1.  Starting Points (Collaboration ; Laboratories ; Research Program)  Physics & Requirements for ULE-HPGe - focus on WIMP Searches  R&D on ULE-HPGe Prototypes  Status & Plans Ultra-Low-Energy HPGe Detector for WIMP Searches @ Henry T. Wong / 王子敬 Academia Sinica / 中央研究院

2. TEXONO Collaboration  Collaboration : Taiwan (AS, INER, KSNPS, NTU) ; China (IHEP, CIAE, THU, NKU, NJU) ; Turkey (METU) ; India (BHU)  Close Partnership with : Korea (KIMS)  Facilities : Kuo-Sheng Reactor Lab (Taiwan) ; Yang-Yang Underground Lab (Korea) ; CIAE Neutron Beam (China)  Program : Low Energy Neutrino & Astroparticle Physics  Goals : Develop O[100 eV threhold  1 kg mass  1 cpd detector] for neutrino physics and dark matter searches

3. Kuo Sheng Reactor Neutrino Laboratory : NPS-IV  28 m from core#1 @ 2.9 GW  Different detectors for different physics

4. R&D :  Coh. ( N)  T < 1 keV Completed Results :   ( e ) : T~1-100 keV [Lin ST’s talk]  Axions : T ~ keV-MeV [Chang HM’s talk ] On-Going Data Taking & Analysis  SM  ( e)  T > 2 MeV Reactor Neutrino Interaction Cross-Sections massqualityDetector requirements 1 counts / kg-keV-day Threshold ~ 100 eV

5.  Operated by KIMS Collaboration, 700 m of rock overburden in east Korea  flagship program on CsI(Tl) for CDM searches [SK Kim’s talk]  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 Yang-Yang Underground Laboratory Y2L

6. “Ultra-Low-Energy” HPGe Detectors  ULEGe – developed for soft X-rays detection ; easy & inexpensive & robust operation  Prototypes built and studied :  5 g @ Y2L  4 X 5 g @ KS/Y2L  10 g @ AS/CIAE  segmented 180 g @ AS/KS  Being built : 500 g single element  Physics for O[100 eV threhold  1 kg mass  1 cpd detector] :  N coherent scattering  Low-mass WIMP searches  This Talk  Improve sensitivities on   Implications on reactor operation monitoring  Open new detector window & detection channel available for surprises

7. Sensitivity Plot for CDM- WIMP direct search Low (<10 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

8. ULEGe-Prototype built & being studied : 5 g 10 g 4 X 5 g Segmented 180 g with dual readout

9.  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 R&D Program towards Realistic O(1 kg) Size Experiments (both N & CDM) :

10. A PSD Scheme to differentiate Physics Vs Noise Events  Calibrations by keV lines & “0” from random trigger  Achieved threshold < 100 eV with Fe55 source : lowest for bulk radiation detectors !  Threshold depends on background level PSD Cut Threshold ~ 100 eV

11. Single Readout Event ID – correlate two channels with different gains & shaping times e.g. Energy as defined by trigger-Channel 1 Sampling of Specific Range for non-trigger-Channel 2 – i.e. look for +ve fluctuations at specific and known times 4 X 5 g Signal Noise Ch #1 : Ch #2 :

12. Evaluation of Selection Efficiency:  Select clean sample of physics events with cosmic-ray and anti-Compton tags  Study survival probabilities of these with the independent selection cuts on Ge-signals Good efficiency > 200 eV for low background KS data with 4X5 g

13. Dual Readout Event ID – correlate anode/cathodes in amplitude & timing SignalNoise Anode : Cathode : e.g. Seg. 180 g Peak Position Correlations between Electrodes

14.  Similar background at KS & Y2L for same detector  Apparent difference between 5 g & 1 kg at T> 5 keV due to scaling with surface area instead, reproduced in simulations  Best Background with 4X5 g comparable to CRESST-1 after corrections due to quenching factor  Intensive studies on background understanding under way 5 g 4 X 5 g 1 kg Sub-keV Background Measurements & Comparisons

15. WIMP Spin-Independent Couplings : Limits & Sensitivities Standard conservative analysis – WIMP rates cannot be higher than total events measured

16. Quenching Factor Measurement for Ge at CIAE’s Neutron Facilities: Goals for Oct 07 Runs :  Use actual ULEGe 100-eV detector  Use lower energy neutron beam with a smaller tandem With 13 MV Tandem

17. Detector Scale-up Plans:  500-g, single-element, modified coaxial HPGe design, inspired by successful demonstration of Chicago group (nucl-ex/0701012)  Dual-electrode readout and ULB specification  Being built, available first-half 2008. S0 S1 p

18. Summary & Outlook (Score Card)  An O[100 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  Mass Scale-Up: recent demonstration of realistic design  Threshold – ~300 eV at hardware level, intensive studies on software techniques to aim at ~100 eV, & on their stabilities and universalities  Prototype data at reactor already provide competitive sensitivities for WIMP search at mass<10 GeV.  Sub-keV Background understanding and suppression – under intensive studies