collimator

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WordNet

a small telescope attached to a large telescope to use in setting the line of the larger one
optical device consisting of a tube containing a convex achromatic lens at one end and a slit at the other with the slit at the focus of the lens; light rays leave the slit as a parallel beam

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/07/14 15:35:31」(JST)

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Example of a particle collimator

A collimator is a device that narrows a beam of particles or waves. To "narrow" can mean either to cause the directions of motion to become more aligned in a specific direction (i.e., collimated or parallel) or to cause the spatial cross section of the beam to become smaller.

History

An English physicist Henry Kater was the inventor of the floating collimator, which rendered a great service to practical astronomy. He reported about his invention in January 1825.^[1] In his report, Kater mentioned previous work in this area by Carl Friedrich Gauss and Friedrich Bessel.

Optical collimators

An example of an optical collimator with a bulb, an aperture (B), and a positive lens (L)

In optics, a collimator may consist of a curved mirror or lens with some type of light source and/or an image at its focus. This can be used to replicate a target at infinity without parallax.

Optical collimators can be used to calibrate other optical devices,^[2] to check if all elements are aligned on the optical axis, to set elements at proper focus, or to align two or more devices such as binoculars and gun barrels/gunsights.^[3] A surveying camera may be collimated by setting its fiduciary markers so that they define the principal point, as in photogrammetry.

Optical collimators are also used as gun sights in the collimator sight, which is a simple optical collimator with a cross hair or some other reticle at its focus. The viewer only sees an image of the reticle and has to use either with both eyes open while one looking into the collimator sight, with one eye open and moving the head to alternately see the sight and then at the target, or use one eye to partially see the sight and target at the same time.^[4] Adding a beam splitter allows the viewer to see the reticle and the field of view, making a reflector sight.

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.

Neutron, X-ray and gamma ray collimators

Collimators used to record gamma rays and neutrons from a nuclear test.

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

Applications

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 may be a sheet of lead or other material opaque to the incoming radiation with many tiny holes bored through it or in the case of neutrons it can be a sandwich arrangement (which can be up to several feet long - see ENGIN-X) with many layers alternating between neutron absorbing material (e.g. Gadolinium) with neutron transmitting material. This can be something simple e.g. air. or if mechanical strength is needed then aluminium may be used. If this forms part of a rotating assembly, the sandwich may be curved. This allows energy selection in addition to collimation - the curvature of the collimator and its rotation will present a straight path only to one energy of neutrons. 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.

For industrial radiography using gamma radiation sources such as Iridium-192 or Cobalt-60, a collimator allows the radiographer to control the exposure of radiation to expose a film and create a radiographic "negative", a.k.a., a radiograph, to inspect materials for defects. A collimator in this instance is most commonly made out of tungsten, and is rated according to how many half value layers it contains, i.e., how many times it reduces undesirable radiation by half. For instance, the thinnest walls on the sides of a 4 HVL tungsten collimator 0.52" thick will reduce the intensity of radiation passing through them by 88.5%. The shape of these collimators allows the radiographer to direct the radiation to the film and away from other workers.

Limitations

Collimator for a neutron stream, University of Washington cyclotron

Although collimators improve resolution, they also reduce intensity by blocking incoming radiation, which is undesirable for remote sensing instruments that require high sensitivity. For this reason, the gamma ray spectrometer on Mars Odyssey is a non-collimated instrument. Most lead collimators let less than 1% of incident photons through. Attempts have been made to replace collimators with electronic analysis.^{[citation needed]}

In radiation therapy

Collimators are used in linear accelerators used for radiotherapy treatments. They help to shape the beam of radiation emerging from the machine and 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 minimize 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.

References

^ The Description of a Floating Collimator. By Captain Henry Kater. Read January 13, 1825. [Phil. Trans. 1825, p. 147.]
^ "Collimators and Auto Collimators" by Ron Dexter
^ WIPO "Magnetic lightweight collimator"
^ Elementary optics and applications to fire control instruments: May, 1921 By United States. Army. Ordnance Dept, page 84

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English Journal

A calibration transmission method to determine the gamma-ray linear attenuation coefficient without a collimator.

