Spectrochimica Acta Part B 58 (2003) 1019–1030

Slow muon experiment by laser resonant ionization method at
RIKEN-RAL muon facility夞
P. Bakulea,b,*, Y. Matsudaa,b, Y. Miyakec, P. Strassera, K. Shimomuraa, S. Makimurac,
K. NagaminecaMuon Science Laboratory, RIKEN (The Institute of Physical and Chemical Research), Wako, Saitama 351-01, Japanb
RIKEN Muon Facility, Rutherford Appleton Laboratory, ISIS R3, Chilton OX110QX, UK
c
Meson Science Laboratory, High Energy Accelerator Research Organization (KEK), 1-1 Oho, Tsukuba, Ibaraki 305-0801, Japan
Received 7 July 2002; accepted 31 January 2003

Abstract
We report first results of the slow muon generation from pulsed surface muon beam using a method of resonant
laser ionization of muonium. The muonium is produced from the muon beam by electron capture near the surface ofthin tungsten foil heated to 2100 K. The ionization is resonantly enhanced by using a vacuum-ultraviolet (VUV)
radiation resonant with 2P state of muonium (l1S–2P s122.09 nm). A pulsed beam at 355 nm is used for an efficientionization from the 2P state. The VUV beam with pulse duration of 4 ns and with bandwidth of over 100 GHz tomatch the Doppler broadening of muonium (200 GHz) is generated by resonant third-order sum-difference frequency
conversion (vVUVs2vRyvT) in krypton gas phase-matched with argon buffer gas. The vR beam has been tuned tothe two-photon resonance of 4p55pw1y2,0x state in krypton to enhance the conversion process. Yield of the VUVradiation has been investigated for different energies and confocal parameters of the incident beams. The wholeapparatus, including a beamline for transport and detection of ionized particles, has been tested by ionizing residualhydrogen atoms at an ultra-high vacuum level of 1.1=10y9 hPa with efficiency better than 10y3 . The yield of slow
muons obtained during the first beamtime was 0.03 mqys.
䊚 2003 Elsevier Science B.V. All rights reserved.
Keywords: Vacuum-ultraviolet beam; Infrared; Optical parametric oscillators; Muon; Muonium

1. Introduction
A slow muon beam with energy below 100 eV
夞 This paper was presented at the International Conferenceon Laser Probing (LAP-2002), held in Leuven, Flanders,Belgium, July 2002, and is published in the Special Issue ofSpectrochimica Acta Part B, dedicated to that conference.
*Corresponding author. Tel.: q44-1235-445256; fax: q441235-446881.
E-mail address: p.bakule@rl.ac.uk (P. Bakule).

that can be accelerated to energies in the range of
2–10 keV can provide an excellent tool for condense matter physics extending the scope of muon
spin relaxation technique from bulk materials tothin samples. The penetration depth of muons atthese very low energies can range from the subnanometre to several nanometre region. This
allows for systematic studies of thin films, multilayered structures, nanomaterials and extremely
small samples. There have been several proposed

0584-8547/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0584-8547Ž03.00062-4

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P. Bakule et al. / Spectrochimica Acta Part B 58 (2003) 1019–1030

methods of slow muon generation w1x. In terms of
the beam intensity, the muon moderation techniquedeveloped at PSI has been so far the most successful w2x. This technique produces polarizedmuons with energies below 30 eV, with a broad
peak centered at 15 eV. An alternative method weare investigating potentially produces muons withenergy as low as 0.2 eV (ultra-slow muons). The
method is based on the laser ionization of muonium atoms (mqey) in ultra-high vacuum. The
muonium can be produced with high conversion
efficiency (1–4%) by stopping high energy,
4 MeV surface muon beam in thin tungsten foil
heated to 2100 K w3x. The muonium is formed byelectron capture near the surface of the hot foil.
The muonium atoms can then escape to the vacuum with thermal velocity.
Using a pulsed Lyman-a (122 nm) laser system
the muonium is resonantly excited from 1S groundstate to 2P state and subsequently photoionizedwith a 355 nm laser beam. The ultra-slow muons
are then extracted and accelerated in a static
electric field. The method of muonium ionization
was first successfully tested at the pulsed proton
beamline at Meson Science laboratory of the HighEnergy Accelerator Research Organization (KEK)in Japan w4x. The thermal muonium was produced
directly in the 500 MeV proton beam from acomposite target consisting of boron nitride pion
production target and a heated tungsten foil. However, there are serious technical issues associatedwith the radiation in the primary proton beamline
limiting more systematic study of optimum conditions for the muonium production. We havetherefore decided to take advantage of a highintensity pulsed surface muon source available at
RIKEN-RAL beamline at ISIS facility of Rutherford Appleton Laboratory, UK. A new experimental setup consisting of a laser system for muonium
ionization operating with a repetition rate of 25
Hz, a muonium production target, and an ultraslow muon beamline was installed at Port 3 of
RIKEN-RAL facility, where 5=105 surface
muonsys are available in a beam pulsed at 50 Hz
and with a pulse duration of 80 ns w5x. In thisprogress report we are presenting the details of ourlaser system and first results of the ultra-slow
muon generation at RIKEN-RAL.

