Cryodetector Readout for Dark Matter Searches

1. 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 CRESST 2007 EDELWEISS II ZEPLIN III XENON10 CDMS SuperCDMS (dashed) [1] Trotta et al. 2008

7. 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

9. 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

10. UV exposure unit Etching bath Laminator 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

11. Etched cables Foil with photo-resist pre-etching 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

12. Heatload • Larger detector mass • Lower heatload / channel • Choice of materials • Practical constraints • Resistivity measurements • Heatload calculations • Etched steel cables offer 20 x lower heatload

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

14. 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?

15. G B A B A B A A A B C D C D C D C C D H 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

16. Cabling design for NTD/Ge • Readout for NTD/Ge • High impedance • Capacitance • Limits bandwidth • Microphonics • Mounted 4K – 10mK • Heatload minimised • Radiopurity

17. 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

18. 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

19. 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

20. Component testing • Performance simulation • Approximate formulae available • Software simulation • Efficiency • Drop voltage • Transformer • Low-T component testing • Transformer • MPP • Capacitors • Polystyrene • Diodes • Silicon 1N4007

21. Time / μs Number of events Pulse height / V Prototype PMT module • Installation of CWG-PMT module • Preliminary 57Co spectra taken at 300K • Detailed study of PMT performance for EURECA WP

22. 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