第二讲 慢正Muon束的产生.pdf

Low energy positive muons beam PDF created with pdfFactory Pro trial version www.pdffactory.com Cryogenic moderator Laser Ionization method PDF created with pdfFactory Pro trial version www.pdffactory.com Slow muon beam l Slow muons : muons which are (re-)accelerated from the muons which are almost at a rest. l Beam energy is tunable, and its spread is very small. a The range in the material is tunable down to sub µm. – Emittance is very small. aSmall sample can be used. lpolarized muons ideal as a microscopic magnetic probe to solid state physics (µSR technique) l Depolarization of the muon spin due to local magnetic fields can be monitored through decay positrons which are emitted preferentially along the spin direction. PDF created with pdfFactory Pro trial version www.pdffactory.com Study of Surface & Interfaces pthin films pnanomaterials pmulti-layered compounds psmall size samples pAtomic Physics. pSurface Chemistry-Catalysis. PDF created with pdfFactory Pro trial version www.pdffactory.com Bulk Thin films PDF created with pdfFactory Pro trial version www.pdffactory.com Muon cooling methods < 1keV range Methods capable of muon cooling below keV within the lifetime of 2.2 µs: 1 keV 10 eV <1 eV Frictional cooling in gasses efficient , problems with muonium µ+ Relatively formation, loss LE muons in extraction from gas cell µ- Cold moderator method (epithermal muons) Uses layer of solid rare gas as a moderator for controlled slowing down emission process. Ideal are perfect insulators with wide band-gap energy (11-22 eV) µ+ Laser ionization of muonium (ultraslow muons) Good quality beam with small emittance BUT: Only pulsed operation < 30 Hz rep µ+ & complex laser required PDF created with pdfFactory Pro trial version www.pdffactory.com Two methods to generate slow muon beam • Cryogenic moderator method – Use a layer of solid rare gas as a moderator. – Initial energy is 10-100eV, and its spread is around 10eV. – Time structure is determined by initial beam. PDF created with pdfFactory Pro trial version www.pdffactory.com • Laser resonant ionization method – Obtain slow muons by ionizing thermal muoniums emitted from a hot tungsten film. – Initial energy is around 0.2eV, and its spread is less than 1eV. – Time structure is determined by laser timing. g Gives better time resolution for pulsed beam. g Possible use for Mu anti-Mu conversion experiment as a sensitive detection method of anti-Mu and background suppression. PDF created with pdfFactory Pro trial version www.pdffactory.com Cryogenic moderator PSI slow muon beam line ISIS EC slow muon beam line l Cryogenic moderator method l Applied cryogenic moderator method to a pulsed muon source. l Yielded about 1 µ/sec (*2) l Pulsed source n Efficiency close to 10−4 n Initial beam energy spread is about 10eV l Yielded about 700 µ/sec (*1) l DC source n A timing counter needs to be inserted in the transport line for µSR study l Successful operation since 1993 PDF created with pdfFactory Pro trial version www.pdffactory.com n Time-of-flight showed time resolution was about 90nsec (FWHM), according to ISIS beam pulse width. n Better energy resolution than PSI thanks to absence of a trigger counter. l Currently defunct. PSI slow muon beam line PDF created with pdfFactory Pro trial version www.pdffactory.com PDF created with pdfFactory Pro trial version www.pdffactory.com PDF created with pdfFactory Pro trial version www.pdffactory.com Moderator efficient • The most efficient moderators to date are s-Ne, s-Ar, and s-N2. Using a flat Al substrate and choosing a momentum bite ∆p/p = 4%(FWHM) of the incoming surface muon beam, we measured the following moderation PDF created with pdfFactory Pro trial version www.pdffactory.com • Energy spectrum of the emitted muons after moderation of surface muons in a solid argon layer. The useful energy interval of epithermal muons is shown. PDF created with pdfFactory Pro trial version www.pdffactory.com PDF created with pdfFactory Pro trial version www.pdffactory.com PDF created with pdfFactory Pro trial version www.pdffactory.com PDF created with pdfFactory Pro trial version www.pdffactory.com PSI Beamline parameters PDF created with pdfFactory Pro trial version www.pdffactory.com Laser Ionization method Alternative: SiO2 powder (3-4% efficiency) PDF created with pdfFactory Pro trial version www.pdffactory.com • Thin W foil used to thermalize surface muon beam owing to inelastic and elastic collisions • Muonium can not be formed inside metal because of the high density of conduction electrons BUT • Muons close to surface can escape the bulk via thermionic emission. Work function for escaping the metal is lower for neutral muonium, hence muonium is efficiently emitted to vacuum. • Heating increases muonium emission • Efficiency determined by how many muons stop within diffusion length from target surface. The beam momentum spread therefore directly affects production efficiency PDF created with pdfFactory Pro trial version www.pdffactory.com PDF created with pdfFactory Pro trial version www.pdffactory.com Muonium n=1 to n=2 transitions Electric dipole selection rules: ∆l = ±1 ∆j = 0, ±1 ∆F = 0, ±1 F=0 ⇒ F=0 2-photon transition: ∆l = 0 ∆F = 0 PDF created with pdfFactory Pro trial version www.pdffactory.com 2S metastable : H (1S-2S): natural linewidth only 1.3 Hz Mu (1S-2S): nat. width 144 kHz 2P dipole allowed transition: H (1S-2P): natural linewidth 100 MHz How to ionize muonium? l Use two-photon ionization of muonium with 122nm and 355nm light. 1S-2P transition is most intense one among muonium’s transitions. l Use a sum-difference frequency mixing method to generate 122nm light. PDF created with pdfFactory Pro trial version www.pdffactory.com What laser parameters are required? • two laser beams necessary to ionise required very broad laser bandwidth due to thermal movement of atoms 1S-2P saturation intensity Esat=Isat .∆tp . A = 37 µJ [=4.6x103 (W/cm2) x 4x10-9 (s) x 2 (cm2) ] Main challenge: to generate VUV @ 122 nm and with 200 GHz ( + 1 ns jitter rel. to ext. trig.) PDF created with pdfFactory Pro trial version www.pdffactory.com l requires simultaneous absorption of two counter-propagation photons l only one laser necessary to ionise low transition probability 1S-2S saturation intensity lIsat ~ 1.6 MW/cm2 l BUT easier to generate high intensity at 244 nm l transition probability strongly depends on laser linewidth Main challenge: to generate pulsed high intensity laser beam with < 10 MHz bandwidth ( + 100 ns jitter rel. to ext. trig.) PDF created with pdfFactory Pro trial version www.pdffactory.com Non-linear frequency upconversion to VUV • Usual aproach of generating tuneable and coherent UV (and even VUV): nonlinear frequency upconversion from laser operation in visible or infrared region. • Nonlinear response of an optical medium to high intensity fields in order to generate new frequencies (eg. SHG ω3=2ω1 ) P=ε0(χ(1)E+χ(2)E2+χ(3)E3+…) Nonzero for piezoelectric crystals with no centre of symmetry (LBO, BBO…) Zero for gases Nonzero for gases e.g. THG ω3=3ω1 And other 4-wave processes. v χ(2) processes in pulsed operation can achieve over 50% conversion efficiency. Usually nonlinear crystals, but none them transparent below 190 nm. v χ(3) processes in pulsed operation – much lower efficiency (10-4 at most). PDF created with pdfFactory Pro trial version www.pdffactory.com l 212.55 nm (single longitudinal mode) tuned to a resonance in Kr - yield resonantly enhanced l 820 nm (844 nm for H) broadband to match Doppler broadening of 200 GHz l tuneable VUV output ~ 122 nm (with 200 GHz bandwidth) PDF created with pdfFactory Pro trial version www.pdffactory.com lShort laser pulses required to increase intensity (~4 ns) lScheme requires relative timing of all laser pulses ~ 1 ns with external trigger (!) lonly possible with OPO lasers pumped by YAG PDF created with pdfFactory Pro trial version www.pdffactory.com Diagram of the laser system All-solid laser system using OPOs and Nd:YAG lasers a Stable operation a Gives good timing (1nsec accuracy) a Good overlapping of 212nm laser and 820nm laser for frequency mixing in Kr gas. a Good overlapping of VUV light and 355nm laser for ionizing muonium. (The lifetime of 2P state is only 1.6nsec.) l PDF created with pdfFactory Pro trial version www.pdffactory.com Schematic view of the slow muon beam line-RIKEN PDF created with pdfFactory Pro trial version www.pdffactory.com PDF created with pdfFactory Pro trial version www.pdffactory.com PDF created with pdfFactory Pro trial version www.pdffactory.com Layout of the RIKEN-RAL muon facility, ISIS, Rutherford Appleton Laboratory, UK PDF created with pdfFactory Pro trial version www.pdffactory.com Schematic view of the slow muon