The muonium ionization efficiency is currently
limited by the availability of sufficiently intense
Lyman-a source. The 1S–2P transition is strongly
allowed electric dipole transition with saturation
intensity of only 2.3 Wycm2 over a natural linewidth of 0.1 GHz. Considering the Doppler widthof the thermal muonium of 200 GHz, Lyman-aintensity of 4.6 kWycm2 is required to excite 25%of the ground state muonium to the 2P state. Thus,
for a typical laser pulse duration of 4 ns and themuonium cloud cross-section of several squarecentimeters, Lyman-a pulse energies of approximately 10–100 mJ are required. Historically, themost commonly used method for generation in thevacuum-ultraviolet (VUV) region is the third harmonic generation (THG) in rare gasses. In general,
however, the conversion efficiency of THG isrelatively low because of the small value of the
non-linear susceptibility for laser frequencies far
from any resonance. Resonantly enhanced
schemes, such as the sum-difference frequency
mixing process, have conversion efficiencies thatare several orders of magnitude higher w6,7x. The
sum-difference frequency mixing process is illustrated in Fig. 1, where VUV is generated in a fourwave mixing process involving two photons at
lRs212.55 nm tuned to the two photon resonanceof 4p55pw1y2, 0x state of Kr and a third photon oftunable wavelength in the infrared (IR) region.
Tuning of the IR laser to lTs820.9 nm or lTs
845.3 nm results in the generation of Lyman-a formuonium or hydrogen, respectively. Using thisscheme it is possible to generate VUV with large
bandwidth using IR laser with large bandwidththat is matched to the 200 GHz doppler width of
the thermal muonium produced from the hot tungsten foil. Recently, VUV pulse energies as high as
7 mJ at frequencies near Lyman-a were generatedusing the sum-difference frequency mixing process
utilizing the two photon enhancement at 193 nm(resonant with 4p56pw3y2,2x state) in Kr gas w8x.
One of the major loss mechanisms in this process
is photoionization of the excited Kr state by a third
193 nm photon or by the generated VUV photon.
Since the photoionization rate from the 4p55pw1y
2, 0x state is lower than from 4p56pw3y2,2x state
it is preferable to use 212.55 nm radiation for the
resonant enhancement in Kr w9,7x.

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Fig. 1. Schematic diagram of resonantly enhanced sum-difference mixing process in Kr used to generate Lyman-a radiation
for muonium and hydrogen.

2. Experimental setup
Owing to the short lifetime of 2P state ofmuonium (1.6 ns) and the nature of the frequency
mixing process in Kr all lasers have to be synchronized to 1 ns accuracy. To achieve this levelof accuracy the laser system is based on opticalparametric oscillators (OPO) that have no intrinsic
build-up time and are pumped by Nd:YAG lasersthat have the build-up time jitter at only 1 ns level.
The pumping Nd:YAG lasers are triggered in twostages. First, the flashlamps are triggered approximately 300 ms in advance of the ISIS muon pulse
by delayed trigger from the previous muon pulse.
The Q-switched pulses of the individual Nd:YAG
lasers are then triggered to better than 1 ns accuracy from the current muon pulse. Flashlamp andQ-switch delays of the individual lasers are computer controlled and are independently adjusted to
optimize the laser performance and timing relative
to the muon pulse. In this way, the pulses ofindividual lasers can be synchronized to 1 ns
accuracy with respect to each other as well as tothe current muon pulse.

