Transcript Possibility on a point positive muon source for a neutrino

Possibility on a point positive muon
source for a neutrino factory by laser
excitation of muonium atoms
Yasuyuki Matsuda (RIKEN)
(for slow muon collaboration)
Introduction : slow muons
 Experiment at the RIKEN-RAL muon facility
 Possibility of application as a point muon source
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4th July, 2002
NuFact 2002 Workshop at Imperial College, London
Collaborators
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Y. Matsuda (RIKEN)
P. Bakule (RIKEN)
P. Strasser (RIKEN)
K. Ishida (RIKEN)
T. Matsuzaki (RIKEN)
M. Iwasaki (RIKEN)
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Y. Miyake (KEK)
K. Shimomura (KEK)
S. Makimura (KEK)
K. Nagamine (KEK)
J.P. Marangos
(Imperial College, UK)
slow muons
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Slow muons : muons which are (re)accelerated from the
muons which are almost in a rest.
Momentum is tunable, and its distribution are very small.
 The range in the material is tunable down to sub mm.
 Small emittance enable us to make small aperture beam.
New application of mSR for thin film, surface/interfaces and nanomaterials, which are scientifically interesting as well as commercially
important.
Possible application towards future muon/neutrino source.
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Two methods to generate slow muon beam
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Cryogenic moderator method
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Successful PSI application.
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
Developed at KEK.
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Obtain slow muons by ionizing thermal
muoniums emitted from a hot tungsten film.
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Initial energy is around 0.2eV, and its spread is
less than 1eV.
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Time structure is determined by laser timing.
g Gives better time resolution for pulsed beam.
g Suitable for high intensity beam.
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Purpose of the experiment
Pros
 Very low emittance.
 Target can cope with high intensity.
Cons
 Low efficiency.
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muongmuonium conversion: a few %.
muonium ionization : a few %? (We need high power VUV light).
Loss due to decay of slow muon.
Needs stable laser operation for reliable beam.
Purpose of the experiment:
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Demonstrate slow muon generation by laser resonant ionization.
Obtain stable and high power VUV light.
Study feasibility for application of slow muon beam.
The RIKEN-RAL Muon Facility
Port 3
Port 4
0
Port 2
Port 1
2m
The RIKEN-RAL Muon Facility
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The world most intense pulsed surface and decay muon
source.
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Surface muon: muons are generated at the surface of the intermediate
target following decay of pions (p+gm+nm). The beam has fixed
momentum (30MeV/c)
Decay muon: muons are generated from in-flight decay of pions in a
superconducting solenoid. Maximum momentum is 120MeV/c.
Repetition rate is 50Hz, each extraction has two pulses with 340ns
separation.
Momentum acceptance about 2% (standard deviation).
Surface muon flux 1x106 muon/sec, beam size about 3cm in diameter.
How to ionize muonium?
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Similar scheme with LIS
(example: COMPLIS at
ISOLDE) but needs much
higher ionization energy.
Use two-photon ionization of
muonium with 122nm and
355nm light. 1S-2P transition
is most intense one.
Use sum-difference
frequency mixing method to
generate 122nm light.
Diagram of the laser system
Good overlapping of 212nm laser and 820nm laser for frequency
mixing in Kr gas is necessary.
 Good overlapping of VUV light and 355nm laser for ionizing
muonium is required. (The lifetime of 2P state is only 1.6nsec.)
g All lasers must be synchronized within 1nsec accuracy.
g All-solid laser system using OPOs and Nd:YAG lasers.
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Schematic view of the slow muon beam line
Slow muon beam line
Lasers in the cabin
Mirage800 laser system which generate
single-mode 850nm light from
frequency-doubled YAG laser (532nm)
Amplifier stage and BBO crystals
which quadruple frequency of laser
The first observation of slow muons
at the RIKEN-RAL muon facility
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A clear peak on TOF spectrum
corresponding to calculated
TOF for slow muon at
accelerating voltage of 7.5kV.
(Lasers are irradiated at
t=120ns.)
Measured magnetic field of
the bending magnet
corresponds to the correct
muon mass.
Count rate was 0.03 m/sec.
Optimum laser delay relative to the muon beam
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Thermal muonium energy ~ 0.17eV g velocity 1.7cm/msec.
Distance between the tungsten film and the extraction lens is ~1cm.
Laser light pass between the film and the lens. g Reasonable
traveling time of muonium atoms from the surface of film to
ionization region.
Tunable laser wavelength dependence
The yield of slow muon peaked when we tune VUV frequency
to the 1S-2P transition of muonium atom.
60
expected peak position
50
Counts [arbitrary unit]
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40
30
20
10
0
820
820.2
820.4
820.6 820.8
Wavelength [nm]
821
Problems
The observed yield, 0.03 m/sec, is lower than our estimation.
Possible reasons are…?
 Smaller intensity of lasers…?
