1. Monochromatization
 
 

High energy neutrons generated from nuclear reactor source of spallation source are moderated by light water (H2O, ∼300K) or liquid hydrogen (H2, ∼20K) and its velocity (or energy) is distributed accroding to the Maxwellian distribution. By selecting necessary energy (or wavelength) range of neutron beam, which is called 'monochromatization', various versatile experiment could be done.
Monochromatization means the process of diffraction toward the special direction (2 ) with characteristic wavelength( ) determined by the spacing of lattice plane and the Bragg condition (2d sin  = n ). The crystal used for this process is called as 'monochromator'.
 

2. Monochromators
 

Flat perfect crystal has a definite wavelength (  =0) but is not practically useful since the integrated intensity is very small. Therefore general requirement for a practical monochromator is obtaining a sufficient integrated intensity by allowing wavelength spread (Δλ≠0).

1) Mosaic monochromator

In a crystal, there exist a intrinsic defects such as dislocation and thus the crystal is divided into very small mosaic blocks (typical size is a few  m) which have small misalignment (mosaicity) of a few arc seconds to a few arc minutes between them. Crystal which has a large mosaicity(∼101 arc min. /by artificial increase) is called as 'mosaic monochromator' and had been used from the beginning of the neutron diffraction experiment.
 
2) Bent Perfect Crystal Monochromator

By elastic deformation of thin (a few mm) slab of perfect crystal, neutron beam can be monochromatized and focused like a cylindrical mirror reflection. Characteristics (Δλ, ΔΘ , focusing, ...) of monochromatic neutron beam can be controlled by changing the bending radius and diffraction geometry. This kind of monochromator, called as BPC(bent perfect crystal) monochromator, is very useful for small sample cases such as single crystal diffractometer and residual stress diffractometer. 
 
 

 
3. Direction of R&D
 
  We are developing both kinds of monochromator and are going to apply to the neutron instruments in HANARO.
For the development of mosaic monochromator, we are focusing to the technologies of large crystal growing, mosaicity increase and evaluation of the monochromator performance.
For BPC monochromator, we are focusing to the technologies of the monochromator designing, crystal cutting and estimation of the characteristics. From the experiments this BPC monochromator is found to be very useful for small sample measurement such as single crystal diffraction, texture measurement and residual stress measurement and it has superior properties than the mosaic germanium monochromator which we have.