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M. Di Marco, P. Peiffer , S. Schönert

LArGe A L iquid Ar gon Ge rmanium hybrid detector system in the framework of the GERDA experiment. M. Di Marco, P. Peiffer , S. Schönert. Thanks to Marik Barnabe Heider. Cryogenic Liquit Detectors for Future Particle Physics workshop, LNGS 13th-14th March 2006. Outline.

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M. Di Marco, P. Peiffer , S. Schönert

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  1. LArGeA Liquid Argon Germanium hybrid detector system in the framework of the GERDA experiment M. Di Marco, P. Peiffer, S. Schönert Thanks to Marik Barnabe Heider Cryogenic Liquit Detectors for Future Particle Physics workshop, LNGS 13th-14th March 2006

  2. Outline • Introduction: GERDA • Energy resolution of bare Ge-diodes in LAr • Experimental Setup of LArGe@MPI-K • DAQ • Operational parameters • Light yield • Background spectrum • Characterization with various -sources • 137Cs, 60Co, 226Ra, 232Th • bkgd suppression in RoI • Outlook on LArGe@LNGS • Conclusions

  3. GERDA – GERmanium Detector Array Physics goal: search for 0ββ-decay   majorana or dirac particle? Method: operate bare, 76Ge enriched, HP-Ge-diodes in LN (or LAr) Signal: single-site events in HP-Ge-diode (Qßß=2039 keV) Background: - compton or summation, µ-induced, ... Physics reach: Phase I: 15 kg*y, existing diodes (HdM, IGEX) sensitivity goal: T1/2 > 3*1025 y mee < 0.24 – 0.77 eV Phase II: 100 kg*y, increased mass, new diodes, additional active background suppression. sensitivity goal: T1/2 > 2*1026 y mee < 0.09 – 0.29 ev H2O LN/LAr Ge GERDA @ LNGS Challenge: reduce background at 2039 keV by ~102 10-3 cts/(kg*keV*y)

  4. Background suppression in GERDA • LN as passive shielding (baseline design) • Cerenkov-muon-veto (Phase I) • Anti-coincidence with adjacent crystals (Phase I) • Pulse shape discrimination (Phase I) • Time correlation between events (Phase I) • Detector-segmentation (Phase II) • LAr scintillation anti-coincidence (option for Phase II) LArGe@MPI-K: R&D experiment operating HP-Ge-diode in LAr. With simultaneous LAr-scintillation-light readout.

  5. 1.33 MeV Energy resolution of a bare 2kg HP-Ge-diode in LAr 1.17 MeV 1.33 MeV FWHM 2.3 keV 40K summation 208Tl  No deterioration of energy-resolution for bare p-type detectors in LAr !

  6. Outline • Introduction: GERDA • Resolution of bare Ge-diodes in LAr • Experimental Setup of LArGe@MPI-K • DAQ • Operational parameters • Light yield • Background spectrum • Characterization with various -sources • 137Cs, 60Co, 226Ra, 232Th • bkgd suppression in RoI • Outlook on LArGe@LNGS • Conclusions

  7. LArGe@MPI-K: Schematic system description • Bare p-type HP-Ge-diode • Dewar ∅29 cm, h=65 cm • Light detection: WLS (VM2000) • + PMT(8“, ETL 9357-KFLB ) • Active volume ∅20 cm, h=43 cm • ≈ 19 kg LAr • Shielding: 5 cm lead • + 15 mwe underground - Measurements: Internal source - Background from crystal holders External source - Background from walls

  8. Electronics Shaping 3 µs Shaping 3 µs Trigger on Ge-signal Record Ge-signal and LAr-signal simultaneously Coincidence time 6 µs Software cut on recorded data LAr

  9. Operational parameters Canberra p-type crystal (390 g) Data taking: Sept. 05 – Dec. 05 • Stability monitored by: • peak position • energy resolution • leakage current Energy resolution: ~4.5 keV FWHM w/o PMT ~5 keV with PMT At 1.33 MeV 60Co-line * Threshold at single pe (~ 2.5 keV) ** Coincidence time: 6 µs Background suppression is not compromised by signal loss due to random coincidences ! Energy resolution limited in this setup.

