Beamline practice at BL12XU (Cheiron school 2011)
Nozomu Hiraoka
National Synchrotron Radiation Research Center (NSRRC), Taiwan

1. Introduction – Inelastic X-ray Scattering Brilliant and energy-tunable third-generation synchrotron radiation sources allow us to carry out various typesof spectroscopic experiments in the hard x-ray region. The undulator beamline BL12XU is primarily designed
for resonant and non-resonant inelastic X-ray scattering (RIXS and NIXS) experiments. Participants at ourcourse will learn basic ideas of inelastic x-ray scattering. In practice, we will perform a NIXS experiment: Wewill measure carbon K-edge (around 284 eV) in diamond, graphite, and C60 fullerene, using 10-keV x-rays.
Figure 1 indicates a difference between (normal) photo-absorption and inelastic scattering. In photoabsorption, an incident photon is totally absorbed and all the energy is transferred to an electron to be excited.
The absorbance is recorded as a function of the photon energy in a spectrum. In the inelastic scattering, theenergy is partially transferred. The electron is excited while another photon is emitted with the rest of theenergy (scattered photon). The scattering intensity is recorded as a function as the transferred energy (E0-E1).
photon-in

Photon-inE

Photoabsorption

photon-outEInelastic
scattering
Eo , ko

Eo , ko

E1 , k1

GG
excitations, ko ~ 0

Excitations, k1 - ko ≠ 0

Elastic (Eo=E1)
Abs.
edge


Edge

Eo

Eo-E1

Advantages of the inelastic scattering are mainly,(i). Absorption edge residing in the soft x-ray region (10 eV < E < 1 keV) can be measured under an extreme
condition such as high pressure (Soft x-ray experiments are incompatible with high-pressure experiments)(ii). Significant momentum (q) is transferred and its magnitude is fully tunable with a scattering angle,
therefore it has a richer information (Inelastic scattering spectrum becomes equivalent to photo-absorption ata small q).

N K-edge

Solid N2
under HP

A disadvantage is a weak intensity: An intense beam is essential

Intensity map of dd
excitons in NiO in 3D qspace2. Rowland-circle type spectrometer
For inelastic experiments, a spectrometer is essential to measure the energy distribution of scattered photons.
The most popular type of the spectrometers is Rowland-circle type, in which the photons are monochromatedand focused into a small spot on a detector. A sample, an analyzer, and a detector are placed on the Rowland
circle defined by the bending radius of the analyzer. To increase the count-rate, multiple analyzers are oftenused.

DCM

DETECTORFM

CM

SOURCESAMPLEANLYZERSDETECTORSAMPLEROWLANDCIRCLESSAMPLE

BEAM

BEAM
B
B

FLAT ANALYZER

BENT ANALYZER

B’• Monochromatic• Focused photons
3. BL Practice
We will try to measure the carbon K-edge in diamond, graphite, and C60-fullerene.
1. Sample alignmentSampleypositioning along the beam-axis (y-axis) by a scope
positioning along the x-z-plane by a PIN diode after the sample
find an orientation of the sample axis by x-ray diffraction

2. MeasurementsEO, kO
E1, k1high-qxLow-qevacuate a sample chamber and set back the detector
measure the elastic line (E0=E1) : N.B. Incident photon energy is scanned in NIXS (unlike RIXS)
measure the C-edge (E0 = E1+284)

If we have a time, we will try high-q (large momentum transfer) to see whether or not the spectra are different.