M. Di Marco, P. Peiffer, S. Schönert LArGe A 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

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

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)

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.

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

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

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

Electronics Trigger on Ge-signal 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

Operational parameters Canberra p-type crystal (390 g) source Ge-rate PMT-rate * Random coinc.** Back-ground 7 Hz 2,1 kHz 1,2 % 60Co int. 600 Bq 17 Hz 2,8 kHz 1,68 % 226Ra int. 1kBq 23 Hz 3,2 kHz 1,92 % 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.

Photo-electron yield in LArGe@MPI-K spe – peak (LED generated) 57Co peak in LAr 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)

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

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

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

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

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

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

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

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

232Th No suppression LAr suppressed RoI: 6% survival

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

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

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

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

60Co MC vs. real

Survival probabilities for LArGe-MPIK setup Source 137Cs 60Co (ext) 1.3 MeV 232Th (ext.) 583 keV 2.6 MeV RoI 60Co (int) 232Th (int) 226Ra (int) 609 keV 2,4 MeV Compton continuum 15% ~ 30% ~ 25 – 33% 12% 6% 19-27% full-E peak 100% ~ 100% 40% 30% 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

(nuclear fuel reprocessing plants) 39Ar, 42Ar and 85Kr Decay mode Source Concentration (STP) 222Rn T1/2 = 3.8 d , ,  Primordial 238U 1 - ?00 Bq/m3 air 85Kr T1/2 = 10.8 y  (687 keV) ,  235U fission (nuclear fuel reprocessing plants) 1.4 Bq/m3 air 1.2 MBq/m3 Kr 39Ar T1/2 = 269 y  (565 keV) Cosmogenic 17 mBq/m3 air 1.8 Bq/m3 Ar 42Ar T1/2 = 32.9 y  (600 keV) 0.5 µBq/m3 air 50 µBq/m3 Ar 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

39Ar and 85Kr in 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