High speed photon energy tuning of X-rays with high duty cycle by use of Clessidra prism arrays

摘要

The refractive index for matter in the hard X-ray range changes rather rapidly with increasing photon energy. In fact away from absorption edges the refractive index decrement varies with the square of the photon energy. As the photon energy dispersion depends on the refractive index decrement, prisms, as used in the visible spectral range, can also be used in the X-ray range for monochromatising or dispersing an incident polychromatic X-ray beam. The dispersion in a single prism is rather small and it is thus mandatory to use ensembles of many prisms for the dispersion process. A particular prism assembly is the Clessidra X-ray lens. The name of the lens, which is Italian for hourglass, describes its appearance as many tiny prisms form two larger prisms with opposing areas. In this form the device can focus an incident plane X-ray wave by refraction. Then the dispersion will make the focusing suffer from chromatic aberrations. The latter can be used in order to operate the structure as a monochromator in combination with an exit slit. For this operation it was found to be advantageous to use a central obstruction in the lens. As the prism array focuses only one-dimensionally the object rotation around an axis, which is perpendicular to the incident X-ray beam and which lies in the dispersion plane, is now an available degree of freedom. In a monochromator one can make good use of it, as it permits us to tune the photon energy in a fixed slit. This contribution will discuss the limits for such an operation in terms of achievable spectral resolution and in terms of rapid tuning.