Possibility on a point positive muon source for

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 n Experiment at the RIKEN-RAL muon facility n Possibility of application as a point muon source n 4 th July, 2002 Nu. Fact 2002 Workshop at Imperial College, London

Collaborators n n n Y. Matsuda (RIKEN) P. Bakule (RIKEN) P. Strasser (RIKEN) K. Ishida (RIKEN) T. Matsuzaki (RIKEN) M. Iwasaki (RIKEN) n n n Y. Miyake (KEK) K. Shimomura (KEK) S. Makimura (KEK) K. Nagamine (KEK) J. P. Marangos (Imperial College, UK)

slow muons n 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. n The range in the material is tunable down to sub mm. n Small emittance enable us to make small aperture beam. è New application of m. SR for thin film, surface/interfaces and nanomaterials, which are scientifically interesting as well as commercially important. è Possible application towards future muon/neutrino source. n

Two methods to generate slow muon beam n Cryogenic moderator method n n n Successful PSI application. Use a layer of solid rare gas as a moderator. Initial energy is 10 -100 e. V, and its spread is around 10 e. V. Time structure is determined by initial beam. Laser resonant ionization method Developed at KEK. n Obtain slow muons by ionizing thermal muoniums emitted from a hot tungsten film. n Initial energy is around 0. 2 e. V, and its spread is less than 1 e. V. n Time structure is determined by laser timing. g Gives better time resolution for pulsed beam. g Suitable for high intensity beam. n

Purpose of the experiment Pros n Very low emittance. n Target can cope with high intensity. Cons n Low efficiency. n n 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: n n n 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 RIKEN-RAL Muon Facility n The world most intense pulsed surface and decay muon source. n n n Surface muon: muons are generated at the surface of the intermediate target following decay of pions (p+gm+nm). The beam has fixed momentum (30 Me. V/c) Decay muon: muons are generated from in-flight decay of pions in a superconducting solenoid. Maximum momentum is 120 Me. V/c. Repetition rate is 50 Hz, each extraction has two pulses with 340 ns separation. Momentum acceptance about 2% (standard deviation). Surface muon flux 1 x 106 muon/sec, beam size about 3 cm in diameter.

How to ionize muonium? n n n Similar scheme with LIS (example: COMPLIS at ISOLDE) but needs much higher ionization energy. Use two-photon ionization of muonium with 122 nm and 355 nm light. 1 S-2 P transition is most intense one. Use sum-difference frequency mixing method to generate 122 nm light.

Diagram of the laser system Good overlapping of 212 nm laser and 820 nm laser for frequency mixing in Kr gas is necessary. n Good overlapping of VUV light and 355 nm laser for ionizing muonium is required. (The lifetime of 2 P state is only 1. 6 nsec. ) g All lasers must be synchronized within 1 nsec accuracy. g All-solid laser system using OPOs and Nd: YAG lasers. n

Schematic view of the slow muon beam line

Slow muon beam line

Lasers in the cabin Mirage 800 laser system which generate single-mode 850 nm light from frequency-doubled YAG laser (532 nm) Amplifier stage and BBO crystals which quadruple frequency of laser

The first observation of slow muons at the RIKEN-RAL muon facility n n n A clear peak on TOF spectrum corresponding to calculated TOF for slow muon at accelerating voltage of 7. 5 k. V. (Lasers are irradiated at t=120 ns. ) 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 n n Thermal muonium energy ~ 0. 17 e. V g velocity 1. 7 cm/msec. Distance between the tungsten film and the extraction lens is ~1 cm. 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 n The yield of slow muon peaked when we tune VUV frequency to the 1 S-2 P transition of muonium atom.

Problems The observed yield, 0. 03 m/sec, is lower than our estimation. Possible reasons are…? n Smaller intensity of lasers…? n n n 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…? n n 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 n Requirement for VUV intensity. n n VUV light with energy of 20 m. J/pulse will be able to excite a quarter of electron in 1 S state to 2 P state. Then slow muon generation efficiency will be 2. 5 x 10 -3. How to achieve it? n n Increase laser power. “phase-matching” in Kr gas with Ar gas. n Farris et al. obtained 7 m. J/pulse at frequencies near 1 S-2 P transition using sum-difference mixing method with phase-matched Kr gas. (J. Opt. Soc. Am. B, Vol. 17 No. 11, p. 1856(2000)) n Marangos et al. reported generation of 11 m. J/pulse of Lyman-a light. (J. Opt. Soc. Am. B. Vol. 7, No. 7 p. 1254(1990))

VUV power vs. laser power n n VUV power ~ ER 0. 75, not ER 2 as expected. VUV power is saturated with ET, while it supposed to show linear dependence. l. R = 212. 55 nm l. T = 844. 9 nm

VUV generation (Kr/Ar mixing) n n 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 80 h. Pa Optimum Kr: Ar ratio 1: 4. 2

VUV generation (Kr/Ar mixing) n n n 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.

Yield estimation of slow muons (with 20 m. J VUV light) n n n Intensity of muons at Port 3 : 5 x 105 m/sec (at 50 Hz) Muon to muonium conversion: 2% laser repetition rate: 25 Hz Number of muoniums emitted from the target : 5 x 103 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 1 mm diameter after acceleration to 10 ke. V. Further focusing depends on how small we can make ionization region. ) â New field of applications of m. SR for thin film, surface/interfaces and nano-materials will be open (with advantage of pulsed muon source).

Possible application for a muon collider? !? n n n 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. n The yield was smaller than expected. â Several improvements for more efficient VUV generation are under way to increase ionization efficiency of muonium. â Measurement of beam profile and emittance is planned, but detectors are not implemented yet. n n n 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=e 0(c(1)E+c(2)E 2+c(3)E 3+…) 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