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High power solid state lasers operating at highrepetition rates suffer from poor beam quality due
to high thermal load on the active medium, leading
to stress induced birefringence and strong thermal
lensing. This currently places a limit on the repetition rate of our laser system that is operating at
only 25 Hz repetition rate, i.e. synchronized with
only every second muon pulse.
A schematic diagram of our laser system isshown in Fig. 2. It consists of single-longitudinalmode (SLM) OPO oscillator with two-multipass
Ti-sapphire amplifiers (Continuum Mirage 800)
pumped by a frequency doubled, diode injection
seeded, SLM Nd:YAG laser (Continuum Powerlite9000). The Mirage 800 laser is tunable near 850
nm with bandwidth below 1 GHz and is producing
output energies of 30–40 mJypulse. The output
beam is further amplified in a 4-pass Ti:sapphire
amplifier, longitudinally pumped by 650 mJ pulses
from a frequency doubled Nd:YAG laser (Continuum Powerlite 9000), giving output energies of
150–200 mJ at 850 nm. After frequency doublingin a 7 mm long b-barium borate (BBO) crystal,pulse energies of up to 70 mJ at 425 nm are
obtained. The wavelength of the 425 nm beam ismonitored by a pulsed wavemeter (Burleigh WA4500). The beam is then expanded with a telescopeand split with a 50y50 beamsplitter before further
frequency doubling in 5 mm long BBO crystals to212.55 nm. The reason for this is twofold. Firstly,
by reducing the incident energy, the heating effects
caused by both linear absorption and non-linear
absorption of 212.55 nm in BBO are reduced. This
allows us to maintain the phase-matching condition
over the cross-section of the beam. Secondly, two
VUV beams can be generated independently in
the Kr cell, where we also observed a strong
saturation of the conversion process. As much as
6 mJypulse with pulse duration of 4 ns at 212.55
nm can be generated in each beam.
A broadband OPO laser coupled with an opticalparametric amplifier (Continuum Mirage 3000)
based on KTP is employed to generate the difference frequency between the excited 4p55pw1y2, 0x
state of Kr and the required Lyman-a frequency.
The output is tunable from 815 to 855 nm allowing
to generate Lyman-a for muonium, hydrogen anddeuterium. The Mirage3000 laser is optically1022

P. Bakule et al. / Spectrochimica Acta Part B 58 (2003) 1019–1030

Fig. 2. Schematic diagram of the laser system. Certain elements such as telescopes and waveplates are for simplicity omitted fromthe diagram.

pumped by a frequency doubled Nd:YAG laser(Continuum Powerlite 7000). The resonator lengthof the OPO, the KDP crystal length as well as thefocusing of the pump beam were optimized togenerate the broad bandwidth output required forthe efficient muonium ionization. The bandwidth,
as measured by the Burleigh WA-4500 pulsed
wavemeter, is typically 160 GHz (FWHM). The
output pulse energy over the tuning range of thelaser is typically 20 mJ with pulse duration of
12 ns.
Another frequency tripled Nd:YAG laser is used
to generate 80 mJ pulses with 10 ns duration at
355 nm required for ionization of muonium fromthe 2P state.
The optical arrangement used for the generationof VUV radiation is shown in Fig. 3 together with
the interaction area in front of the hot tungsten
target. Two pairs of 212.55 nm and IR beams wereused in some of our measurements to generate two
VUV beams separated vertically by 20 mm. For
simplicity only one beam is shown in Fig. 3. The
212.55 nm and IR beams are focussed separately
to the 300 mm long Kr cell by 500 mm focal
length lenses and combined collinearly by adichroic mirror. The generated VUV beam iscollimated in the horizontal plane by fs125 mm

MgF2 cylindrical plano-convex lens separating the
Kr cell from the ultra-high vacuum in the target
chamber of the slow muon beamline. The relative
intensity of the generated VUV is monitored bymeasuring a photoionization current in an NO gas
cell (at a pressure of 6.5 hPa) w11x. The muonium(or hydrogen) atoms excited by the Lyman-abeam to the 2P state are ionized by the pulsed
355 nm beam. This beam is introduced at an angle
from the Lyman-a beam and retro-reflected at asmall angle from the normal incidence to increase
the interaction volume in front of the target. The
total volume in front of the target defined by theoverlap between the VUV and the 355 nm beam
is estimated to be approximately 4 cm3. A remotely
controlled mirror with high reflectivity at 122 nm
located in front of the NO cell can be used toretro-reflect the VUV beam(s).
Two stainless steel plates that can be translated
in the horizontal and vertical direction, respectively, are mounted in front of the NO cell detector todetermine the position and size of the VUV beamusing the knife-edge method.
The surface muon beam with a flux of 5=105qm ys enters the apparatus through a 50 mm thickstainless steel foil that is shielded from the hightemperature tungsten target by a 25 mm thickP. Bakule et al. / Spectrochimica Acta Part B 58 (2003) 1019–1030

