Background rejection techniques in Germanium 0νββ-decay experiments. ν=v

Background rejection techniques in Germanium 0νββ-decay experiments n p ν=v n eep II. Physikalisches Institut Universität Göttingen Institutsseminar des Inst. für Kern- und Teilchenphysik,

Outline Neutrinos in the Standard Model / double beta-decay Experimental aspects of double beta-decay / GERDA Background and background rejection n R&D projects and implications ν=v n Conclusions p eep

Outline Neutrinos in the Standard Model / double beta-decay Experimental aspects of double beta-decay / GERDA Background and background rejection R&D projects and implications Conclusions

Neutrino oscillations Observation: disappearance of solar and atmospheric neutrinos Solution: neutrino mixing and oscillations Mass- and flavor (interaction) eigenstates are not the same νμ ντ flux (x106 cm-2 s-1) Oscillations depend on Δm2 finite neutrino mass νe flux (x106 cm-2 s-1)

Open questions What is the absolute mass scale of the neutrino? What is the neutrino hierarchy? What is the nature of the neutrino? Some mass scales: Top-quark ~ 175 GeV Tritium beta-decay mν < 2.2 ev Cosmology mν < 1.0 ev Double-beta decay mν 0.4 ev Z, W ~ 100 GeV Electron ~ 500 kev Neutrinos < 1 ev Structure formation

Open questions What is the absolute mass scale of the neutrino? What is the neutrino hierarchy? Mass2 What is the nature of the neutrino? ν2 ν1 ν3 Δm 2 21 Δm232 ν2 ν1 Δm221 normal (NH) -Δm232 e μ τ Δm221 = 8.1 10-5 ev2 ν3 Δm232 = 2.2 10-3 ev2 inverted (IH)

Open questions What is the absolute mass scale of the neutrino? What is the neutrino hierarchy? What is the nature of the neutrino? E. Majorana or ν ν P. Dirac ν=ν

Double beta-decay Rare nuclear transition (2nd order weak process): 2νββ: (Z, A) (Z+2, A) + 2 e- + 2 ν ΔL = 0 (T1/2 ~ 1021 y) 0νββ: (Z, A) (Z+2, A) + 2 e- ΔL = 2 (T1/2 > 1025 y) Majorana neutrino (Z, A) euei W eνi νi Uei W Process violates lepton number conservation (Z + 2, A) NUCLEAR PROCESS

Double beta-decay When does double beta-decay occur? Initial nucleus lighter than intermediate nucleus but heavier than final nucleus (single beta-decay forbidden) Even-even nuclei (binding energy) About 35 candidate isotopes A = 76 single β-decay 76 Ge 76As forbidden double β-decay 76 Ge 76Se allowed

Final state: Daughter nucleus (neglect recoil) Electrons Experimental aspects of double beta-decay I 0νββ 2νββ 2 electrons, 2 or 0 neutrinos n n p eν ν ep continous electron spectrum (Te1+Te2 ) / Qββ 0νββ-decay 2νββ-decay Search for a narrow line n p ν n eep line-shaped electron spectrum

Previous (germanium) experiments IGEX (CanFranc) Enriched Ge detectors Total exposure ~8.87 kg y No signal observed T1/2 > 1.57 1025 y Heidelberg-Moscow (INFN) Enriched Ge detectors Total exposure ~71.7 kg y T1/2 > 1.9 1025 y Part of collaboration: claim T = 1.2 1025 y

New experiments Requirements for new experiments: Good energy resolution (0νββ vs. 2νββ) High Q-value (0νββ scales with Q5) Low background in the ROI high Q-value Large target mass: high natural abundance or enrichment High signal efficiency Low overall background in the ROI Background units: counts / (kg kev y) around Qββ total mass

GERDA: Technical realization Clean-room Lock system Water tank (steel) Muon veto (Č) Cryostat (steel + Cu) Liquid argon Detector array

