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Recent development of a point positive muon source by laser excitation of muonium atoms. Yasuyuki Matsuda (Muon Science Laboratory, RIKEN) for slow muon collaboration. Introduction Experiment at the RIKEN-RAL Muon facility Future prospect. Slow muon collaboration. Y. Matsuda (RIKEN)
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Recent development of a point positive muon source by laser excitation of muonium atoms Yasuyuki Matsuda (Muon Science Laboratory, RIKEN) for slow muon collaboration Introduction Experiment at the RIKEN-RAL Muon facility Future prospect
Slow muon collaboration • Y. Matsuda (RIKEN) • P. Bakule (RIKEN) • P. Strasser (RIKEN) • K. Ishida (RIKEN) • T. Matsuzaki (RIKEN) • M. Iwasaki (RIKEN) • Y. Miyake (KEK) • K. Shimomura (KEK) • S. Makimura (KEK) • K. Nagamine (KEK) • J.P. Marangos (Imperial College, UK)
Applications of muon beam • Material Science : • mSR study with surface muon beam (30MeV/c). Very successful. By far, the most popular way to use muon beam. a 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 a 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. a The range in the material is tunable down to sub mm. • Emittance is very small. a Small sample can be used. • New application of mSR 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. g Gives better time resolution for pulsed beam. g 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 m/sec (*1) • DC source • A timing counter needs to be inserted in the transport line for mSR study • Successful operation since 1993 ISIS EC slow muon beam line • Applied cryogenic moderator method to a pulsed muon source. • Yielded about 1 m/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 a Stable operation a Gives good timing (1nsec accuracy) a Good overlapping of 212nm laser and 820nm laser for frequency mixing in Kr gas. a Good overlapping of VUV light and 355nm laser for ionizing muonium. (The lifetime of 2P state is only 1.6nsec.)
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 m/sec. (too small!)
Improved slow muon generation at the RIKEN-RAL muon facility (April 2003) • The yield of slow muon (3.3 slow m/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=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
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 g 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 100mm 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 m/sec (MCP efficiency 66%) a 5.0 m/sec (Decay in flight 43%) a 8.6 m/sec (Transport efficiency unknown. assume 100%) a >8.6 m/sec at the source Initial surface muon beam : 1.0x106m/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 mSR, but is good for Mu 1 anti-Mu oscillation experiment. • Recover muon polarization with polarized laser light?
Future plans • Beam study • Beam profile measurement ( Segmented MCP, Slit, emittance measurement) • mSR study • Scintillator telescopes installation around the MCP chamber. • Helmholtz coil installation • Thinking of fundamental physics… • Mu 1 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.
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 1anti-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.