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Fig. 3. Schematic diagram of the setup used for the VUV generation and of the muonium production target area. The 355 nm beam
introduced from above the Kr cell at 45 degrees from the VUV beam is not shown. In this figure, Kr stands for Kr cell, T fortungsten target, NO for NO ionization cell, KN for motorized ‘knife-edge’ plate.

tungsten foil degrader. The target, a high purity(99.9999%) tungsten foil of 50 mm thickness, islocated at the focus of the muon beam that wascalculated to be of approximately 35 mm diameter.
The target is heated to 2100 K by a d.c. current
to maximize the muonium formation at the target
surface. The laser pulses are triggered with a 0.4
ms delay after the pulse to allow for muonium
propagation several millimetres away from thetarget.
The ionized particles are extracted by an immersion lens and transported through electrostatic
quadrupoles, a bending magnet and electrostatic
bend to a multichannel plate (MCP) detector w10x.
The particle acceleration voltage can be adjusted
between 0.5 and 10 kV. This slow muon transportbeamline is evacuated down to ultra-high vacuumwith three turbo pumps and a cryogenic pump.
The bending magnet in the beamline enables separation of the extracted ions by their mass andthus select the type of particles transported to the
MCP. The timing of the MCP signal, the muon
beam trigger, as well as the timing of the laser
pulses, and other relevant information were recorded with a CAMAC-based data acquisition system.
The time-of-flight (TOF) of the particle throughthe beamline gives also a signature of the particle

identity since particles of different mass will havedifferent velocities at same acceleration voltage.
3. VUV generation in Kr and Ar gas mixtures
The generation of VUV in Kr was investigatedunder various experimental conditions. The confocal parameters of the incident beams could bevaried over a wide range by using different lenses(fs750 mm, fs500 mm, fs360 mm) and two
beam expanding telescopes.
The efficiency of the VUV generation depends
primarily on the phase mismatch per Kr atom
CKrsDkyN (DkskVUVy2kRqkT), where N is thenumber density of Kr atoms. In case of the two
photon resonant difference frequency mixing in
the wavelength region near Lyman-a the mismatch
CKr is negative w4x. An analysis of four wave
mixing with different confocal parameters w12x has
shown that in case of negative wave-vector mismatch (Dk), the maximum VUV energy is generated when Dksy2ybR. Thus, given the confocal
parameter bR of the 212.55 nm beam and the phasemismatch per atom near Lyman-a (CKrsy
1.5=10y17 cm2), the maximum VUV signal
should be observed for an optimum Kr pressure(popt) corresponding to poptsy2 Ky(bR CKr),

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P. Bakule et al. / Spectrochimica Acta Part B 58 (2003) 1019–1030

Fig. 4. Energy of hydrogen Lyman-a radiation generated in pure Kr gas and in phase-matched Kr–Ar mixture. Confocal parameters:bRs3 cm, bTs2 cm; energy of vR beam 3.6 mJ, energy of vT beam 5.6 mJ.

