What is neutron activation?

Neutron activation (bombardment) is the means in which two very important industrial radiographic sources, Cobalt-59 (Co-59) and Iridium-191 (Ir-191), are produced. Cobalt and Iridium exist in nature as stable elements. Cobalt is element 27 and Iridium is element 77 on the periodic table. In the production of a radiographic source the Cobalt and Iridium materials are usually formed into small metal pellets, typically ranging in size from 1 mm by 1 mm to 3 mm by 3 mm, or formed into 0.04 in. by 0.04 in. to 0.12 in. by 0.12 in.. The size of the radioisotope may vary significantly depending on the application.

An example of a very small radioisotope source is those utilized for medical treatment or research. The pellets are encapsulated in stainless steel containers to form radiographic sources. Exposing these elements to a large thermal neutron flux (neutrons with energies less than 0.4 eV) enables the stable element to capture a thermal neutron and thus becoming one mass unit heavier. Immediately after the neutron is absorbed by the nucleus, the energy that binds the neutron is lost as a prompt or capture gamma ray. This is not the gamma ray utilized for radiography but purely a byproduct of the production process. It is the radioactivity that the element now possesses that generates the gamma rays for radiography purposes. Remember, radioactivity is the result of radioactive decay. The following equation expresses one example of CO-59 activation:

Where: Co-60 is the radioisotope, symbol 103 \f "Symbol" \s 12g represents the gamma emission, and symbol 98 \f "Symbol" \s 12b represents the beta emission. Remember, both energy and matter are part of radioactive decay, and due to this decay the element transmutes to another different element. In this case C0-60 transmutes to Nickel-60, (Nickel).

What do all those symbols mean?

Bombardment reactions are commonly designated by the symbols for the incident and emitted matter and energy, such as the above representation for CO-59 The incident matter is the neutron (n), the emitted matter and energy is the beta particle (symbol 98 \f "Symbol" \s 12b), and the gamma ray (symbol 103 \f "Symbol" \s 12g). In bombardment reactions, the atomic mass number of the target nucleus may increase or decrease as illustrated by the CO-59 example. Notice that the atomic number of the isotope is the same as the stable element, but what about the atomic mass number?


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