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Results from LArGe@MPI-K

Results from LArGe@MPI-K. goal: study and quantify background suppression with LAr scintillation. M. Di Marco, P. Peiffer, S. Schönert. Thanks to Davide Franco and Marik Barnabe Heider. Gerda collaboration meeting, Tübingen 9th-11th November 2005. Outline. Resolution of bare Ge in LAr

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Results from LArGe@MPI-K

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  1. Results from LArGe@MPI-K goal: study and quantify background suppression with LAr scintillation M. Di Marco, P. Peiffer, S. Schönert Thanks to Davide Franco and Marik Barnabe Heider Gerda collaboration meeting, Tübingen 9th-11th November 2005

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

  3. Proof of feasibility: bare p-type detectors in LAr Data taken at DSG in Mainz  No deterioration of energy-resolution for p-type detectors in LAr !

  4. Schematic system description System is designed to be air tight to prevent quenching of LAr scintillation by O2 or H2O Continously flushed with gaseous Argon Filling and emptying Ge-crystal (∅ 5.1 cm, h=3.5 cm) LAr in Dewar (∅ 29 cm) WLS and reflector (VM-2000) PMT 5 cm lead + underground lab (15 mwe) Trigger on Ge-signal Record Ge-signal and LAr-signal simultaneously Shaping 3 µs Gate width = 6 µs No hardware veto Monitor filling level (with temperature sensors) Calibrate PMT (trough optical fibre with UV-LED) Internal source External source

  5. Operational parameters PMT threshold set at ~1 single photoelectron (spe) Canberra p-type crystal (390 g) Running stable since several weeks 1 spe ≈ 5 keV energy deposition in LAr • Stability monitoring by: • peak position • resolution • leakage current • Not optimized for energy resolution: • long signal cables • FET outside system • pickup of external noise Energy resolution OK: ~4.5 keV FWHM w/o PMT ~5 keV with PMT At 1,3 MeV 60Co-line Gain in background suppression is not compromised by signal loss due to random coincidences !

  6. Background spectrum 40K 40 counts/h Ge signal (no veto) Ge signal after veto: fraction of the signal which „survives“ the cut 208Tl 10 counts/h energy in Ge (MeV)

  7. Background spectrum 40K 40 counts/h 93% survival 208Tl 10 counts/h 93% survival baseline: 41% survival energy in Ge (MeV)

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

  9. real data 137Cs 662 keV 100% survival Compton continuum: 20% survival • very well reproduced by MaGe : • shape of energy spectrum • peak efficiency • peak/Compton ratio simulations same thing for 60Co (ext), 232Th (int, ext), 226Ra (int)  geometry + basic physics processes well understood 662 keV 100% survival Compton continuum: 20% survival

  10. 137Cs for now, veto simulated as a sharp energy threshold with arbitrary value  suppression by LAr overestimated in more complex cases • next: • proper threshold for spe (Poisson statistics) • calibration of LAr scintillation

  11. Calibration with different sources full energy peaks : no suppression with LAr veto • 60Co : two  lines (1.1 and 1.3 MeV) in 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

  12. 60Co (external) 30% 30% 100% shielding of the source not implemented in MaGe yet ~20% ~20%

  13. 60Co (internal) 40% weak source : 208Tl from bkgd is visible 100% survival 12% 12% summation peak: both  in crystal 100% survival

  14. Calibration with different sources • 137Cs : single  line at 662 keV • 60Co : two  lines (1.1 and 1.3 MeV) in cascade • full-E peak no suppression if external • full-E peak suppressed if internal • 232Th : dominated by 208Tl • 511 keV – 583 keV – 2.6 MeV : prompt cascade • 860 keV – 2.6 MeV : prompt cascade  no suppression if external  suppressed if internal • 226Ra : dominated by 214Bi • 609 keV and 1.120 keV : prompt cascade  suppressed if internal • 1.764 MeV - 2.448 MeV : direct decay  no suppression Compton continuum: suppressed by LAr veto

  15. 232Th (external) 583 keV : 70% 2.6 MeV 83% 33% 25% 25% RoI 208Tl simulated 2.6 MeV 76% 29% 18% 19%

  16. 232Th (internal) weak souce (400 Bq over 3cm)  contribution from 208Tl bkgd in real data  30% 26% (mc 15%) 14% 9,5% 9,5% RoI 208Tl simulated 12% 4% 4%

  17. 226Ra (internal)  92%  30% (mc 23%) 19% 27% 30% RoI 214Bi simulated 13% 28% 30%

  18. Summary of background suppressionfor 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 Suppression factors limited by radius of the active volume. R = 10 cm  significant amount of ‘s escape without depositing energy in LAr

  19. Outlook: LArGe @ Gran Sasso Diameter = 90 cm. No significant escapes. Suppression limited by non-active materials. Examples: Background suppression for contaminations located in detector support Bi-214 Tl-208 survival: 10% LArGe suppression method and segmentation are orthogonal !  Suppression factors multiplicative 3.3·10-3 survival

  20. 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 • Work in progress

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