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

A time-of- flightdevice 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 $\Delta \lambda/\lambda=2.4\cdot 10^{-3}$ at $\lambda=0.89$ nm. The time-of- flightdevice 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 chopperslit in its focus. The device accepts the complete angular divergence of the initial neutron beam. The efficiency of neutron detectionis 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. Measurementshave been performed on the beamlineof the GRANIT experiment at ILL (part of the H172 beamline) on level C, and the first order diffraction peak of the crystal monochromatorused for the GRANIT beamlinewas found to be at $\lambda=0.8961(11)$ nm, and having a width of $\sigma=0.0213(13)$ nm.