cryodetector readout for dark matter searches
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Cryodetector Readout for Dark Matter Searches
Stuart Ingleby
Cryodetectors Group, Oxford
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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%
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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
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Cryodetector experiments
• Detect WIMP scattering– Nuclear recoils
• Extensive shielding– Deep
underground labs• Discrimination
– Exclude electron events
– Determine scattered nucleus
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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
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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
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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
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