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Philip Harris

CryoEDM at ILL. Philip Harris. Overview. Motivation, history and technique CryoEDM: Current status Upgrade Systematic errors Timeline Conclusion. CryoEDM Collaboration.

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Philip Harris

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  1. CryoEDM at ILL Philip Harris

  2. Overview • Motivation, history and technique • CryoEDM: Current status • Upgrade • Systematic errors • Timeline • Conclusion

  3. CryoEDM Collaboration C. Baker, S. Balashov, A. Cottle, V. Francis, P. Geltenbort, M. George, K. Green, M. van der Grinten, M. Hardiman, P. Harris, S. Henry, P. Iaydjiev, S. Ingleby, S. Ivanov, K. Katsika, A. Khazov, H. Kraus, A. Lynch, J.M. Pendlebury, M. Pipe, M. Raso-Barnett, D. Shiers, P. Smith, M. Tucker, I. Wardell, H. Yoshiki, D. Wark, D. van der Werf

  4. Electric Dipole Moments • EDMs are P, T odd • Complementary study of CPv • Constrains models of new physics + E

  5. History Factor 10 every 8 years on average

  6. Measurement principle Use NMR on ultracold neutrons in B,E fields. () – () = – 4 E d/ h with appropriate compensation for any changes in B during measurement period. B0 B0 B0 E E <Sz> = + h/2 h(0) h() h() <Sz> = - h/2

  7. 2 s 2 s 130 s Ramsey method of Separated Oscillating Fields “Spin up” neutron... 1. Apply /2 spin-flip pulse... 2. Free precession... 3. Second /2 spin-flip pulse 4.

  8. Dispersion curve for free neutrons Landau-Feynman dispersion curve for 4He excitations ln = 8.9 Å; E = 1.03 meV UCN production in liquid helium • 1.03 meV (11 K) neutrons downscatter by emission of phonon in liquid helium at 0.5 K • Upscattering suppressed: Boltzmann factor e-E/kT means not many 11 K phonons present • Observed: C.A.Baker et al., Phys.Lett. A308 67-74 (2002) R. Golub and J.M. Pendlebury Phys. Lett. 53A (1975), Phys. Lett. 62A (1977)

  9. CryoEDM overview Neutron beam input Cryogenic Ramsey chamber Transfer section

  10. Sensitivity Achieved 60% polarisation in source, but must improve Successfully produced, transported, stored UCN, but need to reduce losses Successfully applied 10 kV/cm (same as previous expt); aiming for 20-30 kV/cm RT-edm: 130 s. So far we have 62 s cell storage time.

  11. Neutron numbers • Anticipated production rate 1.4 /cc/s • Aperture mask x 0.44 • Entrance window scattering x 0.8 • Beam attenuation x 0.72 • Source storage lifetime 91 s • Incomplete source filling (200 s): x 0.89 Gives expected density in source: 30/cc Source volume 10.5 litres.

  12. Neutron numbers • Measurement has been somewhat indirect (neutrons taking convoluted paths to detectors) but it appears that we are currently down a factor of ~4: Under investigation • Alignment/divergence issue? • Spectrum affected by upstream instruments?

  13. Neutron numbers • Guides and valves not yet optimal. • Ramsey chambers: first attempt yielded storage time 60 s. For next time, improved cleaning; also bakeout. • What is limiting storage lifetimes in source and cells...?

  14. Electric field • See talk by M. Hardiman • Latest feedthru installed designed for 30 kV (6.7 kV/cm); it was run up to 45 kV (10 kV/cm). • We know how to design feed up to ~80 kV, and possibly up to ~150 kV, but... • ... will need mild pressurisation of He.

  15. Detectors • Solid-state detectors developed for use in LHe • Thin surface film of 6LiF: n + 6Li a + 3H; 82% efficient • Fe layer for spin analysis • Currently, a peak hidden under g background • pulse-shape discrimination • Now moving to detector with 10x area, to cover entire guide C.A.Baker et al., NIM A487 511-520 (2002)

  16. Detection of polarised UCN • Observed ~60% polarised downscattered neutrons • Should be able to improve on this - Upcoming measurement of source polarisation

  17. T1 • Longitudinal polarisation T1 is fairly straightforward to hold: field mustn’t change too fast for precession to follow • Issues last time with superconducting material around source/guide region... • Need to watch also sensitive area at entrance to shields, where field is low

  18. T2 • Transverse polarisation T2 is more delicate. Depends largely on variation of Bz within trap volume – causes dephasing: goes as • We are aiming for ~1 nT across the bottle • Currently in commissioning phase. SS plates at end of superfluid containment vessel (SCV) distort field. Modelling suggests that with correction coils we can reach T2 ~ 30 s with current SCV. • Working on non-magnetic SCV.

