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Isotopes of selenium
Isotopes of selenium
Isotopes of selenium
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Isotopes of selenium (34Se)
Main isotopes[1] Decay
Isotope abun­dance half-life (t1/2) mode pro­duct
72Se synth 8.40 d ε 72As
74Se 0.860% stable
75Se synth 119.78 d ε 75As
76Se 9.23% stable
77Se 7.60% stable
78Se 23.7% stable
79Se trace 3.27×105 y β− 79Br
80Se 49.8% stable
82Se 8.82% 8.76×1019 y β−β− 82Kr
Standard atomic weight Ar°(Se)

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,[4] and 82Se, which has the very long half-life of 8.76×1019 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

[edit]
Nuclide
[n 1] 		Z N Isotopic mass (Da)[5]
[n 2][n 3] 		Half-life[1]
[n 4][n 5] 		Decay
mode[1]
[n 6] 		Daughter
isotope
[n 7] 		Spin and
parity[1]
[n 8][n 5] 		Natural abundance (mole fraction)
Excitation energy Normal proportion[1] Range of variation
63Se 34 29 62.98191(54)# 13.2(39) ms β+, p (89%) 62Ge 3/2−#
β+ (11%) 63As
2p? (<0.5%) 61Ge
64Se 34 30 63.97117(54)# 22.6(2) ms β+? 64As 0+
β+, p? 63Ge
65Se 34 31 64.96455(32)# 34.2(7) ms β+, p (87%) 64Ge 3/2−#
β+ (13%) 65As
66Se 34 32 65.95528(22)# 54(4) ms β+ 66As 0+
β+, p? 65Ge
67Se 34 33 66.949994(72) 133(4) ms β+ (99.5%) 67As 5/2−#
β+, p (0.5%) 66Ge
68Se 34 34 67.94182524(53) 35.5(7) s β+ 68As 0+
69Se 34 35 68.9394148(16) 27.4(2) s β+ (99.95%) 69As 1/2−
β+, p (.052%) 68Ge
69m1Se 38.85(22) keV 2.0(2) μs IT 69Se 5/2−
69m2Se 574.0(4) keV 955(16) ns IT 69Se 9/2+
70Se 34 36 69.9335155(17) 41.1(3) min β+ 70As 0+
71Se 34 37 70.9322094(30) 4.74(5) min β+ 71As (5/2−)
71m1Se 48.79(5) keV 5.6(7) μs IT 71Se (1/2−)
71m2Se 260.48(10) keV 19.0(5) μs IT 71Se (9/2+)
72Se 34 38 71.9271405(21) 8.40(8) d EC 72As 0+
73Se 34 39 72.9267549(80) 7.15(9) h β+ 73As 9/2+
73mSe 25.71(4) keV 39.8(17) min IT (72.6%) 73Se 3/2−
β+ (27.4%) 73As
74Se 34 40 73.922475933(15) Observationally Stable[n 9] 0+ 0.0086(3)
75Se 34 41 74.922522870(78) 119.78(3) d EC 75As 5/2+
76Se 34 42 75.919213702(17) Stable 0+ 0.0923(7)
77Se[n 10] 34 43 76.919914150(67) Stable 1/2− 0.0760(7)
77mSe 161.9223(10) keV 17.36(5) s IT 77Se 7/2+
