Results from LArGe@MPI-K

M. Di Marco, P. Peiffer, S. Schönert

Thanks to Davide Franco and Marik Barnabe Heider

Gerda collaboration meeting, Tübingen 9th-11th November 2005

goal: study and quantify background suppression with LAr scintillation

Outline

• Resolution of bare Ge in LAr• Experimental Setup of LArGe@MPI-K

– DAQ– Operational parameters– Background spectrum

• Characterization with various -sources– 137Cs, 60Co, 226Ra, 232Th– bkgd suppression in RoI

• Outlook on LArGe@LNGS• Conclusions

Proof of feasibility: bare p-type detectors in LAr

No deterioration of energy-resolution for p-type detectors in LAr !

Resolution in LN 2.3 keV

Resolution in LAr 2.3 keV

Data taken at DSG in Mainz

Trigger on Ge-signal

Record Ge-signal and LAr-signal simultaneously

Shaping 3 µs

Gate width = 6 µs

No hardware veto

PMT

Ge-crystal ( 5.1 cm, h=3.5 cm)∅

LAr inDewar ( 29 cm)∅

Continously flushed with gaseous Argon

Filling and emptying

Monitor filling level (with temperature sensors)

Calibrate PMT (trough optical fibre with UV-LED)

WLS and reflector (VM-2000)

Internal source

External source

5 cm lead + underground lab (15 mwe)

Schematic system description

System is designed to be air tight to prevent quenching of LAr scintillation by O2 or H2O

Operational parameters

Canberra p-type crystal (390 g)

Running stable since several weeks

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

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 PMTAt 1,3 MeV 60Co-line

PMT threshold set at ~1 single photoelectron (spe)

1 spe ≈ 5 keV energy depositionin LAr

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 %

Gain in background suppression is not compromised by signal loss due to

random coincidences !

Background spectrum

40K

40 counts/h

208Tl

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

Background spectrum

baseline:

41% survival

40K

40 counts/h

93% survival 208Tl

10 counts/h

93% survival

energy in Ge (MeV)

Calibration with different sources

137Cs : single line at 662 keV full energy peak :no suppression with

LAr veto

Compton continuum:suppressed by LAr veto

137Csreal data

simulations

662 keV

100% survival

662 keV

100% survival

Compton continuum:

20% survival

Compton continuum:

20% survival

very well reproduced by MaGe :

shape of energy spectrum

peak efficiency

peak/Compton ratio same thing for 60Co (ext),

232Th (int, ext), 226Ra (int)

geometry + basic physics processes well understood

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

Calibration with different sources

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 peaks :no suppression with

LAr veto

full energy peak :suppressed by LAr veto

Compton continuum:suppressed by LAr veto

60Co (external)

30% 30%

shielding of the source not implemented in MaGe yet

100%

~20% ~20%

60Co (internal)

12% 40% weak source :

208Tl from bkgd is visible

100% survival

summation peak:

both in crystal

100% survival

12%

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

232Th (external)

33%

25% 25%

RoI

2.6 MeV

83%

208Tl simulated

2.6 MeV

76%

583 keV : 70%

29%

18% 19%

232Th (internal)

208Tl simulated

30%

9,5%

14%

9,5%

RoI

26% (mc 15%)

weak souce (400 Bq over 3cm) contribution from

208Tl bkgd in real data

4%

12%

4%

92%

30%

226Ra (internal)

214Bi simulated

27%

RoI

30% (mc 23%)

30%

28%

19%

13%

Summary of background suppressionfor 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-events

Suppression factors 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 20% ~ 30% ~ 25 – 33% 12% 9.5-14% 19-27%

full-E

peak 100% 100% ~ 100% 40% ~ 30%30%

100%

Outlook: LArGe @ Gran Sasso

Bi-214

Tl-208

Examples:Background suppression for contaminations locatedin detector support

3.3·10-3 survival

survival: 10%

LArGe suppression method and segmentation are orthogonal ! Suppression factors multiplicative

Diameter = 90 cm. No significant escapes. Suppression limited by non-active materials.

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– Work in progress