Selenium has six natural isotopes that occur in significant quantities, along with the trace isotope 79Se, which occurs in minute quantities in uranium ores. Five of these isotopes are stable: 74Se, 76Se, 77Se, 78Se, and 80Se. The last three also occur as fission products, along with 79Se, which has a half-life about 330,000 years, and 82Se, which has the very long half-life of years as it decays via double beta decay to krypton-82 and for practical purposes can be considered to be stable. There are 23 other unstable isotopes that have been characterized, the longest-lived after 79Se being 75Se with its half-life 119.78 days, 72Se at 8.40 days, and 73Se at 7.15 hours. The others are all under an hour and most do not exceed 38 seconds.
List of isotopes
|-id=Selenium-63 | rowspan=2|63Se | rowspan=2 style="text-align:right" | 34 | rowspan=2 style="text-align:right" | 29 | rowspan=2 |62.98191(54)# | rowspan=2 style="text-align:center" | 2016 | rowspan=2|13.2(39) ms | β+, p | 62Ge | rowspan=2|3/2−# | rowspan=2| | rowspan=2| |- | 2p? (<0.5%) | 61Ge |-id=Selenium-64 | rowspan=2|64Se | rowspan=2 style="text-align:right" | 34 | rowspan=2 style="text-align:right" | 30 | rowspan=2|63.97117(54)# | rowspan=2 style="text-align:center" | 2005 | rowspan=2|22.6(2) ms | β+? | 64As | rowspan=2|0+ | rowspan=2| | rowspan=2| |- | β+, p? | 63Ge |-id=Selenium-65 | rowspan=2|65Se | rowspan=2 style="text-align:right" | 34 | rowspan=2 style="text-align:right" | 31 | rowspan=2|64.96455(32)# | rowspan=2 style="text-align:center" | 1993 | rowspan=2|34.2(7) ms | β+, p (87%) | 64Ge | rowspan=2|3/2−# | rowspan=2| | rowspan=2| |- | β+ (13%) | 65As |-id=Selenium-66 | rowspan=2|66Se | rowspan=2 style="text-align:right" | 34 | rowspan=2 style="text-align:right" | 32 | rowspan=2|65.95528(22)# | rowspan=2 style="text-align:center" | 1993 | rowspan=2|54(4) ms | β+ | 66As | rowspan=2|0+ | rowspan=2| | rowspan=2| |- | β+, p? | 65Ge |-id=Selenium-67 | rowspan=2|67Se | rowspan=2 style="text-align:right" | 34 | rowspan=2 style="text-align:right" | 33 | rowspan=2|66.949994(72) | rowspan=2 style="text-align:center" | 1991 | rowspan=2|133(4) ms | β+ (99.5%) | 67As | rowspan=2|5/2−# | rowspan=2| | rowspan=2| |- | β+, p (0.5%) | 66Ge |-id=Selenium-68 | 68Se | style="text-align:right" | 34 | style="text-align:right" | 34 | 67.94182524(53) | style="text-align:center" | 1990 | 35.5(7) s | β+ | 68As | 0+
|-id=Selenium-69 | rowspan=2|69Se | rowspan=2 style="text-align:right" | 34 | rowspan=2 style="text-align:right" | 35 | rowspan=2|68.9394148(16) | rowspan=2 style="text-align:center" | 1974 | rowspan=2|27.4(2) s | β+ (99.95%) | 69As | rowspan=2|1/2− | rowspan=2| | rowspan=2| |- | β+, p (.052%) | 68Ge |-id=Selenium-69m1 | style="text-indent:1em" | 69m1Se | colspan="3" style="text-indent:2em" | 38.85(22) keV | style="text-align:center" | 1988 | 2.0(2) μs | IT | 69Se | 5/2−
|-id=Selenium-69m2 | style="text-indent:1em" | 69m2Se | colspan="3" style="text-indent:2em" | 574.0(4) keV | style="text-align:center" | 1988 | 955(16) ns | IT | 69Se | 9/2+
