# Radioactive source
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{{Short description|Radionuclide emitting ionizing radiation}}
{{redirect|Gamma-ray source|celestial gamma-ray sources|Gamma-ray astronomy}}
{{Use dmy dates|date=October 2025}}
[[File:HD.17.095 (11966576463).jpg|thumb|alt=A metal cylinder with a ruler next to it, 3.1 cm high |A new sealed [caesium-137](/source/caesium-137) radiation source as it appears in its final state]]
A '''radioactive source''' is a known quantity of a [radionuclide](/source/radionuclide) which emits [ionizing radiation](/source/ionizing_radiation), typically one or more of the radiation types [gamma ray](/source/gamma_ray)s, [alpha particle](/source/alpha_particle)s, [beta particle](/source/beta_particle)s, and [neutron radiation](/source/neutron_radiation).
Sources can be used for [irradiation](/source/irradiation), where the radiation performs a significant ionising function on a target material, or as a radiation [metrology](/source/metrology) source, which is used for the calibration of radiometric process and [radiation protection](/source/radiation_protection) instrumentation. They are also used for industrial process measurements, such as thickness gauging in the paper and steel industries. Sources can be sealed in a container (highly penetrating radiation) or deposited on a surface (weakly penetrating radiation), or they can be in a fluid.
As an irradiation source they are used in medicine for [radiation therapy](/source/radiation_therapy) and in industry for such as [industrial radiography](/source/industrial_radiography), [food irradiation](/source/food_irradiation), [sterilization](/source/sterilization_(microbiology)), [vermin](/source/vermin) disinfestation, and [irradiation crosslinking](/source/Cross-linked_polyethylene) of PVC.
Radionuclides are chosen according to the type and character of the radiation they emit, intensity of emission, and the [half-life](/source/half-life) of their decay. Common source radionuclides include [cobalt-60](/source/cobalt-60),[{{cite web |url=http://www.nordion.com/gamma-technologies/markets/c-188/ |title=C-188 Cobalt-60 Source |publisher=Nordion Inc |access-date=22 March 2016}}] [iridium-192](/source/iridium-192),[{{cite web |url=http://www.isoflex.com/iridium-192 |title=Iridium-192 |publisher=Isoflex |access-date=22 March 2016}}] and [strontium-90](/source/strontium-90).[{{cite web |url=http://practicalphysics.org/radioactive-sources-isotopes-and-availability.html |title=Radioactive sources: isotopes and availability |access-date=22 March 2016 |archive-date=24 January 2019 |archive-url=https://web.archive.org/web/20190124183607/http://practicalphysics.org/radioactive-sources-isotopes-and-availability.html |url-status=dead }}] The [SI](/source/SI) measurement quantity of source [activity](/source/radioactive_decay) is the [Becquerel](/source/Becquerel), though the historical unit [Curies](/source/Curie_(unit)) is still in partial use, such as in the US, despite their [NIST](/source/National_Institute_of_Standards_and_Technology) strongly advising the use of the SI unit.[{{cite web |url=https://www.nist.gov/pml/pubs/sp811/sec05.cfm#52 |title=NIST Guide to the SI, Chapter 5 (paragraph 5.2) |publisher=NIST |access-date=22 March 2016}}] The SI unit for health purposes is mandatory in the EU.
An irradiation source typically lasts for between 5 and 15 years before its activity drops below useful levels.[{{Cite book|url=http://www-pub.iaea.org/MTCD/Publications/PDF/TRS436_web.pdf|title=Disposal Options for Disused Radioactive Sources|publisher=International Atomic Energy Agency|year=2005|isbn=92-0-100305-6|issn=0074-1914}}] However sources with long half-life radionuclides when used as calibration sources can be used for much longer.
