Generation of low energy muon with laser resonant ionization of muonium atoms Yasuyuki Matsuda



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Generation of low energy muon with laser resonant ionization of muonium atoms

  • Yasuyuki Matsuda

  • (for slow muon collaboration)

  • NuFact05@INFN, Frascati

  • 22nd June 2005


The RIKEN-RAL Muon Facility



SR (Muon spin rotation/resonance/relaxation)

  • Polarized muons are implanted in a sample. Positrons are emitted preferably towards muon spin direction.

  • By observing the change of angular distribution of emitted positrons, we can measure internal magnetic field distributions and their fluctuations.

  • Merits

  • There are no ‘preferred’ nuclei.

  •  Any material can be measured.

  •  NMR, Mossbauer measurement

  • The measurement can be done without external magnetic field and under room temperature.

  • Very sensitive probe

  •  But, its application has been limited to bulk material due to wide momentum dispersion and large beam size.



slow muons

  • Slow muons : muons which are re-accelerated from resting state.

    • Beam energy is tunable, and its spread is small.
    • ⇒ Range can be adjusted from a few nm to a few hundred nm.
    • Beam size is small.
  • ⇒ New applications of SR for thin films, surface/interfaces and nano-materials, which are scientifically interesting as well as commercially important.

  • Cryogenic moderator method (PSI)

  • Laser resonant ionization method (KEK-RIKEN)

    • Obtain ultra 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.


Schematic view of the ultra slow muon beam line



A Picture of the ultra slow muon beam line



Current status of slow muon R&D

  • RIKEN-RAL muon facility : the world’s strongest pulsed muon source

  • PSI : the world’s strongest DC muon source

  • Both facilities are developing slow muon technologies...

  •  “Noblesse Oblige” for muon science community!



Current status of slow muon R&D



Slow muon range measurement



Current status of slow muon R&D

  • Slow muons at RIKEN-RAL muon facility have...

    • Variable implantation energies


Current status of slow muon R&D

  • Slow muons at RIKEN-RAL muon facility have...

    • Variable implantation energies
    • High temporal resolution (as well as high energy resolution)


Temporal resolution of ultra slow muon beam



Current status of slow muon R&D

  • Slow muons at RIKEN-RAL muon facility have...

    • Variable implantation energies
    • High temporal resolution ( as well as high energy resolution)


Current status of slow muon R&D

  • Slow muons at RIKEN-RAL muon facility have...



Beam profile at sample position



Current status of slow muon R&D

  • Slow muons at RIKEN-RAL muon facility have...

    • Variable implantation energies
    • High time resolution ( as well as high energy resolution)
    • Smaller beam size at sample position


Current status of slow muon R&D

  • Slow muons at RIKEN-RAL muon facility have...

    • Variable implantation energies
    • High time resolution ( as well as high energy resolution)
    • Smaller beam size at sample position
    • Lower background


Current status of slow muon R&D



Current status of slow muon R&D

  • Slow muons at RIKEN-RAL muon facility have...

    • Variable implantation energies
    • High time resolution ( as well as high energy resolution)
    • Smaller beam size at sample position
    • Lower background


Current status of slow muon R&D

  • Slow muons at RIKEN-RAL muon facility is...

    • Variable implantation energies
    • High time resolution ( as well as high energy resolution)
    • Smaller beam size at sample position
    • Lower background
  • The best muon beam in the world!



Current status of slow muon R&D

  • Slow muons at RIKEN-RAL muon facility is...

    • Variable implantation energies
    • High time resolution ( as well as high energy resolution)
    • Smaller beam size at sample position
    • Lower background
  • The best muon beam in the world! (except intensity and polarization)



Target studies

  • Increasing conversion efficiency from incident muons to thermal muoniums is a straight-forward way to increase slow muon yield.

  • Micro-fabricating cryogenic moderator increased beam intensity at PSI by 30%

  • Increasing surface area...

  • Etching by chemicals

  • Laser micro-fabrication : 20% increase of surface area expected.

  • (under discussion with Resonetics Ltd.)

  • Micro-fabrication by a diamond cutter : 50% increase of surface area expected.

  • (under discussion with Ohmori Lab., RIKEN)



Target studies

  • Hydrogen solution in metals

  • Extensive studies have been done for the solubility of hydrogen in metals.

  • Large (positive) solution enthalpy means the work function for hydrogen (muonium) to escape from metal is small.

  • But the depth of adsorption energy could play a role, as well as the height of surface barrier energy.

  •  Needs experimental studies!

  • Matsushita et al. studied muonium production from Iridium(Ir)1), Platinum(Pt)2) and Renium(Re)3), and obtained a promising result for Iridium.

  • Ruthenium(Ru) and Molybdenum(Mo) also seem promising.

  • Our system is a very sensitive muonium detector!



