WIN05 Talk

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Ultra-Low Energy Germanium Detectors for
Neutrino-Nucleus Coherent Scattering and Dark
Matter Searches

• Starting Points (Collaboration Laboratory )
• 1-kg HPGe Magnetic Moment Results
• Physics Requirements of ULEGe Detectors
• RD on ULEGe Prototypes
• Results on Cold Dark Matter Searches
  (arXiv0712.1645)
• Status Plans

Henry T. Wong /??? Academia Sinica /?????
_at_
OCPA Workshop July 2008 Hong Kong
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TEXONO Collaboration
Collaboration Taiwan (AS, INER, KSNPS, NTU)
China (IHEP, CIAE, THU, NJU) Turkey (METU)
India (BHU) Program Low Energy Neutrino
Astroparticle Physics
Kuo Sheng (??) Power Reactor
KS NPS-II 2 cores ? 2.9 GW
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Kuo Sheng Reactor Neutrino Laboratory
Front Gate

• 28 m from core1 _at_ 2.9 GW
• Shallow depth 30 meter-water-equivalent
• Reactor Cycle 50 days OFF every 18 months

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Front View (cosmic vetos, shieldings, control
room ..)

• Configuration Modest yet Unique
• Flexible Design Allows different detectors
  conf. for different physics

Inner Target Volume
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Reactor Neutrino Interaction Cross-Sections
1 counts / kg-keV-day
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Reactor Neutrino Spectra Evaluation
Reactor Operation Data
Nuclear Physics
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Neutrino Electromagnetic Properties Magnetic
Moments
requires mn?0
e.g.

• a conceptually rich subject much neutrino
  physics astrophysics can be explored
• n-osc. Dmn , Uij
• 0nbb mn , Uij , nD/nM
• mn mn , Uij , nD/nM , n ? g
• fundamental neutrino properties interaction
  necessary consequences of neutrino masses/
  mixings in principle can differentiate
  Dirac/Majorana neutrinos
• explore roles of neutrinos in astrophysics

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Magnetic Moment Searches _at_ KS

• simple compact all-solid design HPGe (mass 1
  kg) enclosed by active NaI/CsI anti-Compton,
  further by passive shieldings cosmic veto
• selection single-event after cosmic-veto,
  anti-Comp., PSD
• TEXONO data (571/128 days) ON/OFF) PRL 90, 2003
  PRD 75, 2007
• background comparable to underground CDM
  experiment
• 1 day-1keV-1kg-1 (cpd)
• DAQ threshold 5 keV
• analysis threshold 12 keV

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Direct Experiments at Reactors
KS Limit mn(ne) lt 7.4 X 10-11 mB _at_ 90 CL

• Search of mn at low energy
• ? high signal rate robustness
• mngtgtSM decouple irreducible bkg ? unknown
  sources
• T ltlt En ? ds/dT depends on total fn flux but NOT
  spectral shape flux well known 6 fission-n ?
  1.2 238U capture-n per fission

? GEMMA-07 (GeNaI) mn(ne) lt 5.8 X 10-11 mB
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Ultra-Low-Energy HPGe Detectors

• ULEGe developed for soft X-rays detection
  easy inexpensive robust operation
• Prototypes built and studied (starting 2003)
• 5 g _at_ Y2L
• 4 X 5 g _at_ KS/Y2L
• 10 g _at_ AS/CIAE
• segmented 180 g _at_ AS/KS
• Built being studied 500 g single element
• ? Goal O100 eV threhold?1 kg mass?1 cpd
  detector ? physics
• nN coherent scattering
• Low-mass WIMP searches
• Improve sensitivities on mn
• Implications on reactor operation monitoring
• Open new detector window detection channel
  available for surprises

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Neutrino-Nucleus Coherent Scattering

• a fundamental neutrino interaction never been
  experimentally-observed
• s?N2 applicable at Enlt50 MeV where q2r2lt1
• a sensitive test to Stardard Model
• an important interaction/energy loss channel in
  astrophysics media
• a promising new detection channel for neutrinos
  relative compact detectors possible (implications
  to reactor monitoring) the channel for WIMP
  direct detection !
• involves new energy range at low energy, many
  experimental challenges much room to look for
  scientific surprises

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Expected Interaction Rates at KS _at_ different
Quenching Factors
? by-product Tgt500 eV gives mn(ne) ? 10-11
mB at 1 cpd background
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Sensitivity Plot for CDM-WIMP direct search

