LAr Ge A Liquid Argon Germanium hybrid detector
LAr. Ge 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 13 th-14 th March 2006
Outline • Introduction: GERDA • Energy resolution of bare Ge-diodes in LAr • Experimental Setup of LAr. Ge@MPI-K – – DAQ Operational parameters Light yield Background spectrum • Characterization with various -sources – 137 Cs, 60 Co, 226 Ra, 232 Th – bkgd suppression in Ro. I • Outlook on LAr. Ge@LNGS • Conclusions
GERDA – GERmanium Detector Array Physics goal: search for 0 ββ-decay majorana or dirac particle? Method: operate bare, 76 Ge enriched, HP-Ge-diodes in LN (or LAr) Signal: single-site events in HP-Ge-diode (Qßß=2039 ke. V) Background: - compton or summation, µ-induced, . . . H 2 O LN/LAr Ge GERDA @ LNGS Physics reach: Phase I: 15 kg*y, existing diodes (Hd. M, IGEX) sensitivity goal: T 1/2 > 3*1025 y mee < 0. 24 – 0. 77 e. V Phase II: 100 kg*y, increased mass, new diodes, additional active background suppression. sensitivity goal: T 1/2 > 2*1026 y mee < 0. 09 – 0. 29 ev Challenge: reduce background at 2039 ke. V by ~102 10 -3 cts/(kg*ke. V*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) LAr. Ge@MPI-K: R&D experiment operating HP-Ge-diode in LAr. With simultaneous LAr-scintillation-light readout.
Energy resolution of a bare 2 kg HP-Ge-diode in LAr 1. 33 Me. V 1. 17 Me. V 1. 33 Me. V FWHM 2. 3 ke. V 40 K summation 208 Tl Resolution in LN @ 1. 33 Me. V 2. 3 ke. V FWHM Resolution in LAr @ 1. 33 Me. V 2. 3 ke. V FWHM 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 LAr. Ge@MPI-K – – DAQ Operational parameters Light yield Background spectrum • Characterization with various -sources – 137 Cs, 60 Co, 226 Ra, 232 Th – bkgd suppression in Ro. I • Outlook on LAr. Ge@LNGS • Conclusions
LAr. Ge@MPI-K: Schematic system description • Bare p-type HP-Ge-diode • Dewar ∅29 cm, h=65 cm • Light detection: WLS - (VM 2000) + 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 Shaping 3 µs LAr Trigger on Ge-signal Record Ge-signal and LAr-signal simultaneously Coincidence time 6 µs Software cut on recorded data
Operational parameters Canberra p-type crystal (390 g) source Ge-rate Data taking: Sept. 05 – Dec. 05 Stability monitored by: • peak position • energy resolution • leakage current Energy resolution: ~4. 5 ke. V FWHM w/o PMT ~5 ke. V with PMT At 1. 33 Me. V 60 Co-line Energy resolution limited in this setup. Background PMTrate * Random coinc. ** 7 Hz 2, 1 k. Hz 1, 2 % int. 600 Bq 17 Hz 2, 8 k. Hz 1, 68 % 226 Ra 23 Hz 3, 2 k. Hz 1, 92 % 60 Co int. 1 k. Bq * Threshold at single pe (~ 2. 5 ke. V) ** Coincidence time: 6 µs Background suppression is not compromised by signal loss due to random coincidences !
