Towards a Next Generation m SR Facility

1. Towards a Next Generation mSR Facility Bob Cywinski School of Applied Sciences FFAG’08, Manchester, September 2008

2. SR Chemistry Physics Molecular dynamics Oxides Muonium Magnetism Superconductivity Surfaces Fundamental physics Biology Materials Proteins Polymers Semiconductors Hydrogen in metals mSR is a uniquely powerful technique for studying magnetic field distributions and dynamics in condensed matter. mSR is a universal acronym which stands for: MuonSpin Rotation MuonSpin Relaxation MuonSpin Resonance The coherent precession of the muon spin in a transverse applied magnetic field The depolarisation of muon spins in zero or longitudinal applied field The response of the muon spin to pulsed RF fields The technique uses predominantly low energy (keV-MeV) positive muons There are currently ~300-500 muon beam users worldwide

3. Typical mSR experiments Spin dynamics and spin fluctuations Heavy fermion and small moment magnetism Slow spin dynamics, spin glasses and frustration Organic magnets Superconductors Vortex states and the flux line lattice Diffusion, tunneling, and localisation Semiconductor physics Muonium formation and dynamics Molecular dynamics Muon chemistry Low energy muons - surface/near surface studies Muon catalysed fusion Fundamental Physics (1S-2S etc)

4. Muon production ISIS PSI High energy proton C or Be nuclei neutrino Muon (~4MeV) pion  = 26 ns mp=140MeV  = 2.2 s

5. Muon implantation Implantation is rapid and occurs without loss of muon polarisation A “cold” muon technique utilising cryogenically moderated muons has also been developed to probe surface and near surface effects: ~1-3 mm

6. Muon decay ao~0.25 Mass 0.1126xMp Lifetime:2.19714s Charge + (-) Decay asymmetry:W() = 1+a0cos pμ Gyromagnetic ratio: 1.355342x108 x2p s-1T-1

7. Muon precession

8. Muon spin rotation Bx >0 m F B

9. Muon spin rotation – an example l=500nm x= 29nm TC=1.7K Studies of the flux line lattice in Type II superconductors The time evolution of the muon polarisation in a transverse fieldBis wherewL=gB Gx(t)is the Fourier transform of the field distribution averaged over all muon sites.

10. Muon facilities worldwide

11. Parasitic or symbiotic? ISIS - a 50Hz pulsed muon source PSI - a continuous (CW) muon source

12. Graphite targets at PSI Target M Thin graphite (few mm) Target E 4cm graphite “necessary component to expand proton beam before SINQ”

13. PSI muon facilities

14. ISIS muon facilities emu MuSR argus RIKEN-RAL

15. Pulsed or CW? Tw(ns) 1.0 25 50 Relative µSR asymmetry 0.5 100 0.0 0 20 40 60 80 100 Transverse field, mT The finite proton pulse width (80ns at ISIS) limits the dynamic response of a mSR spectrometer at a pulsed source. There are no such limitations in CW Synchrotron operation at 50Hz is inefficient – it provides a measuring window of 20ms whilst only 20ms (ie 10tm) is needed.

16. Pulsed or CW? The finite proton pulse width (80ns at ISIS) limits the dynamic response of a mSRspectrometer at a pulsed source. There are no such limitations in CW Synchrotron operation at 50Hz is inefficient – it provides a measuring window of 20ms whilst only 20ms (ie 10tm) is needed. At a CW source only one muon can be allowed in the sample at a time. PSI spectrometers already count at 25-40KHz – this is the maximum rate possible with CW operation. At a pulsed source the positron count rate is limited only by detector deadtime. Significant increases in countrate can be achieved by increasing source intensity Muons on request (MORE) at PSI

17. A next generation source At existing facilities, muon provision is thus a sub-optimal compromise defined by other users of the proton driver Question: What do muon beam users want ? Answer: Orders of magnitude more muon intensity and smaller muon beam dimensions Why? At current positron count-rates (up to 40kHz) a typical spectrum from a typical sample (of a few cm2) will take ~30min to collect with reasonable statistics Parametric studies (as a function of temperature, field and/or sample concentration) can take days Studies of small (mm2) sample (eg single crystals) can take even longer Low energy muon studies of surfaces can take weeks

