Units Used for Measuring Radiation

There are several different units used to describe the quantities, properties and biological effects associated with ionizing radiation.

Activity

The curie (Ci) and the becquerel (Bq) are used to indicate the activity, or total quantity of a radioactive material, that is present in a particular sample.

The curie is a large unit of activity and is defined as 3.7 x 10¹⁰ disintegrations per second. This means that 3.7 x 10¹⁰ change each second. Most radioactive samples at LLU/LLUMC contain quantities of activity that are more appropriately measured in millicuries (1 x 10^-3 curies) or micro-curies (1 x 10^-6 curies).

The becquerel is a very small unit of activity and is defined as 1 disintegration per second. More appropriate units for expressing the activity of a sample in becquerels are mega-becquerels (1 x 10⁶ becquerels) and giga-becquerels (1 x 10⁹ becquerels).

The activity in a sample decreases predictably with time. The time required for the activity in a sample to decrease to one-half of the initial activity is called the half-life of the isotope. Radioactive half-lives range from millionths of a second to billions of years. The mathematical description of the exponential decay of a radioactive isotope is given as follows and can be used to determine the amount of activity remaining in a particular sample after a certain time t:

where:
	=	the initial activity of the isotope
	=	the activity of the isotope after an elapsed time t
	=	the half-life of the isotope
	=	radioactive decay constant=.693/T
	=	the elapsed time since the initial activity was measured

When using this formula, it is important to express t and T in the same units of time (i.e., days, years, etc.).

Example:

What is the activity remaining after 21 days for a sample of ³²P with an initial activity of 100 Ci (micro-curies)? (The half-life of ³²P is 14.3 days.)

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