Slow muon beam

Low energy positive muons beam

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Cryogenic moderator

Laser Ionization method




Slow muon beaml 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.




Study of Surface & Interfaces

pthin filmspnanomaterialspmulti-layered compoundspsmall size samplespAtomic Physics.pSurface Chemistry-Catalysis.




Thin films

Bulk




Muon cooling methods < 1keV rangeMethods capable of muon cooling below keV within the lifetime of 2.2 µs:

1 keV Frictional cooling in gassesµ+

µ-

Relatively efficient , problems with muoniumformation, loss LE muons in extraction from gas cell

10 eV Cold moderator method(epithermal muons) µ+

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)

<1 eVLaser ionization of

muonium(ultraslow muons)

µ+

Good quality beam with small emittance BUT:Only pulsed operation < 30 Hz rep& complex laser required




Two methods to generateslow 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.




• 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.




Cryogenic moderator

PSI slow muon beam linel Cryogenic moderator methodn Efficiency close to 10−4

n Initial beam energy spread is about 10eV

l Yielded about 700 µ/sec (*1)

l DC sourcen A timing counter needs to be

inserted in the transport line for µSR study

l Successful operation since 1993

ISIS EC slow muon beam linel Applied cryogenic moderator method to

a pulsed muon source.l Yielded about 1 µ/sec (*2)

l Pulsed sourcen 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










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




• 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.













PSI Beamline parameters




Laser Ionization method

Alternative: SiO2 powder(3-4% efficiency)




• 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







Muoniumn=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

2S metastable : H (1S-2S): natural linewidth only 1.3 HzMu (1S-2S): nat. width 144 kHz2P 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.




What laser parameters are required?

1S-2P saturation intensity

• two laser beams necessary to ionise required very broad laser bandwidth due to thermal movement of atoms

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.)




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 nml 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.)




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 gasese.g. THG ω3=3ω1And 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).




l 212.55 nm (single longitudinal mode) tuned to a resonance in Kr -yield resonantly enhancedl 820 nm (844 nm for H) broadband to match Doppler broadening of

200 GHzl tuneable VUV output ~ 122 nm (with 200 GHz bandwidth)




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




Diagram of the laser systemlAll-solid laser system using OPOs and Nd:YAG lasersa Stable operationa 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.)




Schematic view of the slow muonbeam line-RIKEN










Layout of the RIKEN-RAL muon facility, ISIS, Rutherford Appleton Laboratory, UK




Schematic view of the slow muonbeam line

High purity Tungsten film (45µm; 87mg/cm2)Tungsten degrader (20µm; 39mg/cm2)

SUS foil (50µm; 40mg/cm2)

Kapton foils

Ionizing Lasers

Main Chamber

Degrader chamber

(1x10−11 hPa)

(1x10−5 hPa)

surface muons

slow muons

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!)




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.




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 susceptibilityPhase-matching condition: phase velocity of generated light equals to

that of induced nonlinear polarization.

g efficient nonlinear process




Kr-Ar phase matching at muonium 1S-2P frequency

00.5

11.5

22.5

33.5

44.5

5

0 50 100 150 200 250

Ar partial pressure (hPa)

slow

muo

n yi

eld

(cou

nts/

Ksp

ills)

Kr 20hPa + Ar

212.55nm

212.55nm820.9nm

122.09nm

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.




Muon stopping range tuning

0

0.5

1

1.5

2

2.5

3

3.5

0 10 20 30 40 50

Kapton thickness [mg/cm2]

coun

ts/k

spill

s

Preliminary

Kapton foils

Ionizing Lasers

surface muons

slow muons

n The thickness of the degrader is optimized to give maximum yield of slow muons.

n Measured range width (5.5% FWHM) reasonably agrees with expected value.




Laser timing dependence

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

0 1000 2000 3000 4000 5000 6000laser irradiation time from the arrival of the 1st pulse (nsec)

slow

muo

ns/K

spill

s

n 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.

340 ns




Dependence on VUV 122nm laser energy

n 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.

1S

2P

unbound

355nm

122.09nm (Mu)

Muonium

µ e+y = 15.493x 1.3672

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0 0.5 1 1.5 2 2.5 3

VUV 122 nm energy (rel.u.)

µ+ yie

ld /s

strong focusing (5 Apr)

weak cylindrical focus (6 April)

Power (strong focusing (5 Apr))

Note: laser VUV mirror removed (laser intensity halved)




355nm laser power dependence

1S

2P

unbound

355nm

122.09nm (Mu)

Muonium

µ e+

n The power of 355nm laser for 2P g unbound transition is scanned. No saturation is observed up to 1.8W (70mJ/pulse)

n Further increase of slow muon yield is expected with improved 355nm laser power (upgrade to 500 mJ expected in June 2004).

0

1

2

3

4

5

6

7

8

9

0 50 100 150 200 250 300 350 400

355 nm energy [mJ]

µ+ y

ield

/s

YAG5 amp delay change

YAG5 THG misalignment

Linear dependence

Note: VUV retroreflected with 122 nm HR




Size of low energy muon beam at focus point

Measured with Roentdek position sensitive MCP (1 mm resolution)




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…)




Target studies

∆H(eV/atom) Melt point (C)W 0.22 3387Pt 0.20 1772Ir 0.76 2457

Mo 0.53 2610Ru 0.56 2250Rh 0.28 1963Ta −0.37 2996Nb −0.37 2468Ti −0.47 1675V −0.32 1890

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!




• tungsten surface drilled by pulsed laser irradiation




• TOF spectrum of low-energy muons generated by the laser ionization method and accelerated to 7.5 kV in the extraction beam line




TOF spectrum of slow muons obtained with laser resonant ionization method (RIKEN April 2003)

TOF spectrum of slow muons obtained with cryogenic moderator method(Ph.D. Thesis, Dr. K. Trager, 1999)

(a)

(b)

n Laser resonant ionization method makes slow muon beam with better timing resolution.

n Best timing resolution was less than 10nsec (FWHM). When cryogenic moderator method was used in ISIS, the timing resolution was about 100ns.

n Laser ionization allows to trigger LE muon generation by external triggerwith nanosecond resolution.

Comparison of muonium ionization and cryogenic moderator methods on ISIS pulsed source




Present characteristics at RIKEN-RAL

Surface µ+ beam Low energy µ+ beamIntensity 2x106 µ+/sBeamsize (FWHM) 40 mmEnergy 3.55 MeVMomentum p = 27.6 MeV/cMomentum bite ∆p/p ≈ 0.10Pulse 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 mmEnergy at target region 0.2 eVEnergy after re-acceleration 1-10 keVEnergy uncertainty after re-acceleration <50

eVPulse repetition rate 25 HzSingle pulse structure

10 ns (FWHM) at 9.0 keVSpin polarisation ~50%




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




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.




1% efficiency could be achieved

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 productionl Geometry can be changed to include 2nd W target




Current status of slow muon




Thank you!




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Slow muon beam