The High Energy Synchrotron X-ray Diffractometer at BESSRC CAT
Yang Ren
It is well known that physical and chemical properties or biological functions are intimately related to the structures or shapes of matter. To understand the structures and shapes as well as possible phase transformations is an essential step, and one of the most important and challenging tasks in many scientific research areas. Diffraction technique is the most powerful tool to reveal the three dimensional information of structures in atomic scale. X-ray, neutron and electron diffractions are widely utilized in this technique with some advantages and disadvantages between them. Several factors, i.e. available beam intensities, penetration lengths, scattering powers, etc. determine their applications with various experimental conditions.
Recently there has been an increasing demand for high energy synchrotron X-ray diffraction. Why high energy photons? The high energy photons (E > 60 keV) have a large mean free path in materials and corrections for polarization, absorption and extinction are small in general. This makes it easy to perform some complicated experiments under extreme conditions, e.g. in high pressures, at very high/low temperatures, and in situ studies. At high photon energies momentum transfers of Q>45 -1 can be easily achieved. This is very important for studies of amorphous materials. Magnetic X-ray scattering has been developed, both experimentally and theoretically, during the last two decades. Among them, high energy X-ray magnetic diffraction has a unique position. The cross section for magnetic scattering of high energy photons (E > 80 keV) is proportional to the square of the spin component, thus one can determine the absolute spin contribution of the magnetic ions [1]. The high penetration lengths and the high momentum transfers make high energy photons comparable to neutrons, but with higher resolution and flux, thus much smaller samples are required, while keeping the advantage of special sample environments. Combinations of neutron and high energy X-ray diffraction have been demonstrated as very powerful and productive approaches to understanding many complex materials [2].
In this article, we first give a general description of the high energy X-ray diffractometer (11ID-C) at the elliptical multiple wiggler with a critical energy of 32 keV at BESSRC CAT [3]. Detailed information can be found in the paper of Ruett et al. [Diffractometer for high energy X-rays at the APS, published in Nuclear Instruments and Methods in Physics Research A 467-468 (2001) 1026-1029]. As illustrated in the figure, the triple axis diffractometer consists of four towers for monochromator, sample, analyzer and detector in the experimental hutch. The first three are placed on a common optical table, and can be moved along the table on a rail system. The sample tower has a heavy duty z-translation as the base of the rotation tables, which can carry heavy loads up to 200 kg. The detector tower is located behind the optical table. The distance between the detector and the optical table is varied by sliding on a rail system on the ground. Different from other dedicated high energy X-ray diffractometers in the world, the 11ID-C diffractometer is operating in the vertical scattering plane, taking advantage of the narrow vertical beam divergence for high energy resolution.
Three pre-monochromators of annealed Si 111, Si 220 and Si 311 crystals are available at the moment to provide photons with energies of 60, 98 and 115 keV, respectively. The beam spot of 98 keV photons at the sample position is up to 3.5 mm high by 2.5 mm wide with an intensity of 2*1012 photons/sec. The beam size and energy band width can be adapted by a slit directly behind the pre-monochromators. The q-space resolution defined by the FWHM is 10-5 Ǻ-1 and 2*10-4 Ǻ-1 perpendicular and parallel to the reciprocal lattice vector, respectively. And the resolution perpendicular to the scattering plane is determined by the angular acceptance of the detector.
As mentioned above the sample stage was designed for heavy duty use and a superconducting cryomagnet (Oxford Instruments) has been successfully mounted on the diffractometer (see picture). This is probably a unique setup in the world in that it is a combination of high energy X-ray diffraction and superconducting magnet with high magnetic fields up to 8 Tesla in the temperature range from 1.5 to 300 K. Because of low absorption, large penetration length and small scattering angles of high energy photons, single crystals and powder samples as well as thin films can be easily studied with the cryomagnet in transmission geometry. This setup provides a powerful tool for magneto-crystallographic studies of many interesting materials, e.g. the CMR compounds, high Tc superconductors, and magnetic crystals. Recently we have investigated field induced structural changes of various CMR compounds. The results will help us in understanding the real underlying physics of the CMR effect.
The instrument has been extensively used by Argonne scientists as well as researchers from other institutions and universities all over the world. The research subjects cover a very large variety of materials: from crystalline to amorphous; from bulk solids to thin films to nano-scale materials. In the next article, we will show some of very important results obtained with the 11ID-C high energy X-ray diffractometer.
REFERENCES
[1] J. Strempfer et al. Phys. Rev. Lett. 86, 3154(2001).
[2] C. A. Tulk et al. Science, 297, 1320(2002).
[3] U. Ruett et al. Nucl. Inst. Methods Phys. Res. A 467-468, 1026 (2001).