Chapter 1 - Radiological Fundamentals

Atoms may be referred to as Stable or Unstable.

• Stable atoms do not contain excess energy.

• Unstable atoms contain excess energy. This is caused by an imbalance in the ratio of protons to neutrons in the nucleus of the atom. These atoms release their excess energy during the process known as radioactive decay. The energy released in the process is called ionizing radiation.

Radiation (or ionizing radiation) is energy in the form of waves (rays) or particles which can penetrate matter and cause ionization.

Radioactive Material is any material which contains unstable atoms that emit radiation.

Radioactive material may exist in any physical form, such as solids or liquids. Any radioactive material which is in a form that is easily spread or has been transferred to surfaces, liquids, or the atmosphere is known as radioactive contamination. Contamination is a concern due to the potential for its spread to personnel.

A simple definition for Radioactive Contamination:

Radioactive material in an unwanted place.

Radiation may be emitted from sources other than radioactive materials. Certain physical processes (such as those used in x-ray machines) may cause the production of ionizing radiation. The operation of an electron accelerator involves such processes. Due to the high energy nature of the beam, it can also cause the formation of radioactive material. This material is produced in an among the components of the accelerator.

Radiation vs Contamination

If you are not familiar with working around radiation or radioactive material, the terms and concepts may confuse you at first. Let’s look at some of the properties of radiation and radioactive materials.

Radiation – Radiation is energy in the form of waves or particles given off during radioactive decay, or as a consequence of certain physical processes that we can control. Examples of these are x-ray machines and particle accelerators.

• Wave radiations include gamma and x-rays. A common term used to describe this type of radiation is photon radiation

• Particle radiation can consist of charged or uncharged particles which are emitted with very high velocity.

Radioactive material – Any material that contains radioactive (unstable) atoms. Radioactive materials are everywhere. Usually, we only encounter them in very small amounts. Since radioactive material contains unstable atoms, it emits radiation

Radioactive contamination – Not all radioactive material is considered “contamination”. Contamination is radioactive material that is in a form or location which may allow it to be spread to unwanted locations. Many radioactive sources are sealed or are in a form that isolates the material from potential spread. Contamination may be Fixed, Transferable (loose), or Airborne.

It is important to note that exposure to radiation does not result in contamination to the worker. You may become contaminated only through direct contact with material that has removable radioactive material, or by working in areas where this contaminated material is handled.

Radioactivity – Radioactivity is the process of unstable (or radioactive) atoms becoming stable by emitting radiation. The radioactive decay process involves fundamental physical constants which enable us to characterize and measure radioactive materials very accurately.

Radioactive half-life – Radioactive half-life is the time it takes for one half of the radioactive atoms present in a given sample to decay. The half-life of a particular isotope is a constant, and depending on the isotope it may range from a fraction of a second to millions of years. After seven half-lives the activity will be less than 1% of the original activity.

Ionization – The process of ionization is important in understanding radiation, because it is this process that differentiates ionizing radiation from other types. Ionization is the process of removing electrons from atoms. If enough energy is supplied to remove electrons from the atom the remaining atom has a + charge. The positively charged atom and the negatively charged electron are called an ion pair. Ionization should not be confused with radiation. Ions (or ion pairs) can be the result of radiation exposure and allow the detection of radiation.

Units of Measurement

Exposure and Dose

When people are exposed to radiation, the energy of the radiation is deposited in the body. This does not make the person radioactive or cause them to become contaminated.

An analogy would be to shine a bright light upon your body. The body absorbs the light (energy), and in some cases the absorption of the light energy may cause noticeable heating in the body tissue. Your body does not become a light source or emit light now that it has absorbed the light.

In a similar way, when exposed to radiation, your body absorbs the radiation energy. As this absorption takes place, the tissue of your body may be damaged by the penetration and conversion on the radiation energy. Again, your body does not become radioactive or emit radiation due to this energy absorption.

Since absorption of radiation can damage tissue, a way to measure that damage and ensure that it is kept to a minimum is necessary. The amount of radiation energy absorbed in a body is known as dose. The special unit for measuring dose in a person (called dose-equivalent) is the rem.

• The rem is the unit used for equating radiation absorption with biological damage.

• Since the rem is a fairly large unit, radiation exposure is usually recorded in thousandths of a rem – or millirem. 1000 millirem = 1 rem. Millirem is usually abbreviated a mrem.