where KspKr yNKr. This is in very good agreementwith our experimental results when we observed
the maximum VUV signal for bRs4, 2, and 0.5
cm at optimum Kr pressures 1.5, 2.5 and 11 hPa,
respectively. Comparing the relative VUV energy
for different confocal parameters, the largest value
was observed for the weakest focusing tested (bRs
4 cm, bTs5 cm).
The wave vector mismatch limiting the VUVgeneration in pure Kr only to low number density
can be compensated by using a buffer gas with apositive phase mismatch per atom in the Lymana region, such as Ar. In the limiting case of noabsorption and ideal phase matching Dks0, thegenerated VUV energy should theoreticallyincrease as a square of the Kr partial pressure
w8,6x. Experiments performed by Marangos et al.
at low input intensities and narrow bandwidth(ERs0.18 mJ and ETs0.72 mJ in pulses of 20 ns
duration) have yielded an enhancement of the
VUV energy in Kr–Ar mixture compared to optimum energy obtained in pure Kr of a factor of100. Similar enhancement factor at Lyman-a frequency was observed in an experiment by Meyer
et al. w13,8x using a different 2-photon resonance(4p56pw3y2,2x requiring lRs193 nm). In bothcases the increase of the VUV energy was foundto be increasing proportionally to partial Kr pressure. Under our experimental conditions, however,
the enhancement of the VUV energy with Ar gas
phase-matching is observed to be rather modest.
Fig. 4 shows the relative generated energy in pureKr gas and in the Kr–Ar mixture measured athydrogen Lyman-a frequency demonstrating anenhancement of a factor of only 5 due to a strong
saturation effect with increasing pressure of Kr–
Ar mixture. The data for VUV generation in Kr–
Ar mixture at different Kr partial pressures were
measured by preparing a phase-matched mixture
at 60 hPa Kr base pressure and then evacuating
the cell. The optimum phase-matching ratios
between Kr and Ar were found to be 1:4.0 and
1:5.8 for 121.57 nm (hydrogen) and 122.09 nm(muonium), respectively. The VUV energy, as
shown in Fig. 4 was found to have a maximum at
relatively low Kr pressure of 20 hPa.
We have investigated the dependence of thesaturation pressure on some experimental conditions. We have found that the saturation pressure
stays constant over a wide range of the incident
energies of the 212.55 nm beam, from 0.1 to 3.5
mJ. The change of confocal parameters has alsoP. Bakule et al. / Spectrochimica Acta Part B 58 (2003) 1019–1030

had only a limited effect on the value of thesaturation pressure and has not yielded any significant increase in the observed VUV energy.
A measurement of dependence of the VUV yieldon the input energy of vT beam has revealed astrong saturation, which occurs at energies as low
as 1 mJypulse. The saturation energy of vT
depends on the energy of vR and can be increased
by increasing the energy of vR. For example atERs5 mJypulse, bRs0.5 cm and bTs0.4 cm thesaturation energy of vR increases to 7 mJypulse.
Fig. 5 shows the dependence of the VUV signal
on Kr partial pressure in the phase-matched Kr–
Ar mixture measured at low energy of vR andseveral energies of vT. The measurements demonstrate that with increasing vT energy the saturation effects limiting the VUV generation arestronger and that the saturation pressure at which
the VUV output is maximized changes
significantly.
There are several saturation and loss mechanisms which contribute to saturation at high energies, such as 3-photon ionization, two-photon
re-absorption w7x, amplified spontaneous emission
from 4p55pw1y2,0x state w14x, stimulated hyperRaman scattering, self-defocusing w15x and othernon-linear effects. This results in either change of
population distribution and related changes ofrefractive index that can lead to loss of phasematching, or to a direct depletion of the generated
VUV beam or the incident vT and vR beams. At
present we have no model that would consider allof the saturation and phase mismatch effects andexplain the observed saturation behavior. Judging
from the data in Fig. 5 and from the strong
saturation of the VUV yield with increasing vTenergy a large contribution to the saturation is
likely to come from two photon resonant reabsorption of VUV. The sum of the photon energies of the VUV and the vT are inherently equal
to 2vR and the two-photon absorption is thus
resonantly enhanced. The saturation of the VUVsignal with increasing power of vT, which could
be attributed to the two photon absorption, wasalso observed by Hilber et al. w7x in a resonantly
enhanced sum-difference mixing experiment using
4p55pw5y2,2x transition (lRs216.6 nm) in Kr, as
well as in our earlier experiment at KEK w4x.

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Fig. 5. Dependence of the VUV pulse energy on the Kr partialpressure (Kr:Ars1:4) measured at low energy of 212.55 nm
beam (ERs0.2 mJ) and several energies, ET, of the IR beam
at lTs844.9 nm. Confocal parameters of the input beams arebRs0.8 cm, and bTs1 cm. Dashed lines represent fit to thedata in the form of apbKrexp(ygpKr), where a, b, and g are
the fitting parameters.

