4. How Is Ionizing Radiation Measured?

onizing radiation is easy to detect, even in radiation is absorbed by the material or living Ivery small amounts. It can be measured tissue through which the radiation is travel- using a number of different instruments. By ing. It is this absorbed energy which can cause measuring the amount of radiation present, damage. Ionizing radiation that travels people can identify sources of radiation and through material without interacting with any take the necessary steps to avoid or reduce atom, without transferring energy, causes no exposure. damage. Ionizing radiation has the potential to To estimate how much damage ionizing damage living tissue. The extent of the radiation from a particular source might do, it damage depends, in part, on the type and is important to know the amount of energy the amount of ionizing radiation that is absorbed radiation carries. Two types of measurements by the tissue. of the radiation’s energy are generally made: If no radiation energy is absorbed, no the exposure rate, which is the amount of damage is done. Radiation detection is, radiation energy that reaches an object’s therefore, an important part of protecting surface in a given time period, and the people and the environment from radiation. absorbed dose, which is the amount of This Fact Sheet defines radiation dose and radiation energy that is actually absorbed by the units in which dose is generally measured. the material through which it passes. The It also describes some of the more common absorbed dose is multiplied by a factor which types of equipment used to detect radiation. takes into account the different biological effects of the various types of ionizing ➤ Radiation Dose radiation. This converts the absorbed dose to As ionizing radiation passes through material, the equivalent dose, which is of interest to including living tissue, it interacts with atoms, most people, and it is the one that will be transferring some of its energy during each discussed in this Fact Sheet. interaction. The energy lost by the ionizing ➤ Units in which Radiation is Measured Source Dose (per year) The term “dose” or “radiation dose” generally refers to the equivalent dose, which may be Radon 200mrem used for all kinds of ionizing radiations, not Naturally occurring just x- or gamma rays. The equivalent dose is radioactive material in measured in rem or millirem. A millirem is the human body 40mrem one one-thousandth of a rem. In other words, Medical x-rays 39mrem one thousand millirem equal one rem (1000 millirem = 1 rem). Millirem is often abbrevi- Rocks and soil 28mrem ated as mrem. Cosmic radiation 27mrem What radiation dose does the typical person receive? Several sources of radiation Nuclear medicine 14mrem and the dose that each source gives to the Consumer products 10mrem average American each year are listed in Table 1. Table 1. Average Annual Radiation Doses in Federal regulations require that a low-level U.S.A. radioactive waste disposal facility be de- Page 2 ¥ How Is Ionizing Radiation Measured?

Figure 1. Radiation signed, operated, and controlled after closure Continuous sampling monitors can be set Monitors so that no member of the public receives an up to take samples of air or water in and annual dose greater than 25 mrem. No around a low-level radioactive waste disposal existing facility, operating or closed, has facility. Several different types of monitors are resulted in an annual dose of more than 3 used. With one type, the samples are collected mrem to any member of the public. and evaluated periodically to ensure the Federal radiation safety regulations allow a concentration of radioactive material in the worker in a nuclear facility to receive up to 5 air or water is within limits set by federal and rem (5,000 mrem) each year. state regulations. Another type of continuous The radiation dose divided by the time over monitor is designed to emit a signal when the which it is delivered is called the dose rate. amount of radiation present would give a dose The effect of a particular radiation dose on higher than a specified limit. living tissue may depend on the dose rate. It is Personal Dosimeters generally believed that radiation doses ➤ For More Information delivered at low dose rates are less damaging If you want to read more about measuring than the same dose delivered at a high dose radiation, some of the references listed below rate (see, for example, National Research may be helpful. Council report "Biological Effects of Ionizing • Eric J. Hall, Radiation and Life, Fourth Radiation V"). Edition, J. Lippincott Co,, New York, 1994. • Fred A. Mettler, Jr. M.D. and Arthur C. ➤ Devices for Measuring Radiation Upton, M.D., Medical Effects of Ionizing Many different devices are used to measure Radiation, Grune & Stratton, Inc., radiation under a wide range of conditions. Orlando, Florida,1995. Three categories of devices are personal • Jacob Schapiro, Radiation Protection, dosimeters, hand-held detectors, and continu- Harvard University Press, 1990. ous sampling monitors. (See Figure 1) Hand-Held Detector People working in or visiting nuclear • National Research Council, Biological facilities usually wear personal dosimeters on Effects of Ionizing Radiation, 1993. 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 Continuous Sampling with a hand-held detector such as a Geiger Monitor counter.

This series of data sheets is based upon copyright material prepared by Ohio State University Extension under a grant from the Midwest Compact Commission. The material was reviewed and updated at Rutgers University, Department of Environmental Sciences by Alan Appleby, Ph.D., Martin Costello, M.S. and Steven Rose, M.S. Permission to use this material is gratefully acknowledged. November 1996