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1. Half life (t 1/2 ) is the time taken for a radionuclide’s activity to decay to half of it’s original value.

2. It is a fixed characteristic of a particular radionuclide unaffected by temperature, chemical environment, etc.

3. A proportionality can be transformed into an equality by introducing a constant. This constant is called the decay constant (λ): −dN/dt = λN. By substituting A (activity) for −dN/dt: A = λN

4. The minus sign indicates that the number of radioactive atoms decaying per unit time (the decay rate or activity of the sample) decreases with time.

5. The number of atoms decaying per unit time (dN/dt) is proportional to the number of unstable atoms (N) that are present.

6. Derived from the FUNDAMENTAL DECAY EQUATION: N = N 0 /2 n , where N is number of radioactive atoms remaining, N 0 is the initial number of radioactive atoms, and n is the number of half-lives that have elapsed.


1. Bushberg, J. T. (2012). The essential physics of medical imaging. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins. Find it at Amazon
2. Heggie, J. C., Liddell, N. A., & Maher, K. P. (1997). Applied imaging technology. Melbourne: St. Vincents Hospital.

Ⓒ A. Manickam 2018

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