha-veto Feasibility study with Canberra France ongoing Kevin Kröninger PMN07 Blaubeuren, – Example 4: alpha veto veto

Kevin Kröninger PMN07 Blaubeuren, – Summary and outlook Segmented germanium detectors are valuable tool for topological information Prototype detectors (n-type and p-type) work well Monte Carlo predictions agree with data For GERDA: background rejection works reliably well described by Monte Carlo now: background expected to not be dominated by photons Choice of segmentation depends on physics process Rich experimental program ongoing at the MPI für Physik, Munich