Half-Life

The half-life of a radioactive isotope is the amount of time it takes for the isotope to decay so that only half of the original amount of the isotope remains. Half-lives can be measured from smaller than a millionth of a second to more than trillions of years. The shorter the half-life, the more radioactive an isotope is.

Tables

Fraction remaining per half-life elapsed

Half-lives elapsed	Fraction remaining	Percentage of nuclei without disintegrating
0	1/1	100 %
1	1/2	50 %
2	1/4	25 %
3	1/8	12.5 %
4	1/16	6.25 %
5	1/32	3.125 %
6	1/64	1.562 %
7	1/128	0.781 %
...	...	...
n	1/2n	100/(2n)

Half-life of some radionuclides

Uranium-235	7,038·108 years	Uranium-238	4,468·109 years	Potassium-40	1,28·109 years
Rubidium-87	4,88·1010 years	Calcium-41	1,03·105 years	Carbon-14	5760 years
Radium-226	1620 years	Cesium-137	30,07 years	Bismuth-207	31,55 years
Strontium-90	28,90 years	Cobalt-60	5,271 years	Cadmium-109	462,6 days
Iodine-131	8,02 days	Radon-222	3,82 days	Oxygen-15	122 seconds