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.

Stage

Symbol

Element

Radiation

Half-Life

Decay Product1

U-238

Uranium-238

alpha

4,460,000,000 years

Th-234

(

^{92}U_{238 }=^{2}He_{4 }+^{90}Th_{234})2

Th-234

Thorium-234

beta (β)

24.1 days

Pa-234

(

^{90}Th_{234}=^{91}Pa_{234}+^{0}e_{-1}) (see #1)3

Pa-234

Protactinium-234

beta (β)

1.17 minutes

U-234

(

^{91}Pa_{234}=^{92}U_{234}+^{0}e_{-1}) (see #1)4

U-234

Uranium-234

alpha

247,000 years

Th-230

(

^{92}U_{234}=^{90}Th_{230}+^{2}He_{4})5

Th-230

Thorium-230

alpha

80,000 years

Ra-226

(

^{90}Th_{230 }=^{88}Ra_{226}+^{2}He_{4})6

Ra-226

Radium-226

alpha

1,602 years

Rn-222

(

^{88}Ra_{226 }=^{86}Ra_{222}+^{2}He_{4})7

Rn-222

Radon-222

alpha

3.82 days

Po-218

(

^{86}Ra_{222 }=^{84}Po_{218}+^{2}He_{4})8

Po-218

Polonium-218

alpha

3.05 minutes

Pb-214

(

^{84}Po_{218 }=^{82}Pb_{214}+^{2}He_{4}) (see #2)9

Pb-214

Lead-214

beta (β)

27 minutes

Bi-214

(

^{82}Pb_{214}=^{83}Bi_{214}+^{0}e_{-1}) (see #1)10

Bi-214

Bismuth-214

beta (β)

19.7 minutes

Po -214

(

^{83}Bi_{214}=^{84}Po_{214}+^{0}e_{-1}) (see #1 + #3)11

Po-214

Polonium-214

alpha

1 microsecond

Pb-210

(

^{84}Po_{214}_{ }=^{82}Pb_{210}+^{2}He_{4})12

Pb-210

Lead-210

beta (β)

22.3 years

Bi-210

(

^{82}Pb_{210}=^{83}Bi_{210}+^{0}e_{-1}) (see #1)13

Bi-210

Bismuth-210

beta (β)

5.01 days

Po -210

(

^{83}Bi_{210}=^{84}Po_{210}+^{0}e_{-1}) (see #1 + #4)14

Po-210

Polonium-210

alpha

138.4 days

Pb-206

(

^{84}Po_{210}_{ }=^{82}Pb_{206}+^{2}He_{4})Stable Isotope

Pb-206

Lead-206

–

–

–

Key

Key

# 1

Neutron is converted into proton by beta (β−) decay and emits an electron.

# 2

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.

# 3

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.# 4

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