There are three types of machines used for radiation therapy. An Orthovoltage machine uses electricity to produce low energy x-rays. These machines are used primarily for skin tumors and have a short focus to skin distance. A Cobalt-60 machine uses a radioactive isotope that produces gamma photons as the source decays. The cobalt source has a half-life of 5.27 years and should be replaced about every five years. The focus to skin distance for a cobalt machine is usually 80cm.
http://vetmed.illinois.edu/4dvms/documents/imaging/RadTherap...

Cobalt Machine
The radionuclide answer to megavoltage treatment is the cobalt machine. The cobalt source is made by putting a 2cm pellet of cobalt-59 into a nuclear reactor and irradiated it with neutrons.
http://radonc.wikidot.com/cobalt-machine

Introduction to Teletherapy & Cobalt Machine
http://www.slideshare.net/abishadh/cobalt60-external-beam-ra...

What are the uses of cobalt-60?
Cobalt-60 is widely used as a medical and industrial radiation source. Medical use consists primarily of cancer radiotherapy.
Industrial uses include testing of welds and castings, and a large variety of measurement and test instruments including leveling devices and thickness gauges. It is also used to sterilize instruments, and to irradiate food to kill microbes and prevent spoilage.
http://www.epa.gov/superfund/health/contaminants/radiation/p...

What is cobalt-60 used for?
Cobalt-60 is used in many common industrial applications, such as in leveling devices and thickness gauges, and in radiotherapy in hospitals. (...) Cobalt-60 is also used for industrial radiography, a process similar to an x-ray, to detect structural flaws in metal parts. One of its uses is in a medical device for the precise treatment of otherwise inoperable deformities of blood vessels and brain tumors. Radionuclides, such as cobalt-60, that are used in industry or medical treatment are encased in shielded metal containers or housings, and are referred to as radiation ‘sources.' The shielding keeps operators from being exposed to the strong radiation.
http://www.epa.gov/radiation/radionuclides/cobalt.html

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Note added at 9 hrs (2013-05-10 01:47:03 GMT)
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Eszembe jutott, hogy talán jól jöhet legalább egy magyar hivatkozás ezzel kapcsolatban (nem mintha nem tudnál magad is guglizni :)), itt egy kobaltágyú, szép nagy képpel:
http://sugarterapia.blog.hu/tags/kobaltágyú

Magfizikában, részecskefizikában az "ágyú" "gun" szokott lenni, pl. elektronágyú = electron gun, http://en.wikipedia.org/wiki/Electron_gun

Mivel az elv hasonló, csak itt elektron helyett kobaltizotópot használnak, a "cobalt gun" fordítás kézenfekvő. Az interneten számos példát találsz rá; javarészük a kelet-európai vonatkozású. Nem véletlenül, amint az a következő 50 éves dokumentumból is kiderül: http://www.iaea.org/About/Policy/GC/GC07/GC07Records/English...