Method for operating a primary beam stop

A method for operating an X-ray or neutron optical system, the system having an X-ray or a neutron source from which radiation is emitted as a primary beam, and impinges on a sample to be examined, and with an X-ray or neutron detector for detecting radiation refracted or scattered from the sample, wherein the source, the sample and the detector are substantially collinear to define a z-direction, wherein a beam stop is disposed between the sample and the detector, the beam stop having a cross-sectional shape, perpendicular to the z-direction, which is adapted to the cross-section of the primary beam, wherein the beam stop is disposed for displacement along the z-direction to optimize a ratio between a useful beam fraction and an interfering beam fraction impinging on the detector, the method comprising the step of:

• adjusting the X-ray or neutron optical system to time changes in the properties of the beam prior to each measurement of the sample by automatically adjusting a position of the beam stop in the z-direction in such a fashion that, in a plane of the detector, a shadow cast by the beam stop precisely shields the primary beam and parasitic interfering radiation.