Environmental Health and Safety

Radiation Safety Training Manual

Physical Concepts

1. Atomic Structure

Every atom has a nucleus which is composed of elementary particles, protons and neutrons. The protons possess a positive charge while neutrons have no electrical charge. Orbiting the nucleus are very small negatively charged particles called electrons. Neutral atoms have the same number of positively charged protons as negatively charged electrons. Atoms differ from each other in several ways. Elements are distinguished by the number of protons in the nucleus. Each element possesses physical and chemical properties which are unique to that element. Atoms of the same element may vary from each other by the number of neutrons in the nucleus (e.g., 12C vs. 14C). These elemental variations are called isotopes. They cannot be distinguished from other isotopes of the same element by chemical analysis since they react identically. Another possible difference between atoms is the number of electrons orbiting the nucleus. Since neutral atoms have the same number of electrons as protons, an atom which has a different number of protons and electrons is called an ion and may be either positively or negatively charged.

2. Ionizing Radiation

Radiation can either be particles or electromagnetic energy which is emitted or radiated in all directions from a localized source. Two primary types of radiation are ionizing and non-ionizing. Examples of non-ionizing radiation include microwaves, radio waves and visible light. Ionizing radiation is unique in that the emissions are energetic enough to cause ionization by the removal of orbital electrons. The three principal types of ionizing radiation are called alpha, beta and gamma. These three types of radiation are emitted from the nuclei of atoms which are unstable. The transformation to stability is achieved by adjusting the ratio of neutrons to protons within the nucleus of the atom by the emission of particulate radiation. The optimum ratio of neutrons to protons for stability is averaged to be 1.54 to 1.

3. Energy

The energy of ionizing radiation is measured in “electron volts.” One electron volt (eV) is the energy gained by an electron passing through a one volt potential. One eV is equivalent to 1.6 x 10-19 Joules. One electron volt is an extremely small amount of energy. The amount of energy associated with radioactivity is usually expressed in thousands of electron volts (keV) or millions of electron volts (MeV).

4. Radioactive Decay

Radioactive decay is the term used to describe the eventual transformation of radioactive atoms into stable, non-radioactive atoms. As charged particles are emitted, the number of protons and/or neutrons is changed. The resulting atom is called a “daughter product.” The daughter may also be radioactive with its own type and energy of emissions. Sometimes a series of decays must happen before a stable atom is produced. This is known as a decay chain. The formula for determining the amount of radioactivity at any time (t) is:

At= A0 e-λt Where λ = the decay constant for the particular radioisotope. This constant is also equal to ln2/T1/2.

5. Alpha Particles

Alpha particles are positively charged helium nuclei (2 protons and 2 neutrons) which are ejected from heavy atoms having a low neutron to proton ratio. Alpha particles are monoenergetic, in that all alpha emissions from like atoms are ejected with the same kinetic energy. Due to the relatively large size and electrical charge, their range and penetrating ability is very small. Consequently, alpha particles do not pose a hazard outside the body. However, if alpha-emitting material is inhaled or ingested, the alpha particles can cause serious harm to live tissue where the material is deposited.

6. Beta Particles

Beta particles are energetic electrons emitted from the nuclei of atoms with high neutron to proton ratios. The transformation, which occurs in the nucleus, results in the conversion of a neutron into a proton and the ejection of a beta particle. The energy of the beta particle is not monoenergetic as it is for alphas. The beta particle has a range of possible energies from zero to a maximum possible energy which is unique to the radioisotope. Although beta particles are negatively charged, their very small size allows them to travel considerably farther in air than alphas. The ability of beta particles to penetrate solid materials is still very restricted. Only the most energetic beta particles may penetrate thin sheets of paper or plastic. Beta particles pose a significant internal hazard, and also may pose a small hazard to skin surfaces or to the lens of the eye, depending on their energy. A few radioactive isotopes are known as “pure beta” emitters. They are unique because there is no associated gamma ray directly following beta decay. Some examples of pure beta emitters are 3H, 14C, 35S, 99Tc and 32P.

7. Gamma Rays

Gamma rays are essentially light packages or “photons” which are emitted from some unstable atoms which have just undergone a transformation by emitting a charged particle. Depending on the radioisotope, there may be one or several gamma rays emitted with differing energies. In addition the probability or frequency of their emission can vary from 100‰ to well under 1‰ as each transformation or decay occurs. Since gamma rays have no mass or charge, their range and penetrating ability are significant. Gammas have no finite range in any medium, but are best attenuated or shielded by dense material. The thickness of shielding material is cited in half value layers. One half value layer is that thickness of material which reduces the amount of radiation by 50‰. Gamma rays may be an external hazard as well as an internal hazard.

8. X-rays

X-rays are also photons which are capable of causing ionizations and are similar in every way to gamma rays except in their point of origin. Gamma rays are emitted from the nucleus of an atom and x-rays come from the electron orbitals. The most common source of x-rays is from radiation producing machines. Some of the more common x-ray or gamma ray producing radioisotopes are listed below.

Another source of x-rays comes from the abrupt change in velocity of beta particles. X rays generated by this process are referred to as “bremstrahlung” (braking radiation). The creation of these x rays is dependent on the atomic number of the absorbing material. The higher the atomic number the greater the likelihood that an x ray will be produced. The energy of the x ray is directly proportional to the atomic number of the absorber and the energy of the beta particle. Due to bremstrahlung, it is preferable to shield high energy beta particles with materials of low atomic number. Plastic or acrylic materials are generally used for beta shields. Although the production of x rays should be minimized, the resultant exposure rate from bremstrahlung is usually insignificant.

9. Half-Life

One half-life is the length of time required for the amount of radioactivity present to be reduced by 50‰. At each subsequent time interval equaling one half-life, the previous amount of radioactivity is further reduced by half. The half-life can be viewed as the probability of decay. An individual radioactive atom may decay at any time, but for a given quantity of radioactivity, the number of particle emissions per unit time, also referred to as “disintegration rate,” will be a constant.

10. Units

A “Curie” is a unit of measurement which quantifies the amount of radioactivity present as a disintegration rate. One Curie (Ci) is referenced as the amount of radioactivity present in 1 gram of radium and is equivalent to 3.7 x 1010 disintegrations per second (DPS).

A “Bequerrel” is an S.I. unit which describes how much radioactivity is present. One Bequerrel (Bq.) is a very small amount of radioactivity which is equivalent to 1 disintegration per second.

A “Roentgen” is a unit of measurement which quantifies radiation exposure in air from x-rays or gamma rays only. One Roentgen (R) of exposure provides 2.58 x 104 coulombs of charge per kilogram of air.

A “Rad” is a unit of measurement which quantifies the dosage of energy deposited in any medium from any type of ionizing radiation. One rad is equivalent to 100 ergs of energy deposed per gram of absorbing material.

A “Gray” (Gy) is the S.I. unit for absorbed dose. One Gray is equivalent to 100 rads.

A “Rem” is a unit of measurement also known as “dose equivalent” which numerically describes the relative amount of biological damage which occurs from doses of ionizing radiation. The rem is derived by the product of the dose received in rads and a quality factor which is unique to each type of radiation. This equates the effectiveness of each type of radiation to cause biological damage. The rem is used to report doses to persons or organs.

A “Sievert” (Sv) is the S.I. unit for dose equivalent. One Sievert is equivalent to 100 rems.