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NO gas ionization chamber to monitor VUV light’s power gives about
one fifth of the signal we obtained in Japan in commissioning period.
Measured profile of VUV light is much wider than our design. We
may have some misalignment of lenses in our VUV beam path.
Surface muon beam intensity…?
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Collimators with small aperture were in the beam line… loss of beam.
Later (re)calculation showed our target was probably too thick so that
many surface muons stopped in the middle and didn’t come to the
surface of the target.
Towards high intensity VUV light
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Requirement for VUV intensity.
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VUV light with energy of 20mJ/pulse will be able to excite a quarter of
electron in 1S state to 2P state. Then slow muon generation efficiency
will be 2.5x10-3.
How to achieve it?
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Increase laser power.
“phase-matching” in Kr gas with Ar gas.
 Farris et al. obtained 7mJ/pulse at frequencies near 1S-2P transition
using sum-difference mixing method with phase-matched Kr gas.
(J. Opt. Soc. Am. B, Vol. 17 No. 11, p.1856(2000))
 Marangos et al. reported generation of 11mJ/pulse of Lyman-a
light.
(J. Opt. Soc. Am. B. Vol. 7, No.7 p.1254(1990))
VUV power vs. laser power
VUV power ~ ER0.75, not ER2 as expected.
VUV power is saturated with ET, while it supposed to show linear
dependence.
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lT = 844.9 nm
70
20
VUV intensity [arbitrary units]
VUV intensity [arbitrary units]
lR = 212.55 nm
18
16
14
12
10
8
6
4
2
0
60
50
40
30
20
10
0
0
2
4
Energy ER [mJ/pulse]
6
8
0
5
10
Energy ET [mJ/pulse]
15
VUV generation (Kr/Ar mixing)
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We can enhance VUV
generation efficiency in Kr
gas by adding Ar gas. This
is called ‘phase matching’.
The mixing ratio has a
sharp peak. The optimum
ratio depends on the
wavelength of generated
light.
6000
Kr base pressure 80hPa
VUV intensity [relative units] .
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5000
Optimum Kr:Ar ratio 1:4.2
4000
3000
2000
1000
0
0
100
200
300
Ar partial pressure [hPa]
400
VUV generation (Kr/Ar mixing)
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Farris et al. and Marangos et
al. reported an enhancement
of VUV generation of a
factor of 50-100.
Under our conditions, the
enhancement is about a
factor of 5, though.
We suspect impurity in Kr
(and/or Ar) gas and two
photon re-absorption process
in Kr as the reasons of strong
saturation.
1200
1000
VUV intensity [arbitrary units]
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800
600
400
Kr:Ar mixture (1:4)
Pure Kr
200
0
0
10
20
30
40
Kr partial pressure [hPa]
50
60
Yield estimation of slow muons
(with 20mJ VUV light)
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Intensity of muons at Port 3 : 5x105 m/sec (at 50Hz)
Muon to muonium conversion: 2%
laser repetition rate: 25Hz
Number of muoniums emitted from the target : 5x103 m/sec.
Ionization + transportation efficiency 20%
Number of slow muons: 1000 slow m/sec.
(With very small emittance so that we can focus beam to at least 1mm
diameter after acceleration to 10keV. Further focusing depends on how
small we can make ionization region.)
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New field of applications of mSR for thin film,
surface/interfaces and nano-materials will be open (with
advantage of pulsed muon source).
Possible application for a muon collider?!?
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High intensity of beam will deposit large heat on the target.
g the target can cope with it.
Very large momentum dispersion of initial muon beam.
g multi-layers of tungsten films and multi-beam of lasers.
Long time stability of laser operation and high power VUV light are
needed.
g Need to wait developments of new non-linear optical devices.
Initial muon beam time structure.
g Need development of high-repetition laser system? (depends
on accelerator design).
Muon loss due to conversion efficiency of muonium and decay of
(slow) muons before enough acceleration.
g Unavoidable…But better quality will compensate loss,
especially for muon collider??
Summery
We have successfully generated slow muon beam with laser
resonance ionization method at the RIKEN-RAL muon facility.
 The yield was smaller than expected.
 Several improvements for more efficient VUV generation are under
way to increase ionization efficiency of muonium.
 Measurement of beam profile and emittance is planned, but
detectors are not implemented yet.
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With available laser technology, we can generate powerful slow
muon beam for study of material sciences.
There is a possibility for application to neutrino/muon factory, but
its feasibility largely depends on improvements of laser system.
What is phase matching?
P=e0(c(1)E+c(2)E2+c(3)E3+…)
P: polarization (dipole moment per unit volume)
c(1): linear susceptibility
c(2): second order nonlinear susceptibility
c(3): third order nonlinear susceptibility
Phase-matching condition: phase velocity of generated light equals to
that of induced nonlinear polarization.
g efficient nonlinear process