78Se[n 10] 34 44 77.91730924(19) Stable 0+ 0.2369 (22)
79Se[n 11] 34 45 78.91849925(24) 3.27(28)×105 y β− 79Br 7/2+
79mSe 95.77(3) keV 3.900(18) min IT (99.94%) 79Se 1/2−
β− (0.056%) 79Br
80Se[n 10] 34 46 79.9165218(10) Observationally Stable[n 12] 0+ 0.4980(36)
81Se[n 10] 34 47 80.9179930(10) 18.45(12) min β− 81Br 1/2−
81mSe[n 10] 103.00(6) keV 57.28(2) min IT (99.95%) 81Se 7/2+
β− (0.051%) 81Br
82Se[n 10][n 13] 34 48 81.91669953(50) 8.76(15)×1019 y β−β− 82Kr 0+ 0.0882(15)
83Se 34 49 82.9191186(33) 22.25(4) min β− 83Br 9/2+
83mSe 228.92(7) keV 70.1(4) s β− 83Br 1/2−
84Se 34 50 83.9184668(21) 3.26(10) min β− 84Br 0+
85Se 34 51 84.9222608(28) 32.9(3) s β− 85Br (5/2)+
86Se 34 52 85.9243117(27) 14.3(3) s β− 86Br 0+
β−, n? 85Br
87Se 34 53 86.9286886(24) 5.50(6) s β− (99.50%) 87Br (3/2+)
β−, n (0.60%) 86Br
88Se 34 54 87.9314175(36) 1.53(6) s β− (99.01%) 88Br 0+
β−, n (0.99%) 87Br
89Se 34 55 88.9366691(40) 430(50) ms β− (92.2%) 89Br 5/2+#
β−, n (7.8%) 88Br
90Se 34 56 89.94010(35) 210(80) ms β− 90Br 0+
β−, n? 89Br
91Se 34 57 90.94570(47) 270(50) ms β− (79%) 91Br 1/2+#
β−, n (21%) 90Br
β−, 2n? 89Br
92Se 34 58 91.94984(43)# 90# ms [>300 ns] β−? 92Br 0+
β−, n? 91Br
β−, 2n? 90Br
92mSe 3072(2) keV 15.7(7) μs IT 92Se (9−)
93Se 34 59 92.95614(43)# 130# ms [>300 ns] β−? 93Br 1/2+#
β−, n? 92Br
β−, 2n? 91Br
93mSe 678.2(7) keV 420(100) ns IT 93Se
94Se 34 60 93.96049(54)# 50# ms [>300 ns] β−? 94Br 0+
β−, n? 93Br
β−, 2n? 92Br
94mSe 2430.0(6) keV 680(50) ns IT 94Se (7−)
95Se 34 61 94.96730(54)# 70# ms [>400 ns] β−? 95Br 3/2+#
β−, n? 94Br
β−, 2n? 93Br
96Se[6] 34 62
97Se[6] 34 63
This table header & footer:
  1. ^ mSe – Excited nuclear isomer.
  2. ^ ( ) – Uncertainty (1σ) is given in concise form in parentheses after the corresponding last digits.
  3. ^ # – Atomic mass marked #: value and uncertainty derived not from purely experimental data, but at least partly from trends from the Mass Surface (TMS).
  4. ^ Bold half-life – nearly stable, half-life longer than age of universe.
  5. ^ a b # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
  6. ^ Modes of decay:
    EC: Electron capture


