放射性元素的原子核有半數發生衰變時所需要的時間，叫半衰期（Half-life）。隨著放射的不斷進行，放射強度將按指數曲線下降，放射性強度達到原值一半所需要的時間叫做同位素的半衰期。原子核的衰變規律是：N=No*(1/2)^(t/T) 其中：No是指初始時刻（t=0）時的原子核數 t為衰變時間，T為半衰期，N是衰變後留下的原子核數。放射性元素的半衰期長短差別很大，短的遠小於一秒，長的可達數百萬年。

名稱	符號	半衰期	單位
Actinium 227	Ac-227	21.77	Year
Aluminum 26	Al-26	720000	Year
Aluminum 29	Al-29	0.11	Hour
Americium 241	Am-241	432	Year
Antimony 124	Sb-124	60.2	Day
Arsenic 74	As-74	17.9	Day
Astatine 211	At-211	7.21	Hour
Barium 128	Ba-128	2.43	Day
Barium 133	Ba-133	10.5	Year
Barium 140	Ba-140	12.8	Day
Beryllium 7	Be-7	53.3	Day
Bismuth 207	Bi-207	38	Year
Bismuth 210	Bi-210	5.01	Day
Bromine 77	Br-77	57	Hour
Bromine 82	Br-82	35.34	Hour
Cadmium 109	Cd-109	453	Day
Calcium 45	Ca-45	162.61	Day
Carbon 11	C-11	0.34	Hour
Carbon 14	C-14	5730	Year
Cerium 141	Ce-141	32.5	Day
Cerium 144	Ce-144	284	Day
Cesium 134	Cs-134	2.06	Year
Cesium 137	Cs-137	30.17	Year
Chlorine 36	Cl-36	301000	Year
Chromium 51	Cr-51	27.7	Day
Cobalt 57	Co-57	270	Day
Cobalt 58	Co-58	71.3	Day
Cobalt 60	Co-60	5.272	Year
Copper 61	Cu-61	3.32	Hour
Copper 62	Cu-62	0.163	Hour
Copper 64	Cu-64	12.7	Hour
Copper 67	Cu-67	61.88	Hour
Curium 244	Cm-244	18.12	Year
Europium 152	Eu-152	13	Year
Fluorine 18	F-18	1.83	Hour
Gadolinium 153	Gd-153	242	Day
Gallium 67	Ga-67	78.2	Hour
Germanium 68	Ge-68	275	Day
Gold 198	Au-198	2.696	Day
Hafnium 181	Hf-181	42.5	Day
Hydrogen 3	H-3	12.26	Year
Indium 111	In-111	2.81	Day
Indium 113m	In-113m	1.66	Hour
Indium 114m	In-114m	49.51	Day
Iodine 123	I-123	13.3	Hour
Iodine 125	I-125	60.2	Day
Iodine 131	I-131	8.04	Day
Iodine 132	I-132	2.26	Hour
Iridium 192	Ir-192	74.2	Day
Iron 52	Fe-52	8.2	Hour
Iron 55	Fe-55	2.73	Year
Iron 59	Fe-59	44.6	Day
Krypton 85	Kr-85	10.72	Year
Lanthanum 140	La-140	40.3	Hour
Lutetium 177	Lu-177	6.71	Day
Lead 210	Pb-210	22.3	Year
Manganese 52m	Mn-52m	0.352	Hour
Manganese 54	Mn-54	313	Day
Manganese 56	Mn-56	2.578	Hour
Mercury 197	Hg-197	65	Hour
Mercury 203	Hg-203	46.8	Day
Molybdenum 99	Mo-99	66.02	Hour
Neodymium 147	Nd-147	11.1	Day
Neptunium 237	Np-237	2140000	Year
Nickel 63	Ni-63	100.1	Year
Niobium 94	Nb-94	20000	Year
Niobium 95	Nb-95	35.15	Day
Nitrogen 13	N-13	0.166	Hour
Oxygen 15	O-15	0.03417	Hour
Phosphorus 32	P-32	14.26	Day
Phosphorus 33	P-33	25.34	Day
Polonium 208	Po-208	2.9	Year
Polonium 210	Po-210	138	Day
Potassium 38	K-38	0.1285	Hour
Potassium 42	K-42	12.36	Hour
Promethium 147	Pm-147	2.62	Year
Plutonium 238	Pu-238	87.75	Year
Plutonium 239	Pu-239	24100	Year
Plutonium 240	Pu-240	6540	Year
Plutonium 242	Pu-242	380000	Year
Radium 226	Ra-226	1600	Year
Rubidium 82	Rb-82	0.0208	Hour
Rubidium 86	Rb-86	18.7	Day
Rubidium 87	Rb-87	4.75E10	Year
Ruthenium 103	Ru-103	39.5	Day
Ruthenium 106	Ru-106	367	Day
Scandium 46	Sc-46	83.8	Day
Selenium 75	Se-75	118.5	Day
Silver 110	Ag-110	252	Day
Silver 111	Ag-111	7.45	Day
Sodium 22	Na-22	2.602	Year
Sodium 24	Na-24	15.02	Hour
Strontium 82	Sr-82	25	Day
Strontium 83	Sr-83	1.35	Day
Strontium 85	Sr-85	64	Day
Strontium 89	Sr-89	52.7	Day
Strontium 90	Sr-90	28.8	Year
Sulphur 35	S-35	87.2	Day
Technetium 99m	Tc-99m	6.05	Hour
Technetium 99	Tc-99	213000	Year
Thallium 201	Tl-201	72.9	Hour
Thallium 204	Tl-204	3.77	Year
Thorium 228	Th-228	1.913	Year
Thorium 229	Th-229	7340	Year
Thorium 230	Th-230	77000	Year
Thorium 232	Th-232	14000000000	Year
Tin 113	Sn-113	115	Day
Tin 119m	Sn-119m	250	Day
Uranium 233	U-233	159000	Year
Uranium 234	U-234	244000	Year
Uranium 235	U-235	704000000	Year
Uranium 236	U-236	23400000	Year
Uranium 238	U-238	4470000000	Year
Vanadium 48	V-48	16	Day
Xenon 133	Xe-133	5.25	Day
Xenon 135	Xe-135	9.1	Hour
Yttrium 87	Y-87	80	Hour
Yttrium 90	Y-90	64	Hour
Zinc 62	Zn-62	9.13	Hour
Zinc 65	Zn-65	244.1	Day

放射性半衰期計算器公式

N = N0e-(0.693t/T1/2)

N: Activity at time 0

N0: Initial activity

t: decay time

T1/2: half time of the radioactive material