# Presentation: 297135

## Recent development of a point positive muon source by laser excitation of muonium atoms

-  Introduction
-  Experiment at the RIKEN-RAL Muon facility
-  Future prospect

- Yasuyuki Matsuda
- (Muon Science Laboratory, RIKEN)
- for slow muon collaboration

## Slow muon collaboration

- 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)

## Applications of muon beam

- Material Science :
- SR study with surface muon beam (30MeV/c). Very successful. By far, the most popular way to use muon beam.
-  But, its application has been limited to bulk material due to wide momentum dispersion and large beam size.
- Muon-catalyzed fusion
- Fundamental Physics
- Search of rare decay mode and precise measurement of know decay mode
- Muonium and muonic atom for study of symmetries, nuclear structure, etc...
- neutrino source, muon collider
-  requires muon beam with better quality.

## Slow muons

- Slow muons : muons which are (re-)accelerated from the muons which are almost at a rest.
- Beam energy is tunable, and its spread is very small.
-  The range in the material is tunable down to sub m.
- Emittance is very small.
-  Small sample can be used.
- New application of SR for thin films, surface/interfaces and nano-materials, which are scientifically interesting as well as commercially important.
- 
- Possible application towards future muon/neutrino sources

## Two methods to generate slow muon beam

- Cryogenic moderator method
- 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
- 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.
-  Possible use for Mu anti-Mu conversion experiment as a sensitive detection method of anti-Mu and background suppression.

## Previous slow muon experiments

- PSI slow muon beam line
- Cryogenic moderator method
- Efficiency close to 104
- Initial beam energy spread is about 10eV
- Yielded about 700 /sec (*1)
- DC source
- A timing counter needs to be inserted in the transport line for SR study
- Successful operation since 1993

- ISIS EC slow muon beam line
- Applied cryogenic moderator method to a pulsed muon source.
- Yielded about 1 /sec (*2)
- Pulsed source
- Time-of-flight showed time resolution was about 90nsec (FWHM), according to ISIS beam pulse width.
- Better energy resolution than PSI thanks to absence of a trigger counter.
- Currently defunct.

- (*1)  E. Morenzoni et al. Physica B 326(2003)196
- (*2)  K. Trager et al. Physica B 289-290(2000)662

## PSI slow muon beam line

- E. Morenzoni et al. Hyperfine Interactions 106(1997)229

## Laser Ionization method : How to ionize muonium?

- Similar to 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 among muonium’s transitions.
- Use a sum-difference frequency mixing method to generate 122nm light.

## Diagram of the laser system

-  All-solid laser system using OPOs and Nd:YAG lasers
-  Stable operation
-  Gives good timing (1nsec accuracy)
-  Good overlapping of 212nm laser and 820nm laser for frequency mixing in Kr gas.
-  Good overlapping of VUV light and 355nm laser for ionizing muonium. (The lifetime of 2P state is only 1.6nsec.)

## Schematic view of the slow muon beam line

## Schematic view of the slow muon beam line

## The first observation of slow muons at the RIKEN-RAL muon facility (July 2001)

- A clear peak on TOF spectrum was observed at the position which corresponds slow muon at the accelerating voltage of 7.5kV.
- Measured magnetic field of the bending magnet corresponds to the correct muon mass.
- Count rate was 0.03 /sec. (too small!)

## Improved slow muon generation  at the RIKEN-RAL muon facility (April 2003)

- The yield of slow muon (3.3 slow /sec)  is 100 times more than that obtained in July 2001, and larger than the previous experiment at the EC muon beam line with cryogenic moderator method.
- The time diversion of slow muon beam is about 10ns (FWHM), whereas the cryogenic moderator experiment gave about 100nsec.

## Improvements during 2001/02

- laser system improvement
- Improved VUV laser intensity by phase-matching.
- Installed image relays for pumping YAG resulted an improved profile.
- Installed an image relay for 355nm laser, eliminating hot spots.
- Installed a cylindrical telescope to convert 355ns laser’s beam profile to perpendicular shape (3x12mm) to maximize overlapping between 355nm and VUV lasers.
- beam line improvement
- Removed a small aperture, which was reducing initial beam intensity.
- Target thickness was reduced so that we can maximize the number of muons stopped at the rear-surface of the target.
- Beam line magnet was tuned to maximize Port 3 intensity while keeping same intensity at Port 2.

