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
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 - PowerPoint PPT Presentation
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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
Kevin Kröninger PMN07 Blaubeuren, 01.07. – 05.07.2007
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
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
60 l dewar with lN2
Pre-amplifiers and filters
I. A
bt
et a
l. N
IMA
577
(20
07)
574
Kevin Kröninger PMN07 Blaubeuren, 01.07. – 05.07.2007
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
et a
l. N
IMA
577
(20
07)
574
Kevin Kröninger PMN07 Blaubeuren, 01.07. – 05.07.2007
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
Kevin Kröninger PMN07 Blaubeuren, 01.07. – 05.07.2007
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
Kevin Kröninger PMN07 Blaubeuren, 01.07. – 05.07.2007
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 :
Kevin Kröninger PMN07 Blaubeuren, 01.07. – 05.07.2007
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
Kevin Kröninger PMN07 Blaubeuren, 01.07. – 05.07.2007
Example 2: neutron scattering
74Ge(n, n‘ γ)
require Nseg = 2 and Eany = 596 keV
208Tl 214Bi
Eseg2 [keV]
to be published
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
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
Kevin Kröninger PMN07 Blaubeuren, 01.07. – 05.07.2007
Example 4: alpha veto
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