Mechanical velocity selector, neutron flux and Q-range for the small Angle Neutron Scattering facility at Risø

A mechanical velocity selector (MVS) was placed at the beam hole at the end of the neutron guide in the "Neutron House" at Risø. A 240 cm long evacuated tube with "pin hole" collimation was mounted after the velocity selector in order to simulate the geometry of the proposed small angle neutron scattering (SANS) instrument. At the end of the tube, corresponding to the sample position, count rates and absolute fluxes were measured with a monitor and Au foil activation, respectively. Likewise, Au foil flux measurements were performed immediately before and after the MVS. At the peak position K = 1.9 Å^-1 (λ = 3.3 Å, E = 7.2 meV) of the cold source reactor spectrum and at bandwith Δk/k = 0.20 the flux at the sample position was measured to φ =1.0•10⁶n/cm²sec. This is about 5 times lower than the corresponding flux obtained at the D11A SANS facility at ILL, Grenoble. A lower limit of mommomentum transfer for the proposed SANS instrument is estimated to Ω_min∼1•10^-3Å^-1