Collimator

A collimator is a specialized device used to narrow a beam of particles or waves.

This narrowing occurs in two distinct ways. First, it can align the directions of motion so that rays become parallel. This is often called making light collimated. Second, it can act as a beam limiting device to reduce the spatial cross section of the beam. By controlling these beams, scientists can focus energy and improve the clarity of images and measurements.

In the field of optics, a collimator often uses a curved mirror or a lens. The device typically includes a light source or an image placed at its focus. This setup allows the device to replicate a target focused at infinity with very little parallax.

Optical collimators are also used for calibration. They help check if optical elements are aligned on the optical axis. They can also set elements at the proper focus. This is useful for aligning devices like binoculars or gun sights. Some collimator sights use a reticle, which is a crosshair, at the focus so the viewer sees an image of it.

When dealing with X-rays, gamma rays, or neutrons, the mechanism changes. It is difficult to use lenses to focus these types of radiation. Instead, a collimator acts as a filter to allow only rays traveling in a specific direction to pass through.

For example, a Söller collimator might use a sheet of lead with many tiny holes bored through it. Only rays traveling nearly parallel to these holes can pass through. Any other rays will hit the sides of the holes and be absorbed. This prevents rays from all directions from being recorded, which would create a blurry and useless image.

For neutrons, a collimator might use a sandwich arrangement. This can be several feet long and consists of alternating layers. One layer is a neutron-absorbing material, such as gadolinium. The other layer is a neutron-transmitting material, like air or aluminum. If the assembly rotates, the curvature can allow only one specific energy of neutrons to pass. This process allows for both energy selection and collimation, ensuring rays are recorded in their proper place.

History shows the long-standing importance of these tools in science. The English physicist Henry Kater invented the floating collimator. He reported his invention in January 1825. This device provided a great service to the field of practical astronomy. Kater noted that his work built upon previous efforts by Carl Friedrich Gauss and Friedrich Bessel. Later, in May 1921, the United States Army Ordnance Department documented the use of collimators in fire control instruments.

Today, collimators are vital in industrial and medical settings. In industrial radiography, workers use tungsten collimators to inspect materials for defects. These devices use gamma radiation sources like iridium-192 or cobalt-60. A collimator is rated by how many half value layers, or HVL, it contains. These layers reduce undesirable radiation by half. For instance, a 4 HVL tungsten collimator has walls that reduce radiation intensity by 88.5%.

In hospitals, linear accelerators use collimators for radiation therapy. These machines use primary and secondary collimators to shape the beam. Some advanced systems use multileaf collimators, or MLCs, which contain 50 to 120 heavy-metal plates that slide to form a specific shape.

Despite their benefits, collimators have a significant limitation. While they improve resolution, they also reduce the intensity of the beam. This happens because they must block many incoming rays to keep the beam straight. Most lead collimators allow less than 1% of incident photons to pass through. Because of this loss of strength, some remote sensing instruments avoid them. For example, the gamma ray spectrometer on the Mars Odyssey is a non-collimated instrument to maintain high sensitivity.