    IT: Isomeric transition
    n: Neutron emission
    p: Proton emission
  7. ^ Bold symbol as daughter – Daughter product is stable.
  8. ^ ( ) spin value – Indicates spin with weak assignment arguments.
  9. ^ Theoretically capable of β+β+ decay to 74Ge; half-life is above 2.3×1018 y.
  10. ^ a b c d e f Fission product
  11. ^ Long-lived fission product
  12. ^ Theoretically capable of β−β− decay to 80Kr
  13. ^ Primordial radionuclide

Use of radioisotopes

[edit]

The isotope selenium-75 has radiopharmaceutical uses. For example, it is used in high-dose-rate endorectal brachytherapy, as an alternative to iridium-192.[7]

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.[8][9]

See also

[edit]

Daughter products other than selenium

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Isotopes of selenium

View on Grokipedia
from Grokipedia
Selenium (atomic number 34) has six naturally occurring stable isotopes—^{74}Se, ^{76}Se, ^{77}Se, ^{78}Se, ^{80}Se, and ^{82}Se—which account for all terrestrial selenium, with ^{80}Se being the most abundant at 49.61%. These isotopes exhibit relative natural abundances of 0.89% for ^{74}Se, 9.37% for ^{76}Se, 7.63% for ^{77}Se, 23.77% for ^{78}Se, and 8.73% for ^{82}Se, contributing to the element's standard atomic weight of 78.971. In addition to these stable nuclides, selenium has about 25 known radioactive isotopes, ranging from the short-lived ^{65}Se (half-life 33 ms) to the longer-lived ^{79}Se (half-life 3.27 × 10^5 years), many of which are produced artificially through neutron activation or as fission products in nuclear reactors. Among the radioactive isotopes, ^{75}Se is particularly notable for its practical applications, with a half-life of 119.8 days and decay primarily via , emitting gamma rays including at 0.265 MeV that enable its use in for weld inspections and in diagnostics. ^{79}Se, a beta emitter with maximum beta energy of 0.151 MeV and no significant gamma , arises as a low-yield fission product (approximately 0.04%) from fission, posing an internal health hazard due to its long and potential for , though its environmental concentrations remain trace. Other radioactive selenium isotopes, such as ^{81}Se ( 18.5 minutes) and ^{83}Se ( 22.3 minutes), are typically short-lived and studied in for their decay properties and role in understanding , but they have limited practical utility outside research settings. The isotopic composition of influences its geochemical behavior, with stable isotope ratios used as tracers in environmental and biological studies to track cycling in ecosystems and food chains.

Overview

Natural abundance

Selenium occurs naturally with six stable isotopes: ^{74}Se, ^{76}Se, ^{77}Se, ^{78}Se, ^{80}Se, and ^{82}Se. Their relative abundances in standard atomic weight measurements are as follows:
IsotopeNatural Abundance (%)
^{74}Se0.89(4)
^{76}Se9.37(29)
^{77}Se7.63(16)
^{78}Se23.77(28)
^{80}Se49.61(41)
^{82}Se8.73(22)
These values reflect the primordial distribution established during nucleosynthesis, with minor adjustments from cosmogenic contributions. In Earth's environment, selenium primarily originates from primordial sources and is concentrated in sulfide minerals such as pyrite and chalcopyrite, as well as in soils derived from sedimentary rocks and shales; it is also present in seawater at concentrations of 0.04 to 0.12 μg/L. The total cosmic abundance of selenium is estimated at 62 atoms per 10^6 atoms, based on analyses of meteoritic material representing solar system compositions. Isotopic ratios of exhibit slight variations from primordial values due to geological processes, including transformations and biological cycling, which can fractionate isotopes by up to several permil in sedimentary environments. These abundances were first precisely measured in the mid-20th century using techniques, with key determinations reported in 1948 and refined through the .

Synthetic production

Synthetic production of selenium isotopes began in the post-World War II era, with the first reactor-produced radioisotopes distributed from (ORNL) starting in 1946, following the Project's infrastructure development for nuclear research. Facilities like ORNL's Reactor and later the (HFIR) enabled the of stable targets to generate radioactive isotopes, marking the transition from wartime applications to peacetime scientific and medical uses. The primary method for producing radioactive selenium isotopes involves on stable selenium targets within nuclear reactors. For instance, selenium-75 is synthesized via the reaction 74^{74}Se(n,γ\gamma)75^{75}Se by irradiating enriched selenium-74 targets in high-flux reactors such as HFIR at ORNL, followed by chemical dissolution to yield the isotope in solution. This process has been applied to produce at least 16 radioactive selenium isotopes from stable precursors, with selenium-75 being one of the most commonly generated for due to its suitable and gamma emissions. Accelerator-based production offers an alternative for short-lived selenium isotopes, particularly through proton of suitable targets. Proton of bromine targets, such as via nat^{nat}Br(p,x)73,75^{73,75}Se reactions, generates isotopes like and with high , often using beams at currents around 5 μ\muA for durations of about one hour. Similarly, can be produced by proton-induced reactions on targets, 79,81^{^{79,81}}Br(p,x)72^{72}Se, providing a route for positron-emitting isotopes used in . For stable selenium isotopes, enrichment techniques enhance their purity beyond natural abundances for research applications, contrasting with the baseline primordial distribution. Electromagnetic isotope separation (EMIS), utilizing technology at ORNL since the late 1940s, has been employed to produce enriched stable selenium isotopes from multi-isotopic mixtures. Centrifugal methods, involving gaseous selenium hexafluoride (SeF6_6) in gas centrifuges, allow high-level enrichment of all selenium isotopes to near-pure compositions. isotope separation, leveraging resonance ionization spectroscopy to selectively excite and ionize specific isotopes, is explored for precise, small-scale enrichment of selenium in advanced research settings.

