LArGe A Liquid Argon Germanium hybrid detector system in the framework of the GERDA experiment

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
2
Outline

• Introduction GERDA
• Energy resolution of bare Ge-diodes in LAr
• Experimental Setup of LArGe_at_MPI-K
• DAQ
• Operational parameters
• Light yield
• Background spectrum
• Characterization with various ?-sources
• 137Cs, 60Co, 226Ra, 232Th
• bkgd suppression in RoI
• Outlook on LArGe_at_LNGS
• Conclusions

3
GERDA GERmanium Detector Array
Physics goal search for 0?ßß-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 (Qßß2039 keV) Background ? -
compton or summation, µ-induced, ...
Physics reach Phase I 15 kgy, existing diodes
(HdM, IGEX) sensitivity goal T1/2 gt 31025
y mee lt 0.24 0.77 eV Phase II 100 kgy,
increased mass, new diodes, additional active
background suppression. sensitivity goal T1/2
gt 21026 y mee lt 0.09 0.29 ev
H2O
LN/LAr
Ge
GERDA _at_ LNGS
Challenge reduce background at 2039 keV by 102
? 10-3 cts/(kgkeVy)
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_at_MPI-K RD experiment operating
HP-Ge-diode in LAr. With simultaneous
LAr-scintillation-light readout.
5
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 _at_ 1.33 MeV 2.3 keV FWHM
Resolution in LAr _at_ 1.33 MeV 2.3 keV FWHM
? No deterioration of energy-resolution for bare
p-type detectors in LAr !
6
Outline

• Introduction GERDA
• Resolution of bare Ge-diodes in LAr
• Experimental Setup of LArGe_at_MPI-K
• DAQ
• Operational parameters
• Light yield
• Background spectrum
• Characterization with various ?-sources
• 137Cs, 60Co, 226Ra, 232Th
• bkgd suppression in RoI
• Outlook on LArGe_at_LNGS
• Conclusions

7
LArGe_at_MPI-K Schematic system description

• Bare p-type HP-Ge-diode
• Dewar Ø29 cm, h65 cm
• Light detection WLS (VM2000)
• PMT(8, ETL 9357-KFLB )
• Active volume Ø20 cm, h43 cm
• 19 kg LAr
• Shielding 5 cm lead
• 15 mwe underground

-
Measurements
Internal source - Background from crystal holders
External source - Background from walls
8
Electronics
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
9
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.
10
(No Transcript)
11
Photo-electron yield in LArGe_at_MPI-K
57Co peak in LAr
spe peak (LED generated)
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)

12
Background spectrum (LArGe_at_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
13
Outline

• Introduction GERDA
• Resolution of bare Ge-diodes in LAr
• Experimental Setup of LArGe_at_MPI-K
• DAQ
• Operational parameters
• Light yield
• Background spectrum
• Characterization with various ?-sources
• 137Cs, 60Co, 226Ra, 232Th
• bkgd suppression in RoI
• Outlook on LArGe_at_LNGS
• Conclusions

14
Characterization with different sources
full energy peak no suppression with LAr veto

• 137Cs single ? line at 662 keV

Compton continuum suppressed by LAr veto
15
137Cs
real data
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
16
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
17
60Co peak suppression
external source
internal source
1.5 m
18
226Ra real vs. MC
No suppression
RoI (Qßß2039 keV) 20 survival
LAr suppressed
19
232Th real vs. MC (208Tl228Ac)
No suppression
228Ac contribution ? 228Ac not in secular
equilibrium with 228Th
LAr suppressed
RoI 6 survival
20
232Th
No suppression
LAr suppressed
RoI 6 survival
21
Outline

• Introduction GERDA
• Resolution of bare Ge-diodes in LAr
• Experimental Setup of LArGe_at_MPI-K
• DAQ
• Operational parameters
• Light yield
• Background spectrum
• Characterization with various ?-sources
• 137Cs, 60Co, 226Ra, 232Th
• bkgd suppression in RoI
• Outlook on LArGe_at_LNGS
• Conclusions

22
Outlook LArGe _at_ Gran Sasso
Active volume Ø20 cm ? supression limited by
escapes Active volume Ø90 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
310²
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 0?ßß signal
• Results will be improved in larger setup _at_LNGS
• MaGe simulations reproduce well the data

24
137Cs effective veto threshold
No suppression
LAr suppressed
LAr-veto threshold 1pe 2.5 keV
25
60Co MC vs. real
26
Survival probabilitiesfor LArGe-MPIK setup
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
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 lt 2
Background suppression limited by radius of the
active volume. R 10 cm ? significant amount of
?s escape without depositing energy in LAr
27
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) Cosmogenic 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

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