How Is Ionizing Radiation Measured?

RER-23

Audeen W. Fentiman
Joyce E. Meredith
Ronald J. Veley

Ionizing radiation is easy to detect, even in very small amounts. It can be measured using a number of different kinds of instruments. By measuring the amount of radiation present, people can identify sources of radiation and take the necessary steps to avoid them.

Ionizing radiation has the potential to damage living tissue. The extent of the damage depends, in part, on the amount of ionizing radiation that is absorbed by the tissue (i.e., the dose). If no radiation energy is absorbed, no damage is done. Radiation detection is, therefore, an important part of protecting people and the environment from radiation.

This fact sheet defines radiation dose and the units in which dose is generally measured. It also describes some of the more common types of equipment used to detect radiation.

Radiation Dose

As ionizing radiation passes through material, including living tissue, it interacts with atoms, transferring some of its energy during each interaction. The energy lost by the ionizing radiation is absorbed by the material or living tissue through which the radiation is traveling. It is this absorbed energy which can cause damage. Any ionizing radiation that travels through material without interacting with any atom, without transferring any energy, causes no damage.

To estimate how much damage ionizing radiation from a particular source might do, it is important to know the amount of energy the radiation carries. Two types of measurements of the radiation's energy are generally made: the exposure rate, i.e., the amount of radiation energy that reaches an object's surface in a given time period, and the absorbed dose which is the amount of radiation energy that is actually absorbed by the material through which it passes. The absorbed dose is multiplied by a factor which takes into account the different biological effects of the various types of ionizing radiation. This converts the absorbed dose to the equivalent dose which is of interest to most people, and it is the one that will be discussed in the remainder of this fact sheet.

Units in which Radiation Doses are Measured

The term "dose" or "radiation dose" generally refers to the equivalent dose. The equivalent dose is measured in rem or millirem. A millirem is one one-thousandth of a rem. In other words, one thousand millirem equal one rem (1000 millirem = 1 rem). Millirem is often abbreviated as mrem or sometimes as mr.

A different, standard international unit of radiation dose has been adopted. It is the Sievert (Sv). One Sievert equals 100 rem. In most non-technical articles, however, the units used are rem and millirem.

What radiation dose does the typical person receive? Several sources of radiation and the dose that each source gives to the average American each year are listed in Table 1.

Regulations require that a low-level radioactive waste facility be designed, operated, and controlled after closure so that no member of the public receives an annual dose greater than 25 mrem. No existing low-level waste facility, operating or closed, has resulted in an annual dose of more than 3 mrem to any person beyond the facility fence line.

Federal radiation safety regulations allow a worker in a nuclear facility to receive up to 5 rem (5,000 mrem) each year. A lethal dose of radiation is about 400-600 rem (400,000 to 600,000 mrem) over a very short time, such as a few minutes.

The radiation dose divided by the time over which it is delivered is called the dose rate. The effect of a particular radiation dose on living tissue may depend on the dose rate.

Devices for Measuring Radiation

Many different devices are used to measure radiation under a wide range of conditions. Three categories of devices are (a.) personal dosimeters, (b.) hand-held detectors, and (c.) continuous sampling monitors. Pictures of these three types of devices are shown in Figure 1.

Figure 1. Radiation Monitors

a. Personal Dosimeters

b. Hand-Held Detector

c. Continuous Sampling Monitor

People working in or visiting nuclear facilities usually wear personal dosimeters on their clothing. These devices measure the radiation dose a person receives while in the facility. A film badge is an example of a personal dosimeter.

Hand-held detectors are used to measure the exposure rate from a specific object. The exposure rate from a package of radioactive waste or a piece of granite can be measured with a hand-held detector such as a Geiger counter. Continuous sampling monitors can be set up to take samples of air or water in and around a nuclear facility. Several different types of monitors are used. With one type, the samples are collected and evaluated periodically to be sure the concentration of radioactive material in the air or water is within acceptable limits. Another type of continuous monitor is designed to emit a signal when the amount of radiation present would give a dose higher than a specified limit.

For More Information

If you want to read more about measuring radiation, some of the references listed below may be helpful.

Eric J. Hall, Radiation and Life, Second Edition, Pergamon Press, New York, 1984.

Fred A. Mettler, Jr. and Robert D. Moseley, Jr., Medical Effects of Ionizing Radiation, Grune & Stratton, Inc., Orlando, Florida, 1985.

Author Notes:

Dr. Audeen W. Fentiman is an Assistant Professor in Nuclear Engineering at The Ohio State University. Joyce E. Meredith and Ronald J. Veley are Graduate Research Associates, Ohio State University Extension.