Possibility on a point positive muon source for a neutrino factory by laser excitation of muonium at

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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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Collaborators

• Y. Miyake (KEK)
• K. Shimomura (KEK)
• S. Makimura (KEK)
• K. Nagamine (KEK)
• J.P. Marangos (Imperial College, UK)

• Y. Matsuda (RIKEN)
• P. Bakule (RIKEN)
• P. Strasser (RIKEN)
• K. Ishida (RIKEN)
• T. Matsuzaki (RIKEN)
• M. Iwasaki (RIKEN)

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slow muons

• 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 nano-materials, 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

• Cryogenic moderator method
• 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.
• 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 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.
• 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
• Demonstrate slow muon generation by laser
  resonant ionization.
• Obtain stable and high power VUV light.
• Study feasibility for application of slow muon
  beam.

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The RIKEN-RAL Muon Facility
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The RIKEN-RAL Muon Facility

• The world most intense pulsed surface and decay
  muon source.
• Surface muon muons are generated at the surface
  of the intermediate target following decay of
  pions (pgmnm). 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.

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How to ionize muonium?

• 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.

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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 NdYAG
  lasers.

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Schematic view of the slow muon beam line
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Slow muon beam line
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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
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The first observation of slow muons at the
RIKEN-RAL muon facility

• A clear peak on TOF spectrum corresponding to
  calculated TOF for slow muon at accelerating
  voltage of 7.5kV. (Lasers are irradiated at
  t120ns.)
• Measured magnetic field of the bending magnet
  corresponds to the correct muon mass.
• Count rate was 0.03 m/sec.

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Optimum laser delay relative to the muon beam

• 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.

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Tunable laser wavelength dependence

• The yield of slow muon peaked when we tune VUV
  frequency to the 1S-2P transition of muonium atom.

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Problems

• The observed yield, 0.03 m/sec, is lower than our
  estimation.
• Possible reasons are?
• Smaller intensity of lasers?
• NO gas ionization chamber to monitor VUV lights
  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?
• 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 didnt come to the
  surface of the target.

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Towards high intensity VUV light

• Requirement for VUV intensity.
• 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?
• 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))

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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.

lR 212.55 nm
lT 844.9 nm
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VUV generation (Kr/Ar mixing)

• 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.

Kr base pressure 80hPa
Optimum KrAr ratio 14.2
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VUV generation (Kr/Ar mixing)

• 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.

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Yield estimation of slow muons (with 20mJ VUV
light)

• 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.)
• New field of applications of mSR for thin film,
  surface/interfaces and nano-materials will be
  open (with advantage of pulsed muon source).

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Possible application for a muon collider?!?

• 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 UnavoidableBut better quality will
  compensate loss, especially for muon collider??

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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.
• 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.

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What is phase matching?

• Pe0(c(1)Ec(2)E2c(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