Induced radioactivity
Encyclopedia
Induced radioactivity occurs when a previously stable material has been made radioactive by exposure to specific radiation
. Most radioactivity does not induce other material to become radioactive.
Neutron activation
is the main form of induced radioactivity, which happens when free neutron
s are captured by nuclei. This new heavier isotope can be stable or unstable (radioactive) depending on the chemical element
involved.
Because free neutrons disintegrate within minutes outside of an atomic nucleus
, neutron radiation
can be obtained only from nuclear disintegrations, nuclear reaction
s, and high-energy reactions (such as in cosmic radiation showers or particle accelerator
collisions). Neutrons that have been slowed down through a neutron moderator
(thermal neutrons) are more likely to be captured by nuclei than fast neutrons.
A less common form involves removing a neutron via photodisintegration
. In this reaction, a high energy photon (gamma ray
) strikes a nucleus with an energy greater than the binding energy
of the atom
, releasing a neutron. This reaction has a minimum cutoff of 2 MeV
(for hydrogen
) and around 10 MeV for most heavy nuclei. Many radionuclides do not produce gamma rays with energy high enough to induce this reaction.
The isotope
s used in food irradiation
(cobalt
-60, caesium
-137) both have energy peaks below this cutoff and thus cannot induce radioactivity in the food.
Some induced radioactivity is produced by background radiation
, which is mostly natural. However, since natural radiation is not very intense in most places on Earth
, the amount of induced radioactivity in a single location is usually very small.
The conditions inside certain types of nuclear reactor
s with high neutron flux
can cause induced radioactivity. The components in those reactors may become highly radioactive from the radiation to which they are exposed. Induced radioactivity increases the amount of nuclear waste that must eventually be disposed, but it is not referred to as radioactive contamination
unless it is uncontrolled.
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
. Most radioactivity does not induce other material to become radioactive.
Neutron activation
Neutron activation
Neutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus often decays immediately by emitting particles such as neutrons, protons, or alpha...
is the main form of induced radioactivity, which happens when free neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s are captured by nuclei. This new heavier isotope can be stable or unstable (radioactive) depending on the chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
involved.
Because free neutrons disintegrate within minutes outside of an atomic nucleus
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
, neutron radiation
Neutron radiation
Neutron radiation is a kind of ionizing radiation which consists of free neutrons. A result of nuclear fission or nuclear fusion, it consists of the release of free neutrons from atoms, and these free neutrons react with nuclei of other atoms to form new isotopes, which, in turn, may produce...
can be obtained only from nuclear disintegrations, nuclear reaction
Nuclear reaction
In nuclear physics and nuclear chemistry, a nuclear reaction is semantically considered to be the process in which two nuclei, or else a nucleus of an atom and a subatomic particle from outside the atom, collide to produce products different from the initial particles...
s, and high-energy reactions (such as in cosmic radiation showers or particle accelerator
Particle accelerator
A particle accelerator is a device that uses electromagnetic fields to propel charged particles to high speeds and to contain them in well-defined beams. An ordinary CRT television set is a simple form of accelerator. There are two basic types: electrostatic and oscillating field accelerators.In...
collisions). Neutrons that have been slowed down through a neutron moderator
Neutron moderator
In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....
(thermal neutrons) are more likely to be captured by nuclei than fast neutrons.
A less common form involves removing a neutron via photodisintegration
Photodisintegration
Photodisintegration is a physical process in which an extremely high energy gamma ray interacts with an atomic nucleus and causes it to enter an excited state, which immediately decays by emitting a subatomic particle. A single proton or neutron is effectively knocked out of the nucleus by the...
. In this reaction, a high energy photon (gamma ray
Gamma ray
Gamma radiation, also known as gamma rays or hyphenated as gamma-rays and denoted as γ, is electromagnetic radiation of high frequency . Gamma rays are usually naturally produced on Earth by decay of high energy states in atomic nuclei...
) strikes a nucleus with an energy greater than the binding energy
Binding energy
Binding energy is the mechanical energy required to disassemble a whole into separate parts. A bound system typically has a lower potential energy than its constituent parts; this is what keeps the system together—often this means that energy is released upon the creation of a bound state...
of the atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
, releasing a neutron. This reaction has a minimum cutoff of 2 MeV
Electronvolt
In physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
(for hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
) and around 10 MeV for most heavy nuclei. Many radionuclides do not produce gamma rays with energy high enough to induce this reaction.
The isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
s used in food irradiation
Food irradiation
Food irradiation is the process of exposing food to ionizing radiation to destroy microorganisms, bacteria, viruses, or insects that might be present in the food. Further applications include sprout inhibition, delay of ripening, increase of juice yield, and improvement of re-hydration...
(cobalt
Cobalt
Cobalt is a chemical element with symbol Co and atomic number 27. It is found naturally only in chemically combined form. The free element, produced by reductive smelting, is a hard, lustrous, silver-gray metal....
-60, caesium
Caesium
Caesium or cesium is the chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-gold alkali metal with a melting point of 28 °C , which makes it one of only five elemental metals that are liquid at room temperature...
-137) both have energy peaks below this cutoff and thus cannot induce radioactivity in the food.
Some induced radioactivity is produced by background radiation
Background radiation
Background radiation is the ionizing radiation constantly present in the natural environment of the Earth, which is emitted by natural and artificial sources.-Overview:Both Natural and human-made background radiation varies by location....
, which is mostly natural. However, since natural radiation is not very intense in most places on Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, the amount of induced radioactivity in a single location is usually very small.
The conditions inside certain types of nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
s with high neutron flux
Neutron flux
The neutron flux is a quantity used in reactor physics corresponding to the total length travelled by all neutrons per unit time and volume . The neutron fluence is defined as the neutron flux integrated over a certain time period....
can cause induced radioactivity. The components in those reactors may become highly radioactive from the radiation to which they are exposed. Induced radioactivity increases the amount of nuclear waste that must eventually be disposed, but it is not referred to as radioactive contamination
Radioactive contamination
Radioactive contamination, also called radiological contamination, is radioactive substances on surfaces, or within solids, liquids or gases , where their presence is unintended or undesirable, or the process giving rise to their presence in such places...
unless it is uncontrolled.
See also
- Neutron activationNeutron activationNeutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus often decays immediately by emitting particles such as neutrons, protons, or alpha...
- Radioactive decayRadioactive decayRadioactive 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...
- Radioactivity
- Slow neutron