1. GERDA Test Stands for Segmented Germanium Detectors
Good Morning altogether, I’m Manuela Jelen from the GERDA Collaboration. will tell you about T.S. that Gerda operates in Munich.
Iris Abt, Allen Caldwell, Manuela Jelen, Kevin Kroeninger, Jing Liu, Xiang Liu, Bela Majorovits, Jens Schubert, Franz Stelzer
DPG, March 2007
GERDA

2. Outline Motivation Test Stands with Detectors on Cold Finger
Test Stands with Detectors in Cryo-Liquid
Conclusions
Outline: Motivation: what do we want to measure, why do we want to measure it and how we do it
Gerda has 2 types of T.S. in Munich, I will tell you about them: T.S. with D. on Cold Finger and we have some with D. submerged directly in Cryo Liquid
Conclusions, what we learn from it for large experiment
DPG, March 2007
GERDA

3. Ge: good energy resolution → background reduction
Motivation
GERDA is looking for 0νββ-decay
→ n Dirac or Majorana
particle?
→ total mass
(upper limit if no discovery)
0νββ: rare process
→ low background required
(<10-3 cnts/kg/y/keV)
counts [arbitrary units]
2-neutrino-double-beta-decay: usual process, neutrinoless-double-beta-decay,
neutrino Dirac type (no discovery) or Majorana type (its own antiparticle; discovery) ->Jing!
total mass scale; upper limit 0.2eV
rare process -> low background required; Gerda Phase II: 35kg Ge, run 3y ->1bg event/10keV
E-spectrum: sum electron energies: 2 neutrinos; can carry away energy -> energy distribution; 0 neutrinos-> only 1 peak (no possibility to get rid of energy)
76Ge good res -> peak and distrib. can be separated. bg reduction
(sum of electron energies)/Qββ
Ge: good energy resolution → background reduction
DPG, March 2007
GERDA

4. Motivation Background reduction:
GERDA in underground lab (cosmic rays)
Water Tank (neutrons)
GERDA Detectors operated directly in LAr (γ-background)
Test Stands in Munich:
LN2/LAr and Cold Finger
More bg red.:…underground lab in Italy; Gran Sasso mountain
strings: 30g copper and support structure; overall idea is to have as little high-Z material near detectors to minimize background. emphasis on bg reduction
T.S. in Munich: both kinds to study advantages of different setups and to learn about detectors for large experiment GERDA
DPG, March 2007
GERDA

5. Segmented Germanium Detectors
detector material: high purity Germanium
detectors segmented 6-fold in φ and 3-fold in height
segmentation helps with background rejection
signal: 1 segment hit
background event: more segments hit
Ge: semiconductor detector; works better in cool environment
segmentation: signal we expect -> 2 electrons ->local deposition of E (mean free path only mm) -> 1 segment;
bg mainly photons -> mean free path longer than for electrons (a few cm) ->more than 1 seg. -> “segmentation was optimized with respect to the mean free path of photons” -> Kevin!
DPG, March 2007
GERDA

6. Test Stands with Detectors on Cold Finger
classical cryostat
‘Galatea’
Vacuum teststands traditionally used, left one: Galatea: sources can be moved around 360deg, complete height and radius
both: cryo liquid in tank, detectors cooled via a copper cold finger
LN2
DPG, March 2007
GERDA

7. Test Stands with Detectors on Cold Finger
classical cryostat
cryostat for first n-type 18-fold segmented detector
detector cooled indirectly via copper cold finger
sources put outside aluminum vacuum tank
first n-type 18-f. seg. det. -> Kevin will explain setup more closely!
{show source}: sources can be placed all around detector, but only outside aluminum cap
DPG, March 2007
GERDA

8. Test Stands with Detectors on Cold Finger –
Galatea
Sources movable 360° in φ- and whole range in z-and r- direction
→ 3-D scan; surface and
bulk properties,
dead layers
no material between
source and detector
→ ideal for α-/
β-sources/laser
Segment resolution, segment boundaries and cross-talk
→ pulse shape studies and background suppression
High Precision
Tool
sources can be moved around 360deg, complete height and radius. 3-D: especially thickness of dead layers!
Absolutely no material between source and detector!!! ->alpha, beta sources, laser cannot be used in cryo liquid ->use here,
visible tanks only thermal shields; whole setup in big vacuum tank.
Scan segment boundaries (what happens if an event takes place directly in seg boundary?), resolution in seg, cross-talk in between seg, pulse shape: important for background suppression;
high precision tool, more precise than any other test stand for that purpose!
DPG, March 2007
GERDA

9. Test Stands with Detectors in Cryo-Liquid
‘GERDAlinchen II’
‘GERDAlinchen’
‘Milchkanne’
LN2 or LAr
second group of T.S.:
new Test stand milchkanne and gerdalinchen mimic naked crystals in cryo liquid ….working on Gerdalinchen II (strings for gerda); LN2/LAr in tank together with crystals
DPG, March 2007
GERDA

10. Test Stands with Detectors in Cryo-Liquid
‘Milchkanne’
>30 cooling cycles:
1) submerge crystal in LN2 or LAr
2) measurement (resolution)
3) warm crystal to 300K and put back in vacuum storage
→ operational routine
→ detector stands alternating environments
top view 2
currently: detector Mikesch n-type det. in LN2, see top of naked crystal,
the way it’s lowered into milchkanne
cooling cycle: submerge in LN2 - measure(~2 hours) - warm up and put back into vacuum storage facility
deceloped operational routine (always same conditions), detector stands all so far, lets hope also in the future
measured europium spectrum -> resolution is fine for both cryo liquids
DPG, March 2007
GERDA

11. Mounting of a 6-fold segmented p-type detector
Test Stands with Detectors in Cryo-Liquid
‘GERDAlinchen’
Mounting of a 6-fold segmented p-type detector
103
102
Leakage Current [pA] 10 1
Gerdalinchen: first attempt of mimicing large GERDA; IR shield on top of det.
in reality: that’s the mounting procedure for a 6-fold segmented p-type detector; lowered in dewar -> fill cryo liquid
test IR influence on detector; knew that IR influences the det., wanted to find out how bad it really is
Bias Voltage [V]
DPG, March 2007
GERDA

12. Test Stands with Detectors in Cryo-Liquid
‘GERDAlinchen II’
Gerdalinchen II is the T.S. with the same string setup as gerda, our final string design,
up to date photo: that’s how far we’ve come
DPG, March 2007
GERDA

13. Conclusions
Detectors can be operated in vacuum or in cryogenic liquids, LN2 or LAr
Stable operation of Ge detectors in LN2/LAr
New vacuum test stand ‘Galatea’: precision tool to scan crystal in 3-D
New test stand with naked crystal in LN2 : ‘GERDAlinchen II’ → test for GERDA setup of detector strings
operation possible in both considered cryo liquids, LN2 and LAr (important because want to use LAr) …
Galatea will help us to understand crystals more precisely (especially surface/seg boundaries)
GII: setup of det strings like in gerda: important to see if structure works (also with more than 1 det)
DPG, March 2007
GERDA