For example, if you receive a chest x-ray, the amount of exposure – or dose – would be approximately 10 mrem (0.010 rem). This same amount of dose of biological harm, could be received from making two or three coast to coast airline flights – each round trip involves about 5 mrem (From elevated cosmic radiation levels in the upper atmosphere). The important point is, the source of the exposure is relatively unimportant, once the dose has been measured in a standard unit, it can be compared to other doses, added to other doses, or used in risk comparisons regarding non-radiation risks. We will make some of these comparisons later.

Other related units are used to make radiation measurements. You will hear several terms such as “exposure”, “dose” or “absorbed dose” associated with some of these units. Check the glossary for more information on these terms. Since these units and terms are used frequently, and have similar meanings they are mentioned here for comparison. The following units are among the most common English units used. The limitations of each of the units are described. The units (the ‘R’, the ‘rad’, and the ‘rem’) are often interchanged, but each has a specific definition.

• Roentgen – abbreviated ‘R’, pronounced “renken”

The roentgen is a unit for measuring exposure. It is defined only for effect on air. The roentgen is essentially a measure of how many ion pairs are formed in a given volume of air when it is exposed to radiation. Therefore it is not a measure of energy absorbed, or dose. It applies only to gamma and x-rays. It does not related the amount of exposure to biological effects of radiation in the human body.

1 R (Roentgen) – 1000mR (milliRoentgen)

• Rad (Radiation Absorbed Dose)

The rad is a unit for measuring absorbed dose in any material. Absorbed dose results from energy being deposited by the radiation. It is defined for any material. It applies to all types of radiation. It does not take into account the potential effect that different types of radiation have on the body. Therefore, it can be used as a measure of energy absorbed by the body, but not as a measure of the relative biological effect (harm or risk) to the body.

1 rad = 1000 millirad (mrad)

• Rem (Roentgen equivalent man)

As stated above, the rem is the unit for measuring the special quantity called dose equivalent. The rem takes into account the energy absorbed (dose) and the relative biological effect on the body due to the different types of radiation (expressed as the “quality factor” of the radiation). It is therefore a measure of the relative harm or risk caused by a given dose of radiation when compared to any other doses of radiation of any type. Occupational radiation exposure is recorded in rems.

• The rem can be thought of as the unit of biological hazard.

Note: For purposes of this training, the term dose will be used to mean dose-equivalent.

• Dose is the amount of radiation you receive. Dose Rate indicates how fast you receive the dose. Dose rate is the intensity of the radiation.

For example: Dose – usually measured in mrem

Dose rate – usually measured in mrem/hr

Note: The units of R, rad, and rem can sometimes be acceptably interchanged. For instance, for gamma radiation, an exposure of 1 R causes an absorbed dose in a person of about 1 rad, which results in a dose equivalent of 1 rem. This is due to the basis for the definitions of the units and the relative biological effectiveness of gamma radiation. An absorbed dose of 1 rad from fast neutrons, however would result in a dose equivalent of about 10 rem.

Measuring Radioactivity/Contamination

The amount of radioactive material in a given object or sample can be visualized by thinking of the unstable atoms in the material. These atoms are continuously decaying, so the more unstable atoms there are, the greater the decay rate. This rate of decay is measured in units of Curies. Curies are related to the decay rate, or disintegration rate, as follows:

One curie = 1,200,000,000,000 disintegrations per minute or 2.2 X 10¹² dpm

Since the curie is a large number of disintegrations per minute, sub-units of the curie such as the microcurie or millicurie are often used. Also, the dpm is used when measuring surface contamination. For example, when using a frisker (contamination monitoring instrument) to measure contamination, the instrument reading – in counts of minute (cpm) – is converted to dpm by a simple conversion of 1 cpm → 10 dpm. Therefore if the frisker reads 100cpm, the contamination level is 1000 dpm. This is a very small amount or radioactivity.

If your work involves working around contamination, you will receive training on the use of a frisker or other contamination monitoring instruments.

Types of Ionizing Radiation

The four basic types of ionizing radiation of concern in most radiological work situations are alpha particles, beta particles, gamma rays and neutron particles. These may exist in various amounts, depending on the exact location and nature of the work. We will examine each type of radiation for its characteristics here.