Basic Radiological Emergency Response Training: General Education


The basic building block of all matter is the atom. Atoms are very, very small. Human beings are made up of billions and billions of atoms. The center of an atom is made up of a nucleus that consists of neutrons and protons. Electrons orbit the nucleus much the same way the planets orbit the sun.

Atoms with the same number of protons are called elements. For example, all iodine atoms have 53 protons and all uranium atoms have 92 protons.

Atoms with the same number of protons but different number of neutrons are called isotopes. The isotope Iodine-131 has 53 protons and 78 neutrons while Iodine-133 has the same number of protons but 80 neutrons. Even though there are 103 known elements there are thousands of isotopes.


In nature atoms are mostly stable with small portions that are unstable. An unstable atom has too much energy or is excited and wants to become a stable atom. Radioactivity is a collection of unstable atoms that emit ionizing radiation to become stable atoms.

When an unstable atom releases energy or a particle from its nucleus, that energy or particle (know as radiation) has the ability to remove (or ionize) the electron from another atom. Or, the radiation may be so strong that it alters another atom's nucleus. Fission occurs when a neutron is absorbed into the nucleus of an atom and causes it to split.


One aspect of radioactivity is that atoms produce radiation to become stable over time. In other word, atoms of the same radioactive isotope do not emit radiation instantly. Half-life is the time in which half the atoms of a particular radioactive isotope decay into another atomic form. Radioactive half-lives may range from fractions of seconds for some radioactive isotopes to thousands of years for others. Even half-lives of radioactive isotopes of the same elements differ. For example, the half-life for Iodine-131 is 8 days while the half-life for Iodine-133 is approximately 21 hours. The chart below tracks the half-life of Iodine-131.

In the above graph, 100 units of Iodine-131 decreases or decays to 50 units in the first half-life of about 8 days. The radioactivity will be about one-fourth, or 25 units, of the original activity in two half-lives (16.1 days).

Another factor is that there are different types of radiation known as alpha, beta, and gamma. Different types of radiation are shielded different ways. This chart shows the penetrating distance of radiation.

Atoms are very small. It takes millions of atoms to form the head of a straight pin. When compared to the size of atoms, radiation is a fraction of an atom's size.Radiation loses energy when it ionizes electrons of atoms. This transfer of energy is similar to a cue ball hitting other billiard balls. As the cue ball hits other balls its energy is transferred to other balls until the cue ball comes to rest. The difference between radiation and billiard balls is that radiation will be absorbed into another atom when it loses all of its energy while a cue ball will come to rest intact.

The alpha particle is relatively big and needs very little material to be shielded. The gamma ray is pure energy and a dense material like lead is needed to absorb its energy.

Information courtesy of the CDC - educational material.