Design and test of a compact and high-resolution time-of-flight measurement device for cold neutron beams

Abstract

A time-of-flight device was developed to characterize wavelength distribution and uniformity of a cold neutron beam. This device is very compact---the distance of flight is 60 cm---but achieves very high resolution---the intrinsic resolution $\mathrm{\ensuremath{\Delta}}\ensuremath{\lambda}/\ensuremath{\lambda}=2.4\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ at $\ensuremath{\lambda}=0.89\text{ }\text{ }\mathrm{nm}$. The time-of-flight device is composed of a fixed slit, a disk rotating up to 216 Hz, and a neutron detector with a thin spherical conversion layer with the chopper slit in its focus. The device accepts the complete angular divergence of the initial neutron beam. The efficiency of neutron detection is constant over the detector area. Systematic corrections caused by neutron scattering in air are minimized due to the reduction of the time-of-flight length. Measurements have been performed on the beamline of the GRANIT experiment at ILL (part of the H172 beamline) on level C, and the first order diffraction peak of the crystal monochromator used for the GRANIT beamline was found to be at $\ensuremath{\lambda}=0.8961(11)\text{ }\text{ }\mathrm{nm}$, and having a width of $\ensuremath{\sigma}=0.0213(13)\text{ }\text{ }\mathrm{nm}$.