Cheiron School 2014
Probing atomic vibration using nuclear resonant inelastic scattering
BL09XU: Nuclear Resonant Scattering Beamline
1. Introduction
BL09XU is an X-ray undulator beamline constructed for the research using nuclear
resonant scattering (NRS), which is caused by the nuclear level as shown in Fig. 1.
Usually X-rays are scattered by electrons and nuclear Thomson scattering is negligible.
But when the energy of the incident X-ray coincides with that of the nuclear level, its
scattering cross-section gets larger. The nuclear levels of various isotopes are differentfrom each other and its energy width is extremely narrow, typically 10-6 to 10-9 eV. For
example, the resonant energy of 57Fe is 14.4 keV and the energy width is 4.7 x 10-9 eV.
The half lifetime of the 57Fe excited state is 98 nsec, which follows the Heisenberg
uncertainty principle.
NRS of synchrotron radiation has specific features compared with conventional
Mössbauer spectroscopy. Synchrotron radiation can produce a well-collimated and smallbeam, which allows the study of materials under extreme conditions and under diffraction
conditions including grazing incident geometry. Another important technique is nuclear
resonant inelastic scattering (NRIS) to study the localized vibrational density of states ofthe Mössbauer isotopes. Quantized vibrational motion is called a phonon, which isinvestigated by several spectroscopic methods such as Raman spectroscopy, inelastic
X-ray scattering and neutron scattering. Its typical energy range is ~100 meV. Everyisotope has different nuclear level, so NRIS has distinguished feature of just probing aspecific isotope. Recently biochemical materials such as an enzyme have been
intensively studied by NRIS to focus on its active center in the complicated system.

Fig. 1 Nuclear level scheme of 57Fe and nuclear resonant scattering
2. BL09XU

BL09XU is a standard X-ray beamline in SPring-8 with a 32mm period linear
undulator and a cryogenic cooling high-heat load monochromator. We have two
experimental hutches as shown in Fig. 2 after one optics hutch where the high-heat loadmonochromator is located. High-resolution monochromators are usually arranged inExperimental Hutch 1. Samples are set with temperature control in Experimental Hutch
2.

Fig. 2 schematic view of experimental hutches at BL09XU
2.1 High-resolution monochromator
A high-resolution monochromator (HRM) is one of the most important tools for theNRS study using synchrotron radiation. It is used to change the energy of the incident
X-rays in case of the study of the vibrational states of materials using NRIS. The
resolution of the obtained energy spectrum is determined by that of HRM. The 3
bounce-type HRM for 57Fe with 0.8 meV resolution is shown in Fig. 3. It is composed ofone germanium crystal and two silicon crystals.

Fig. 3. Schematic view of HRM arrangements with resolution of 0.8 meV2.2 Fast timing discrimination
The time structure of the incident X-ray is quite important for the time-resolved
experiments such as a nuclear resonant scattering. The storage ring of SPring-8 has a
harmonic number of 2436 and an R.F. frequency of 508.58 MHz. So the bunch interval is1.966 nsec and it takes 4.8

sec for electrons to turn around. Several kinds of bunchmodes are prepared at SPring-8 as shown in the following web site, to fulfill the timestructures user experiments require.
http://www.spring8.or.jp/en/users/operation_status/schedule/bunch_mode

An avalanche photo diode (APD) detector with 1 nsec resolution and fast electric circuits
are used to discriminate the prompt electronic scattering noise from the delayed signal,
which originates from the nuclear resonant scattering.

2.3 Cryostat
A Continuous flow cryostat for X-ray diffraction, Oxford CF1104 is used for thetemperature control of samples. Liquid nitrogen or liquid helium can be selected as
flowing gas according to the controlled temperature of the sample. An outer vacuum
chamber was specially designed for nuclear inelastic scattering experiments to make thedistance between the sample and a detector as short as possible, because delayed signals
are scattered into the whole solid angle after the nuclear inelastic absorption. The sample
is mounted on the sample holder attached to the cryohead so as not to touch the top Be
window as shown in Fig. 4.

Fig. 4 Cryostat designed for nuclear resonant inelastic scattering experiments.

3. Practical
The following practical will be done after the introduction of the beamline and NRS.
1) HRM setting to get the beam with narrow energy width
Adjust the undulator gap and high heat-load monochromator to get the beam with ~2eV resolution before HRM at Experimental Hutch 1. Tune the high heat-load
monochromator to get the beam with ~ 0.8 meV through the HRM.
2) Fast electronics adjustment to get the delayed signal
Adjust the fast electric circuits after setting the 57Fe foil and APD detector todiscriminate the prompt electronic scattering noise from the delayed signal.
3) Energy tuning to find the resonance of 57Fe nuclear level
Tune the X-ray energy from the HRM in to the energy of the 57Fe nuclear level. The
X-ray energy is changed by means of rotating the one of the crystals of the HRM.
4) NRIS measurement at room temperature
Mount the model sample in the cryostat and start the NRIS measurement at room
temperature counting the delayed signal with the finite energy step.
5) NRIS measurement at low temperature
Start the flow of liquid He and mount the model sample again at low temperature.
Start the NRIS measurement at low temperature. Compare the data between at room
temperature and at low temperature.

References
1) “Nuclear Resonant Scattering Beamline at SPring-8”, Y. Yoda et. al., Nucl. Instrum.
Methods A, 467-468 (2001) 715-718.
2)”Observation of Nuclear Resonant Scattering accompanied by Phonon Excitation using
Synchrotron Radiation”, M. Seto et. al., Phys. Rev. Lett., 74 (1995) 3828