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

How a Söller collimator filters a stream of rays. Top: without a collimator. Bottom: with a collimator.

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—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.

Notes

• Collimators are used in linear accelerators used for radiotherapy treatments. They help to shape the beam of radiation emerging from the machine, they can limit the maximum field size of a beam.

The treatment head of a linear accelerator consists of both a primary and secondary collimator. The primary collimator is positioned after the electron beam has reached a vertical orientation. When using photons, it is placed after the beam has passed through the X-ray target. The secondary collimator is positioned after either a flattening filter (for photon therapy) or a scattering foil (for electron therapy). The secondary collimator consists of two jaws which can be moved to either enlarge or reduce the size of the treatment field. New systems involving multileaf collimators (MLCs) are used to further shape a beam to localise treatment fields in radiotherapy. MLCs consist of approximately 50–120 leaves of heavy, metal collimator plates which slide into place to form the desired field shape.

• Collimators are also used with radiation detectors in nuclear power stations for monitoring sources of radioactivity.

• In optics, a collimating lens is one that produces parallel rays of light. Collimators may be used with laser diodes and CO2 cutting lasers.

• Proper collimation of a laser source with long enough coherence length can be verified with a shearing interferometer.

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