|-id=Selenium-70 | 70Se | style="text-align:right" | 34 | style="text-align:right" | 36 | 69.9335155(17) | style="text-align:center" | 1950 | 41.1(3) min | β+ | 70As | 0+
|-id=Selenium-71 | 71Se | style="text-align:right" | 34 | style="text-align:right" | 37 | 70.9322094(30) | style="text-align:center" | 1957 | 4.74(5) min | β+ | 71As | (5/2−)
|-id=Selenium-71m1 | style="text-indent:1em" | 71m1Se | colspan="3" style="text-indent:2em" | 48.79(5) keV | style="text-align:center" | 1982 | 5.6(7) μs | IT | 71Se | (1/2−)
|-id=Selenium-71m2 | style="text-indent:1em" | 71m2Se | colspan="3" style="text-indent:2em" | 260.48(10) keV | style="text-align:center" | 1982 | 19.0(5) μs | IT | 71Se | (9/2+)
|-id=Selenium-72 | 72Se | style="text-align:right" | 34 | style="text-align:right" | 38 | 71.9271405(21) | style="text-align:center" | 1948 | 8.40(8) d | EC | 72As | 0+
|-id=Selenium-73 | 73Se | style="text-align:right" | 34 | style="text-align:right" | 39 | 72.9267549(80) | style="text-align:center" | 1948 | 7.15(9) h | β+ | 73As | 9/2+
|-id=Selenium-73m | rowspan=2 style="text-indent:1em" | 73mSe | rowspan=2 colspan="3" style="text-indent:2em" | 25.71(4) keV | rowspan=2 style="text-align:center" | 1953 | rowspan=2|39.8(17) min | IT (72.6%) | 73Se | rowspan=2|3/2− | rowspan=2| | rowspan=2| |- | β+ (27.4%) | 73As |-id=Selenium-74 | 74Se | style="text-align:right" | 34 | style="text-align:right" | 40 | 73.922475933(15) | style="text-align:center" | 1922 | colspan=3 align=center|Observationally StableTheoretically capable of β+β+ decay to 74Ge; half-life is above . | 0+ | 0.0086(3) | |-id=Selenium-75 | 75Se | style="text-align:right" | 34 | style="text-align:right" | 41 | 74.922522870(78) | style="text-align:center" | 1947 | 119.78(3) d | EC | 75As | 5/2+
|-id=Selenium-76 | 76Se | style="text-align:right" | 34 | style="text-align:right" | 42 | 75.919213702(17) | style="text-align:center" | 1922 | colspan=3 align=center|Stable | 0+ | 0.0923(7) | |-id=Selenium-77 | 77SeFission product | style="text-align:right" | 34 | style="text-align:right" | 43 | 76.919914150(67) | style="text-align:center" | 1922 | colspan=3 align=center|Stable | 1/2− | 0.0760(7) | |-id=Selenium-77m | style="text-indent:1em" | 77mSe | colspan="3" style="text-indent:2em" | 161.9223(10) keV | style="text-align:center" | 1947 | 17.36(5) s | IT | 77Se | 7/2+
|-id=Selenium-78 | 78Se | style="text-align:right" | 34 | style="text-align:right" | 44 | 77.91730924(19) | style="text-align:center" | 1922 | colspan=3 align=center|Stable | 0+ | 0.2369 (22) | |-id=Selenium-79 | 79SeLong-lived fission product | style="text-align:right" | 34 | style="text-align:right" | 45 | 78.91849925(24) | style="text-align:center" | 1950 | 3.27(28)×105 y | β− | 79Br | 7/2+
