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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

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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