• Low (lt10 GeV) WIMP Mass / Sub-keV Recoil Energy
• Not favored by the most-explored specific models
  on galactic-bound SUSY-neutralinos as CDM still
  allowed by generic SUSY
• Solar-system bound WIMPs require lower recoil
  energy detection
• Other candidates favoring low recoils exist e.g.
  non-pointlike SUSY Q-balls.
• Less explored experimentally

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ULEGe-Prototype built studied
5 g
10 g
4 X 5 g
Segmented 180 g with dual readout
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RD Program towards Realistic O(1 kg) Size
Experiments (both nN CDM)

• measure study background at sub-keV range at KS
  Y2L design of active passive shielding
  based on this.
• compare performance and devise event-ID (PSD
  coincidence) strategies of various prototypes
• devise calibration efficiency evaluation
  schemes applicable to sub-keV range
• measure quenching factor of Ge with neutron beam
• study scale-up options ULEGe-detector
• Keep other detector options open

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Yang-Yang Underground Laboratory

• Operated by KIMS Collaboration, 700 m of rock
  overburden in east Korea
• flagship program on CsI(Tl) for CDM searches
• TEXONO ? Install 5 g ULB-ULEGe at Y2L Study
  background and feasibility for CDM searches may
  evolve into a full-scale O(1 kg) CDM experiment

Y2L
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Single Readout Event ID correlate channels
with different gains shaping times
e.g.
4 X 5 g
Sampling of Specific Range for non-trigger-Channel
2 i.e. look for ve fluctuations at specific
and known times
Energy as defined by trigger-Channel 1
Signal
Noise
Ch 1
Ch 2
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Dual Readout Event ID correlate anode/cathodes
in amplitude timing
Peak PositionAmplitue Correlations in Electrodes
Calibration spectrum (55Fe) Raw/PSDs
Threshold 250 eV
Seg. 180 g
Signal
Noise
Cathode
Anode
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• KS In Situ Data - Calibration, PSD Efficiency
  Evaluation
• Linear 0-60 keV, better than 1
• Stability better than 5
• Threshold 220 eV at 50 efficiency by two
  independent methods

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Sub-keV Background Measurements Comparisons
4 X 5 g (1 yr apart)
1 kg
5 g

• Similar background at KS Y2L for same detector
• Apparent difference between 5 g 1 kg at Tgt 5
  keV due to scaling with surface area instead,
  reproduced in simulations
• Best Background with 4X5 g comparable to CRESST-1
• Intensive studies on background understanding
  under way

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• Some first understanding on background
• Expect self-shielding effects when detector
  scales up from O(10 g) to O(1 kg) IF bkg is
  external
• g-induced bkg is flat while n-induced bkg
  increase with lower recoil energy.

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From Background to Limits WIMP Spin-Independent
Couplings
Standard conservative analysis WIMP rates
cannot be higher than total events measured
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WIMP Spin-dependent Couplings
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Road Map Threshold Vs Background
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Quenching Factor Measurement for Ge at CIAEs
Neutron Facilities
With 13 MV Tandem

• Goals for 2008 Runs
• Use actual ULEGe 100-eV detector
• Use lower energy neutron beam (860 keV?50 keV)
  with a pulsed-LE tandem

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Detector Scale-up Plans

• 500-g, single-element, modified coaxial HPGe
  design, following successful demonstration of
  Chicago group
• Position-sensitive from drift-profile pulse shape
• Dual-electrode readout and ULB specification
• Delivered, being studied.

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• Another Possible Design for O(1 kg) Segmented
  ULEGe
• 3X3X5 elements _at_ 20 g each (i.e. 900 g)
• Dual readout per element
• veto ring ? lids

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Summary Outlook

• An O100 eV threshold?1 kg mass?1 cpd detector
  has interesting applications in neutrino and dark
  matter physics, also in reactor monitoring
• Open new detector window detection channel
  potentials for surprise
• Dont know what to expect what are expected
• Mass Scale-Up recent demonstration of realistic
  design
• Threshold 300 eV at hardware level 200 eV
  demonstrated with software intensive studies
  under way ? goal 100 eV
• Prototype data at reactor already provide
  competitive sensitivities for WIMP search at
  masslt10 GeV .
• Sub-keV Background understanding and suppression
  under intensive studies