|-id=Selenium-79m | rowspan=2 style="text-indent:1em" | 79mSe | rowspan=2 colspan="3" style="text-indent:2em" | 95.77(3) keV | rowspan=2 style="text-align:center" | 1950 | rowspan=2|3.900(18) min | IT (99.94%) | 79Se | rowspan=2|1/2− | rowspan=2| | rowspan=2| |- | β− (0.056%) | 79Br |-id=Selenium-80 | 80Se | style="text-align:right" | 34 | style="text-align:right" | 46 | 79.9165218(10) | style="text-align:center" | 1922 | colspan=3 align=center|Observationally StableTheoretically capable of β−β− decay to 80Kr | 0+ | 0.4980(36) | |-id=Selenium-81 | 81Se | style="text-align:right" | 34 | style="text-align:right" | 47 | 80.9179930(10) | style="text-align:center" | 1949 | 18.45(12) min | β− | 81Br | 1/2−
|-id=Selenium-81m | rowspan=2 style="text-indent:1em" | 81mSe | rowspan=2 colspan="3" style="text-indent:2em" | 103.00(6) keV | rowspan=2 style="text-align:center" | 1949 | rowspan=2|57.28(2) min | IT (99.95%) | 81Se | rowspan=2|7/2+ | rowspan=2| | rowspan=2| |- | β− (0.051%) | 81Br |-id=Selenium-82 | 82SePrimordial radionuclide | style="text-align:right" | 34 | style="text-align:right" | 48 | 81.91669953(50) | style="text-align:center" | 1922 | 8.76(15)×1019 y | β−β− | 82Kr | 0+ | 0.0882(15) | |-id=Selenium-83 | 83Se | style="text-align:right" | 34 | style="text-align:right" | 49 | 82.9191186(33) | style="text-align:center" | 1937 | 22.25(4) min | β− | 83Br | 9/2+
|-id=Selenium-83m | style="text-indent:1em" | 83mSe | colspan="3" style="text-indent:2em" | 228.92(7) keV | style="text-align:center" | 1947 | 70.1(4) s | β− | 83Br | 1/2−
|-id=Selenium-84 | 84Se | style="text-align:right" | 34 | style="text-align:right" | 50 | 83.9184668(21) | style="text-align:center" | 1960 | 3.26(10) min | β− | 84Br | 0+
|-id=Selenium-85 | 85Se | style="text-align:right" | 34 | style="text-align:right" | 51 | 84.9222608(28) | style="text-align:center" | 1960 | 32.9(3) s | β− | 85Br | (5/2)+
|-id=Selenium-86 | rowspan=2|86Se | rowspan=2 style="text-align:right" | 34 | rowspan=2 style="text-align:right" | 52 | rowspan=2|85.9243117(27) | rowspan=2 style="text-align:center" | 1973 | rowspan=2|14.3(3) s | β− | 86Br | rowspan=2|0+ | rowspan=2| | rowspan=2| |- | β−, n? | 85Br |-id=Selenium-87 | rowspan=2|87Se | rowspan=2 style="text-align:right" | 34 | rowspan=2 style="text-align:right" | 53 | rowspan=2|86.9286886(24) | rowspan=2 style="text-align:center" | 1968 | rowspan=2|5.50(6) s | β− (99.50%) | 87Br | rowspan=2|(3/2+) | rowspan=2| | rowspan=2| |- | β−, n (0.60%) | 86Br |-id=Selenium-88 | rowspan=2|88Se | rowspan=2 style="text-align:right" | 34 | rowspan=2 style="text-align:right" | 54 | rowspan=2|87.9314175(36) | rowspan=2 style="text-align:center" | 1970 | rowspan=2|1.53(6) s | β− (99.01%) | 88Br | rowspan=2|0+ | rowspan=2| | rowspan=2| |- | β−, n (0.99%) | 87Br |-id=Selenium-89 | rowspan=2|89Se | rowspan=2 style="text-align:right" | 34 | rowspan=2 style="text-align:right" | 55 | rowspan=2|88.9366691(40) | rowspan=2 style="text-align:center" | 1971 | rowspan=2|430(50) ms | β− (92.2%) | 89Br | rowspan=2|5/2+# | rowspan=2| | rowspan=2| |- | β−, n (7.8%) | 88Br |-id=Selenium-90 | rowspan=2|90Se | rowspan=2 style="text-align:right" | 34 | rowspan=2 