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Surface events suppression in the germanium bolometers EDELWEISS experiment

Surface events suppression in the germanium bolometers EDELWEISS experiment. Xavier-François Navick (CEA Dapnia) TAUP2007 - Sendai September 07. +++. - - -. Ionization-heat detectors. T = 20 mK. Thermal Sensor (NTD Ge). Al electrode. WIMP neutron X or   rays. Biais amplifier.

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Surface events suppression in the germanium bolometers EDELWEISS experiment

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  1. Surface events suppression in the germanium bolometers EDELWEISS experiment Xavier-François Navick(CEA Dapnia) TAUP2007 - Sendai September 07

  2. +++ --- Ionization-heat detectors T = 20 mK Thermal Sensor (NTD Ge) Al electrode WIMPneutronX orrays Biais amplifier V = 1-6 Volts D = 2 cm Charge amplifier Al electrodes (charge collection ) HPGesingle crystal TAUP2007 X-F Navick September 2007

  3. Rejection power Simultaneous measurement of ionization and heat => Evt per evt identification Q = Eionisation / Erecul Q = 1for electronic recoil (ambiant radioactivity) Q  0.3for nuclear recoil (WIMP and neutron)discrimination g/n > 99.9% pour Er> 15keV TAUP2007 X-F Navick September 2007

  4. Edelweiss-II Ge/NTD detectors • Developed by CEA Saclay and Canberra • Optimized NTD size in collaboration with LBNL for sub keV resolution • New holder and connectors (Teflon and copper only) TAUP2007 X-F Navick September 2007

  5. Incomplete charge collection • Amorphous layers (Ge or Si) improve the charge collection • At very low T and V : electric field is screened => diffusion phase • Some carriers are trapped in the “wrong” electrode => incomplete collection TAUP2007 X-F Navick September 2007

  6. Effect of the amorphous layers TAUP2007 X-F Navick September 2007

  7. Edelweiss-I data with phonon trigger 2003 I TAUP2007 X-F Navick September 2007

  8. Surface contamination E = 5.3 MeV, Q = 0.3 • a’s from 210Po (Ea=5.3 MeV) • Q=0.3  a decays near surfaces • Rate ~ 400 /m²/d • As expected, non-fiducial part more exposed • 210Pb on Cu covers or Ge surfaces • Should see Pb recoils and b’s • No 206Pb recoil peak at 100 keV observed as heat-only events: 210Pb implanted in Cu, not Ge. • Rate of 0.3 < Q < 1.0 events at low energy consistent with expected surface b’s • does not exclude contribution from 14C • By removing Cu between detectors, these events should disappear, or ID by coincidences TAUP2007 X-F Navick September 2007

  9. Edelweiss-II first data taking TAUP2007 X-F Navick September 2007

  10. Edelweiss-II first data taking 100 keV recoils, no ionization 5.3 MeV alpha TAUP2007 X-F Navick September 2007

  11. Surface events suppression Initial effort : improve the cupper treatment (Rn contamination study) and reduce the surface exposure to radon  Passive rejection : improve the charge collection for surface event Physics of the Ge and Si amorphous underlayer Detectors with thick electrodes Active rejection : identification of the surface events Interleaved electrodes localization of the event  Pulse shape analysis of the charge signals Detectors sensitive to athermal phonons Ge/NbSi detectors TAUP2007 X-F Navick September 2007

  12. aGe:H aGe:H Al Ge Al Passive protection • Amorphous layers: Al / aGe:H / cGe / aGe:H / Al => GGA 20mK --- E +++ • Thick electrodes (high entrance window) TAUP2007 X-F Navick September 2007

  13. 24 mm NTD Ge thermometer electrodes guard ring Cross-section Top view Interleaved electrodes design • 200 g Ge crystal • Hydrogenated a-Ge underlayer for improved charge collection • Annular aluminum electrodes, interconnected by ultrasonic bonding • (strip width: 200 mm; pitch: 2 mm) • 7 measurement channels: 6 charge (7 MHz bandwidth) + heat (Ge NTD) TAUP2007 X-F Navick September 2007

  14. Mode of operation channel b (Vb) channel a (Va) Bulk events: Qa = Qc = 0 Qb = - Qd Surface events (top surface): Qc = Qd = 0 Qa0 & Qb0 (bottom surface): Qa = Qb = 0 Qc0 & Qd0 guard g (Vg) near- surface event low-field area Type III: low-field area bulk event NTD Ge thermometer guard h (Vh) channel c (Vc) channel d (Vd) * Voltage biases: Va = 1V, Vb = 2V, Vc = -1V, Vd = -2V, Vg = 0.5V, Vh = -0.5V TAUP2007 X-F Navick September 2007

  15. Rejection of the surface event 241AmgsourceCuts: |Qa| > 2 keV& |Qb + Qd| | > 2 keV (e.e.) event rejected before selection: 3596 evts after selection: 1134 evts Ionisation yield Ionisation yield Events of incomplete charge collection Erecoil(keV) Erecoil(keV) Voltage biases: Va= -0.25V, Vb= 2V, Vc= 0.25V, Vd= - 2V, Vg= 0.5V, Vh= - 0.5V Trigger threshold in ionization: 12 keV (e.e.) Baseline energy resolution of the ionization channels: 1 keV (e.e.) Heat channel resolution: 4 keV (FWHM) T = 17 mK TAUP2007 X-F Navick September 2007

  16. Reduction of the fiducial volume 241Am g source + 252Cf neutron source before selection: 3472 evts after selection: 1058 evts Ionisation yield Ionisation yield 38 evts 32 evts Erecoil(keV) Erecoil(keV) Loss in fiducial volume: (38-32) / 38~ 15 % TAUP2007 X-F Navick September 2007

  17. 1000 ns Near-surface, 122 keVgevent: centre electrode signal inred guard electrode signal inblue best fit simulation in black Localization by ionization shape analysis Test at LSM on a 320g Edw-I detector * Experimental setup (-1 V collection voltage) Event location Due to electronic noise, surface event rejection by pulse-shape analysis is limited in practice to high-energy events only (>50 keV e.e.) TAUP2007 X-F Navick September 2007

  18. Sensor A Sensor A Sensor A sensor A’s response Responses of sensors A and B Sensor B Sensor B sensor B’s response Ge detectors with NbSi sensors Surface event discrimination: Nb-Si thin films as athermal phonon sensors developed in CSNSM in Orsay Surface event discrimination: Nb-Si thin films as athermal phonon sensors developed in CSNSM in Orsay See Claudia Nones’s talk Surface event Bulk event Thermal component (energy-sensitive only) Athermal phonon (position-sensitive) component of signals Signal amplitude (a.u.) Time (s) TAUP2007 X-F Navick September 2007

  19. 200g Ge detector with NbSi sensors in the Edw-I set-up at LSM 57Co g source (a) Ionization yield vs. recoil energy (thermal component of heat signals only): note the large population of surface events of poor charge collection (b) With the proper cuts in the athermal component of phonon signals to remove surface events Ionisation yield (a.u.) Recoil energy (keV) Recoil energy (keV) TAUP2007 X-F Navick September 2007

  20. Summary The surface events appears to limit the sensitivity of ionization-heat germanium bolometers to WIMP. Different techniques are under development in Edelweiss to reduce or reject these events. The most promising ones lead to an active discrimination by athermal phonon measurement with NbSi thin film or by charge collection on interleaved electrodes. We are starting to apply these techniques in LSM with prototype detectors. In the next step of Edelweiss-II we will produce massive bolometers with active rejection. TAUP2007 X-F Navick September 2007

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