Byun JI1, Yun JY2.
Applied radiation and isotopes : including data, instrumentation and methods for use in agriculture, industry and medicine.Appl Radiat Isot.2015 Aug;102:70-3. doi: 10.1016/j.apradiso.2015.05.006. Epub 2015 May 12.
It is shown that the gamma-ray linear attenuation coefficient of a sample with unknown chemical composition can be determined through a systematic calibration of the correlation between the linear attenuation coefficient, gamma-ray energy and the relative degree of attenuation. For calibration, H2O,
PMID 25997111

Improved Detection of Sentinel Lymph Nodes in SPECT/CT Images Acquired Using a Low- to Medium-Energy General-Purpose Collimator: Reply to the Editor.

Yoneyama H1.
Clinical nuclear medicine.Clin Nucl Med.2015 Jul;40(7):612-3. doi: 10.1097/RLU.0000000000000725.
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Development of a 3D optical scanner for evaluating patient-specific dose distributions.

Chang KH1, Lee S2, Jung H3, Choo YW3, Cao YJ1, Shim JB1, Kim KH4, Lee NK1, Park YJ1, Kim CY1, Cho SJ5, Lee SH6, Min CK7, Kim WC7, Cho KH7, Huh HD8, Lim S9.
Physica medica : PM : an international journal devoted to the applications of physics to medicine and biology : official journal of the Italian Association of Biomedical Physics (AIFB).Phys Med.2015 Jul;31(5):553-9. doi: 10.1016/j.ejmp.2015.05.009. Epub 2015 Jun 3.
PURPOSE: This paper describes the hardware and software characteristics of a 3D optical scanner (P3DS) developed in-house. The P3DS consists of an LED light source, diffuse screen, step motor, CCD camera, and scanner management software with 3D reconstructed software.MATERIALS AND METHOD: We perform
PMID 26048682

Reduced Lung Function in Children Associated with 137Cesium Body Burden.

Svendsen ER1, Kolpakov IE2, Karmaus WJ3, Mohr LC4, Vdovenko VY5, McMahon DM6, Jelin BA7, Stepanova YI8.
Annals of the American Thoracic Society.Ann Am Thorac Soc.2015 Jun 13. [Epub ahead of print]
RATIONALE: We previously reported that obstructive and restrictive lung function findings were associated with radioactive 137Cesium (137Cs) soil contamination from the 1986 Chernobyl disaster in a pediatric cohort residing in the Narodichesky district of Ukraine from 1993-98.OBJECTIVES: To determ
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Japanese Journal

Multicenter cross-calibration of I-123 metaiodobenzylguanidine heart-tomediastinum ratios to overcome camera-collimator variations

Nakajima Kenichi,Okuda Koichi,Yoshimura Mana,Matsuo Shinro,Wakabayashi Hiroshi,Imanishi Yasuhiro,Kinuya Seigo
Journal of Nuclear Cardiology 21(5), 970-978, 2014-10
… Background The heart-to-mediastinum ratio (HMR) of 123I-metaiodobenzylguanidine (MIBG) showed variations among institutions and needs to be standardized among various scinticamera-collimator combinations. … Based on phantom studies, a conversion coefficient for each camera-collimator system was created, including low-energy (LE, n = 125) and a medium-energy (ME, n = 100) collimators. …
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見田秀次,大西英雄,中本健太
日本放射線技術学会雑誌 70(7), 662-669, 2014-07
NAID 40020172846

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Synthesiology = 構成学 7(2), 118-128, 2014-05
NAID 40020165459

Performance of the beam collimation system in the J-PARC RCS (The 12th International Conference on Radiation Shielding (ICRS-12) and the 17th Topical Meeting of the Radiation Protection and Shielding Division of ANS (RPSD-2012)) -- (Accelerator Facilities)