style="text-align:right" | 56 | rowspan=2|89.94010(35) | rowspan=2 style="text-align:center" | 1994 | rowspan=2|210(80) ms | β− | 90Br | rowspan=2|0+ | rowspan=2| | rowspan=2| |- | β−, n? | 89Br |-id=Selenium-91 | rowspan=3|91Se | rowspan=3 style="text-align:right" | 34 | rowspan=3 style="text-align:right" | 57 | rowspan=3|90.94570(47) | rowspan=3 style="text-align:center" | 1975 | rowspan=3|270(50) ms | β− (79%) | 91Br | rowspan=3|1/2+# | rowspan=3| | rowspan=3| |- | β−, n (21%) | 90Br |- | β−, 2n? | 89Br |-id=Selenium-92 | rowspan=3|92Se | rowspan=3 style="text-align:right" | 34 | rowspan=3 style="text-align:right" | 58 | rowspan=3|91.94984(43)# | rowspan=3 style="text-align:center" | 1997 | rowspan=3|90# ms [>300 ns] | β−? | 92Br | rowspan=3|0+ | rowspan=3| | rowspan=3| |- | β−, n? | 91Br |- | β−, 2n? | 90Br |-id=Selenium-92m | style="text-indent:1em" | 92mSe | colspan="3" style="text-indent:2em" | 3072(2) keV | style="text-align:center" | 2012 | 15.7(7) μs | IT | 92Se | (9−)
|-id=Selenium-93 | rowspan=3|93Se | rowspan=3 style="text-align:right" | 34 | rowspan=3 style="text-align:right" | 59 | rowspan=3|92.95614(43)# | rowspan=3 style="text-align:center" | 1997 | rowspan=3|130# ms [>300 ns] | β−? | 93Br | rowspan=3|1/2+# | rowspan=3| | rowspan=3| |- | β−, n? | 92Br |- | β−, 2n? | 91Br |-id=Selenium-93m | style="text-indent:1em" | 93mSe | colspan="3" style="text-indent:2em" | 678.2(7) keV | style="text-align:center" | 2012 | 420(100) ns | IT | 93Se
|-id=Selenium-94 | rowspan=3|94Se | rowspan=3 style="text-align:right" | 34 | rowspan=3 style="text-align:right" | 60 | rowspan=3|93.96049(54)# | rowspan=3 style="text-align:center" | 1997 | rowspan=3|50# ms [>300 ns] | β−? | 94Br | rowspan=3|0+ | rowspan=3| | rowspan=3| |- | β−, n? | 93Br |- | β−, 2n? | 92Br |-id=Selenium-94m | style="text-indent:1em" | 94mSe | colspan="3" style="text-indent:2em" | 2430.0(6) keV | style="text-align:center" | 2020 | 680(50) ns | IT | 94Se | (7−)
|-id=Selenium-95 | rowspan=3|95Se | rowspan=3 style="text-align:right" | 34 | rowspan=3 style="text-align:right" | 61 | rowspan=3|94.96730(54)# | rowspan=3 style="text-align:center" | 2010 | rowspan=3|70# ms [>400 ns] | β−? | 95Br | rowspan=3|3/2+# | rowspan=3| | rowspan=3| |- | β−, n? | 94Br |- | β−, 2n? | 93Br |-id=Selenium-96 | 96Se | style="text-align:right" | 34 | style="text-align:right" | 62 | |style="text-align:center" | 2024
|-id=Selenium-97 | 97Se | style="text-align:right" | 34 | style="text-align:right" | 63 | |style="text-align:center" | 2024
Use of radioisotopes The isotope selenium-75 has radiopharmaceutical uses. For example, it is used in high-dose-rate endorectal brachytherapy, as an alternative to iridium-192.
In paleobiogeochemistry, the ratio in amount of selenium-82 to selenium-76 (i.e, the value of δ82/76Se) can be used to track down the redox conditions on Earth during the Neoproterozoic era in order to gain a deeper understanding of the rapid oxygenation that trigger the emergence of complex organisms.