The D20 instrument at the ILL: a versatile high-intensity two-axis neutron diffractometer

D20 is a medium to high resolution two-axis diffractometer capable of producing a neutron flux of 10(8) s(-1) cm(-2) at the sample position. The 1536 detection cells of its curved linear position sensitive detector (PSD) cover a continuous 2 theta. range of 153.6 degrees. over a total solid angle of 0.27 sr. This combination of a high incident neutron flux and a large detector solid angle provides D20 with the fastest counting rate, at a given resolution, of any reactor-based neutron diffractometer. Different monochromators and take-off angles, plus optional Soller collimators and secondary slits, permit a wide choice in the Q-space range, wavelength, resolution and flux. A high-resolution configuration offers Delta d/d similar to 2 x 10(-3). Fast modern counting electronics allow in situ time-resolved experiments at the timescale of a few tens of milliseconds. In addition, a variety of sample environments, including an optional radial oscillating collimator for suppressing parasitic scattering, contribute to a rich scientific programme.