Nuclear Resonant Scattering of Synchrotron Radiation

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Nuclear Resonant Scattering of Synchrotron
Radiation

• Dénes Lajos Nagy

KFKI Research Institute for Particle and Nuclear
Physicsand Eötvös Loránd University , Budapest,
Hungary
Thin Films as Seen by Local Probes ERASMUS
Intensive Programme Frostavallen (Höör), Sweden,
2-12 May, 2002
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Outline

• Synchrotron Radiation (SR)
• History
• The machine
• SR sources
• Properties of SR

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Outline

• Nuclear Resonant Scattering of SR Theory
• Conventional Mössbauer spectroscopy
• Nuclear resonant forward scattering
• Nuclear Resonant Scattering of SR Experiment
• The experimental setup
• The transverse coherence length
• Nuclear resonant inelastic scattering
• Problems

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Synchrotron radiation History

• SR polarised electromagnetic radiation produced
  in particle accelerators or storage rings when
  relativistic electrons or positrons are deflected
  in magnetic fields
• Elder et al. (1947) first observation of SR at a
  70-MeV synchrotron
• Tomboulian, Hartman (1956) first spectroscopic
  studies at a 300-MeV machine
• First-generation SR sources (1965-1980)
  machines built for particle physics, SR produced
  at bending magnets is used in parasitic regime

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Synchrotron radiation History

• Second-generation SR sources (1970-1990)
  machines dedicated to the applications of SR,
  radiation produced at bending magnets
• Third-generation SR sources (1990-)machines
  dedicated to the applications of SR, radiation
  produced both at bending magnets and at insertion
  devices
• ESRF (Grenoble, France) 6 GeV
• APS (Argone, USA) 7 GeV
• SPring8 (Harima, Japan) 8GeV
• The future x-ray free-electron lasers (XFEL)

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Technical aspects (example ESRF)

• Pre-accelerators
• LINAC 100 keV electron gun 200 MeV
• booster synchrotron 200 MeV 6 GeV
• The storage ring
• circumference 845 m
• number of electron buckets up to 992
• electron bunch length 6 mm pulse duration 20
  ps and 100 ps at bending magnets and insertion
  devices, respectively
• re-acceleration power at I 100 mA 650 kW.

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Technical aspects (example ESRF)

• Critical wavelength of SR lc (4p/3)(R/c3),
  i.e., lcÅ 5.59 (Rm/EGeV3)where R is
  the radius of the electron orbit in the bending
  magnet or in the insertion device.
• Spectral brilliance of a SR source (bending
  magnet or insertion device)
  photons/s/mm2/mrad2/0.1 energy bandwidth

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Technical aspects (example ESRF)

• Insertion devices wigglers and undulators. These
  are two arrays of N permanent magnets above and
  below the electron (positron) beam. The SR is
  generated through the sinusoidal motion of the
  particles in the alternating magnetic field.
• Wigglers strong magnetic field, broad-band
  radiation from the individual poles is
  incoherently added. Intensity N. Horizontal
  beam divergence gtgt 1/c.

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Technical aspects (example ESRF)

• Undulators weak magnetic field, narrow-band
  radiation from the individual poles is coherently
  added at the undulator maxima. Intensity N2.
  Horizontal beam divergence 1/c.

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Properties of SR

• Tunable energy
• High degree of polarisation
• High brilliance
• Small beamsize
• Small beam divergence
• Pulsed time structure

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• Only one transition is excited at the same time,
  therefore the resultant spectrum is the
  incoherent sum of the indivitual transitions (the
  intensities are added).

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Nuclear resonant scattering of SRMössbauer
effect with SR

• E. Gerdau et al. (1984) first observation of
  delayed photons from nuclear resonant scattering
  of SR (at beamline F4 of HASYLAB).
• Basic problem huge background from prompt
  non-resonant photons. The solution
• monochromatisation of the primary SR,
• suppression of electronic scattering by using
  electronically forbidden Bragg reflections (out
  of date),
• fast detectors and electronics.

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Nuclear resonant scattering of SRMössbauer
effect with SR

• Bergmann et al. (1994) first observation of
  delayed photons from nuclear resonant forward
  scattering of SR.
• The bandwidth of SR is much larger than the
  hyperfine splitting. Þ All transitions are
  excited at the same time. Therefore the resultant
  time response is the coherent sum of the
  indivitual transitions (the amplitudes are added).

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Nuclear resonant scattering of SRMössbauer
effect with SR

• Not only the different transitions of the same
  nucleus but also transitions of different nuclei
  within the coherence length are excited
  simultaneously and the scattering takes place
  coherently.
• The longitudinal coherence length of the resonant
  radiation is ctn 42 m for 57Fe.

