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Segmented germanium detectors in 0 νββ -decay experiments. Outline: Segmentation in 0 νββ -decay experiments Example 1: photons vs. electrons Example 2: neutron scattering Example 3: 2 νββ -decay into excited states Example 4: alpha veto Summary and outlook. Kevin Kröninger
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Segmented germanium detectors in 0νββ-decay experiments • 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 (Max-Planck-Institut für Physik, München) PMN07 Blaubeuren 01.07. - 05.07.2007
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, 01.07. – 05.07.2007
Example 1: electrons vs. photons I Range log(R [mm]) • 0νββ-decay has two electrons (only) in the final state • Sum of the kinetic energies at Q-value (2 039 keV for 76Ge) • 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) PSA Segm. Single crystal Kevin Kröninger PMN07 Blaubeuren, 01.07. – 05.07.2007
Example 1: electrons vs. photons II 18-fold segmented n-type detector Pre-amplifiers and filters Pre-amplifiers and filters I. Abt et al. NIMA 577 (2007) 574 60 l dewar with lN2 Kevin Kröninger PMN07 Blaubeuren, 01.07. – 05.07.2007
Example 1: electrons vs. photons II 18-fold segmented n-type detector 3-fold segment in height I. Abt et al. NIMA 577 (2007) 574 6-fold segmented in azimuthal angle Kevin Kröninger PMN07 Blaubeuren, 01.07. – 05.07.2007
Example 1: electrons vs. photons III I. Abt et al. arxiv:nucl-ex/0701005 Channel ID • Core electrode spectrum (60Co) • 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 Kevin Kröninger PMN07 Blaubeuren, 01.07. – 05.07.2007
Example 1: electrons vs. photons IV I. Abt et al. arxiv:nucl-ex/0701005 • Suppression factor SFL= N (all) / N (single segment) • Data and MC agree (dev. <5%) • Add segment energies to study effective segmentation • 18-fold segmentation best Kevin Kröninger PMN07 Blaubeuren, 01.07. – 05.07.2007
Example 1: electrons vs. photons V GERDA expectation: 21 detectors with 18-fold segmentation I. Abt et al. NIMA 570 (2007) 479 Monte Carlo study: segmentation improves background rejection by up to an order of magnitude, depending on source Kevin Kröninger PMN07 Blaubeuren, 01.07. – 05.07.2007
Example 2: neutron scattering • Study AmBe neutron source with 18-fold prototype detector • Segmentation allows to observe recoil spectrum • Example: inelastic scattering : 74Ge(n, n‘ γ) γ Event selection: Nseg = 2 Eany = 596 keV (select photon) recoiling nucleus n Kevin Kröninger PMN07 Blaubeuren, 01.07. – 05.07.2007
Example 2: neutron scattering • Study AmBe neutron source with 18-fold prototype detector • Segmentation allows to observe recoil spectrum 74Ge(n, n‘ γ) 208Tl 214Bi Eseg2 [keV] to be published require Nseg = 2 and Eany = 596 keV Kevin Kröninger PMN07 Blaubeuren, 01.07. – 05.07.2007
Example 3: 2νββ into excited states I • Double beta-decay of 76Ge can populate excited states of 76Se • Observation could help check reliability of nuclear models • Signature for 01+ 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/0702030 Kevin Kröninger PMN07 Blaubeuren, 01.07. – 05.07.2007
Example 3: 2νββ into excited states II • GERDA: Monte Carlo simulation of the decay and possible background contributions (60Co, 68Ge, 2νββ, ...) • Sensitivity (for 100 kg·y): • Two orders of magnitude above current limit of T1/2 > 6.2·1021 y • Allows testing of predictions with T1/2 ~ 7.5·1021 y – 3.1·1023 y KK, L. Pandola, V. Tretyak arxiv:nucl-ex/0702030 Kevin Kröninger PMN07 Blaubeuren, 01.07. – 05.07.2007
Example 4: alpha veto • Evidence for surface contaminations in previous experiments • Alpha decays of 210Pb daughter can cause energy depositionin 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 veto Kevin Kröninger PMN07 Blaubeuren, 01.07. – 05.07.2007
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 Kevin Kröninger PMN07 Blaubeuren, 01.07. – 05.07.2007
