Page 1
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
Page 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
Page 3
GERDA – GERmanium Detector Array
GERDA @ LNGS
Physics goal: search for 0ββ-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 ymee < 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
Challenge: reduce background at 2039 keV by ~102 10-3 cts/(kg*keV*y)
Ge
LN/LAr
H2O
Page 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.
Page 5
Energy resolution of a bare 2kg HP-Ge-diode in LAr
No deterioration of energy-resolution for bare p-type detectors in LAr !
Resolution in LN @ 1.33 MeV 2.3 keV FWHM
Resolution in LAr @ 1.33 MeV 2.3 keV FWHM
FWHM2.3 keV
1.33 MeV
1.17MeV
1.33MeV
40K summation
208Tl
Page 6
Outline
• Introduction: GERDA
• Resolution of bare Ge-diodes in LAr• Experimental Setup of LArGe@MPI-KExperimental Setup of LArGe@MPI-K
– DAQDAQ– Operational parametersOperational parameters– Light yieldLight yield– Background spectrumBackground spectrum
• Characterization with various -sources– 137Cs, 60Co, 226Ra, 232Th– bkgd suppression in RoI
• Outlook on LArGe@LNGS• Conclusions
Page 7
LArGe@MPI-K: Schematic system description
Internal source- Background from crystal holders
External source- Background from walls
• 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:
-
Page 8
Electronics
Trigger on Ge-signal
Record Ge-signal and LAr-signal simultaneously
Coincidence time 6 µs
Software cut on recorded data
Shaping 3 µs
Shaping 3 µs
LAr
Page 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 PMTAt 1.33 MeV 60Co-line
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 %
Background suppression is not compromised by signal loss due to
random coincidences !
Energy resolution limited in this setup.
* Threshold at single pe (~ 2.5 keV)** Coincidence time: 6 µs
Page 11
Photo-electron yield in LArGe@MPI-K
57Co peak in LArspe – peak
(LED generated)
- 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)
122 keV - 86%136 keV - 11%
Source position:
Page 12
Background spectrum (LArGe@MPI-K)
40K
40 counts/h208Tl
10 counts/h
energy in Ge (MeV)
Ge signal Ge signal
(no veto)(no veto)
Ge signal after veto:Ge signal after veto:
fraction of the signal fraction of the signal which „survives“ the cutwhich „survives“ the cut
Time of data taking: 2 days
Page 13
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 Characterization with various -sources-sources– 137137Cs, Cs, 6060Co, Co, 226226Ra, Ra, 232232ThTh– bkgd suppression in RoIbkgd suppression in RoI
• Outlook on LArGe@LNGS• Conclusions
Page 14
Characterization with different sources
137Cs : single line at 662 keV full energy peak :no suppression with
LAr veto
Compton continuum:suppressed by LAr veto
Page 15
137Csreal data
simulations
662 keV
~ 100% survival
662 keV
100% survival
Compton continuum:
20% survival
Compton continuum:
20% survival
very well reproduced by MC(MaGe):
shape of energy spectrum
peak efficiency
peak/Compton ratio
survival probability
Page 16
Characterization with different sources
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 peaks :no suppression with
LAr veto
full energy peak :suppressed by LAr veto
Compton continuum:suppressed by LAr veto
Page 17
1.5 m
100%
60Co peak suppression external sourceinternal source
40%
Page 18
226Ra real vs. MCNo suppression
LAr suppressedRoI (Qββ=2039 keV)
20% survival
Page 19
232Th real vs. MC (208Tl+228Ac)
RoI: 6% survival
No suppression
LAr suppressed
228Ac – contribution 228Ac not in secular equilibrium with 228Th
Page 20
232Th
RoI: 6% survival
No suppression
LAr suppressed
Page 21
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@LNGSOutlook on LArGe@LNGS• ConclusionsConclusions
Page 22
Outlook: LArGe @ Gran Sasso
Bi-214
Tl-208
Exapmles (MC):Background suppression for contaminations locatedin detector support
3·10²
factor: 10
LArGe suppression and segmentationare orthogonal ! Suppression factors multiplicative
Active volume ∅20 cm supression limited by escapes
Active volume ∅90 cm No significant escapes. Suppression limited by non-active materials.
Page 23
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 0ßß signal
• Results will be improved in larger setup @LNGS
• MaGe simulations reproduce well the data
Page 24
137Cs – effective veto threshold
LAr-veto threshold ~ 1pe = 2.5 keV
No suppression
LAr suppressed
Page 26
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 0ßß-eventsRandom 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
Source 137Cs 60Co (ext)1.3 MeV
232Th (ext.)583 keV
2.6 MeV
RoI
60Co (int) 1.3 MeV
232Th (int) 583 keV
2.6 MeV
RoI
226Ra (int)609 keV
2,4 MeV
RoI
Compton
continuum 15% ~ 30% ~ 25 – 33% 12% 6% 19-27%
full-E
peak 100% 100% ~ 100% 40% ~ 30%30%
100%
Page 27
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
Decay mode Source Concentration (STP)
222RnT1/2 = 3.8 d
, , Primordial 238U 1 - ?00 Bq/m3 air
85KrT1/2 = 10.8 y
(687 keV) , 235U fission
(nuclear fuel reprocessing plants)
1.4 Bq/m3 air1.2 MBq/m3 Kr
39ArT1/2 = 269 y (565 keV)
Cosmogenic17 mBq/m3 air1.8 Bq/m3 Ar
42ArT1/2 = 32.9 y
(600 keV)Cosmogenic
0.5 µBq/m3 air50 µBq/m3 Ar
Page 28
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