Effects of the sample environment and collimation in the background measurement

Most of diffractometers have collimators in front of detectors either to reduce the background contribution or to increase the angular resolution. These collimation devices are designed to collect only the neutrons coming from the sample position. When a sample environment is required, its walls are close to the sample position and produce some unwanted effects on the diffractograms. A simple geometrical model allows a description of the observed patterns in the two-axis diffractometer D4 (ILL, Grenoble, France). The calculated effects are compared with experimental data for a vanadium bar as measured in the standard cryostat of that instrument. This kind of measurements allows the experimental determination of the sample transmission.