  19. Sensitivity summary: Current • Room-temperature expt final sensitivity ~2E-25 ecm/day • Took 12 years of incremental developments from known technology • Systematics limited (geometric phase effect) • We can come within factor 4-5 of this in 2013 by • increasing detector area x10: technology now proved • refurbishing damaged detector-valve: in hand • applying ~70 kV (previously ~40 kV): should be straightforward • opening beam aperture from 43 to 50 mm: depends on radiation levels • retaining polarisation: superconducting material has been removed • There may be additional improvements beyond this • a peak above background (detector improvement) • Polarisation to 60% or more (improved guide field) • Increasing cell storage lifetime (insulator bakeout) (we will achieve these by 2014)

  20. Shutdown, move to new beamline Mid-2013: Have to vacate current location. ILL to shut down for a year; we move to new dedicated beamline. • New beam 4x more intense; and dedicated • Due to become operational mid-2014 • Beam must then be characterised (9A flux, divergence, stability, polarisation) • We will then have access to the area (late 2014) to move our apparatus into it.

  21. Upgrade 2013-15 • Not yet fully costed • Major upgrade to experiment: • Cryogenics design changes: • Pressurise the liquid helium: increase E field x 2-3 • Upgrade to back-to-back cells (or possibly 4 cells) • 2 x neutrons • Cancellation of some systematic effects • Installation of inner superconducting magnetic shield • B-field stability improves x500, for systematics • Construction of non-magnetic SCV • Improves depolarisation: better T2 • Overcome geometric-phase systematic error • Net result: • Order of magnitude improvement in sensitivity • Commensurate improvement in systematics

  22. Systematics: General • Systematics minimised by highly symmetric data taking: • B and E field reversals • Alternating either side and above/below middle of central Ramsey fringe • Upgrade: opposite E in adjacent cells • Possibly also neutron magnetometers in adjacent (outer) cells, for 4-cell system

  23. Systematics: B field fluctuations • At present, Pb shield too short: flux lines clip coil end, inducing current in whole coil • Introduces common-mode noise, limiting sensitivity to 1E-27 e.cm Figure: JMP

  24. Systematics: B field fluctuations • We plan to add an inner superconducting shield. Scale model work in lab (MH) suggests that this can bring increased shielding factor ~500. ISS Figure: JMP

  25. Systematics: B field fluctuations • Can also (or instead) add Pb end caps, calculated to give factor ~250 improvement Figure: JMP

  26. Bnet Br Bnet Bv Bv Bv Bnet Br Bnet Systematics: Geometric phase ... so particle sees additional rotating field Bottle (top view) Frequency shift  E Looks like an EDM, but scales with dB/dz

  27. Systematics: Geometric phase • For neutrons, • Scales as 1/B2; increase B 5x to obtain factor 25 protection • <1 nT/m  3E-29 e.cm

  28. Systematics: E x v • Translational: • Vibrations may warm UCN, cause CM to rise ~1 mm in 300 s  3E-6 m/s • If E, B misaligned 0.05 rad., gives 2E-29 e.cm • Rotational: • Net rotation damped quickly (~1 s): matt walls • Delay before NMR pulses allows rotation to die away • Neutrons enter E-field cells centred horizontally; no preferred rotation • Below 1E-29 e.cm

  29. Systematics: 2nd order E x v • Perpendicular component, adds in quadrature to B. • Prop. to E2; gives signal if E reversal is asymmetric • Cancellations (back-to-back cells; B reversals) reduce effect to < 3E-29 e.cm

  30. Systematics: m metal hysterisis • Room-temp expt: Pickup in B coil from E field reversals; return flux causes hysterisis in m metal • Coil here is SC, not power-supply driven • Inner shield is SC also • Small effect from trim coils, enhanced by any misalignments • Net estimate < 1E-30 e.cm

  31. Systematics: E induced cell movement • Electrostatic forces of order 1 N;  E2 • Asymmetry perhaps ~1% of this • Radial gradients of order 3 nT/m • Must keep radial displacement on E reversal symmetric to ~ 0.01 mm • Cancellation with double cell • Symmetric voltages to ~2% • Net effect < 1E-28 e.cm

  32. Systematics: Leakage currents • Azimuthal current components generate axial contributions to B • Cancellation in adjacent cells • Conservative estimate: 1 nA  5E-29 e.cm • In reality LHe should keep currents much below this? • New source of current: ionisation from UCN decay electrons (10-100 pA?, but preferentially axial)

  33. Systematics: HV supply contamination • HV circuit isolated as far as possible to minimise earth contamination. Feedback line far from cells. Separate computer control. • 10 kHz ripple on HV line can “pull” resonant freq. Estimate 1E-30 e.cm • Likewise 50 Hz ripple: estimate ~1E-29 e.cm • Directly generated AC B fields negligible

  34. Systematics: Summary

  35. Sensitivity timeline

  36. New collaborators • Swansea is interested in joining us soon. • There is still plenty of room for new collaborators! Grants panel would like to see us recruiting from overseas.

  37. Conclusions • CryoEDM is now commissioning • New beamline 2014 • Aim to start running ~2015 • No “showstoppers” evident • Goal (for now) ~3E-27 e.cm • There’s still room on board!

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