## What is phase matching? 

- P=0((1)E+(2)E2+(3)E3+...)
- 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

## Kr-Ar phase matching at muonium 1S-2P frequency

- Enhancement of VUV generation due to phase matching of Kr gas with Ar buffer gas was observed with slow muon yield.
- The ratio of Kr and Ar buffer gas (1:6) agrees with theoretical calculation.

## Muon stopping range tuning

- The thickness of the degrader is optimized to give maximum yield of slow muons.
- Measured range width (5.5% FWHM) reasonably agrees with expected value.

## Laser timing dependence

- Relative timing between the initial beam’s arrival time and laser irradiation time was scanned.
- A double-pulse structure of the initial muon beam is clearly observed.

## 355nm laser power dependence

- The power of 355nm laser for 2P  unbound transition is scanned. No saturation is observed up to 1.8W (70mJ/pulse)
- Previously observed saturation at 1.5W was eliminated by improving beam profile of the laser.

## 820nm laser frequency dependence

- The differential frequency in the sum-difference mixing is scanned. The slow muon yield peaked at the calculated frequency based on calibration measurements with hydrogen.
- Corresponding VUV bandwidth is 0.02nm for 122.115nm

## Measurement of MCP’s efficiency

- A vacuum flange at the end of the MCP chamber was replaced with a flange with 100m SUS film.
- A scintillator telescope was installed nearby the film to detect decay positrons from the film.
- By measuring “enhancement” in scintillator’s spectrum, the number of slow muons stopped at the film was estimated.
- The efficiency of MCP was evaluated to be about 66%.

## Efficiency of slow muon generation

- Observed slow muon signal : 	3.3 /sec
- (MCP efficiency 66%)  	 5.0 /sec
- (Decay in flight 43%)		 8.6 /sec
- (Transport efficiency unknown. assume 100%)
-  >8.6 /sec at the source
- Initial surface muon beam : 1.0x106 /sec
- Efficiency 8.6/1.0x106 = 8.6x106  (still low...)

## Issues to be addressed

- Improvement of muonium ionization efficiency.
- Improvement in laser power
-  both 355nm and VUV lasers are not saturating muonium transition.
- Improvement of pumping laser’s profile will benefit frequency-quadrupling with BBO crystals
- Improvement of OPO frequency conversion system
- Stable operation of laser system is the key for the application of slow muon beam.
- Improvement of muon to muonium conversion efficiency.
- optimizing target material, its thickness, temperature...
- Possible use of cyclotron trap with a thinner tungsten film

## Cyclotron Trap

- PSI & LEAR application
- Winding up the range path of stopping particles inside a weak focusing cyclotron field. It has been used for producing low energy negative muons beams, pions and anti-protons.
- Moderator can be gas (typically ~1mbar Hydrogen), or thin metal foils.
- Application for positive muons have been limited because of high capture rate of electron.
- Cyclotron trap + laser ionization
- Can expect increase of muon stopping density at the near-surface of Tungsten foil.
- We will lose muon polarization. This will limit application for SR, but is good for Mu  anti-Mu oscillation experiment.
- Recover muon polarization with polarized laser light?

## Future plans

- Beam study
- Beam profile measurement ( Segmented MCP, Slit, emittance measurement)
- SR study
- Scintillator telescopes installation around the MCP chamber.
- Helmholtz coil installation
- Thinking of fundamental physics...
- Mu  anti-Mu conversion experiment : double coincidence between laser irradiation and anti-Mu detection will reduce background significantly.
- PSI experiment accumurated 5.7x1010 muonium decays. We need significant improvement of slow muon yield.
**Muon intensity improvement : J-PARC, new proton driver, new design of capture channel.**
- Muon to muonium conversion improvement : cyclotron trap
- Muonium ionization improvement : laser performance.

## Time table (optimistic)

## Summery

- We have successfully generated slow muon beam with laser resonance ionization method at the RIKEN-RAL muon facility. The yield has improved by 100 times compared to previous result obtained at July 2001.
- Several improvement plans for more efficient VUV generation are under way to increase efficiency of slow muon conversion, which will make slow muons available to material sciences.
- Slow muon setup can be used as a prototype of Mu  anti-Mu  experiment. We can expect reduced background by coincidence between laser irradiation and anti-Mu detection.
- Measurement of beam profile and emittance is planned during 2003/2004.
- There is a possibility for application to neutrino/muon factory, but its feasibility largely depends on improvements of laser system in terms of stability and efficiency.

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