Photo-electron yield in LAr. Ge@MPI-K 57 Co spe – peak (LED generated) - 57 Co-peak in LAr 122 ke. V - 86% 136 ke. V - 11% at ch 2153, peak energy 123. 5 ke. V - spe-peak at ch (122. 4 ± 3), pedestal at ch 81 photo-electron yield L = (407 ± 10) pe/Me. V - Possible to improve light yield with TPB ( WARP) Source position:
Background spectrum (LAr. Ge@MPI-K) Ge signal (no veto) 40 K 40 counts/h 208 Tl 10 counts/h Ge signal after veto: fraction of the signal which „survives“ the cut energy in Ge (Me. V) Time of data taking: 2 days
Outline • Introduction: GERDA • Resolution of bare Ge-diodes in LAr • Experimental Setup of LAr. Ge@MPI-K – – DAQ Operational parameters Light yield Background spectrum • Characterization with various -sources – 137 Cs, 60 Co, 226 Ra, 232 Th – bkgd suppression in Ro. I • Outlook on LAr. Ge@LNGS • Conclusions
Characterization with different sources Ø 137 Cs : single line at 662 ke. V full energy peak : no suppression with LAr veto Compton continuum: suppressed by LAr veto
real data 137 Cs 662 ke. V Compton continuum: ~ 100% survival 20% survival simulations very well reproduced by MC(Ma. Ge): ü shape of energy spectrum ü peak efficiency ü peak/Compton ratio ü survival probability Compton continuum: 20% survival 662 ke. V 100% survival
Characterization with different sources Ø 60 Co : two lines (1. 1 and 1. 3 Me. V) in a cascade § external : high probability that only 1 reaches the crystal acts as 2 single lines § internal : if one reaches the crystal, 2 nd 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 peak suppression internal externalsource 1. 5 m 100% 40%
226 Ra real vs. MC No suppression LAr suppressed Ro. I (Qββ=2039 ke. V) 20% survival
232 Th real vs. MC (208 Tl+228 Ac) No suppression 228 Ac – contribution 228 Ac not in secular equilibrium with 228 Th LAr suppressed Ro. I: 6% survival
232 Th No suppression LAr suppressed Ro. I: 6% survival
Outline • Introduction: GERDA • Resolution of bare Ge-diodes in LAr • Experimental Setup of LAr. Ge@MPI-K – – DAQ Operational parameters Light yield Background spectrum • Characterization with various -sources – 137 Cs, 60 Co, 226 Ra, 232 Th – bkgd suppression in Ro. I • Outlook on LAr. Ge@LNGS • Conclusions
Outlook: LAr. Ge @ Gran Sasso Active volume ∅20 cm supression limited by escapes Active volume ∅90 cm No significant escapes. Suppression limited by non-active materials. Bi-214 Exapmles (MC): Background suppression for contaminations located in detector support Tl-208 factor: 10 LAr. Ge suppression and segmentation are orthogonal ! Suppression factors multiplicative 3· 10²
Conclusions • LAr does not deteriorate resolution of ptype 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 • Ma. Ge simulations reproduce well the data
137 Cs – effective veto threshold No suppression LAr suppressed LAr-veto threshold ~ 1 pe = 2. 5 ke. V
60 Co MC vs. real
Survival probabilities for LAr. Ge-MPIK setup Source Compton continuum full-E peak 137 Cs 15% 100% 60 Co (ext) 232 Th (ext. ) 60 Co (int) 232 Th (int) 226 Ra (int) 1. 3 Me. V 583 ke. V 2. 6 Me. V Ro. I 609 ke. V 2, 4 Me. V Ro. I ~ 30% ~ 25 – 33% 12% 6% 19 -27% ~ 100% 40% ~ 30% 100% 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 k. Bq 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
39 Ar, 42 Ar and 85 Kr Decay mode Source Concentration (STP) 222 Rn T 1/2 = 3. 8 d , , Primordial 238 U 1 - ? 00 Bq/m 3 air 85 Kr T 1/2 = 10. 8 y (687 ke. V) , 39 Ar T 1/2 = 269 y (565 ke. V) 42 Ar T 1/2 = 32. 9 y (600 ke. V) 235 U fission (nuclear fuel reprocessing plants) 1. 4 Bq/m 3 air 1. 2 MBq/m 3 Kr Cosmogenic 17 m. Bq/m 3 air 1. 8 Bq/m 3 Ar Cosmogenic 0. 5 µBq/m 3 air 50 µBq/m 3 Ar • Q-value of 39 Ar and 85 Kr below 700 ke. V – relevant in case of dark matter detection • Dead-time could be a problem when Ar scintillation is used (slow decay time: ~ 1µs) • 42 Ar is naturally low
39 Ar and 85 Kr in argon Dead time: Assume 10 m 3 active volume rate: 15 k. Hz 1. 5 % Fine! 85 Kr rate not higher ≤ 0. 3 ppm Kr required – 39 Ar – Results from a 2. 3 kg WARP test stand : ~ 0. 6 ppm
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