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Paul Scherrer Institut Thomas Prokscha, Laboratory for Muon Spin Spectroscopy

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Paul Scherrer Institut Thomas Prokscha, Laboratory for Muon Spin Spectroscopy Wir schaffen Wissen – heute für morgen Paul Scherrer Institut Thomas Prokscha, Laboratory for Muon Spin Spectroscopy The PSI continuous muon beam and low-energy µSR facilities ISIS Muon Spectroscopy Training School, March 21, 2012 RAL/ISIS PSI EL issF Sw n SINQ µSµS γ SLS PSI proton accelerator complex SINQ, neutron spallation source Dolly, µSR, GPS clone Low-energy muon beam facility (1-30 keV), LEM GPD, high pressure µSR High-Field (9.5 T, 20 mK) µSR, 2013 GPS/LTF, general purpose and low T (20 mK) µSR facility Ultra-cold neutrons UCN proton therapy, irradiation facility 50 MHz proton cyclotron, 2.2 mA, 590 MeV, 1.3 MW beam power (2.4 mA, 1.4 MW test operation); Comet cyclotron (superconducting), most powerful cyclotron worldwide and most 250 MeV, 500 nA, 72.8 MHz intense muon beams (quasi-continuous); The PSI isochronous cyclotron 2.2 mA: ~1.4 x 1016 protons/sec @ 590 MeV (1.3 MW on 5x5 mm2 = 50 kW/mm2, stainless steel melts in ~0.1 ms; electric power demand of 1000 households) A MW proton beam allows to generate 100% polarized 4-MeV µ+ beams with rates >108/sec Larmor frequency of protons: q/(2πm) = 15.25 MHz/T ν = q/(2πγm)∙B, γ = E /mc2 0 tot ν = n∙ν , frequency of accelerating radio-frequency rf 0 Isochronous cyclotron: B (R) ~ γ(R), constant ν ! 0 rf PSI cyclotron: B = 0.554 T, ν = 8.45 MHz, n = 6, 0 0 → ν = 50.7 MHz rf “Decay” and “Surface” muon beams Jess Brewer, UBC & TRIUMF: PSI muon beam time structure Pion decay time of 26 ns is “smearing” the proton beam structure in the muon beam. This results in a “continuous” muon beam. A µ+ rate R of 105/s means: average time between to µ+ is 1/R = 10 µs. Probability p to have the next µ+ at time t: p = 1 – exp(-Rt) (“pile-up”) Single µ+ can be detected with very good time resolution (< 1ns), compared to a bunch width of 50-100 ns at pulsed beams. --> measurement of GHz frequencies and fast relaxation rates (>100 µs-1). But “acci- dental” background in µ-decay histograms. Continuous beam µSR experiment ωµ = γµ · Βµ ( γ µ = 135.5 M H z/T) P ( t = 0 ) P ( t ) + s µ p µ e µ + M y o n - C o u n t e r P o s i t r o n - S a m p l e B e x t P o s i t r o n - C o u n t e r C o u n t e r Accepted rate as a function of µ rate Continuous versus pulsed muon beams ● time resolution ≤ 1 ns, ν > 100 MHz Observable asymmetry as a function of time resolution σ : t ● fast depolarization observable µ -1 ω ω 2σ 2 ( >> 10 s ) A( ) = A0 x exp(- t /2) ● no segmentation/position information for e+ necessary ● start counter needed ● random background (can be reduced by „Muons On REquest“, MORE) ● limited time window ● separator required for surface muons µSR instruments at PSI ● GPS: General Purpose Surface muon instrument; πM3 permanent installation, shared with LTF; 4 MeV µ+, Separator/Spin Rotator (6 – 60o); 0 – 0.6 T, 1.8 – 1000 K; Muons On REquest (MORE). ● LTF: Low Temperature Facility; πM3 permanent installation, shared with GPS; 0 – 3 T, 0.01 K – 10 K. ● Dolly: GPS „clone“; πE1 permanent installation in 2012; 4 MeV µ+; Separator/Spin Rotator (6 – 45o); 0 – 0.5 T; 0.3 – 300 K. ● GPD: General Purpose Decay channel instrument; µE1 beam; 15 – 60 MeV µ+ and µ-; 0 – 0.65 T, 0.3 – 500 K; high pressure up to 26 kbar. ● ALC: Avoided Level Crossing instrument; 4 MeV µ+; 0 – 5 T. Not in operation. ● High-Field µSR: πE3 permanent installation; 4 MeV µ+; 2 Spin Rotators (0 – 90o); 0.1 – 9.5 T; 0.02 – 300 K. ● LEM: Low Energy Muon beam & µSR instrument; µE4 beam; 1 – 30 keV µ+ (controllable implantation depth 5 – 300 nm), 0 – 0.3 T, 2.3 – 320 K. Muons On Request (MORE) ● GPS “tells” kicker to send a muon to GPS ● If a muon is detected in GPS “tell” kicker to send the beam to LTF ● no random beam background in GPS ● if decay e+ is detected in GPS the next muon is requested from the kicker Kicker (two electrodes, +-5kV) “normal” MORE pulsed B /N [10-5] 660 9 1 0 0 ∆t [ns] <1 <1 80 Events [106/h] 12 20 20-40 MORE example: UPt 3 MORE opens the possibility ● of studying slow relaxation phenomena at high magnetic