In nuclear physics the charged-particle equilibrium occurs when the number charged particle

Charged particle

In physics, a charged particle is a particle with an electric charge. It may be either a subatomic particle or an ion. A collection of charged particles, or even a gas containing a proportion of charged particles, is called a plasma, which is called the fourth state of matter because its...

s, of each type, leaving a volume are equal to the number entering. This equilibrium exists for a volume in a medium when, for a given volume of medium, every charged particle leaving is countered by another charged particle of the same kind and energy entering. This equilibrium value can vary within a medium.

This equilibrium occurs when a material is under 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...

 and results in a constant charge

Charge (physics)

In physics, a charge may refer to one of many different quantities, such as the electric charge in electromagnetism or the color charge in quantum chromodynamics. Charges are associated with conserved quantum numbers.-Formal definition:...

.