PMN07 Blaubeuren 01.07. - 05.07.2007 Segmented germanium detectors in 0νββ-decay experiments...

PMN07 Blaubeuren 01.07. - 05.07.2007

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, 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


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 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) Range log(R [mm])

PSA Segm.

Single crystal


Example 1: electrons vs. photons II

18-fold segmented n-type detector

Pre-amplifiers and filters

60 l dewar with lN2

Pre-amplifiers and filters

I. A

bt e

t al.

NIM

A 5

77 (2

007)

574


Example 1: electrons vs. photons II

18-fold segmented n-type detector

6-fold segmented in azimuthal angle

3-fold segment in height

I. A

bt e

t al.

NIM

A 5

77 (2

007)

574


Example 1: electrons vs. photons III

• 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

Channel ID

I. Abt et al. arxiv:nucl-ex/0701005


Example 1: electrons vs. photons IV

• 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

I. Abt et al. arxiv:nucl-ex/0701005


Example 1: electrons vs. photons V

Part Source SFC SFS

Detector Co-60 3.2 ± 0.1 38.3 ± 1.0

Ge-68 2.4 ± 0.1 18.0 ± 1.4

Holder 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

Electronics Tl-208 1.5 ± 0.3 2.9 ± 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 :


Example 2: neutron scattering

74Ge(n, n‘ γ)

nrecoilingnucleus

γEvent selection:

Nseg = 2

Eany = 596 keV

(select photon)

• Study AmBe neutron source with 18-fold prototype detector

• Segmentation allows to observe recoil spectrum


Example 2: neutron scattering

74Ge(n, n‘ γ)

require Nseg = 2 and Eany = 596 keV

208Tl 214Bi

Eseg2 [keV]to be published


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


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

Detector array T1/2 limit (90% prob.)

21 unseg. detectors 2.2·1023 y

21 18-fold segm. det. 5.6·1023 y

KK, L. Pandola, V. Tretyak arxiv:nucl-ex/0702030

• Evidence for surface contaminations in previous experiments

• Alpha decays of 210Pb 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


Example 4: alpha veto

veto


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

PMN07 Blaubeuren 01.07. - 05.07.2007 Segmented germanium detectors in 0νββ-decay experiments...

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