Radiation Safety Office
Laboratory Worker Training Manual
RADIOACTIVE DECAY AND UNITS
When the neutrons and protons
of the nucleus of the atom are in the right proportion, the nucleus is
considered stable. For light nuclei (1 < Z < 20) there is
an equal number of protons and neutrons. For heavier nuclei (Z>20)
more neutrons must be added for each added proton and therefore heavier
nuclei are more likely to be unstable. All
elements with very high atomic numbers (greater than 81) are naturally
unstable. In order to become stable, the nuclei emit radiation.
This is known as radioactive decay. The radiation which is emitted
is in the form of alpha particles, beta particles and/or gamma rays.
The nuclei may change only one time and become stable or many times before
becoming stable. However, each time it changes, some type of radiation
is emitted. The figure at the left shows the decay scheme for Co-60.
Each transformation releases one beta (0.314 MeV) and two gammas (1.173
MeV and 1.332 MeV), the total number of radiations is 3.
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Different isotopes are transformed at different rates, and each isotope has its own characteristic transformation rate. No operation, either chemical or physical, is known that will change the transformation rate of the isotope. This transformation rate is known as the half-life, the time in which half the atoms of a radionuclide are transformed through radioactive decay. Half-lives of some common isotopes found on campus at Emory are shown at the left. |
The activity of the radioactive material refers to the rate of its transformation or its decay. The activity is dependent on the number of disintegrations and not the number of emissions. As shown previously in the case of 60Co, each transformation resulted in the emission of three radiations.
1 Ci = 3.7 (1010) dps = 2.22 (1012) dpm The SI unit for measuring activity is the Becquerel (Bq) which is
1 mCi = 3.7 (107) dps = 2.22 (109) dpm 1 disintegration per second (1dps). However, in the United States,
1 uCi = 3.7 (104) dps = 2.22 (106) dpm the common unit of activity is the Curie (Ci).
The Curie is a very large unit, so for our purposes at Emory, we commonly use the millicurie (10-3 Ci) and the microcurie (10-6 Ci) which are abbreviated mCi and uCi respectively. The above table shows conversions from Curie to Becquerel and dpm (disintegrations per minute).
In order to calculate the activity of the radioactive material after decay, it is necessary to know the half-life of the isotope, the original activity and the elapsed time. The mathematical equation for activity is shown as the equation :
For example, a shipment of 3H contains 5000 uCi. How much activity will remain after 2.5 years? Ao is 5000, L is 0.693/12.3 and t is 2.5. Equation 2 shows the result of the calculation.
The axis for each decay table will vary depending on the half-life of the isotope. For example, the decay table for 125I (see below) has a horizontal axis in days and a vertical axis in days also. If we were to calculate the final activity of a stock vial which had an activity of 5 mCi one month ago, we would follow the 20 day row across to the 10 day column (a total of 30 days) and find the decay fraction to be .707. The activity after 30 days of decay would be 3.54 mCi.
Finally, the 32P decay table has a horizontal axis of hours and a vertical axis of days. If 3265 uCi of 32P was placed in a radioactive waste container, how much activity will remain at pick-up time, 10 days later? By following the row for 8 days to the column for 48 hours (10 days total), the decay fraction is determined to be .617. Therefore, 2014.5 uCi of 32P will remain in the container at pick-up time.
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