A Bonse/Hart camera has been built [*] in order to measure ultra small angle X-ray scattering (What is measured in a Small Angle X-ray Scattering (SAXS) ?). A mirror installed between the source and the first slits increases the incident flux on the monochromator : a factor of ten has been gained. Fifty millions of photons per second actually arrive on the sample. In 2007, mirror was replaced by a Xenocs collimated monochromatror, which enable us to use generator at low power without losing flux. Two channel-cut Ge (111) crystals are used in the Bonse/Hart geometry to collimate the beam and obtain a very high q-resolution. The first one plays the rôle of a monochromator, selecting the incident wave-vector with a resolution of 80 mrad. The second one is used in Bragg geometry as an "analyser" crystal. The photons are then detected by a high count-rate scintillator associated to a photomultiplier. The ensemble analyser-detector and is under vacuum and can rotate around the sample axis with a precision of few microradians.
· Ultra Small Angles. The value of qmin is fixed by the angular acceptance of the crystals, and not by geometric conditions as the beam stop size in 2D cameras. Instead of few radians, one can then measure a signal at few micro-radians from the center of the beam : the very high selectivity of the analyser crystal allows to measure the scattered intensity inside the direct beam ! This is the reason why this camera is called an ultra small angle spectrometer.
· High resolution. The resolution is excellent : it is not determined by any pixel size or collimated beam size as in 2D cameras but by the crystal angular acceptance. Measurements can be performed with an accuracy of 80 mrad, i.e. q>4.10-4 Å-1.
The specificity of the camera is double :
The rocking curve (scattering signal measured without sample) is shown on figure 3. It indicates the limits of the spectrometer : direct beam size, experiment background, maximal flux and rejection rate. These useful quantities are also summed up in the table.
The camera is adjustable to the largest q-range available with a laboratory instrument. It is particularly well adapted to investigate materials as colloidal crystals, clays, glasses, latex suspensions, etc.. Moreover, after desearing the intensity can be obtained in absolute units (cm-1) : the sample transmission is carefully measured with an ionisation chamber.
[*] J. Lambard, P. Lessieur and Th. Zemb, J. Phys. I France 2 (1992) p. 1191-1213