Ultra slow muon generation and applications
P. Bakule (RIKEN)
Y. Matsuda (RIKEN)
M. Iwasaki (RIKEN)
Former colleagues:
R. Scheuermann (PSI)
Y. Ikedo (TOYOTA)

Y. Miyake (KEK)K. Nagamine (KEK)
P. Strasser (KEK)K. Shimomura (KEK)
S. Makimura (KEK)Contents
Part 1 : Development of low energy muon source at RIKEN RAL
Method of low energy muon (LE-µ+) generation
Beam and spectrometer characteristics
Control over the implantation energyEfficiency of LE-µ+ generation
Summary of the current status and comparison with LE-µ+ beam at PSIPart 2 : Laser applications at RIKEN RAL beamlinesApplications for LE-µ+ beamµSR experiments with laser irradiated samples
Construction of new laser laboratory at Port 2

Looking back over past 10 years and looking forward to the future laser
experiments at RIKEN RAL

µSR with low energy muons
For “surface muons” with energy of 4 MeV thestopping range in a solid varies from 0.1 - 1 mm witha straggling of about 20% of the mean value.
Beam size 40-50 mm (FWHM)

source: E. Morenzoni, PSI

For “low energy muons” with energy 0.01-30 keV
the stopping range in a solid varies from 1 - 200 nm.
Implantation depth easily controlled on nm scale.
Beam size is small 4-5 mm (FWHM)- allows investigations of near-surface regions, thin films, interfaces and multilayers, nanomaterials and of samples which can be grown only as thin films.
- allows to make depth resolved measurements.

Methods of LE-muon generation
1) Cold Moderator Method (@PSI)• ideal for continuous muon source• layer of solid rare gas as a moderator• conversion efficiency up to 10-5
• 92 % Polarization• 10-100eV Kinetic Energy• DC, Requiring a start trigger (->5 ns resolution)• Time structure determined by initial muon beam

50 eV
4 MeV
Ar, N2, Kr2) Laser Resonant Ionization of Muonium (@RIKEN-RAL)• ideal for pulsed muon source• 1% efficiency of conversion to thermal muonium i.e.
potentially much higher conversion efficiency to LEmuons• 50 % Polarization reduction• potentially 0.2eV Monochromatic beam
• Time structure determined by laser pulse(~10 ns) synchronized with pulsed muon beam
• external trigger allows synchronisation
with sample excitation

4 MeV

0.2 eVPrinciple of ultra low energy muon generation
4 MeV muons

2%0.2 eV thermal Mu

0.2 eV µ+122.09 nm (Mu)121.57 nm (H)
121.53 nm (D)• two laser beams necessary for resonant ionization• required very broad laser bandwidth due to thermal
movement of atoms

1S-2P saturation intensity
Isat=2.3 W/cm2
monochromatic<100MHz

Isat=4.6 kW/cm2(Doppler 200GHz)

Main challenge: to generate VUV @ 122 nm
and with 200 GHz ( + 1 ns jitter rel. to ext. trig.)Lyman-α generation(sum-difference frequency mixing in Kr gas)

• 212.55 nm (single longitudinal mode)
tuned to a resonance in Kr - yieldresonantly enhanced• 820 nm (844 nm for H or D)
broadband to match Doppler
broadening of 200 GHz• tuneable VUV output ~ 122 nm(with 200 GHz bandwidth)• Short laser pulses required to increase intensity (~4 ns)• Scheme requires relative timing of all laser pulses ~ 1 ns with external trigger (!)⇒ possible with OPO lasers pumped by YAG

Schematic diagram of the laser system
25 Hz operation
Output synchronised to 1 ns (!)
High stability : 20 days continuous 24/7
operation

Solid State Laser parameters:
212.55 nm (single mode, tuned to Kr resonance):
Energy: 10-15 mJ /pulse x 2 beams
Pulse duration: 4 ns
800-880 nm (tunable broadband output)
Energy: 25 mJ
Pulse duration: 8 ns
Bandwidth: 160 GHz
355 nm (multimode output)
energy 380 mJ, 10 ns

Transport beamline for low energy µ+Laser beam overlap with muoniumIn laser beam (per pulse):
Mu: 1-10 atoms
Deuterium: 103-104 atoms

µSR setup for LE-muon experiment
Solenoid type magnet(transverse field < 60 mT)

Diamagnetic asymmetryin Ag sample:
10.1% ± 0.2%sLE-µ+

Iwatani two-stage cryostat
cooling power 0.5W @4K

3-axis fluxgate magnetometers

Scintillation counters(8 telescope pairs)
solid angle coverage
80% of 4π sr.

