What is Ionizing Radiation?

Introduction - Waves and Particles

Energy emitted from a source is generally referred to as radiation. Examples include heat or light from the sun, microwaves from an oven, X rays from an X-ray tube, and gamma rays from radioactive elements

Ionizing Radiation

Ionizing radiation is radiation with enough energy so that during an interaction with an atom, it can remove tightly bound electrons from the orbit of an atom, causing the atom to become charged or ionized.

Here we are concerned with only one type of radiation, ionizing radiation, which occurs in two forms - waves or particles.

Forms of electromagnetic radiation. These differ only in frequency and wave length.

• Heat waves
• Radiowaves
• Infrared light
• Visible light
• Ultraviolet light
• X rays
• Gamma rays

Longer wave length, lower frequency waves (heat and radio) have less energy than shorter wave length, higher frequency waves (X and gamma rays). Not all electromagnetic (EM) radiation is ionizing. Only the high frequency portion of the electromagnetic spectrum which includes X rays and gamma rays is ionizing.

Waves

Most of the more familiar types of electromagnetic radiation (e.g. visible light, radio waves) exhibit “wave-like” behavior in their interaction with matter (e.g. diffraction patterns, transmission and detection of radio signals). The best way to think of electromagnetic radiation is a wave packet called a photon. Photons are chargeless bundles of energy that travel in a vacuum at the velocity of light, which is 300 000 km/sec.

Particulate

Specific forms of ionizing radiation:
Particulate radiation, consisting of atomic or subatomic particles (electrons, protons, etc.) which carry energy in the form of kinetic energy or mass in motion.

Electromagnetic radiation, in which energy is carried by oscillating electrical and magnetic fields traveling through space at the speed of light.

Alpha particles and beta particles are considered directly ionizing because they carry a charge and can, therefore, interact directly with atomic electrons through coulombic forces (i.e. like charges repel each other; opposite charges attract each other).

The neutron is an indirectly ionizing particle. It is indirectly ionizing because it does not carry an electrical charge. Ionization is caused by charged particles, which are produced during collisions with atomic nuclei.

The third type of ionizing radiation includes gamma and X rays, which are electromagnetic, indirectly ionizing radiation. These are indirectly ionizing because they are electrically neutral (as are all electromagnetic radiations) and do not interact with atomic electrons through coulombic forces.

Isotopes and Activity

Isotopes

Atoms in their normal state are electrically neutral because the total negative charge of electrons outside the nucleus equals the total positive charge of the nucleus .

Atoms with the same number of protons and different number of neutrons are called ISOTOPES. An isotope may be defined as one or two or more forms of the same element having the same atomic number (Z), differing mass numbers (A), and the same chemical properties.

These different forms of an element may be stable or unstable (radioactive). However, since they are forms of the same element, they possess identical chemical and biological properties.

The simplest atom is the hydrogen atom. It has one electron orbiting a nucleus on one proton. Any atom which has one proton in the nucleus is a hydrogen atom, like both of the ones shown here. Hydrogen-2 is called deuterium, hydrogen-3 is called tritium. However, while their chemical properties are identical their nuclear properties are quite different as only tritium is radioactive.

Activity

• The activity of a radioisotope is simply a measure of how many atoms undergo radioactive decay per a unit of time.
• The SI unit for measuring the rate of nuclear transformations is the becquerel (Bq). The becquerel is defined as 1 radioactive disintegration per second.
• The old unit for this is the curie (Ci), in honour of Pierre and Marie Curie who discovered radium and polonium. The curie is based on the activity of 1 gram of radium-226, i.e. 3.7 x 1010 radioactive disintegrations per second.

Dose and Source

Dose

• Only the amount of energy of any type of ionizing radiation that imparted to (or absorbed by) the human body can cause harm to health.
• To look at biological effects, we must know (estimate) how much energy is deposited per unit mass of the part (or whole) of our body with which the radiation is interacting.
• The international (SI) unit of measure for absorbed dose is the gray (Gy), which is defined as 1 joule of energy deposited in 1 kilogram of mass. The old unit of measure for this is the rad, which stands for "radiation absorbed dose." - 1 Gy = 100 rad.
• Equivalent dose – the biological effect depends not only on the amount of the absorbed dose but also on the intensity of ionisation in living cells caused by different type of radiations.
• Neutron, proton and alpha radiation can cause 5-20 times more harm then the same amount of the absorbed dose of beta or gamma radiation.
• The unit of equivalent dose is the sievert (Sv). The old unit of measure is the rem. - 1 Sv = 100 rem.

Sources of Radiation Exposure

• Radiation is permanently present throughout the environment, in the air, water, food, soil and in all living organisms.
• Large proportion of the average annual radiation dose received by people results from natural environmental sources.
• Each member of the world population is exposed, on average, to 2.4 mSv/yr of ionizing radiation from natural sources.
• In some areas (in different countries of the world) the natural radiation dose may be 5 to 10-times higher to large number of people.