An orphan source is a self-contained radioactive

Radioactive decay

Radioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...

 source that is no longer under proper regulatory control.

The United States Nuclear Regulatory Commission

Nuclear Regulatory Commission

The Nuclear Regulatory Commission is an independent agency of the United States government that was established by the Energy Reorganization Act of 1974 from the United States Atomic Energy Commission, and was first opened January 19, 1975...

 defines an orphan source more exactly as:

...a sealed source of radioactive material contained in a small volume--but not radioactively contaminated soils and bulk metals--in any one or more of the following conditions

• In an uncontrolled condition that requires removal to protect public health and safety from a radiological threat
• Controlled or uncontrolled, but for which a responsible party cannot be readily identified
• Controlled, but the material's continued security cannot be assured. If held by a licensee, the licensee has few or no options for, or is incapable of providing for, the safe disposition of the material
• In the possession of a person, not licensed to possess the material, who did not seek to possess the material
• In the possession of a State radiological protection program for the sole purpose of mitigating a radiological threat because the orphan source is in one of the conditions described in one of the first four bullets and for which the State does not have a means to provide for the material's appropriate disposition

Orphan source incidents

Most known orphan sources were, generally, small radioactive sources produced legitimately under governmental regulation and put into service for radiography

Radiography

Radiography is the use of X-rays to view a non-uniformly composed material such as the human body. By using the physical properties of the ray an image can be developed which displays areas of different density and composition....

, generating electricity in radioisotope thermoelectric generator

Radioisotope thermoelectric generator

A radioisotope thermoelectric generator is an electrical generator that obtains its power from radioactive decay. In such a device, the heat released by the decay of a suitable radioactive material is converted into electricity by the Seebeck effect using an array of thermocouples.RTGs can be...

s, medical radiotherapy or irradiation

Irradiation

Irradiation is the process by which an object is exposed to radiation. The exposure can originate from various sources, including natural sources. Most frequently the term refers to ionizing radiation, and to a level of radiation that will serve a specific purpose, rather than radiation exposure to...

. These sources were then "abandoned, lost, misplaced or stolen" and so no longer subject to proper regulation.

For example, in different incidents, various orphan sources have been:

• A caesium-137 based teletherapy unit left behind at Goiânia’s Instituto Goiano de Radioterapia (IGR)[3], located 1 km northwest of Praça Cívica, in 1985. One of the most disastrous orphan source incidents, this event is known as the Goiânia accident
  Goiânia accident
  The Goiânia accident was a radioactive contamination accident that occurred on September 13, 1987, at Goiânia, in the Brazilian State of Goiás after an old radiotherapy source was taken from an abandoned hospital site in the city...
  
  .
• stolen from an RTG
  Radioisotope thermoelectric generator
  A radioisotope thermoelectric generator is an electrical generator that obtains its power from radioactive decay. In such a device, the heat released by the decay of a suitable radioactive material is converted into electricity by the Seebeck effect using an array of thermocouples.RTGs can be...
  
   in a Russian lighthouse and then recovered 50 kilometres away at a bus station (Leningrad, Russia, 1999)
• lost during radiography and taken home by other people who initially failed to recognize the source (Morocco
  Morocco
  Morocco , officially the Kingdom of Morocco , is a country located in North Africa. It has a population of more than 32 million and an area of 710,850 km², and also primarily administers the disputed region of the Western Sahara...
  
  , 1984)
• lost during radiography at a power plant and found by an unsuspecting worker who put the source in his chest pocket, Gilan, Iran, 1996
• source sold to a junkyard for its lead
  Lead
  Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
  
   container, İkitelli, in the western outskirts of Istanbul
  Istanbul
  Istanbul , historically known as Byzantium and Constantinople , is the largest city of Turkey. Istanbul metropolitan province had 13.26 million people living in it as of December, 2010, which is 18% of Turkey's population and the 3rd largest metropolitan area in Europe after London and...
  
  , Turkey, 1999
• source taken home by the person who found the source after it was lost, Mit Halfa, 15km north of Cairo
  Cairo
  Cairo , is the capital of Egypt and the largest city in the Arab world and Africa, and the 16th largest metropolitan area in the world. Nicknamed "The City of a Thousand Minarets" for its preponderance of Islamic architecture, Cairo has long been a centre of the region's political and cultural life...
  
  , Qaluobiya, Egypt, 2000
• an orphan source was discovered in Karachi, Pakistan, within the vicinity of the OGDCL (Oil & Gas Development Company Limited) in 2008. Two containers were found buried which were suspected to be left over from Soviet oil drilling operations before the OGDCL took over in late 60s.
• an orphan source caused the death of one worker and irradiated seven others in a scrap yards at Mayapuri in India.