(A secret plan) Recovery of muonium polarization

  • Currently, muonium are generated under no magnetic field, resulting loss of polarization because triplet states are mixed up.

  • Applying magnetic field to muonium would resolve degenerated levels.

  • → less depolarization of muonium at triplet state

  • → 100% polarization of muonium(Overcoming our weak point)

  • Needs careful study for beam transportation, though.



The goal of our R&D

  • Slow muons at RIKEN-RAL muon facility will have...

    • Variable implantation energies
    • High time resolution ( as well as high energy resolution)
    • Smaller beam size at sample position
    • Lower background
    • 100% polarization


Slow muons at -Factory

  • Slow muons at factory will have...

    • Variable implantation energies
    • High time resolution ( as well as high energy resolution)
    • Smaller beam size at sample position
    • Lower background
    • 100% polarization
  • The best muon beam in the world to open many possibilities!



Summary

  • We have successfully generated slow muon beam at the RIKEN-RAL muon facility by laser resonant ionization method.

  • Slow muon beam gives depth-resolution to mSR technique, which is very sensitive tool to investigate magnetic property of materials.

  • This demonstration shows that laser resonant ionization method is ideally suited to intense pulsed muon source.

  • R&D work is in progress to increase conversion efficiency further and to recover muon’s polarization to nearly 100%.

  • There would be a strong case for intense pulsed proton beam (at factory) from material scientists, who would like to have intense pulsed low-energy muon beam (and intense pulsed neutron beam).



Collaborators



---spare OHPs---

  • --- Spare OHPs ---



Laser resonant ionization method

  • Ionization energy of muonium is 13.6eV (corresponding wavelength is 90nm)

  •  Single photon ionization is difficult.

  • Use two-photon resonant ionization with 122nm and 355nm photons. Since the 1S-2P transition is a strongly allowed transition, high efficiency is expected.

  • But generation of 122nm photon is challenging. Conventional non-linear medium (like BBO crystal) can not be used in this wave length region. Need to use gaseous medium.

  • Use a resonant sum-difference frequency mixing method in Kr gas to generate 122nm light. 102~103 enhancement can be expected compared to third harmonic generation in gaseous medium.



Time Schedule (FY2005)



Laser studies

  • Laser intensities of both lyman and 355nm not saturated for muonium ionization.

  • We are currently generating lyman photons using resonant-sum-difference method at Kr 4p55p[1/2,0] transition. According to our experience, lyman intensity linearly increases as 212.55nm intensity increases.

  • Pursue brighter 212.55nm output using different conversion scheme.

  • Investigate alternative schemes to generate lyman.



Diagram of the laser system



SR spectrometer

  • Large solid angle covered

  • Zero-Field measurement and Transverse-Field measurement (~600G) can be done.

  • Longitudinal-Field measurement under consideration.

  •  installation finished in December 2005 (except ZF coils).



PSI slow muon beam line



Depth-resolved profile of the magnetic field beneath the surface of a superconducter with a few nm resolution

  • T.J. Jackson, et al. PRL84(2000)4958

  • A magnetic field was applied parallel to the surface of a superconducting YBCO film (thikness 700nm).

  • The variation of the magnetic field below the surface was directly measured by stopping polarized muons at different implantation depth.



Direct observation of the oxygen isotope effect on the in-plane magnetic field penetration depth in optimally doped YBCO



Direct observation of nonlocal effects in a superconductor



Observation of the conduction electron spin polarizaion in the Ag spacer of a Fe/Ag/Fe trilayer

  • H. Kuetkens, et al. PRL91(2003)017204

  • Muons are implanted in a intermediate Ag layer of 20nm thickness, sandwiched by Fe layer. External magnetic field of 8.8mT was applied.

  • Obtained mSR spectrum was fourier transformed to give the profile of magnetic field in the intermediate Ag layer. The peaks around 8.8mT is due to hyperfine interaction between polarized electron in Ag and muons. This spectrum can be explained if conduction electron are polarized with oscillating behaviour.



An example of possible SR studies  towards “spintronics”

  • “Spintronics” is a recent buzz word; the idea is to control electronic property by manipulating electron spin. Examples include...

  • Multilayer composed of alternating ferromagnetic metal and non-ferromagnetic spacer layers.

    • Giant Magneto-Resistance (GMR) effect; changes in resistance exceeding 100% is observed when an external magnetic field is applied.
    • Strong industrial applications (example : recent HDD)
  • Conjunction of ferromagnetic metal and semiconductor

    • Electron spin is induced to semiconductors from spin-polarized metal.
    • Industrial applications expected (example : spin FET)
  • The understanding of electron spin state in the non-magnetic intermediate layer is the key for studies of these systems, but direct measurement is difficult, depth-resolved measurement is further difficult.

  • Slow muons could change that!





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