[[File:Teletherapy Capsule2.svg|thumb|alt=A cutaway diagram of a teletherapy capsule |A cutaway diagram of a radioactive source used for teletherapy ([external beam radiotherapy](/source/external_beam_radiotherapy)): A key to the lettering can be found on the file page]]
==Sealed sources==
Many radioactive sources are sealed, meaning they are permanently either completely contained in a capsule or firmly bonded solid to a surface. Capsules are usually made of [stainless steel](/source/stainless_steel), [titanium](/source/titanium), [platinum](/source/platinum) or another inert metal.[{{cite web |url=http://www.onr.org.uk/hass.htm |title=Implementation of the Control of High-activity Sealed Radioactive Sources and Orphan Sources (HASS) directive for nuclear licensed sites |access-date=22 March 2016}}] but the container is not intended to attenuate radiation, so further shielding is required for radiation protection.[{{cite web |url=https://www.iaea.org/OurWork/ST/NE/NEFW/Technical-Areas/WTS/sealedsources-sealedsources.html |title=Disused Sealed Source Management |publisher=International Atomic Energy Agency |access-date=22 March 2016}}] Sealed sources are used in almost all applications where the source does not need to be chemically or physically included in a liquid or gas.
=== Categorisation of sealed sources ===
[[File:Logo iso radiation.svg|thumb|2007 ISO radioactivity danger symbol intended for IAEA Category 1, 2 and 3 sources defined as dangerous sources capable of causing death or serious injury.[[http://www.iaea.org/newscenter/news/2007/radiationsymbol.html IAEA news release Feb 2007]]]]
Source:[{{Cite book|url=http://www-pub.iaea.org/MTCD/publications/PDF/Pub1578_web-57265295.pdf|title=Radiation protection and safety of radiation sources : International basic safety standards|publisher=International Atomic Energy Agency|year=2014|isbn=978-92-0-135310-8|location=Vienna|issn=1020-525X}}]
Sealed sources are categorised by the [IAEA](/source/IAEA) according to their activity in relation to a minimum dangerous source (where a dangerous source is one that could cause significant injury to humans). The ratio used is A/D, where A is the activity of the source and D is the minimum dangerous activity.
{| class="wikitable"
!Category
!A/D
|-
|1
|≥1000
|-
|2
|10–1000
|-
|3
|1–10
|-
|4
|0.01–1
|-
|5
|<0.01
|}
Note that sources with sufficiently low radioactive output (such as those used in [Smoke detector](/source/Smoke_detector)s) as to not cause harm to humans are not categorised.
==Calibration sources==
thumb|Hand-held large area alpha scintillation probe under calibration using a plate source
Calibration sources are used primarily for the calibration of radiometric instrumentation, which is used on process monitoring or in radiological protection.
Capsule sources, where the radiation effectively emits from a point, are used for beta, gamma and X-ray instrument calibration. High level sources are normally used in a calibration cell: a room with thick walls to protect the operator and the provision of remote operation of the source exposure.
The plate source is in common use for the [calibration](/source/Counts_per_minute) of radioactive contamination instruments. This has a known amount of radioactive material fixed to its surface, such as an alpha and/or beta emitter, to allow the calibration of large area radiation detectors used for contamination surveys and personnel monitoring. Such measurements are typically counts per unit time received by the detector, such as [counts per minute](/source/counts_per_minute) or counts per second.
Unlike the capsule source, the plate source emitting material must be on the surface to prevent attenuation by a container or self-shielding due to the material itself. This is particularly important with alpha particles which are easily stopped by a small mass. The [Bragg curve](/source/Bragg_curve) shows the attenuation effect in free air.
==Unsealed sources==
[Unsealed sources](/source/Unsealed_source_radiotherapy) are sources that are not in a permanently sealed container, and are used extensively for medical purposes.[{{cite web |url=http://www.ionactive.co.uk/glossary/Unsealed_Source.html |title=Radiation Protection Glossary |access-date=22 March 2016 |archive-date=5 October 2017 |archive-url=https://web.archive.org/web/20171005174853/http://www.ionactive.co.uk/glossary/Unsealed_Source.html |url-status=dead }}] They are used when the source needs to be dissolved in a liquid for injection into a patient or ingestion by the patient. Unsealed sources are also used in industry in a similar manner for leak detection as a [radioactive tracer](/source/radioactive_tracer).
== Disposal ==
Disposal of expired radioactive sources presents similar challenges to the disposal of other [nuclear waste](/source/Radioactive_waste), although to a lesser degree. Spent low level sources will sometimes be sufficiently inactive that they are suitable for disposal via normal waste disposal methods – usually landfill. Other disposal methods are similar to those for higher-level radioactive waste, using various depths of [borehole](/source/borehole) depending on the activity of the waste.