Physics potential / sensitivity Phase I: 18 kg germanium Phase III 20 kg y exposure 10-2 counts/(kg kev y) Phase II: Phase II 35 kg germanium 100 kg y exposure 10-3 counts/(kg kev y) Phase I Phase III: 500 kg germanium <10-3 counts/(kg kev y) A. Caldwell, KK, Phys. Rev. D 74 (2006) 092003

Sources of background Background: processes which cause energy deposition at Q-value

Sources of background Cosmic muons Rad. isotopes Neutrons (rock) Rad. isotopes Photons (far) Rad. isotopes Photons (near) Rad. isotopes Electrons Muon induced neutrons (l. Ar/N) Alphas in surface contaminations Cosmogenic production (detector)

Sources of background Cosmic muons Rad. isotopes Neutrons (rock) Rad. isotopes Photons (far)! Rad. isotopes Photons (near) Rad. isotopes Electrons Muon induced neutrons (l. Ar/N) Alphas in surface contaminations Cosmogenic production (detector)

Background rejection techniques

Background rejection strategies Purity: Reduce amount of radioactive isotopes Reduce amount of dangerous isotopes Example: Majorana electroformed copper Goal: < 0.3 μbq/kg 208Tl Achieved: < 2-4 μbq/kg 232Th

Background rejection strategies Shielding: Cosmic muons Steel/Water/Copper/Argon shell 1400 m ~ 3.800 m.w.e

Background rejection strategies Granularity: photon / electron separation Tracking of particles Detector array Segmentation NEMO COBRA MPI f. Physik, Munich GERDA

Background rejection strategies Analysis of the time structure of the detector response: Pulse shape analysis: pulse contains information on position of energy deposition Used e.g. in AGATA (gamma-ray tracking) MPI f. Physik, Munich

Background rejection strategies Active muon veto: Scintillator plates Water Cherenkov Detector reduction by 2 orders of magnitude Univ. of Tübingen

Background rejection strategies Active veto using scintillation light from liquid Argon: Equip inner vessel with PMT Photon can scatter (energy deposition inside the Ge-detectors and liquid Argon) MPI f. Kernphysik, Heidelberg

Pu rit y Sh ie ld Tr. ac k. PS A Ve to Ti m in l. g A r Sources of background Cosmic muons Rad. isotopes Neutrons (rock) Rad. isotopes Photons (far) Rad. isotopes Photons (near) Rad. isotopes Electrons Muon induced neutrons (l. Ar/N) Alphas in surface contaminations Cosmogenic production (detector)

Techniques to distinguish electrons from photons

Electron/photon discrimination 0νββ-decay has two electrons (only) in the final state Photons with MeV-energies mostly Compton-scatter Sum of the kinetic energies at Q-value (2 039 kev for 76Ge) Electrons of O(1) MeV have a Range of photons O(1-5) cm in germanium Multi-site events range of ~ 1 mm in Ge Single-site events Aim: Distinguish between single-site (electrons) and multi-site events (photons) Single crystal Range log(r [mm]) I. Abt et al. Nucl.Instrum.Meth.A 570 (2007) 479

Electron/photon discrimination 0νββ-decay has two electrons (only) in the final state Photons with MeV-energies mostly Compton-scatter Sum of the kinetic energies at Q-value (2 039 kev for 76Ge) Electrons of O(1) MeV have a Range of photons O(1-5) cm in germanium Multi-site events range of ~ 1 mm in Ge Single-site events Aim: Distinguish between single-site (electrons) and multi-site events (photons) Segm. Single crystal Range log(r [mm]) I. Abt et al. Nucl.Instrum.Meth.A 570 (2007) 479

Electron/photon discrimination 0νββ-decay has two electrons (only) in the final state Photons with MeV-energies mostly Compton-scatter Sum of the kinetic energies at Q-value (2 039 kev for 76Ge) Electrons of O(1) MeV have a Range of photons O(1-5) cm in germanium Multi-site events range of ~ 1 mm in Ge Single-site events Aim: Distinguish between single-site (electrons) and multi-site events (photons) PSA Segm. Single crystal Range log(r [mm]) I. Abt et al. Nucl.Instrum.Meth.A 570 (2007) 479

