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Ionizing & Non-Ionizing Radiation
Understanding Radiation:  

Other Modes of Radioactive Decay

Ionizing & Non-Ionizing Radiation

Although alpha, beta, and gamma radiation are the most common methods of radioactive decay, unstable nuclei throw off excess energy in other ways as well.

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Neutron Radiation

Neutron radiation is energy released from an atom in the form of neutral particles called neutrons. Neutrons are part of the basic building blocks of atoms. They have no charge and are about the same mass as a proton. Due to ion-producing collisions with matter and absorption/decay processes, neutrons are a type of ionizing radiation. They were discovered by James Chadwick, who received the 1935 Nobel Prize in physics for his work.

During the fission process, as well as certain decay processes, neutrons are emitted from the nucleus of an atom. This is neutron radiation. Combining alpha-emitting isotopes with beryllium produces a neutron source. Accelerators are another means of producing neutron radiation. In the upper atmosphere, the interaction of cosmic radiation with air also produces neutron radiation.

Neutron radiation is used by researchers to investigate the sub-atomic structure of matter. It has been used in the security arena as part of a technology that can investigate the existence of explosives and other dangerous items. The medical community relies on neutron radiation for production of medical isotopes and certain direct therapies.

Significant neutron radiation in the human environment is rare. Possibilities include an improperly handled neutron source and an unexpected criticality accident. Neither of these occurrences are likely in an area of human population due to the secure locations where these materials are handled.

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Positron Decay

A positron is a particle that has the same mass as an electron but has a positive charge. Positron decay may occur when there are too many protons in the nucleus

In this case, a proton is converted into a neutron, and nucleus emits a positron. This increases the number of neutrons by one, decreases the number of protons by one, and leaves the atomic mass unchanged. By changing the numbers of protons, however, positron decay transforms the nuclide into a different element.

Electron Capture

Electron capture will occur when there are too many protons in the nucleus, and there isn't enough energy to emit a positron.

In this case, one of the orbital electrons is captured by a proton in the nucleus, forming a neutron. Since the proton is essentially changed to a neutron, the number of neutrons increases by 1, the number of protons decreases by 1, and the atomic mass remains unchanged. By changing the number or protons, electron capture transforms the nuclide into a new element.

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Isomeric Transition

If, after previous attempts at stabilization, the nucleus still has excess energy, it can emit energy without changing the number of protons or neutrons. Processes that accomplish this are called "isomeric transition".

Isomeric transition can occur through the emission of a gamma ray or through internal conversion. Internal conversion is an alternate mechanism of shedding excess energy. In internal conversion, the excess energy of the nucleus is transmitted to one of the orbital electrons, and the electron may be ejected from the atom. This process usually competes with gamma radiation. Internal conversion can occur only if the amount of energy given to the orbital electron exceeds its binding energy.

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