Refractive beryllium X-ray lens with variable focal length

A refractive lens for hard X-rays comprising two saw-tooth profiles is presented. This lens has the same focusing properties as a parabolic compound refractive lens. One advantage is the remarkably low fabrication cost, since curved surfaces are replaced by planar ones. In addition, the focal length of the lens can be easily varied by adjusting the angle between the two halves. Since the index of refraction depends on the X-ray energy, the lens is chromatic and acts as a band-pass filter for a broad energy spectrum. Combined with the tunability of the focal length, this allows versatile spectral shaping of the X-ray beam. Calculations and numerical examples of the focusing properties are presented. Due to its low atomic number, beryllium is an excellent choice for refractive optics and a prototype in beryllium has been fabricated using diamond turning technique. Surface metrology shows a deviation from the ideal shape of about 400 nm rms, indicating a loss of intensity of between 20% and 50%, depending on the geometry and X-ray energy.