TF measurement to 60 mT
ZF compensation to 0.1 µT

LE-µ+ decay spectrum

RIKEN-RAL LE-µ+

Background suppressed below
0.01 counts over 15 µs period
after slow µ+ arrival.
Background further reduced bysubtracting “laser off” events

Background much lower
than at continuous muonsource -> much wider time
window for measurement

10ns – 15µs

Size of low energy muon beam at sample
Measured with Roentdek
position sensitive MCP(0.8 mm resolution)
4.1 mm~ 100 times smaller crosssection than incident
surface muon beam.

3.3 mmAllows us to measure small
samples of 10-20 mm diameterwith excellent S/N ratio

Muon implantation with external trigger
340 ns
90 ns

LE muon TOF

laser pulse

ISIS muon pulse structure

The timing between
muon injection to thetungsten film and laser
ionization was scanned
to find optimum timing.

Proportional to muonium
density as a function oftime.

Double-pulsed structureof initial muon beamvisible from this chart.

Comparison of muonium ionization and cryogenic
moderator methods on ISIS pulsed source
laser resonant
ionization method(2006)

cryogenic moderator
method(K. Trager 1999)

Laser resonant ionization method
makes slow muon beam with goodtiming resolution.

Time resolution is 7.5 nsec(FWHM). When cryogenic
moderator method was used in
ISIS, the time resolution was about
100ns.

Laser ionization allows to trigger
LE muon generation by external
trigger with nanosecond resolution→ synchronization with pulsed
fieldsLE-µSR: frequency response plot

Higher frequency limit forpulsed µSR is significantlyextended.
ISIS 82 ns pulse

Measured using muonium spin precession

Muon implantation energy
Implantation energy range of
0-18 keV controlled by applying
potential on sample
Implantation depth in
Au : 0-55 nm
Cu : 0-73 nm
Al : 0-135 nm-9.0 kV to +9 kV

9.0 kV

Energy resolution of the LE-µ+ beam
Initially only 0.2eV (thermal energy)
Energy resolution at sample determined by extraction
i.e. differences in potential seen by individual muons:- Width of the laser ionization region (σE=~13 eV)- Uneven distance between W and S1 (σE=~4 eV)- Differences due to laser beam alignment (σE=~4 eV)

σE = 14 eV (33 eV at FWHM)

Energy dependence of Aµ in Al(40 nm) on SiO2
We have demonstrated that we can control muon implantation range within 10nm
resolution by changing energy of LE-muons.
→ provides magnetic probe with depth resolution on nm scale

The LE muons are transported through the LE muon beamline at 9 keV. Muon energy iscontrolled by applying a potential on the sample in the range of 9.0 kV to -9.0 kV giving
control over the implantation energy in the range of 0 – 18 keV.

Muon implantation at very low energies
E. Morenzoni et al., NIM B 192 (2002) 254–266

Reflection from Au

Au sample

Initial assymmetry [%]At low incident energies(E < 3 keV) :• large fraction is reflected• nearly all reflectedmuons form muonium

3 nm 6 nm

8 nm

14 nm

22nm

30 nm

11.00

8.25

5.50

TRIM.SP calculation for Au
RAL data
PSI data (scaled)

2.75

00

4.625

9.250

Implantation energy [keV]

13.87518.500Efficiency of LE muon generation
RIKEN-RAL

PSI(muonium ionization)(cryogenic moderator)

Surface muon beam
intensity

1.2x106 µ+/sec (50 Hz)
6x105 µ+/sec (25 Hz)

2x108 µ+/sec (new beamline)

LE µ+/ intensity at sample

20 µ+/sec

8000 µ+/sec

Overall efficiency

3x10-54x10-5

Muonium ionization method is capable of much higher efficiency– potentially up to 10-3 level!