Analysis chain I: energy cut Simulation 50 kg y, 10-3 counts/(kg kev y) Region of interest (ROI) Require energy 2 039 ± 5 kev

Analysis chain II: crystal anti-coincidence detectors electrons photons Select events with one crystal hit

Analysis chain III: segment anti-coincidence segments electrons photons Select events with one segment hit

Analysis chain IV: pulse shape analysis multiple interactions Neural network analysis

Example: 0νββ and Co-60 MC simulation of 21 detectors with 18-fold segmentation I. Abt et al. Nucl.Instrum.Meth.A 570 (2007) 479 I. Abt et al. Nucl.Instrum.Meth.A 570 (2007) 479

Test stand program at MPI Munich Prototype development Test operation of n-type and p-type detectors inside cryoliquid Test segmentation schemes Monte Carlo verification

Siegfried I. Abt et al. Nucl.Instrum.Meth.A 577 (2007) 574 18-fold segmented n-type detector Pre-amplifiers and filters Pre-amplifiers and filters 60 l dewar with ln2

Siegfried I. Abt et al. Nucl.Instrum.Meth.A 577 (2007) 574 18-fold segmented n-type detector 3-fold segment in height 6-fold segmented in azimuthal angle

Data to Monte Carlo comparison (Co-60) Channel ID I. Abt et al. Nucl.Instrum.Meth. A 583 (2007) 332 Core electrode spectrum (60Co) Average deviation ~5% Some features not in MC Substructure due to drift anisotropy of charge carriers Effective model in MC

Segment (anti-)coincidence I. Abt et al. Nucl.Instrum.Meth. A 583 (2007) 332 SFS = N (all) / N (single segment) study effective segmentation Geometry dependent Data and MC agree within <5% Add segment energies to 18-fold segmentation best

Pulse shape analysis DEP Tl-208 (SSE) Methods I. Abt et al. Eur.Phys.J. C 52 (2007) 19 I. Abt et al. Eur.Phys.J. C 54 (2008) 425 Library method PSA: Neural network output Likelihood method Distinction between single- Neural network Bi-212 photons (MSE) site and multi-site events

Example: Th-228 spectrum DEP Tl-208 Core energy spectrum Correct identification of final Segmentation reduces states Compton-background in ROI Bi-212 photons PSA confirms signal

Implications for GERDA

Monte Carlo simulation GEANT4-based Monte Carlo simulation (MaGe): Idealized Phase II setup 21 18-fold segmented detectors Liquid nitrogen (original proposal) (Hopefully) complete list of background components Impurities from screening experiments (ongoing) No pulse shape simulation / analysis included yet

Background reduction summary MC simulation of 21 detectors with 18-fold segmentation Part Source Detector Co-60 3.2 ± 0.1 38.3 ± 1.0 Ge-68 2.4 ± 0.1 18.0 ± 1.4 Tl-208 2.2 ± 0.4 4.6 ± 0.9 Bi-214 2.8 ± 0.5 6.0 ± 1.4 Co-60 6.7 ± 0.2 157 ± 27 Electronic Tl-208 1.5 ± 0.3 2.9 ± 0.6 Holder SFC SFS Segmentation improves background rejection by up to an order of magnitude

Background reduction summary Part Background contribution [10-4 counts/(kg kev y)] Detector Holder 68 Ge 10.8 (4.3) 60 Co 0.3 (0.3) Bulk 3.0 Surf. 3.5 Cu 1.4 Teflon 2.0 Cabling 7.6 Electronics 3.5 Liquid nitrogen 0.1 Infrastructure 2.9 Muons and neutrons 2.0 Total 37.1 Gain through segmentation factor ~10-30 Reducible via PSA Redesign: Reduce mass Will change with argon (30.6)

Conclusion Neutrinoless double beta-decay opens an exciting window to neutrino properties Experiments (GERDA) place emphasis on low background Identification and reduction of background processes important need complementary techniques Segmented germanium detectors have a great potential to distinguish between electron and photon induced events: Segment (anti-)coincidences Pulse shape analysis Rich experimental program ongoing Awaiting GERDA data...