Energy, Activity, Intensity and Exposure

Different radioactive materials and X-ray generators produce radiation at different energy levels and at different rates. It is important to understand the terms used to describe the energy and intensity of the radiation. The four terms used most for this purpose are: energy, activity, intensity and exposure.

Radiation EnergyAs mentioned previously, the energy of the radiation is responsible for its ability to penetrate matter. Higher energy radiation can penetrate more and higher density matter than low energy radiation. The energy of ionizing radiation is measured in electronvolts (eV). One electronvolt is an extremely small amount of energy so it is common to use kiloelectronvolts (keV) and megaelectronvolt (MeV). An electronvolt is a measure of energy, which is different from a volt which is a measure of the electrical potential between two positions. Specifically, an electronvolt is the kinetic energy gained by an electron passing through a potential difference of one volt. X-ray generators have a control to adjust the keV or the kV.

The energy of a radioisotope is a characteristic of the atomic structure of the material. Consider, for example, Iridium-192 and Cobalt-60, which are two of the more common industrial Gamma ray sources. These isotopes emit radiation in two or three discreet wavelengths. Cobalt-60 will emit 1.33 and 1.17 MeV Gamma rays, and Iridium-192 will emit 0.31, 0.47, and 0.60 MeV Gamma rays. It can be seen from these values that the energy of radiation coming from Co-60 is about twice the energy of the radiation coming from the Ir-192. From a radiation safety point of view, this difference in energy is important because the Co-60 has more material penetrating power and, therefore, is more dangerous and requires more shielding.

ActivityThe strength of a radioactive source is called its activity, which is defined as the rate at which the isotope decays. Specifically, it is the number of atoms that decay and emit radiation in one second. Radioactivity may be thought of as the volume of radiation produced in a given amount of time. It is similar to the current control on a X-ray generator. The International System (SI) unit for activity is the becquerel (Bq), which is that quantity of radioactive material in which one atom transforms per second. The becquerel is a small unit. In practical situations, radioactivity is often quantified in kilobecqerels (kBq) or megabecquerels (MBq). The curie (Ci) is also commonly used as the unit for activity of a particular source material. The curie is a quantity of radioactive material in which 3.7 x 10¹⁰ atoms disintegrate per second. This is approximately the amount of radioactivity emitted by one gram (1 g) of Radium 226. One curie equals approximately 37,037 MBq.  New sources of cobalt will have an activity of 20 to over 100 curies, and new sources of iridium will have an activity of similar amounts.

Once a radioactive nucleus decays, it is no longer possible for it to emit the same radiation again. Therefore, the activity of radioactive sources decrease with time and the activity of a given amount of radioactive material does not depend upon the mass of material present. Additionally, two one-curie sources of Cs-137 might have very different masses depending upon the relative proportion of non-radioactive atoms present in each source. The concentration of radioactivity, or the relationship between the mass of radioactive material and the activity, is called the specific activity. Specific activity is expressed as the number of curies or becquerels per unit mass or volume. The higher the specific activity of a material, the smaller the physical size of the source is likely to be.

IntensityRadiation intensity is the amount of energy passing through a given area that is perpendicular to the direction of radiation travel in a given unit of time. The intensity of an X-ray or gamma-ray source can easily be measured with the right detector. Since it is difficult to measure the strength of a radioactive source based on its activity, which is the number of atoms that decay and emit radiation in one second, the strength of a source is often referred to in terms of its intensity. Measuring the intensity of a source is sampling the number of photons emitted from the source in some particular time period, which is directly related to the number of disintegrations in the same time period (the activity).

ExposureOne way to measure the intensity of x-rays or gamma rays is to measure the amount of ionization they cause in air. The amount of ionization in air produced by the radiation is called the exposure. Exposure is expressed in terms of a scientific unit called a roentgen (R or r). The unit roentgen is equal to the amount of radiation that produces in one cubic centimeter of dry air at 0°C and standard atmospheric pressure ionization of either sign equal to one electrostatic unit of charge. Most portable radiation detection safety devices used by a radiographer measure exposure and present the reading in terms of roentgens or roentgens/hour, which is known as the dose rate.