Decay Chains is the name given to the stages of radioactive decay of unstable isotopes or elements. Some radioactive isotopes decay into stable isotopes directly but some unstable or radioactive isotopes decay into other unstable isotopes many times before becoming stable and these stages are explained in the ‘decay chain’. These decay chains can be useful when radioactively dating samples by comparing the ratios of the isotopes in the chain.
Decay Chain of Uranium-238
Below is the decay chain of Uranium-238 which is the most common isotope on earth. The stages shown in the table are the most likely outcome of radioactive decay naturally however at some stages it is possible for some of the elements to succumb to either alpha or beta decay which obviously changes the outcome. The possible variations and their likely hood and are shown in the key below.
(92U238 = 2He4 + 90Th234)
(90Th234 = 91Pa234 + 0e-1) (see #1)
(91Pa234 = 92U234 + 0e-1) (see #1)
(92U234 = 90Th230 + 2He4)
(90Th230 = 88Ra226 + 2He4)
(88Ra226 = 86Ra222 + 2He4)
(86Ra222 = 84Po218 + 2He4)
(84Po218 = 82Pb214 + 2He4) (see #2)
(82Pb214 = 83Bi214 + 0e-1) (see #1)
(83Bi214 = 84Po214 + 0e-1) (see #1 + #3)
(84Po214 = 82Pb210 + 2He4)
(82Pb210 = 83Bi210 + 0e-1) (see #1)
(83Bi210 = 84Po210 + 0e-1) (see #1 + #4)
(84Po210 = 82Pb206 + 2He4)
Neutron is converted into proton by beta (β−) decay and emits an electron.
99.98% of the time Polonium-218 will decay by alpha decay to become lead-214 but the remaining 0.02% of the time it will decay with beta radiation and become Astatine-218 (same half life of 3.05 minutes. Astatine-218 in 99.9% of the time will decay by alpha radiation to become Bismuth-214 (half life of 3.10mins) and continue from stage 13 on table.
The other 0.1% of the time Astatine will succumb to beta (β−) decay and become Radon-218 (half life of 1.5 seconds). The Radon-218 then succumbs to alpha decay and becomes Polonium-214 (half life of 35 mille-seconds) which continues from stage 11 on table.
99.979% of the time Bismuth-214 will decay by beta– decay to become Polonium-214 but the remaining 0.0211% of the time it will decay with alpha radiation and become Thallium-210 (same half life of 19.7 minutes). In this instance Thallium-210 will then decay by (β−) radiation to become Lead-210 and continue to stage 12 on table.
99.99987% of the time Bismuth-210 will decay by beta– decay to become Polonium-210 but the remaining 0.00013% of the time it will decay with alpha radiation and become Thallium-206 (same half life of 5.01 days). In this instance Thallium-206 will then decay by (β−) radiation to become Lead-206 (half life of 4.199 minutes) which is the final stable isotope of Uranium-238