Nuclear characteristics

Stability criteria

Selenium occupies Z = 34 in the periodic table, positioning its stable isotopes within the valley of stability for medium-mass nuclei, where the neutron-to-proton ratio N/Z approximates 1.3 in the mass region around A ≈ 80. This ratio balances the competing effects of the nuclear strong force, which favors neutron-proton symmetry, and the repulsion among protons, which necessitates excess neutrons for stability in nuclei with Z > 20. The (SEMF) provides a theoretical framework for assessing the stability of s by estimating their total B through five key terms: the volume term representing the bulk , the surface term accounting for reduced binding at the nuclear surface, the term for electrostatic repulsion, the asymmetry term penalizing deviations from N ≈ Z, and the pairing term favoring paired nucleons. For isotopes, these terms collectively determine whether a achieves sufficient binding to resist decay, with optimal stability occurring when the asymmetry and contributions are minimized relative to the attractive volume and pairing effects. The per , a key indicator of stability, is approximated by the SEMF as BA=av−asA−1/3−acZ(Z−1)A−4/3−aa(A−2Z)2A2±δA,\frac{B}{A} = a_v - a_s A^{-1/3} - a_c Z(Z-1) A^{-4/3} - a_a \frac{(A - 2Z)^2}{A^2} \pm \frac{\delta}{A}, where typical coefficients for medium-mass nuclei like those of are av≈15.5a_v \approx 15.5 MeV (), as≈16.8a_s \approx 16.8 MeV (surface), ac≈0.72a_c \approx 0.72 MeV (), aa≈23.3a_a \approx 23.3 MeV (), and δ≈11.2A−1/2\delta \approx 11.2 A^{-1/2} MeV (pairing, with the sign positive for even-even, zero for odd-A, and negative for odd-odd configurations). Higher values of B/AB/A (peaking around 8-9 MeV for this region) correspond to greater stability, as nuclides with lower binding energies relative to neighbors are prone to decay. For , isotopes with mass numbers A = 74–82 achieve stability primarily due to even-even configurations that maximize the positive pairing term, enhancing their , while odd-A neighbors outside this range lack this advantage and exhibit . These even-even nuclides lie at the point where the asymmetry term is balanced against the increasing term, ensuring they reside near the maximum of the curve.

Decay processes

Radioactive isotopes of primarily undergo or , depending on whether they are neutron-rich or proton-rich relative to the line of stability. Neutron-rich isotopes, such as those with mass numbers greater than 82, predominantly decay via beta-minus (β⁻) emission, converting a into a proton, an , and an antineutrino, thereby increasing the to . This mode is favored in these nuclides due to their excess neutrons, which drive the nucleus toward stability by adjusting the neutron-to-proton ratio. For example, in ⁷⁹Se, the decay proceeds 100% via β⁻ to ⁷⁹Br, with a Q-value of 150.9 ± 1.7 keV and a maximum (E_max) of approximately 151 keV. In contrast, proton-rich isotopes, such as those with mass numbers less than 74, typically decay via (EC) or, less commonly, positron emission (β⁺), reducing the to . involves the nucleus capturing an inner-shell , leading to neutrino emission and often characteristic X-rays or Auger electrons from atomic rearrangement. Proton-rich isotopes favor EC over β⁺ due to the lower for proximity in lighter nuclei. is rare and not observed as a significant mode in isotopes, as the relatively low mass and binding energies make it energetically unfavorable compared to beta processes. Following or , the nucleus is often left in an , leading to deexcitation via gamma (γ) emission or . Gamma rays carry away the excess as photons, with energies corresponding to the differences between nuclear energy levels. In many cases, this is accompanied by isomeric transitions, where long-lived excited states (isomers) decay to lower levels. For instance, in the decay of ⁷⁵Se, which undergoes 100% EC to ⁷⁵As with a Q-value of 863.4 ± 0.3 keV, the process populates excited states in ⁷⁵As, resulting in prominent gamma emissions at 66.0 keV (3.4%), 121.0 keV (17.4%), 136.0 keV (59.3%), and 265.0 keV (58.7%), among others; branching ratios to specific levels in ⁷⁵As are approximately 28% to the and 72% to excited states, influencing the observed gamma spectrum. These cascades provide insights into the nuclear of the nuclei. Similar gamma emissions occur after β⁻ decays in neutron-rich isotopes, though pure ground-state-to-ground-state transitions, like in ⁷⁹Se, produce no accompanying gamma rays. The preference for specific decay modes in selenium isotopes is influenced by the nuclear shell structure, particularly near magic neutron numbers like N=50 (for example, in neutron-rich isotopes approaching ⁸⁴Se). The closed neutron shell at N=50 enhances stability, leading to suppressed β⁻ decay rates for neutron-rich isotopes approaching this closure due to reduced phase space and forbidden transitions between shell-model configurations. This shell effect alters branching ratios and Q-values, making decays to low-lying states in daughter nuclei more probable while inhibiting high-energy transitions. For proton-rich isotopes near Z=34 and N approaching 50 from below, shell closures contribute to favored EC branches by stabilizing the initial and final states. These influences are evident in systematic studies of beta-decay properties across the region.

