Development of a TOF reflectometer at the Greek Research Reactor

The Greek research reactor GRR-1 is a continuous neutron source of 5 MW thermal power but without a cold source and without a neutron guide system. The principles of the new time-of-flight (TOF) reflectometer under installation are described. Different design parameters for the reflectometer are considered and optimized with the aim to obtain the best performance, satisfying different experimental needs and under the constraints of cost and local conditions. For measuring liquid samples the in-pile collimation system has been designed, such as the emerging from the reactor neutron beam is inclined by -0.8 deg. with respect to the horizontal axis. Thus beam inclination for the desirable θ range is achieved by simply moving the apertures and the sample vertically of maximum value of -1.6 deg. Different wavelength ranges are available and all of them have the same minimum wavelength of 0.15 nm utilizing, thus, the wavelengths with the maximum flux. The resolution is also a variable parameter in order to satisfy experimental needs for either high resolution or lower resolution but higher counting rate experiments. Pulse overlap is avoided by the employment of a supermirror. Since there are no guides and it is essential to reduce the background, a sapphire filter is used for the removal of the fast neutron component. For achieving an efficient removal of the fast neutrons without considerable reduction of the neutron flux in the wavelength range from 0.15 to 1.0 nm, the optimum sapphire thickness has been calculated by MCNP code. Further, towards a low background instrument, optimization of the shielding has been made by MCNP calculations. (author)