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Basics - Poster 5
Radioactivity Units & Half-Life
The nucleus of a radioactive atom is in an unstable , energetic state . When it releases its potential energy by radioactive decay , it can transform into a lower energy state .
The basic unit of radioactivity is the Becquerel ( named after Henri Becquerel ) and is 1 disintegration per second . This is a very small unit and for the amounts of radioactivity commonly encountered , it is necessary to use prefixes that represent multiples .
For example there is about 4 kBq of naturally occurring potassium-40 in our bodies : whilst activity levels commonly found in industrial , medical and research uses range from MBq to TBq .
Definition
Activity
Activity is the average number of unstable nuclei that disintegrate ( or decay ) per second
Unit Becquerel ( Bq ) 1 Bq = 1 disintegration per second
Radioactive atoms decay independently of each other and whether a specific atom decays is a matter of probability . However , collectively , radioactive atoms follow an exponential decay law such that the time taken for half the atoms to decay is always the same for a specific radioisotope .
This is called the half-life and is shown graphically below for Tellurium-131 , which has a half life of 30 hours . The graph on the left is drawn with linear axes . Whilst the one on the right has a logarithmic y axis and a linear x axis . Both show the same relationship .
Concept of Half-life tellurium-131 , T = 30h
½
Graphical Representation of Half-Life
1
1
0.8
0.8
0.6
0.1
0.5
0.4
3xT 1/2
90 hours T 1/2 30 hours
0.25
0.2
0.125
0
0 20 40 60 80 100 120 140 hours
0.01
0 20 40 60 80 100 120 140
Half-lives range from less than a second to millions of years . The list on the right shows the half-lives for some of the commonly used radionuclides .
Use of a particular radionuclide is usually driven by other properties such as the types of radiation it emits and their energies ( and hence penetrating power ). However the half-life can be a factor . For example sodium-24 is used in tracer experiments , and for some of these it is beneficial , from a safety perspective , that it has decayed away in a relatively short time .