Backscattering Spectrometer - Equipment

The spectrum of neutrons in each pulse of the beam is modified by choppers. This beamline utilizes three bandwidth choppers, which consist of a disc that is suspended from kinematic mounts and rotates so that the outer edge passes through the path of the beam.

The installation of chopper 2 is shown in the picture to the left. When the chopper is lowered into the beam guide, neutrons can only pass through when the window (aligned down in the picture) has rotated into the path of the beam. The rest of the chopper disc acts as an absorber for the neutrons.

The heart of the work in a typical experiment is setting up the sample in the desired environment. A typical neutron sample ranging from a mm to a few cm is placed in a specialized cylindrical can and sealed. For liquids, the backscattering instrument often uses an annular can, created by placing a smaller can within a larger can and inserting the liquid sample between the two cans. This picture shows a Helium dewer cooling the environment encompassing the sample can, which has been lowered into the beam from the top of the scattering tank.

The backscattering spectrometer is defined by the reflection of specific neutron wavelengths by the analyzer crystals arrayed perpendicular to the paths of the scattered neutrons. Thin hand-sized hexagons of crystal which have been machined along a specified crystalline face are pressed and glued to a panel of spherical radius. The wall of the scattering tank is lined with the panels to form a spherical shell that Bragg reflects neutrons whose wavelengths match the crystalline spacing of the crystal chosen. One half of the backscattering tank is lined with 2mm thick silicon crystals cut along the 111 crystal plane.

Detector tubes filled with ³He are positioned along a cylindrical shell above and below the sample position at the end of the beam guide (entering from the right) to catch the neutrons backscattered from the crystals (behind the camera) that line the wall of the scattering tank. The picture shows the intial installation of the detectors above and below the sample position on one half of the tank. In the background to the right, a larger pack of detectors can be seen set up for a diffraction measurement in the empty half of the tank.

The experiment is controlled from a room at the top of the tank. The picture shows the real-time display of various views of detector information.