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

Outline Resolution of bare Ge in LAr Experimental Setup of –DAQ –Operational parameters –Background spectrum Characterization with various  -sources – 137 Cs, 60 Co, 226 Ra, 232 Th –bkgd suppression in RoI Outlook on Conclusions

Proof of feasibility: bare p-type detectors in LAr  No deterioration of energy-resolution for p-type detectors in LAr ! Resolution in LN2.3 keV Resolution in LAr2.3 keV Data taken at DSG in Mainz

Trigger on Ge-signal Record Ge-signal and LAr-signal simultaneously Shaping 3 µs Gate width = 6 µs No hardware veto PMT Ge-crystal ( ∅ 5.1 cm, h=3.5 cm) LAr in Dewar ( ∅ 29 cm) Continously flushed with gaseous Argon Filling and emptying Monitor filling level (with temperature sensors) Calibrate PMT (trough optical fibre with UV-LED) WLS and reflector (VM-2000) Internal source External source 5 cm lead + underground lab (15 mwe) Schematic system description System is designed to be air tight to prevent quenching of LAr scintillation by O 2 or H 2 O

Operational parameters Canberra p-type crystal (390 g) Running stable since several weeks 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 60 Co-line PMT threshold set at ~1 single photoelectron (spe) 1 spe ≈ 5 keV energy deposition in LAr sourceGe-rateLAr-rateRandom coinc. Back- ground 7 Hz2,1 kHz1,2 % 60 Co int. 600 Bq 17 Hz2,8 kHz1,68 % 226 Ra int. 1kBq 23 Hz3,2 kHz1,92 % Gain in background suppression is not compromised by signal loss due to random coincidences !

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

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

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

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

137 Cs 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

Calibration with different sources  60 Co : 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 peaks : no suppression with LAr veto full energy peak : suppressed by LAr veto Compton continuum: suppressed by LAr veto

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

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

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

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

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

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

Summary of background suppression for 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 Source 137 Cs 60 Co (ext) 1.3 MeV 232 Th (ext.) 583 keV 2.6 MeV RoI 60 Co (int) 1.3 MeV 232 Th (int) 583 keV 2.6 MeV RoI 226 Ra (int) 609 keV 2,4 MeV RoI Compton continuum 20%~ 30%~ 25 – 33%12%9.5-14%19-27% full-E peak 100% ~ 100% 40%~ 30% 30% 100%

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

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 MaGe simulations reproduce well the data –Work in progress