7C2, the new neutron diffractometer for liquids and disordered materials at LLB

The disordered-materials diffractometer 7C2 at the Laboratoire Leon Brillouin (LLB), Saclay, France has been upgraded through the replacement of its detector. The old one, a banana like BF₃ detector, has been replaced by a new ensemble of ³He tubes arranged on a cylindrical surface concentric with the sample position, covering an angular range of 133°, with an angular step of 0.52°. At the shortest wavelength, 0.57 Å, this represents a maximum momentum transfer of 20 Å^-1. The gas pressure in the ³He tubes is 30 bars, which increases the efficiency to 80% for 0.72 Å neutrons, compared to 17% for the previous detector. A larger solid angle of detection and an improved efficiency have increased the total counting rate by a factor of 22, thereby reducing random error in the diffraction measurements. The banana-like multidetector gives to 7C2 the possibility of registering a complete diffractogram over the whole angular range in one shot, which is a comparative advantage with respect to other hot-neutrons two-axis diffractometers.