Collimator

In neutron, X-ray and gamma ray optics, a collimator is a device that filters a stream of rays so that only those traveling parallel to a specified direction are allowed through. Collimators are used in neutron, X-ray, and gamma-ray optics because it is not yet possible to focus radiation with such short wavelengths into an image through the use of lenses as is routine with electromagnetic radiation at optical or near-optical wavelengths.

Overview


The figure to the right illustrates how a Söller collimator is used in neutron and X-ray machines. The upper panel shows a situation where a collimator is not used, while the lower panel introduces a collimator. In both panels the source of radiation is to the right, and the image is recorded on the gray plate at the left of the panels.

Without a collimator rays from all directions will be recorded; for example, a ray that has passed through the top of the specimen (to the right of the diagram) but happens to be travelling in a downwards direction may be recorded at the bottom of the plate. The resultant image will be so blurred and indistinct as to be useless.

In the lower panel of the figure, a collimator has been added (blue bars). This is a sheet of lead or other material opaque to the incoming radiation with many tiny holes bored through it. Only rays that are travelling nearly parallel to the holes will pass through them&mdash;any others will be absorbed by hitting the plate surface or the side of a hole. This ensures that rays are recorded in their proper place on the plate, producing a clear image.

Although collimators improve the resolution, by blocking incoming radiation they also reduce the intensity of the signal, a property that would not be desired for remote sensing instruments that are detecting very small signals as it is. For that reason, the gamma ray spectrometer on Mars Odyssey which is currently orbiting over Mars, for example, is a non-collimated instrument. Most lead collimators let less than 1% of incident photons through. For this reason, attempts have been made to replace collimators with electronic analysis.