Considering the relatively broad bandwidth ofour vT beam, the VUV generation can be, in
principle, also limited by low acceptance bandwidth of the Kr–Ar mixture at higher pressures.
The actual acceptance bandwidth depends on the
shape of the phase matching integral that has
maximum at Dks0 and can be approximated asexp(bRDk) for Dk(0 and as exp(ybTDk) for
Dk00 w8x. The acceptance bandwidth for large
confocal parameters of the input beams could be
expected to be of a comparable size to the vT
bandwidth at relatively low Kr pressure. To check
whether this is the case, we have measured the
VUV tuning range at the Kr partial pressure of
50 hPa in the phase-matched mixture near 212.5
nm (Fig. 6). Though at this pressure the VUVsignal was strongly saturated, the tuning range
measured at full-width half-maximum was 0.45
nm. This is more than an order of magnitude larger
than the bandwidth of the vT beam (or the VUVbeam) and we can conclude that the VUV yield

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P. Bakule et al. / Spectrochimica Acta Part B 58 (2003) 1019–1030

Fig. 6. Measured tuning range near hydrogen Lyman-a wavelength using a phase-matched Kr–Ar mixture at a Kr partial pressureof 50 hPa. The dashed line represents a fit to the data with an approximated phase matching integral—see text. Confocal parameters:bRs0.8 cm, bTs1 cm; energy of vR beam 3.6 mJ, energy of vT beam 5.6 mJ.

saturation at this pressure is not due to the narrow
acceptance bandwidth for phase-matching. The fulltuning range on the high frequency side could not
be measured due to the limited tuning range ofour broadband OPO laser. The phase matching
profile was fitted with the expected shape of thephase matching integral discussed above. The ratioof the confocal parameters giving the best fit to
the data is bR ybTs0.9.
4. Hydrogen ionization
The whole experimental setup, including thelaser system, ion transport beamline, and MCP
detector, was tested by ionizing hydrogen atoms
in front of the tungsten target. Given the capabilityof the setup to detect single particles, the experiment was performed using residual hydrogen in
the ultra-high vacuum without introducing any
additional H2 gas. The hydrogen atoms are produced by dissociation from H2 on the surface ofthe heated tungsten target with maximum efficiency at target temperatures above 2000 K w4x. Hqions were transported to the MCP with a characteristic TOF delay relative to the laser pulses andcharacteristic mass corresponding to protons. Up

to 105 Hq were observed per laser pulse from an
ionization volume of approximately 4 cm3 at ultrahigh vacuum pressures of only 1.1=10y9 hPa.
Only one VUV beam without the VUV retroreflecting mirror was used for these measurements.
The VUV intensity can be increased by a factorof three if both VUV beams and the retro-reflection
mirror are used.
We have also measured the value of the Lymana wavelength to check calibration of our wavemeters. Fig. 7 shows the measured yield of laser
ionized hydrogen as a function of VUV wavelength near 121.6 nm. The VUV wavelength was
determined from direct measurements of vR y2 andvT using a pulsed wavemeter. The measured center
wavelength of 121.568 nm is in very good agreement with the theoretical Lyman-a wavelength(121.5676 nm). The measured line profile is a
convolution of the Doppler profile of hydrogen at1900 K, with FWHM of 78 GHz, and of theLorentzian profile of the OPO laser output. The
data were therefore fitted with a Voigt profile. The
linewidth measured at FWHM is 122 GHz. Thisis lower than the bandwidth of the OPO laser(160 GHz) indicating possible line narrowing inthe VUV generation process.

P. Bakule et al. / Spectrochimica Acta Part B 58 (2003) 1019–1030

1027

Fig. 7. Yield of laser ionized hydrogen detected on the MCP as a function of wavelength near 121.6 nm. Solid line represents a
Voigt profile fit to the measured data.