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Nuclear resonant scattering of SRMössbauer
effect with SR

• The temporal interference of the amplitudes
  scattered from different hyperfine-split
  transitions leads to quantum beats. The strength
  of the hyperfine interaction (e.g. magnetic
  field) is reflected in the frequency/frequencies
  of the quantum beats.
• The orientation of the magnetic field and of the
  electric field gradient is reflected in the
  intensities of the different frequency components
  and in the depth of the beating.

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Nuclear resonant scattering of SRMössbauer
effect with SR

• Due to the full linear polarisation of SR, the
  nuclear resonant scattering of SR is extremely
  sensitive to the orientation of the hyperfine
  magnetic field.

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Measurement of the isomer shift

• The NRS time response depends only on the
  differences of the resonance line energies.
  Therefore the isomer shift has no influence to
  the quantum-beat pattern.
• The isomer shift can be measured by inserting a
  single-line absorber to the photon beam within
  the longitudinal coherence length.

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NRS vs. conventional MS

• NRS is not just a repetition of conventional
  energy-domain Mössbauer spectroscopy the two
  methods are complementary. It should be applied
  when unique properties of SR are used
• small solid angle is available (e.g., at
  grazing-incidence experiments in thin films),
• small samples are available (small single
  crystals, high-pressure experiments, biological
  samples),
• linear polarised radiation is advantageous
  (determination of the hyperfine field direction),
• etc.

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Nuclear inelastic scattering experiment

• The pulsed SR beam is monochromatized to a meV
  energy band with the high-resolution
  monochromator before it penetrates the ionization
  chamber (IC) and the sample.
• The radiative decay of the resonant nuclei in the
  sample is measured with two APD detectors one in
  forward direction (NFS), which collects data only
  from a small solid angle (top) and one at 90
  (NIS) which collects data in a large solid angle
  (bottom).

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Nuclear inelastic scattering experiment

• At exact resonance energy (ED 0) the NFS
  detector collects the time-depending NFS.
• During scanning the energy of the incident beam
  by detuning the HRM the time-integrated signal of
  the NFS detector shows a sharp peak at ED 0
  which represents the energy resolution of the
  monochromator system.

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Nuclear inelastic scattering experiment

• The time-integrated signal of the NIS detector
  shows for the same energy scan a high central
  peak at ED 0 and peaks apart from the resonance
  energy, depending on the sample. This energy
  spectrum represents the probability of resonance
  absorption with recoil overlapped by the signal
  at ED 0 produced by subsequent processes of the
  internal conversion. The time dependence of the
  NIS signal shows an exponential decay after
  excitation, since the data are collected
  angle-integrated.

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Lattice dynamics in an icosahedral
Al62Cu25.5Fe12.5 quasicrystal (A. Chumakov)
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Inelastic x-ray scattering with nuclear resonant
anayser
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Inelastic x-ray scattering with nuclear resonant
anayser
Chumakov et al., Phys. Rev. Lett. 76, 4258 (1996)
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Problems

• Bunch modes at ESRF
• uniform filling 992 bunches uniformly
  distributed in the storage ring,
• 1/3 filling 331 bunches filling 1/3 of the ring,
• single-bunch filling 1 bunch in the ring,
• 16-bunch filling 16 bunches uniformly
  distributed in the storage ring,
• hybrid filling 331 bunches filling 1/3 of the
  ring 1 bunch in front of the 331 bunches.
• Which modes are suitable for nuclear
  resonant forward scattering experiments on 57Fe
  (nuclear lifetime of the resonant level 141 ns)?
  And for inelastic scattering experiments on the
  same nucleus?

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Problems

1. Explain qualitatively, why no quantum beats but
   an exponential decay is observed when the axially
   symmetric EFG axis is perpendicular both to k and
   E. (1/2 3/2 transition).
2. A 57Fe foil is randomly vibrating along the
   photon beam with an average frequency n 10 Hz
   and an amplitudea 5 mm. Describe qualitatively
   the conventional energy-domain Mössbauer spectrum
   as compared with the case of the static foil! Do
   the same for the nuclear resonant forward
   scattering of SR!

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Problems

1. A resonant photon beam is passing two subsequent
   57Fe foils. Both foils are magnetised to
   saturation in high magnetic fields perpendicular
   to the sample plane, i.e., along the photon beam.
   Both energy- and time-domain Mössbauer
   experiments are performed for a) parallel b)
   antiparallel magnetisations of the two foils.
   Describe qualitatively the results of both pairs
   of experiments!