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

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

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

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.

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

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

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:

-

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

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

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:

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

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

Characterization 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 MC(MaGe):

shape of energy spectrum

peak efficiency

peak/Compton ratio

survival probability

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

1.5 m

100%

60Co peak suppression external sourceinternal source

40%

226Ra real vs. MCNo suppression

LAr suppressedRoI (Qββ=2039 keV)

20% survival

232Th real vs. MC (208Tl+228Ac)

RoI: 6% survival

No suppression

LAr suppressed

228Ac – contribution 228Ac not in secular equilibrium with 228Th

232Th

RoI: 6% survival

No suppression

LAr suppressed

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

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.

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

137Cs – effective veto threshold

LAr-veto threshold ~ 1pe = 2.5 keV

No suppression

LAr suppressed

60Co MC vs. real

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%

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

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