5. Slow muon generation
The slow muon generation experiment was carried out in the summer of 2001 for 5 days. The
surface of the tungsten target was cleaned inoxygen atmosphere with 4=10y7 hPa pressure at
1800 K for 8 h before the start of the experimentwhen the temperature was increased to 2000 K forthe muonium production.
The muon pulse repetition rate at the ISIS
facility is 50 Hz, twice the repetition rate of thelaser system. This allowed us to make easily
measurements with and without the laser ionization. An observed TOF spectrum is shown in Fig.
8. A clear signal appeared only when the VUVlight was introduced in front of the target, and theobserved peak was confirmed to correspond to thecalculated TOF for slow muons at the acceleration
voltage of 7.5 kV. Time ts0 in Fig. 8 was chosen
to be 120 ns prior to the laser Q-switch trigger, sothat the Q-switch trigger could be recorded simultaneously. Time jitter of 20 ns originating in thedata acquisition system was observed on both theQ-switch trigger signal and the slow muon signal.
The measured width of the slow muon peak in the
TOF spectrum is limited by this jitter. The real
width is expected to be about a factor of two
smaller. The electric field gradient of 100 Vycm
in front of the target introduces an additional
uncertainty on the energy of the particles detected
at the MCP position. The extraction energy of themq depends on the distance from the target atwhich the muonium is ionized. For the VUV beam
diameter of 8 mm this effect introduces an energy
uncertainty of 80 eV and respective TOF uncertainty of 7.5 ns.
If required, this energy uncertainty can be
reduced by reducing the size of the VUV beamand by reducing the electric field gradient in frontof the target. Reduction of the energy uncertainty
to below 10 eV should be achievable without
significant loss in the number of slow muons thatare generated and extracted from the target area.
The VUV pulse energies generated by the laser

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P. Bakule et al. / Spectrochimica Acta Part B 58 (2003) 1019–1030

Fig. 8. Comparison of slow muon TOF spectrum measured with and without the laser ionization. The beamline acceleration voltage
was 7.5 kV.

system are currently not high enough to saturate
the 1S–2P transition. Therefore, the reduction of
the number muonium atoms in a more focusedlaser beam would be compensated by the increased
ionization efficiency.
The muonium yield was also measured as afunction of the difference wavelength lT. Maximum yield of slow muons was observed for lTs
821.1 nm corresponding to the generated VUVwavelength of 122.08 nm. This is in agreementwith the expected 1S–2P transition frequency inmuonium (122.0891 nm). We have also scanned
the timing of laser pulses relative to the incoming
muon pulse. It was found that the yield is maximized when the laser is introduced 0.4 ms after
the muon pulse. This agrees with previous exper-

iments on muonium production from hot metal
surfaces w3x.
6. Conclusion
Generation of ultra-slow muon beam frompulsed surface muon beam using two photon resonant laser ionization of muonium was demonstrated. The measured yield of 0.03 mq ys obtained
during the first beamtime is currently below thedesign value for this apparatus. Unfortunately, thedesign of our apparatus does not currently allow
us to measure the absolute muonium production
and the absolute VUV energy independently. It istherefore difficult to determine the exact reason
for the low slow muon yield.

P. Bakule et al. / Spectrochimica Acta Part B 58 (2003) 1019–1030

The hydrogen ionization measurements haveshown that up to 105 Hq per pulse can be obtained
at a vacuum level 1.1=10y9 hPa. Assuming thatthe pressure corresponds to free hydrogen atoms
only, we can estimate the combined ionization anddetection efficiency to be better than 10y3. Assuming the surface muon beam intensity of 5=105
mq ys, the efficiency of conversion to muonium of
2%, fraction of the muonium in the laser beam of
50%, and decay of 50% of the muons in flight,
we can then expect slow muon yield of the order
1 mq ys.
The discrepancy is likely to be due to low
efficiency of muonium production. An improvement can be expected from experimental tuning of
the momentum of the initial muon beam to matchthe thickness of the target. Relative muonium yield
as a function of the muon beam momentum canbe, in principle, measured by the laser ionizationin our apparatus. We have also found that a beamcollimator placed on the input of our apparatus
was partly blocking the muon beam and contributed to the reduction of the observed slow muon
signal. The slow muon signal showed almost linear
dependence on energies of both VUV and the355 nm pulses, suggesting that significant
improvement can be gained by improving theefficiency of VUV generation.
Further work is in progress to better understandthe saturation processes involved in the generationof the intense VUV radiation in phase-matchedgas mixtures of Kr and Ar and to install a calibrated VUV detector to measure the absoluteenergy of the generated VUV pulses.
Acknowledgments
The authors would like to acknowledge Drs K.
Ishida, T. Matsuzaki, and I. Watanabe for theircontributions to the construction of the experimental set-up, and Dr Marangos for helpful discussions
on the subject of VUV generation. The authors
would like to thank Mr Fujino and Mr Shiraishi
for their contribution to the building of the laser
system. This research was partially supported by
the Japanese Ministry of Education, Science,

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Sports and Culture, Grant-in-Aid for Scientific
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