fields ● to resolve close spin-precession frequencies In UPt : 2 Knight shift components, very close 3 Dramatic and opposite T dependence around TN (6K). UPt3 is intrinsically inhomogenous. A. Yaouanc et al, PRL84, 2702 (2000). The new high-field µSR facility at SµS Beamline: surface muon beam Δp/p < 5% FWHM: πE3 90° spin rotation! Magnet: max. field ~10 T, homogeneity: 10 ppm, field drift < 1 ppm/hr. ν σ -1 µ ~1.35 GHz < 0.1 µs @ 10 T Detector system: time resolution δt < 140 ps (RMS) for TF Sample Environment: 2 K cryostat & DR, with integrated ‘Veto’ system Difficulties: charged particles with finite momentum distribution (spiraling radius of 30 MeV positron: 1 cm in 10 T) Detector dimensions for a 10 T spectrometer ν Sample Scintillators ø µ+ e+ Veto Ø (mm) N A F counter Muon ν counter 14 36’400 0.31 ± 0.01 59.1 60 20 30’800 0.33 ± 0.01 57.9 50 40 24 26’300 0.34 ± 0.01 55.1 30 30 19’900 0.39 ± 0.01 55.0 20 40 12’000 0.41 ± 0.01 44.9 10 0 50 7’300 0.41 ± 0.01 35.0 10 20 30 40 50 60 60 4’400 0.29 ± 0.01 19.1 Oxford Instrument 8/9.5 T Recondensing System BF-H400 0.01 – 4 K Commissioning 2012, user operation 2013 Range of muons in matter mm 1 102 Cu “Surface Muons” from π+ bulk decay at rest ( ~ 4 MeV) µm 10 thin films, e generally used for bulk g 1 n multilayers.. studies: no depth resolu- a 2 R 10 tion nm 10 1 1 10 102 103 104 Energy [keV] . “Low-energy muons”: 0.5 – 30 keV . Allows depth-dependent µSR investigations ( ~ 1 – 300 nm) . Extends the use of µSR to new objects of investigtions . New magnetic/spin probe for thin films, multilayers, surface regions, buried layers Implantation Profiles of Low Energy Muons m 200 Stopping profiles calculated with Monte n Range Carlo code Trim.SP by W. Eckstein, MPI Variance 150 Garching, Germany. 100 Experimentally tested for muons: 50 E. Morenzoni, H. Glückler, T. Prokscha, R. YBa2Cu3O7 Khasanov, H. Luetkens, M. Birke, E. M. 0 Forgan, Ch. Niedermayer, M. Pleines, 0 5 10 15 20 25 30 35 NIM B192, 254 (2002). Energy [keV] 0,05 3.4 keV y t i s YBa2Cu3O7 n 0,04 6.9 keV e D 0,03 15.9 keV g 20.9 keV n i 24.9 keV p 0,02 p o 29.4 keV t S 0,01 0,00 0 50 100 150 200 Depth [nm] Generation of thermal µ+ at a pulsed muon beam Pulsed beam (25Hz) @ RIKEN-RAL: P. Bakule,Y.Matsuda,Y.Miyake, K. Intensity: ~15 LE-µ +/sec Nagamine, M. Iwasaki, Y. Ikedo, K. Polarization: ~ 50% Shimomura, P. Strasser, S. Makimura Nucl. Instr Meth. B 266, 335 (2008) Beam spot: 4 mm Generation of polarized epithermal muons „Surface“ Muons ~ 4 MeV ~ 100% polarized Energy spectrum after a degrader Solid line: muon energy spectrum Solid circles: energy spectrum of muonium ~100 µm Ag Using a proper moderator: motivated by experiments for positron moderation, a solid film of a rare-gas should work! T. Prokscha et al., Phys. Rev. A58, 3739 (1998). Generation of polarized epithermal muons „Surface“ Muons ~ 4 MeV ~ 100% polarized ~100 µm Ag ~500 nm s-Ne, Ar, 6 K s-N2 motivated by experiments for positron moderation T. Prokscha et al., Appl. Surf. Sci. 172, 235 (2001). T. Prokscha et al., Phys. Rev. A58, 3739 (1998). E. Morenzoni et al,. J. Appl. Phys. 81, 3340 (1997). D. Harshmann et al., Phys. Rev. B36, 8850 (1987). Characteristics of epithermal muons y r t e m m y ) s t ( A L P A P(0) ≅1 T i m e [ µ s ] E. Morenzoni, T. Prokscha, A. Suter, H. Luetkens, R. E. Morenzoni, F. Kottmann, D. Maden, B. Matthias, M. Meyberg, Khasanov, J.Phys.: Cond. Matt. 16, S4583 (2004). T. Prokscha, T. Wutzke, U. Zimmermann, PRL 72, 2793 (1994). suppression of electronic energy loss for E > Eg, large band gap Eg (10-20 eV) „soft, perfect“ insulators large escape depth L (10-100 nm), no loss of polarization during moderation (~10 ps) moderation efficiency is low (requires high intensity µ + beam, > 108 µ+/s): ε ∆Ω ∆ ∆ -4 -5 µ+ = Nepith/N4MeV ≈ (1-FMu ) L/ R ≈ 0.25 L/ R ≈ 10 – 10 ∆Ω: probability to escape into vacuum (~50% for isotropic angular distribution) F : muonium formation probability Mu page 28 LE-µ+ Apparatus at µE4 beam line At 2.2 mA proton current: ~4.6 • 108 µ +/s total, ∆p/p = 9.5% (FWHM) ~1.9 • 108 µ+/s on LEM moderator ~1.1 • 104 µ +/s moderated (solid Ar) T. Prokscha, E. Morenzoni, K. Deiters, F. Foroughi, D. George, R. Kobler, A. Suter and V.
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