<P> For neutron diffraction vanadium cylinders are used as sample holders . Vanadium has a negligible absorption and coherent scattering cross section for neutrons and is hence nearly invisible in a powder diffraction experiment . Vanadium does however have a considerable incoherent scattering cross section which may cause problems for more sensitive techniques such as neutron inelastic scattering . </P> <P> A later development in X-ray cameras is the Guinier camera . It is built around a focusing bent crystal monochromator . The sample is usually placed in the focusing beam, e.g. as a dusting on a piece of sticky tape . A cylindrical piece of film (or electronic multichannel detector) is put on the focusing circle, but the incident beam prevented from reaching the detector to prevent damage from its high intensity . </P> <P> Diffractometers can be operated both in transmission and in reflection configurations . The reflection one is more common . The powder sample is filled in a small disc - like container and its surface carefully flattened . The disc is put on one axis of the diffractometer and tilted by an angle θ while a detector (scintillation counter) rotates around it on an arm at twice this angle . This configuration is known under the name Bragg--Brentano theta - 2 theta . </P> <P> Another configuration is the Bragg--Brentano theta - theta configuration in which the sample is stationary while the X-ray tube and the detector are rotated around it . The angle formed between the tube and the detector is 2theta . This configuration is most convenient for loose powders . </P>

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