beam line slow muons Main Chamber (1x10−11 hPa) Ionizing Lasers High purity Tungsten film (45µm; 87mg/cm2) Tungsten degrader (20µm; 39mg/cm2) SUS foil (50µm; 40mg/cm2) Kapton foils Degrader chamber (1x10−5 hPa) surface muons PDF created with pdfFactory Pro trial version www.pdffactory.com Port 3 beam line (1x10−6 hPa) The first observation of slow muons at the RIKEN-RAL muon facility (July 2001) • A clear peak on TOF spectrum was observed at the position which corresponds slow muon at the accelerating voltage of 7.5kV. • Measured magnetic field of the bending magnet corresponds to the correct muon mass. • Count rate was 0.03 µ/sec. (too small!) PDF created with pdfFactory Pro trial version www.pdffactory.com Improved slow muon generation at the RIKEN-RAL muon facility (April 2003) • The yield of slow muon (3.3 slow µ/sec) is 100 times more than that obtained in July 2001, and larger than the previous experiment at the EC muon beam line with cryogenic moderator method. • The time diversion of slow muon beam is about 10ns (FWHM), whereas the cryogenic moderator experiment gave about 100nsec. PDF created with pdfFactory Pro trial version www.pdffactory.com What is phase matching? P=ε0(χ(1)E+χ(2)E2+χ(3)E3+…) P: polarization (dipole moment per unit volume) χ(1): linear susceptibility χ(2): second order nonlinear susceptibility χ(3): third order nonlinear susceptibility Phase-matching condition: phase velocity of generated light equals to that of induced nonlinear polarization. g efficient nonlinear process PDF created with pdfFactory Pro trial version www.pdffactory.com slow muon yield (counts/Kspills) Kr-Ar phase matching at muonium 1S2P frequency 5 4.5 Kr 20hPa + Ar 820.9nm 4 212.55nm 3.5 3 2.5 2 1.5 212.55nm 1 122.09nm 0.5 0 0 50 100 150 Ar partial pressure (hPa) 200 250 Kr / Kr+Ar • Enhancement of VUV generation due to phase matching of Kr gas with Ar buffer gas was observed with slow muon yield. • The ratio of Kr and Ar buffer gas (1:6) agrees with theoretical calculation. PDF created with pdfFactory Pro trial version www.pdffactory.com Muon stopping range tuning 3.5 slow muons Preliminary counts/kspills 3 2.5 Ionizing Lasers 2 1.5 1 Kapton foils 0.5 0 0 10 20 30 Kapton thickness [mg/cm2] n n 40 50 surface muons The thickness of the degrader is optimized to give maximum yield of slow muons. Measured range width (5.5% FWHM) reasonably agrees with expected value. PDF created with pdfFactory Pro trial version www.pdffactory.com Laser timing dependence 340 ns Relative timing between the initial beam’s arrival time and laser irradiation time was scanned. n A double-pulse structure of the initial muon beam is clearly observed. n 35.0 slow muons/Kspills 30.0 25.0 20.0 15.0 10.0 5.0 0.0 0 1000 2000 3000 4000 5000 6000 laser irradiation time from the arrival of the 1st pulse (nsec) PDF created with pdfFactory Pro trial version www.pdffactory.com Dependence on VUV 122nm laser energy 5 y = 15.493x 1.3672 Note: laser VUV m irror rem oved (laser intens ity halved) unbound µ+ e 4.5 4 355nm 3 2P 2.5 + µ yield /s 3.5 2 1.5 strong focusing (5 Apr) 1 122.09nm (Mu) weak cylindrical focus (6 April) 0.5 Power (strong focusing (5 Apr)) 0 0 0.5 1 1.5 2 VUV 122 nm e nergy (rel.u.) 2.5 3 Muonium 1S We measured dependence of slow µ+ yield on VUV (122 nm) energy for 1S g 2P transition. As expected the transition is far from saturation point. n VUV energy is currently in µJ range. While one of the brightest Lyman-α sources available there is still huge scope for improvement. n PDF created with pdfFactory Pro trial version www.pdffactory.com 355nm laser power dependence 9 unbound µ+ e Note: VUV retroreflected with 122 nm HR 8 355nm 7 + µ yield/s 6 2P 5 4 3 122.09nm (Mu) 2 YAG5 amp delay change 1 YAG5 THG misalignment Linear dependence 0 0 50 100 150 200 250 300 355 nm energy [mJ] n n 350 400 Muonium 1S The power of 355nm laser for 2P g unbound transition is scanned. No saturation is observed up to 1.8W (70mJ/pulse) Further increase of slow muon yield is expected with improved 355nm laser power (upgrade to 500 mJ expected in June 2004). PDF created with pdfFactory Pro trial version www.pdffactory.com Size of low energy muon beam at focus point Measured with Roentdek position sensitive MCP (1 mm resolution) PDF created with pdfFactory Pro trial version www.pdffactory.com Efficiency of slow muon generation Observed slow muon signal : 3.3 