Unlike contamination, radiation cannot be spread by any medium. It travels through materials until it loses its energy. Exposure to ionizing does not necessary mean, that the object becomes radioactive (except very rare neutron radiation). In order to become radioactive, you have to contain some radioactive material. In general, there are two ways, how to become radioactive:
- Exposure to neutron radiation. Neutrons interact only with atomic nuclei and may be captured by target nucleus. This reaction is known as the radiative capture and this reaction makes matter radioactive. The radioactive decay of these produced radionuclides is specific for each element (nuclide). Each nuclide emits the characteristic gamma rays. Significant neutron radiation is generally only found inside nuclear reactors. But it does not mean, you cannot meet neutrons in everyday life. Neutrons are also produced in the upper atmosphere. Cosmic rays interact with nuclei in the atmosphere, and produce also high-energy neutrons. According to UNSCEAR the fluency of neutrons is 0.0123 cm-2s–1 at sea level for a geomagnetic latitude of 45 N. Based on this, the effective annual dose from neutrons at sea level and at 50 degree latitude is estimated to be 0.08 mSv (8 mrem).
- Intake of radioactive material. The intake of radioactive material can occur through various pathways such as ingestion of radioactive contamination in food or liquids. This is the principle behind the medical use (nuclear medicine) of many radioactive materials, as it aids in imaging, diagnosis, and other areas. Nuclear medicine uses a very small amounts of radioactive materials called radiotracers, which are taken internally, for example, intravenously or orally. Between the short half-lives of the elements involved and the body’s natural means of disposing of many radioactive elements, a person’s individual radioactivity is usually short-lived.
What’s the answer? A person becomes ‘radioactive’ if dust particles containing various radioisotopes land on the person’s skin or garments. This is contamination. Once a person has been decontaminated by clothes removal and dermal scrubbing, all of the particulate radioactivity sources are eliminated, and the individual is no longer contaminated. But a person exposed to radiation will not become radioactive. Alpha, beta, gamma radiations cannot activate target nuclei, since they interact primarily with atomic electrons. Therefore, most types of radiation cannot activate any material. Radiation travels through materials until it loses its energy. After this, materials remain inactive. Though, as it is said. The human body is already radioactive.
Another very important point is that all people also have some radioactive isotopes inside their bodies from birth. These isotopes are especially potassium-40, carbon-14 and also the isotopes of uranium and thorium. The variation in radiation dose from one person to another is not as great as the variation in dose from cosmic and terrestrial sources. The average annual radiation dose to a person from internal radioactive materials other than radon is about 0.3 mSv/year of which:
- 2 mSv/year comes from potassium-40,
- 0.12 mSv/year comes from the uranium and thorium series,
- 12 μSv/year comes from carbon-40.
As can be seen, major contributor is potassium-40. The potassium concentration in the human body is strictly based on the homeostatic principle. Potassium is more or less distributed in the body (especially in soft tissues) following intake in foods. A 70-kg man contains about 126 g of potassium (0.18%), most of that is located in muscles. The daily consumption of potassium is approximately 2.5 gram. Hence the concentration of potassium-40 is nearly stable in all persons at a level of about 55 Bq/kg (3850 Bq in total), which corresponds to the annual effective dose of 0.2 mSv. In addition, sleeping next to someone also causes a radiation dose of about 0.05 µSv.
See also: Internal Source of Radiation