Digital Pulse-Shape Discrimination

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Digital Pulse-Shape Discrimination Detector
Segmentation for the Majorana Project

• C. E. Aalseth
• National Security Division,
• Radiological Chemical Sciences Group
• Pacific Northwest National Laboratory

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Majorana Overview(http//majorana.pnl.gov)

• 0nbb decay of 76Ge potentially measured at 2039
  keV
• Sensitive to effective Majorana n mass as low as
  0.02-0.07 eV
• Based on well-known 76Ge detector technology
  plus
• Pulse-shape analysis
• Detector segmentation
• Ready to begin now
• Requires
• Deep underground location
• 500 kg enriched 85 76Ge
• 210 crystals, 8 segments each
• Advanced signal processing
• Special materials (low bkg)

Baseline Concept
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Physics Motivation for Background Rejection

• Interaction multiplicity varies with energy, type
  of radiation
• Multiple interactions change shape of induced
  detector current

• Multi-site events
• Full-energy gamma signals gt500 keV
• Sum-energy peak signals
• Single-site events
• Gamma signals lt150 keV
• 72Ge(n, n e-) fast neutrons
• Double-escape peaks
• Internal beta-decay (no g) events

G. F. Knoll. Radiation detection and measurement.
John Wiley Sons, Inc., second edition, 1989.
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Why Now is a good time for PSD.

• Commercial digital spectroscopy hardware is
  available with fast (40 MHz), high-resolution
  (14-bit) digitization
• Significant developments in pulse-shape
  discrimination techniques for HPGe have been made
  in the past 10 years and are ready to apply to
  new hardware

Full-energy 1621-keV g (top) and 1592-keV DEP
(bottom) reconstructed current pulses from 120
P-type Ortec HPGe detector (experimental data)
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Starting Background Estimate

• International Germanium Experiment (IGEX)
  achieved between 0.1-0.3 cts/keV/kg/y
• Documented experiences with cosmic secondary
  neutron production of isotopes

Calculated Experimental
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Low-Background Electroformed CopperKey
Technologies

• Semiconductor-grade acids
• Glassware-free handling
• Copper sulfate purified by recrystallization
• Baths circulated with continuous microfiltration
  to remove oxides and precipitates
• Continuous barium scavenge removes radium
• Cover gas in plating tanks reduces oxide
  formation
• Periodic surface machining during production
  minimizes dendritic growth

Low-background detector and electroformed
cryostat during assembly
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Low-Background Electroformed Copper Examples

• Purity established with International Germanium
  Experiment (IGEX) experience
• Strength equal to OFHC
• Can be easily formed into thin, low-mass parts
• Parts shown are for the MEGA array

Electroformed cups shown have wall thickness of
only 250 mm!
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Backgrounds 68Ge
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Backgrounds 60Co
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Single-site interaction example
Monte-Carlo 2038-keV deposition from 0n bb-decay
of 76Ge
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Multiple-site interaction example
Monte-Carlo 2038-keV deposition from
multi-Compton of 2615-keV 208Tl g
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PNNLMulti-parametric Discriminator

• Extracts key parameters from each preamplifier
  output pulse
• Sensitive to radial location of interactions and
  interaction multiplicity
• Self-calibrating allows optimal discrimination
  for each detector
• Discriminator can be recalibrated for changing
  bias voltage or other variables
• Method is computationally cheap, requiring no
  computed libraries-of-pulses

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PSD can reject multiple-site backgrounds (like
68Ge and 60Co)
Keeps 80 of the single-site DEP (double escape
peak)
Experimental Data
Rejects 74 of the multi-site backgrounds (use
212Bi peak as indicator)
Original spectrum
Scaled PSD result
Improves T1/2 limit by 56
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Fast Neutron Signature

• Reaction of interest is 72Ge(n,ne-)
• Signature is single-site (electron nuclear
  recoil)
• Sensitive to useful range of fast-neutron energies

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252Cf data result

• Result is virtually free of gamma interference
• Efficiency of PSD for the fast neutron signal is
  91
• Fast neutron peak/continuum ratio improved by a
  factor of 11
• Sensitivity increase of 300

total
Final result
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Detector Segmentation

• Sensitive to axial and azimuthal separation of
  depositions
• Perkin-Elmer design with six azimuthal and two
  axial contacts
• Projected efficacy of this design is excellent
  with expected backgrounds

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Monte-Carlo Example
Segment multiplicity at 2039 keV
Sensitive to z and phi separation of depositions
0nbb efficiency 91 internal 60Co efficiency
14 Improves T1/2 limit by 140
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Projected Sensitivity

• GIVEN
• Background at 2038 keV 0.2 cts/keV/kg/y
• 68Ge decay 10x reduction
• 60Co decay/self shielding/less copper mass 2x
  reduction
• 500 kg 86 76Ge x 10 years
• PSDSegmentation FOM 1.6 x 2.4 3.8
• RESULT
• 0n T½ 4.0 x 1027 y
• ltmngt 0.020 0.068 eV

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Conclusions

• Unprecedented confluence
• Krasnoyarsk availability/Neutrino mass interest/
  Underground development/Crystal capacity
• High Density
• reduced shielding and footprint
• Low Risk
• proven technology / modular instrument /
  relocatable
• Experienced Collaboration
• long bb track record
• Neutrino mass sensitivity
• potential for discovery