18. Low energy muons Muons are cryogenically moderated and energy-selected (MeV→eV) to tune localisation depth within the sample: Incident muon rates remain relatively low (~500µ/s) Cooling efficiency ~ 10-5

19. A stand-alone facility? It has been suggested that high intensity muon facilities could be incorporated into the SNS and ESS designs as they have been at J-PARC This would be yet another compromise both for neutron and muon beam users – and would also be expensive An EU funded workshop on Future Developments of Muon Sources in November 2006 concluded that a stand-alone muon facility may be the best way forward – if economically viable Physics World, December 2006

20. ....and the follow up ....an nmi3 funded workshop at the Cockcroft Institute, April 2008

21. nmi3 workshop summary Muon production rates Substantial (two orders of magnitude) gain in muon intensity can only be achieved in pulsed source operation Gains of 30 could be achieved at ISIS x3 by increasing target thickness x10 by increasing solid angle acceptance of muons from pion target Both are unacceptable by the neutron users of the facility A gain of 100 could be achieved by making the above optimisations, and increasing the power of the proton driver by a factor of 3 This in turn implies that a fully optimised muon source requires a pulsed proton driver of at least 500kW, ie 1mA at 500meV or 0.5mA at 1GeV For optimal efficiency the pulse repetition rate should approach 25kHz

22. nmi3 workshop summary Pulse width considerations For pulsed surface muons, the pion lifetime (26ns) itself sets a lower intrinsic time resolution However, narrower pulses can be achieved by electrostatically trimming the pion decay tail, and even substantially wider pulses (80ns) can be shaped with muon pulse conditioning post-production, as demonstrated at ISIS kicker chops within a pulse Such conditioning presents the opportunity of almost instantaneously trading absolute count rate against frequency response on the same muon spectrometer. At the extreme, quasi-CW could be possible through “muon on request” techniques, offering intensities close to those at PSI

23. nmi3 workshop summary The proton driver In order to be world leading a next generation muon source should therefore be driven by a 500kW proton driver and be capable of delivering three modes of operation – 1. A pure pulsed mode (ideally 25kHz) with an integrated count rate of at least x100 ISIS and with better frequency response than ISIS (30ns pulse width) 2. An electrostatically-tailored pulse mode (~5ns, 25KHz) with an order of magnitude higher count rate than ISIS but with significantly improved frequency response 3. A quasi-CW mode – which at least matches the experimental count rate at PSI A dedicated proton driver unconstrained by parasitic uses of the proton beam will enable precise tailoring of beam/target assemblies, allowing smaller proton/muon beams, and more efficient pion/muon collection and will also facilitate the implementation of multiplemuon production targets.

24. The proton driver

25. An FFAG driven muon source? A FFAG appears to satisfy most of the requirements demanded of an optimised proton driver for a stand-alone muon facility. Although repetition rates of 25kHz may not be achievable, even 2kHz offers considerable advantages FFAG technology is also likely to keep costs within acceptable limits The CONFORM project is currently exploring this possibility

26. Acknowledgements Prof Roger Barlow Cockcroft Institute/Manchester University, UK Dr Adriana Bungau University of Huddersfield, UK Dr Kurt Clausen Paul Scherrer Institut, Switzerland Dr Pierre Dalmas de Reotier CEA Grenoble, France Dr Rob Edgecock, Rutherford Appleton Laboratory, UK Dr Philip King ISIS Facility (RAL), UK Dr James Lord ISIS Facility (RAL), UK Prof Mike Poole ASTeC (STFC) Daresbury Laboratory, UK Dr Francis Pratt ISIS Facility (RAL), UK Dr Toni Shiroka sμs, Paul ScherrerInstitut, Switherland Dr Sue Smith ASTec, (STFC) Daresbury Laboratory, UK