cryodetector readout for dark matter searches
DESCRIPTION
Cryodetector Readout for Dark Matter Searches. Stuart Ingleby Cryodetectors Group, Oxford. Cryodetector readout. Direct dark matter searches Liquid noble gas Cryodetectors Cryogenic readout techniques Low impedance – SQUIDs High impedance – NTD/Ge sensors Light detectors. - PowerPoint PPT PresentationTRANSCRIPT
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Cryodetector Readout for Dark Matter Searches
Stuart Ingleby
Cryodetectors Group, Oxford
2
Cryodetector readout
• Direct dark matter searches– Liquid noble gas– Cryodetectors
• Cryogenic readout techniques– Low impedance – SQUIDs– High impedance – NTD/Ge sensors– Light detectors
3
Cosmological evidence of dark matter
• Baryon-to-photon ratio constrained– BBN– CMB power
spectrum• Matter density
constrained– Supernova redshift– CMB– Baryon acoustic
oscillations• Overall
– Baryons ~4%– Dark matter ~23%– Dark energy ~73%
4
Astronomical evidence of dark matter• Galactic rotation curves
– Expect 1/√r velocity curve
– Observe ~linear• ‘Halo’ of DM• Alternative gravities
• Bullet cluster– Collision of clusters– Observe galaxies, gas
and overall mass separately
• Consistent with CDM model
5
Cryodetector experiments
• Detect WIMP scattering– Nuclear recoils
• Extensive shielding– Deep
underground labs• Discrimination
– Exclude electron events
– Determine scattered nucleus
6
Recent results
• Exclusion plot– Long exposures and
low event rate– Exclude more
parameter space
• Constrained MSSM theory– Filled area [1]
• Aim 10-10pb (=10-46cm2)
– Larger detectors– Lower backgrounds
[1] Trotta et al. 2008
CRESST 2007
EDELWEISS II
ZEPLIN III
XENON10
CDMS
SuperCDMS (dashed)
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CRESST methods• 300g CaWO4 crystal• Phonons & scintillation
at ~10mK– Light absorbed in
separate silicon/sapphire wafer
• Tungsten SPT in s/c transition
• Coincident measurement of phonon & light– Recoils identified by
quenching factor
8
Low-impedance readout:SQUIDs
• SQUID– Parallel Josephson
junctions– V proportional to
flux enclosed– Input coil
• Current meter
• S/C film stabilised within transition– Current biased– Small ∆T; large ∆I
• Current read out using SQUID– SQUID voltage
channel low-impedance
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Cryogenic cabling• CRESST SQUID cabling
– Bespoke twisted-wire woven cables (right)
– £400 / channel• Etched metal foil cabling
– Conducting track defined by photolithography (below)
– £60 / channel
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Etched metal foil cabling• Oxford Physics
Photofabrication Unit• Phototool masks area to be
etched– UV exposure– Developed to produce
photo-resist layer• Etching removes resist-free
areas– Max width 40cm– Max length 3m
• New 1.2m laminator– Extra length can be
achieved with multiple pressing- lower yield
UV exposure unit
Etching bath
Laminator
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Cable design for SQUID readout• Maximum yield
– Even track width– Radiussed tracks– Teardropped contacts– 15 cables / etched sheet
• Simplicity– Surface mount
connectors
• Durability – Laminate cover layer– Straight fold-free cables– Reinforcement of
vulnerable areas
Etched cables
Foil with photo-resist pre-etching
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Heatload
• Larger detector mass– Lower heatload /
channel• Choice of
materials– Practical
constraints• Resistivity
measurements– Heatload
calculations– Etched steel
cables offer 20 x lower heatload
Mean Resistivity of Steel Foil Samples
7.0E-07
7.5E-07
8.0E-07
8.5E-07
9.0E-07
9.5E-07
1.0E-06
0 50 100 150 200 250 300
Temperature / K
Res
istiv
ity/O
hm
.m
Mean resistivity of copper foil samples
0.0E+00
5.0E-09
1.0E-08
1.5E-08
2.0E-08
2.5E-08
3.0E-08
0 50 100 150 200 250 300
Temperature / K
Res
istiv
ity/O
hm
.m
13
Installation in K400• Cryodetectors Lab
Oxford• 6-channel SQUID
system– Mounted at 4K– 2 x 12-channel
etched foil cable• Custom hardware
– Compact SQUID mount
• Built around existing readout
– Copper baffles for etched foil cables
– SCSI connector box
• Vacuum tight PCB flange with high channel density
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Low noise SQUID readout
• SQUID baseline noise– Testing
cryostat in Oxford
• Intrinsic SQUID noise ~1 pA/√Hz
(=1.2 μV/√Hz)
– CRESST cables
1.55 pA/√Hz – Steel foil cables
~2.5 pA/√Hz • Extra noise
– Nyquist noise on voltage channel?
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EDELWEISS method
• Ge crystal 320g– 20mK operation
• Phonon signal– NTD/Ge
thermometer
• Ionisation signal – ‘ID’ detector– Interleaved
electrodes for charge capture
– Fiducial volume• Reject surface
events
A A A A AB B B B
C C C C CD D D D
G
H
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Cabling design for NTD/Ge
• Readout for NTD/Ge– High impedance
• Capacitance– Limits bandwidth– Microphonics
• Mounted 4K – 10mK– Heatload
minimised– Radiopurity
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Radiopurity measurements
• Radiopurity tests– On samples of materials used– From GERDA, NEMO, CUORE experiments
• Kapton has high 40K content• Steel wiring does not appear significant
– 7.1% steel by mass• Polyethylene napthalate (PEN) suitable alternative
– Prototyping and testing
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Light Detectors• CRESST light detector
– Silicon on sapphire wafer– Cryodetector
• Separate SQUID readout– Stabilised separately to
phonon detector– High sensitivity
• 20eV
• Photomultiplier tube– Operated within cryostat– Simple high-impedance readout– Radiopurity
• Light guides
– HV supply• Voltage divider• Voltage multiplier
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HV supply for cold PMT• Cockcroft-Walton voltage
multiplier– As seen in particle
accelerators• Resistive voltage divider
– Dissipative components add heatload
– Possible noise on DC HV• Voltage multiplier chain
– Can be designed and run efficiently at optimum frequency
– Single-frequency supply can be chosen outside signal range
• 2.9kV generated at 4K from 15V supply
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Component testing• Performance
simulation – Approximate
formulae available– Software simulation
• Efficiency– Drop voltage– Transformer
• Low-T component testing– Transformer
• MPP– Capacitors
• Polystyrene– Diodes
• Silicon 1N40071
10
100
1000
0 50 100 150 200 250 300
Temperature / K
Ca
paci
tanc
e /
nF
0
0.5
1
1.5
2
2.5
1 10 100 1000
0
1
2
3
4
5
6
0 100 200 300 400 500 600 700 800 900 1000
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Prototype PMT module
• Installation of CWG-PMT module
• Preliminary 57Co spectra taken at 300K
• Detailed study of PMT performance for EURECA WP
-20 0 20 40 60 80 100
1
10
100
0.5 1.0 1.5 2.0 2.50.1
1
10Nu
mb
er
of
eve
nts
Pulse height
Time, sTime / μs
Pulse height / VN
umbe
r of
eve
nts
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Future cryodetectors
• Ton-scale experiments– EURECA
• Greater exposure– Larger detector mass
• Lower cost readout per module• Lower heatload per readout channel
– Simplicity & reproducibility for mass production• Excellent discrimination
– Ionisation • EDELWEISS ID detectors
– Scintillation • Low-temperature light detectors