Results from LArGe@MPI-K

1
Results from LArGe_at_MPI-K
goal study and quantify background suppression
with LAr scintillation

• M. Di Marco, P. Peiffer, S. Schönert

Thanks to Davide Franco and Marik Barnabe Heider
Gerda collaboration meeting, Tübingen 9th-11th
November 2005
2
Outline

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

3
Proof of feasibility bare p-type detectors in LAr
Data taken at DSG in Mainz
Resolution in LN 2.3 keV
Resolution in LAr 2.3 keV
? No deterioration of energy-resolution for
p-type detectors in LAr !
4
Schematic system description
System is designed to be air tight to prevent
quenching of LAr scintillation by O2 or H2O
Continously flushed with gaseous Argon
Filling and emptying
Ge-crystal (Ø 5.1 cm, h3.5 cm)
LAr in
Dewar (Ø 29 cm)
WLS and reflector (VM-2000)
PMT
5 cm lead underground lab (15 mwe)
Trigger on Ge-signal Record Ge-signal and
LAr-signal simultaneously Shaping 3 µs Gate width
6 µs No hardware veto
Monitor filling level (with temperature sensors)
Calibrate PMT (trough optical fibre with UV-LED)
Internal source
External source
5
Operational parameters
PMT threshold set at 1 single photoelectron (spe)
Canberra p-type crystal (390 g)
Running stable since several weeks
1 spe 5 keV energy deposition in LAr

• Stability monitoring by
• peak position
• resolution
• leakage current

source Ge-rate LAr-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

• 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 60Co-line
Gain in background suppression is not compromised
by signal loss due to random coincidences !
6
(No Transcript)
7
Background spectrum
40K 40 counts/h
Ge signal (no veto)
Ge signal after veto fraction of the signal
which survives the cut
208Tl 10 counts/h
energy in Ge (MeV)
8
Background spectrum
40K 40 counts/h 93 survival
208Tl 10 counts/h 93 survival
baseline 41 survival
energy in Ge (MeV)
9
Calibration with different sources
full energy peak no suppression with LAr veto

• 137Cs single ? line at 662 keV

Compton continuum suppressed by LAr veto
10
137Cs
real data
662 keV 100 survival
Compton continuum 20 survival

• very well reproduced by MaGe
• shape of energy spectrum
• peak efficiency
• peak/Compton ratio

simulations
same thing for 60Co (ext), 232Th (int, ext),
226Ra (int) ? geometry basic physics processes
well understood
662 keV 100 survival
Compton continuum 20 survival
11
137Cs
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

12
Calibration with different sources
full energy peaks no suppression with LAr veto

• 60Co 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 peak suppressed by LAr veto
Compton continuum suppressed by LAr veto
13
60Co (external)
30
30
100
shielding of the source not implemented in MaGe
yet
20
20
14
60Co (internal)
40
weak source 208Tl from bkgd is visible 100
survival
12
12
summation peak both ? in crystal 100 survival
15
Calibration with different sources

• 137Cs single ? line at 662 keV
• 60Co two ? lines (1.1 and 1.3 MeV) in cascade
• full-E peak no suppression if external
• full-E peak suppressed if internal
• 232Th dominated by 208Tl
• 511 keV 583 keV 2.6 MeV prompt cascade
• 860 keV 2.6 MeV prompt cascade
• ? no suppression if external
• ? suppressed if internal
• 226Ra dominated by 214Bi
• 609 keV and 1.120 keV prompt cascade
• ? suppressed if internal
• 1.764 MeV - 2.448 MeV direct decay
• ? no suppression

Compton continuum suppressed by LAr veto
16
232Th (external)
583 keV 70
2.6 MeV 83
33
25
25
RoI
208Tl simulated
2.6 MeV 76
29
18
19
17
232Th (internal)
weak souce (400 Bq over 3cm) ?
contribution from 208Tl bkgd in real data
? 30
26 (mc 15)
14
9,5
9,5
RoI
208Tl simulated
12
4
4
18
226Ra (internal)
? 92
? 30 (mc 23)
19
27
30
RoI
214Bi simulated
13
28
30
19
Summary of background suppressionfor 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 20 30 25 33 12 9.5-14 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
Suppression factors limited by radius of the
active volume.
R 10 cm ? significant amount of ?s escape
without depositing energy in LAr
20
Outlook LArGe _at_ Gran Sasso
Diameter 90 cm. No significant escapes.
Suppression limited by non-active materials.
Examples Background suppression for
contaminations located in detector support
Bi-214
Tl-208
survival 10
LArGe suppression method and segmentation are
orthogonal ! ? Suppression factors multiplicative
3.310-3 survival
21
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
• Work in progress

22
(No Transcript)
23
(No Transcript)
24
(No Transcript)