  10. Photo-electron yield in LArGe@MPI-K 57Co peak in LAr spe – peak (LED generated) 122 keV - 86% 136 keV - 11% Source position: • 57Co-peak at ch 2153, peak energy 123.5 keV • spe-peak at ch (122.4 ± 3), pedestal at ch 81 • photo-electron yield L = (407 ± 10) pe/MeV - Possible to improve light yield with TPB (WARP)

  11. Background spectrum (LArGe@MPI-K) Ge signal (no veto) 40K 40 counts/h 208Tl 10 counts/h Ge signal after veto: fraction of the signal which „survives“ the cut energy in Ge (MeV) Time of data taking: 2 days

  12. Outline • Introduction: GERDA • Resolution of bare Ge-diodes in LAr • Experimental Setup of LArGe@MPI-K • DAQ • Operational parameters • Light yield • Background spectrum • Characterization with various -sources • 137Cs, 60Co, 226Ra, 232Th • bkgd suppression in RoI • Outlook on LArGe@LNGS • Conclusions

  13. Characterization with different sources full energy peak : no suppression with LAr veto • 137Cs : single  line at 662 keV Compton continuum: suppressed by LAr veto

  14. real data 137Cs 662 keV ~ 100% survival Compton continuum: 20% survival • very well reproduced by MC(MaGe): • shape of energy spectrum • peak efficiency • peak/Compton ratio • survival probability simulations 662 keV 100% survival Compton continuum: 20% survival

  15. Characterization with different sources full energy peaks : no suppression with LAr veto • 60Co : two  lines (1.1 and 1.3 MeV) in a cascade • external : high probability that only 1  reaches the crystal  acts as 2 single  lines • internal : if one  reaches the crystal, 2nd  will deposit its energy in LAr full energy peak : suppressed by LAr veto Compton continuum: suppressed by LAr veto

  16. 100% 40% 60Co peak suppression internal source external source 1.5 m

  17. 226Ra real vs. MC No suppression RoI (Qββ=2039 keV) 20% survival LAr suppressed

  18. 232Th real vs. MC (208Tl+228Ac) No suppression 228Ac – contribution  228Ac not in secular equilibrium with 228Th LAr suppressed RoI: 6% survival

  19. 232Th No suppression LAr suppressed RoI: 6% survival

  20. Outline • Introduction: GERDA • Resolution of bare Ge-diodes in LAr • Experimental Setup of LArGe@MPI-K • DAQ • Operational parameters • Light yield • Background spectrum • Characterization with various -sources • 137Cs, 60Co, 226Ra, 232Th • bkgd suppression in RoI • Outlook on LArGe@LNGS • Conclusions

  21. Outlook: LArGe @ Gran Sasso Active volume ∅20 cm  supression limited by escapes Active volume ∅90 cm  No significant escapes. Suppression limited by non-active materials. Exapmles (MC): Background suppression for contaminations located in detector support Bi-214 Tl-208 factor: 10 LArGe suppression and segmentation are orthogonal !  Suppression factors multiplicative 3·10²

  22. Conclusions • LAr does not deteriorate resolution of p-type crystals • Experimental data shows that • LAr veto is a powerful method for background suppression • No relevant loss of 0ßß signal • Results will be improved in larger setup @LNGS • MaGe simulations reproduce well the data

  23. 137Cs – effective veto threshold No suppression LAr suppressed LAr-veto threshold ~ 1pe = 2.5 keV

  24. 60Co MC vs. real

  25. Survival probabilitiesfor LArGe-MPIK setup full energy peak : no suppression by LAr veto Compton continuum: suppressed by LAr veto full energy peak : suppressed by LAr veto No efficiency loss expected for 0ßß-events Random coincidence even for 1 kBq source next to the crystal: < 2% Background suppression limited by radius of the active volume. R = 10 cm  significant amount of ‘s escape without depositing energy in LAr

  26. 39Ar, 42Ar and 85Kr • Q-value of 39Ar and 85Kr below 700 keV – relevant in case of dark matter detection • Dead-time could be a problem when Ar scintillation is used (slow decay time: ~ 1µs) • 42Ar is naturally low

  27. 39Ar and 85Krin argon Dead time: Assume 10 m3 active volume • 39Ar rate: 15 kHz  1.5 % Fine! • 85Kr rate not higher  ≤ 0.3 ppm Kr required Results from a 2.3 kg WARP test stand : ~ 0.6 ppm

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