µ/sec (MCP efficiency 66%) a 5.0 µ/sec (Decay in flight 43%) a 8.6 µ/sec (Transport efficiency unknown. assume 100%) a >8.6 µ/sec at the source Initial surface muon beam : 1.0x106 µ/sec Efficiency 8.6/1.0x106 = 8.6x10−6 (still low…) PDF created with pdfFactory Pro trial version www.pdffactory.com Target studies Hydrogen solution in metals • • • Extensive studies have been done for the solubility of hydrogen in metals. Large (positive) solution enthalpy means the work function for hydrogen (muonium) to escape from metal is small. But the depth of adsorption energy could play a role, as well as the height of surface barrier energy. g Needs experimental studies! • • • Matsushita et al. studied muonium production from Iridium(Ir)1), Platinum(Pt)2) and Renium(Re)3), and obtained a promising result for Iridium. Ruthenium(Ru) and Molybdenum(Mo) also seem promising. Our system is a very sensitive muonium detector! PDF created with pdfFactory Pro trial version www.pdffactory.com W Pt Ir Mo Ru Rh Ta Nb Ti V ∆H(eV/atom) 0.22 0.20 0.76 0.53 0.56 0.28 −0.37 −0.37 −0.47 −0.32 Melt point (C) 3387 1772 2457 2610 2250 1963 2996 2468 1675 1890 • tungsten surface drilled by pulsed laser irradiation PDF created with pdfFactory Pro trial version www.pdffactory.com • TOF spectrum of low-energy muons generated by the laser ionization method and accelerated to 7.5 kV in the extraction beam line PDF created with pdfFactory Pro trial version www.pdffactory.com Comparison of muonium ionization and cryogenic moderator methods on ISIS pulsed source n n (a) TOF spectrum of slow muons obtained with laser resonant ionization method (RIKEN April 2003) n (b) TOF spectrum of slow muons obtained with cryogenic moderator method (Ph.D. Thesis, Dr. K. Trager, 1999) PDF created with pdfFactory Pro trial version www.pdffactory.com Laser resonant ionization method makes slow muon beam with better timing resolution. Best timing resolution was less than 10nsec (FWHM). When cryogenic moderator method was used in ISIS, the timing resolution was about 100ns. Laser ionization allows to trigger LE muon generation by external trigger with nanosecond resolution. Present characteristics at RIKEN-RAL Surface µ+ beam Low energy µ+ beam Intensity 2x106 µ+/s Beamsize (FWHM) 40 mm Energy 3.55 MeV Momentum p = 27.6 MeV/c Momentum bite ∆p/p ≈ 0.10 Pulse repetition rate 50 Hz, Double pulse structure 80 ns (FWHM) separated by 350 ns. Spin polarisation 100% Intensity at sample ~ 16 µ+/s (25 µ+/s expected soon ) Beamsize (FWHM): 4 mm Energy at target region 0.2 eV Energy after re-acceleration 1-10 keV Energy uncertainty after re-acceleration <50 eV Pulse repetition rate 25 Hz Single pulse structure 10 ns (FWHM) at 9.0 keV Spin polarisation ~50% PDF created with pdfFactory Pro trial version www.pdffactory.com Main features of the of this method • Positive - Timing determined by laser pulse, which is externally triggered → “muons on demand” to ns accuracy ; time resolution comparable to muons from continuous source such as @PSI - Low muon energy – in principle as low as 0.2 eV - Efficiency of conversion from surface muon beam can be, in principle, as high as 1 %. • Negative - Only suitable for pulsed sources with low repetition rate - Inherent loss of muon polarization (50%) - Laser system currently too complex PDF created with pdfFactory Pro trial version www.pdffactory.com Efficiency of LE muon generation • Efficiency at RAL currently 1x10-5, i.e. similar to cryogenic moderator method. - 25 Hz operation means ½ of muons “wasted” - overlap of laser with muonium cloud is not ideal. 1) Increase VUV laser pulse energy In principle muonium can be ionized with 100% efficiency, if ~ 100 µJ at 122 nm could be produced. Such system not yet available. PDF created with pdfFactory Pro trial version www.pdffactory.com 2) Increase muonium density l Tighter focusing of the incident muon beam would allow better overlap with laser l “pre-cooling” of incident beam would lead to higher fraction of muons stopping close to W target surface and therefore higher muonium production l Geometry can be changed to include 2nd W target 1% efficiency could be achieved PDF created with pdfFactory Pro trial version www.pdffactory.com Current status of slow muon PDF created with pdfFactory Pro trial version www.pdffactory.com Thank you! PDF created with pdfFactory Pro trial version www.pdffactory.com