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Union University Department of Physics

The Science Guys

Science Guys > September 2003

September 2003

What units are used to measure nuclear radiation released by radioactive material from a bomb or nuclear reactor?

Radiation workers must understand several quantities describing radioactivity and the unit for each measurement. This is a tough demand because there are several measurements used with radiation.

The nuclei of many elements are unstable and spontaneously emit alpha particles (helium nuclei), beta particles (electrons), or gamma rays (ultra-high energy "light"). These emissions are collectively called radioactivity, sometimes just called radiation. Such radiation is often denoted as ionizing radiation to distinguish it from radiation given off by regular light sources, because visible light usually doesn’t produce ionization (produce an electric charge in matter). By contrast, when radioactivity passes through matter, the matter is ionized.

The different quantities used to describe radiation are called activity, exposure, absorbed dose, and relative effective dose. Some of these are often called by other names - it can get confusing, but let’s give it a shot.

Activity tells us what is happening in a sample (how many emissions or rays a sample is giving off each second). The old unit for activity, the Curie, represents 37 billion emissions per second. The new unit for activity is called the Becquerel, which is one emission per second.

Exposure tells how effectively a radioactive sample ionizes, or induces an electric charge, in dry air. The amount of exposure is measured in Roentgens. A Roentgen of exposure produces one electrostatic unit of charge in 0.001293 grams of dry air.

Absorbed Dose is the radiation energy absorbed by some material. The rad is the old unit for absorbed dose and is 0.01 Joule of energy absorbed per kilogram of material. The new unit, a Gray, is 1 Joule per kilogram. The type of material is important, because materials don’t absorb radiation equally well.

Relative Equivalent Dose (or the biological effectiveness dose) is related to the energy deposited in human tissue. The equivalent dose is the absorbed dose multiplied by weighting factors that take into account the type of radiation absorbed and the particular tissue that absorbed the radiation. The equivalent dose takes into account the biological effects of the absorbed dose. The same absorbed dose delivered by different types of radiation (alpha, beta, or gamma) may result in different biological effects. Furthermore, radiating different types of tissue may result in different effects. For example, when thyroid tissue is radiated, the biological effect is greater then when muscle tissue is radiated. Thus, thyroid tissue has a greater weighting factor.

The old unit for equivalent dose is the rem, which stands for Roentgen Equivalent Man. The new unit is the Seivert, which is 100 rems. The units carry the same units as the rad, that is, Joules of energy per kilogram of tissue.

So as you see there are numerous terms used in describing radiation. This may seem too detailed but science must be very specific.