Stable isotopes

Individual properties

Selenium has six stable isotopes: ^{74}Se, ^{76}Se, ^{77}Se, ^{78}Se, ^{80}Se, and ^{82}Se. Their nuclear properties, including atomic masses, mass excesses, nuclear spins, and natural abundances, are precisely determined from experimental data evaluated in the Atomic Mass Evaluation 2020 (AME2020). Atomic masses and mass excesses reflect the binding energies of these nuclei, with uncertainties indicating measurement precision. Nuclear spins arise from unpaired nucleons, resulting in zero spin for even-mass isotopes and half-integer spin for the odd-mass ^{77}Se. Natural abundances vary slightly in terrestrial samples due to minor isotopic fractionation during geological processes, but standard values are well-established. The following table summarizes key properties for these isotopes, with atomic masses in unified atomic mass units (u), mass excesses in keV, spins including parity, and magnetic dipole moments in nuclear magnetons (μ_N) where applicable (zero for even-mass isotopes due to paired nucleons; for ^{77}Se, the value is measured via hyperfine interactions). Quadrupole moments are not applicable for spin-1/2 nuclei like ^{77}Se. Data are from AME2020 for masses and excesses, and standard nuclear data compilations for spins and moments. Abundances are standard terrestrial values from IUPAC evaluations.
IsotopeAtomic Mass (u)Mass Excess (keV)Nuclear Spin (J^π)Magnetic Moment (μ/μ_N)Natural Abundance (%)
^{74}Se73.922475933(16)-72213.210(15)0^+00.86(3)
^{76}Se75.919213702(17)-75251.959(16)0^+09.23(7)
^{77}Se76.919914150(66)-74599.497(62)1/2^-+0.53504(6)7.60(7)
^{78}Se77.917309244(192)-77025.952(179)0^+023.69(22)
^{80}Se79.916521761(1015)-77759.487(947)0^+049.80(36)
^{82}Se81.916699531(50)-77593.895(46)0^+08.82(15)
These properties confirm the stability of isotopes, with no observed channels under terrestrial conditions (though ^{82}Se undergoes extremely slow ). The mass excesses decrease with increasing neutron number, consistent with nuclear shell effects near N=50. In natural samples, isotopic ratios like ^{82}Se/^{76}Se show variations up to several per mil (δ^{82/76}Se ≈ -13‰ to +5‰) due to redox-dependent during biogeochemical cycling, but these do not alter the bulk elemental composition significantly.

