4th July, 2002NuFact 2002 Workshop at Imperial College, London Possibility on a point positive muon source for a neutrino factory by laser excitation of muonium atoms Introduction : slow muons Experiment at the RIKEN-RAL muon facility Possibility of application as a point muon source Yasuyuki Matsuda (RIKEN) (for slow muon collaboration)

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)

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  m. Small emittance enable us to make small aperture beam.  New application of  SR for thin film, surface/interfaces and nano- materials, which are scientifically interesting as well as commercially important.  Possible application towards future muon/neutrino source.

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 eV, 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.  Gives better time resolution for pulsed beam.  Suitable for high intensity beam.

Purpose of the experiment Pros Very low emittance. Target can cope with high intensity. Cons Low efficiency. muon  muonium 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.

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 (       ). 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 1x10 6 muon/sec, beam size about 3cm in diameter.

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.

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.)  All lasers must be synchronized within 1nsec accuracy.  All-solid laser system using OPOs and Nd:YAG lasers.

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 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  /sec.

Optimum laser delay relative to the muon beam Thermal muonium energy ~ 0.17eV  velocity 1.7cm/  sec. Distance between the tungsten film and the extraction lens is ~1cm. Laser light pass between the film and the lens.  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.

Problems The observed yield, 0.03  /sec, is lower than our estimation. Possible reasons are…? Smaller intensity of lasers…? 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…? 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 Requirement for VUV intensity. VUV light with energy of 20  J/pulse will be able to excite a quarter of electron in 1S state to 2P state. Then slow muon generation efficiency will be 2.5x How to achieve it? Increase laser power. “phase-matching” in Kr gas with Ar gas. Farris et al. obtained 7  J/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 11  J/pulse of Lyman-  light. (J. Opt. Soc. Am. B. Vol. 7, No.7 p.1254(1990))

VUV power vs. laser power VUV power ~ E R 0.75, not E R 2 as expected. VUV power is saturated with E T, while it supposed to show linear dependence. R = nm T = nm

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 Kr:Ar ratio 1:4.2

VUV generation (Kr/Ar mixing) Farris et al. and Marangos et al. reported an enhancement of VUV generation of a factor of 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.

Yield estimation of slow muons (with 20  J VUV light) Intensity of muons at Port 3 : 5x10 5  /sec (at 50Hz) Muon to muonium conversion: 2% laser repetition rate: 25Hz Number of muoniums emitted from the target : 5x10 3  /sec. Ionization + transportation efficiency 20% Number of slow muons: 1000 slow  /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  SR for thin film, surface/interfaces and nano-materials will be open (with advantage of pulsed muon source).

Possible application for a muon collider?!? High intensity of beam will deposit large heat on the target.  the target can cope with it. Very large momentum dispersion of initial muon beam.  multi-layers of tungsten films and multi-beam of lasers. Long time stability of laser operation and high power VUV light are needed.   Need to wait developments of new non-linear optical devices. Initial muon beam time structure.   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.  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. 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=  0 (  (1) E+  (2) E 2 +  (3) E 3 +…) P: polarization (dipole moment per unit volume)  (1) : linear susceptibility  (2) : second order nonlinear susceptibility  (3) : third order nonlinear susceptibility Phase-matching condition: phase velocity of generated light equals to that of induced nonlinear polarization.  efficient nonlinear process