Ionization - DOE-HDBK-1109-97

DOE-HDBK-1109-97

Radiological Safety Training for Radiation-Producing (X-Ray) Devices

Student's Guide

IONIZATION

An atom usually has a number of electrons equal to the number of protons in its nucleus so

that the atom is electrically neutral. A charged atom, called an ion, can have a positive or

negative charge. Free electrons also are called ions. An ion is formed when ionizing radiation

interacts with an orbiting electron and causes it to be ejected from its orbit, a process called

ionization. This leaves a positively charged atom (or molecule) and a free electron.

RADIATION

Radiation as used here means alpha particles, beta particles, gamma rays, X-rays, neutrons,

high-speed electrons, high-speed protons, and other particles capable of producing ions.

Radiation with enough energy to cause ionization is referred to as ionizing radiation.

Radiation that lacks the energy to cause ionization is referred to as non-ionizing radiation.

Examples of non-ionizing radiation include radio waves, microwaves, and visible light.

For radiation-protection purposes, ionization is important because it affects chemical and

biological processes and allows the detection of radiation.

For most radiation-protection situations, ionizing radiation takes the form of alpha, beta, and

neutron particles, and gamma and X-ray photons.

X-rays and gamma rays are a form of electromagnetic radiation. X-rays differ from gamma

rays in their point of origin. Gamma rays originate from within the atomic nucleus, whereas

X-rays originate from the electrons outside the nucleus and from free electrons decelerating in

the vicinity of atoms (i.e., bremsstrahlung). Module 102 discusses how X-rays are produced.

UNITS

Roentgen (R), a measure of radiation exposure, is defined by ionization in air.

Rad, a measure of the energy absorbed per unit mass. It is defined for any absorbing

material.