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High pressure measurements in Diamond Anvil Cells

A high proportion of experiments at synchrotrons use X-ray diffraction to study a sample's structure on an atomic scale. Using hard X-rays for these experiments makes it possible to gain higher spatial resolution, to study large samples or samples made of heavier elements, and to probe samples while they are contained in sample environments. A good example of the latter would be extreme conditions experiments. By compressing a sample inside a diamond anvil cell while heating it with a laser, it is possible to recreate the extremely high pressures and temperatures found inside of planets or in the outer layers of stars. If a highly focused X-ray beam is fired through the sample, the X-rays will be diffracted, and from the diffraction pattern it is possible to infer the atomic-level structure. High-speed photon counting detectors like LAMDBA can then allow scientists to study the rapid changes that occur in a sample when the pressure and temperature are changed. To investigate this possibility, the GaAs LAMBDA detector was tested at PETRA-III beamline P02.2, which is used for extreme conditions experiments [1]. A sample of a common test standard, CeO 2 , was placed inside a diamond anvil cell in the X-ray beam. The X-ray beam had a photon energy of 42 keV; at this energy, the photoelectric absorption efficiency of 500 µm-thick GaAs will be 74%, compared to 4.6% for the same thickness of silicon. The LAMBDA GaAs detector was placed at a distance of 35 cm, with a horizontal offset of 4 cm between the X-ray beam and the edge of the sensitive area. A series of images were then taken with different shutter times. Figure A shows an image taken with a shutter time of 1 ms. Because the sample is a coarse powder, the diffraction pattern consists of rings of diffraction spots, with each spot produced by scattering from a single grain of the material. The angle and intensity of these rings convey information about the crystalline structure of the material. Due to the short shutter time, the rings are faint, with most pixels only detecting a few photons. Nevertheless, due to the effectively noise-free behaviour of the detector, when the photon hits in the 1 ms image are integrated as a function of scattering angle a clear diffraction signal is obtained, with the rings clearly distinguished from the background level, as shown in Fig B. This demonstrates that the detector could be used to measure structural changes in a sample on a timescale of milliseconds. More recently, two 3-module LAMBDA 2M GaAs systems have been delivered and are now in routine use for diamond anvil cell experiments at 2 kHz frame rates. [1] http://photon-science.desy.de/facilities/petra_iii/beamlines/p02_hard_x_ray_diffraction_beamline/ecb/index_eng.html 
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(A) Diffraction pattern obtained in 1ms with the GaAs LAMBDA detector. The sample was CeO2 powder in a diamond anvil cell, illuminated by a 42 keV synchrotron X-ray beam at PETRA-III P02.2.(B) The diffracted X-ray intensity integrated as a function of scattering angle, using the single 1ms image shown in A.