Dependence of yield on laser pulse energy
122 nm355 nmVUV energy is currently in µJ range. While one of the brightest
Lyman-α sources available there is still large scope for improvement!
Muonium ionization efficiency
1) Increase VUV laser pulse energy
50 mm

535nm

We expect modest improvements to VUV energy : 50%(In principle muonium can be ionized with close to 100%efficiency, with ~ 100 µJ at 122 nm)

122 nmW122 nm2) Increase muonium density• Tighter focusing of the incident muon beam would
allow better overlap with laser• increasing W target surface area (laser drilled or
porous W, tungsten coated aerogel)• SiO2 aerogel

Other factors increasing the number of LE muons available at sample:• Planned upgrade of ISIS proton current from 200 µA to 300 µA → immediate 50% increase• Increasing the thickness of muon production target from 10 to 15 mm• Increasing the acceleration voltage in LE muon beamline from 9.0 kV to 18.0 kV(TOF reduced by ~400 ns i.e. 16% increase in µ+ on sample)

More intense VUV?
Can we get more intense 122 nm beam from different laser system?
C. Dölle et al., Appl. Phys. B 75, 629–634 (2002)
Generation of 100 µJ pulses at 82.8 nm by frequency tripling of sub-picosecond KrF laser radiation

Non-linear conversion efficiency in gases is typically 10-4 to 10-7 but in this case
it is claimed to be 0.7% !
100 µJ pulses at 82.8 nm generated by frequency tripling (249 nm) in Ar gas jet.
On the other hand: Ganeev RA, Usmanov T, J. OPTICS A 2 (6): 550-556 NOV 2000:
350 nm ps pulses converted to 116.6 nm with 8x10-4 efficiency (max. 2.4 µJ )If this conversion efficiency can be reproduced with
0.5 ps pulses at 366.27 nm it could:- increase the muonium ionization efficiency tonearly 100% ( with 100 µJ pulses )- greatly simplify the laser system (only one
wavelength needed & need to overlap several laser
beams is eliminated)- automatically match the Doppler broadenedbandwidth of Mu since the transform limit would beabout 300 GHz- time resolution of LE-µ+ would be reduced to ~ 1 ns
(limited by extraction ion optics)

366.27 nm generated by SHG
solid-state system at 732.54 nm(e.g. Ti:Sapphire)

Main features of the of this methodPositive- Timing determined by laser pulse, which is externally triggered- Pulse duration only 7.5 ns (comparable to continuous source) and independent of-

the surface muon pulse structure
Good energy resolution ~ 14 eV– (in principle as low as 0.2 eV)
Extremely low background
Small beam spot size
Efficiency of conversion from surface muon beam can be, in principle,
as high as 10-3.

Negative- Only suitable for pulsed sources with low repetition rate
- Inherent loss of muon polarization (50%) - BUT can be recoveredSummary - Present characteristics
Low energy µ+ beam

µSR spectrometer

Intensity at sample ~ 15-20 µ+/s
Beam diameter (FWHM): 4 mm
Energy at target region 0.2 eV
Energy after re-acceleration 0.1-18 keVEnergy uncertainty
after re-acceleration ~14 eV
Pulse repetition rate 25 Hz
Single pulse structure
7.5 ns (FWHM) at 9.0 keVSpin polarisation ~50%

Background: <0.01 per 15 µs after µ+ pulse
Count rates: ~ 50 kev/hour(compared to 20-50 Mev/hour @ bulk µSR atISIS)TF : < 60 mT
ZF compensation to 0.1 µT
Sample temperature: 10K-300K
External LE-µ+ trigger

J-PARC facility– projected muon intensity ~ 108 µ+/s (comparable to current PSI beam)– projected smaller diameter of the surface muon beam
– 25 Hz operation (double pulse structure – 600 ns separation)
We can expect more than 104 LE-µ+/s in <10 ns pulse

Comparison with PSI LE-muon beam