The design of a high heat-load monochromator for nuclear resonance scattering applications is advisable to have the following characteristics: 

  • short and long term angular stability
  • high accuracy and resolution of angular positioning
  • reliable energy defining element
  • fixed exit 
  • matching the divergence of the source and the angular acceptance of the monochromator
  • ideal propagation of the x-ray wave field

Based on these demands a fixed exit liquid nitrogen cooled high heat-load monochomator was designed consisting of two separate Si(111) crystals on two separate mechanical stages. The crystals reside in UHV conditions (10-8 mbar). The first crystal is mounted on a translation stage to allow for fixed exit operation. The second crystal is equipped with a high resolution encoder system and defines the chosen energy. The crystals are cut symmetrically. Tilt stages allow for crystal alignment. The offset between incoming and outgoing beam is 16mm allowing for an energy range between 6 and 100 keV using the Si(111) reflection. The monochromator has been purchased from KOHZU, Japan.

The high heat-load monochromator of ID14 operates in horizontal scattering geometry thanks to the reduced horizontal emittance of the extremely brilliant synchrotron radiation storage ring. In this configuration the distortion of the wavefront induced by the distorted (mainly the first) crystals of the high heat-load monochromator does not affect the downstream high-resolution optics that scatter in vertical geometry.

The KOHZU mechanics provides perfect angular resolution (100 nrad) and angular stability (mean deviation of about 1 urad per month). This provides an enhanced energy stability of the monochromatized beam (mean deviation of about 0.3 eV per month). Due to the efficient cooling, proper thermal isolation, and shielding of the second crystal, the monochromator has no "transition time" after refill: it provides a proper flux at proper energy within a  minute after the beam is back to user mode.