Geochemical significance

Stable selenium isotopes exhibit mass-dependent during geochemical cycling, primarily driven by redox transformations such as microbial reduction and inorganic precipitation, which preferentially incorporate lighter isotopes into reduced species like (Se(-II)) and heavier isotopes into oxidized forms like selenate (Se(VI)). This is quantified using the δ82/76Se notation, defined as δ82/76Se = [(82Se/76Se)sample / (82Se/76Se)standard - 1] × 1000, where the standard is NIST SRM 3149. Typical δ82/76Se values in natural samples range from approximately -13‰ to +5‰ in selenium-rich ores, such as the Yutangba deposit, reflecting intense during enrichment processes. In contrast, marine shales and sediments show narrower ranges of -3‰ to +3‰, while surface waters and soils often fall between -2‰ and +1‰, influenced by biological uptake and effects. Selenium isotope ratios serve as effective tracers for reconstructing conditions in ancient oceans, where variations in δ82/76Se reflect shifts between oxic, suboxic, and anoxic environments. For instance, lighter δ82/76Se values (down to -5‰) in early black shales indicate ferruginous (iron-rich anoxic) seawater, with enrichment linked to quantitative removal under low-oxygen conditions. Similarly, progressive oxidation during the and is evidenced by trends toward heavier δ82/76Se in marine sediments, signaling increased oceanic oxygenation and partial Se removal in oxygenated surface waters. These proxies complement other indicators like and isotopes, providing insights into the timing and extent of Earth's oxygenation events. Selenium isotopes in meteorites reveal cosmogenic and nucleosynthetic effects, with minor contributions to 78Se production from interactions during space exposure, aiding studies of pre-atmospheric size and exposure . High-precision measurements show small δ82/76Se anomalies in chondrites, attributed to cosmogenic alterations, which help distinguish between genetic sources of volatiles in planetary materials. For example, CI chondrites exhibit δ82/76Se values consistent with outer solar system delivery to , with 78Se serving as a reference in ratio calculations to quantify these effects. Recent studies since 2010 have applied selenium isotopes to track volcanic emissions and environmental pollution sources. In volcanic systems, δ82/76Se variations during magma degassing on Mount Etna demonstrate kinetic fractionation, with erupted lavas showing lighter values (-0.5‰ to -1.5‰) compared to undegassed melts, enabling quantification of volatile loss in subduction zones. For pollution tracking, selenium isotopes distinguish anthropogenic inputs from coal mining, where mine drainage exhibits distinct δ82/76Se signatures (-1‰ to +2‰) relative to natural backgrounds, facilitating long-range transport assessments over hundreds of kilometers in river systems. These applications highlight Se isotopes' utility in monitoring remediation efficacy, as isotopic shifts indicate permanent removal versus temporary attenuation in contaminated watersheds.

Radioactive isotopes

Key examples

One of the most studied radioactive isotopes of selenium is ^{75}Se, which has a of 119.8 days and primarily undergoes decay to ^{75}As. This isotope emits characteristic gamma rays at energies of 136 keV, 265 keV, and 401 keV, originating from the de-excitation of the daughter nucleus. It was first reported in through early nuclear research efforts. Another notable long-lived radioactive isotope is ^{79}Se, with a half-life of 3.27 × 10^5 years, decaying via β⁻ emission to stable ^{79}Br. This isotope exhibits low specific activity and appears as a minor fission product in waste, contributing minimally to overall due to its extended lifetime and lack of gamma emission. Among shorter-lived examples, ^{73}Se has a of 7.15 hours and decays primarily by to ^{73}As. In contrast, the neutron-rich ^{81}Se is highly unstable, with a of 18.5 minutes, undergoing rapid β⁻ decay to ^{81}Br. Selenium isotopes also feature isomeric states, such as ^{77m}Se, which has a brief of 17.5 seconds and decays via isomeric transition to the of ^{77}Se.

Production and half-lives

Radioactive isotopes, such as ^{75}Se, are synthesized primarily through reactions on isotopes in nuclear reactors. The isotope ^{75}Se is produced via the reaction ^{74}Se(n,\gamma)^{75}Se, which has a cross-section of 66.8 \pm 1.4 barns. In a typical high-flux with a on the order of 10^{14} n/cm²/s, the production yield for ^{75}Se can reach several curies per milligram of enriched ^{74}Se target after extended , depending on the time and target enrichment (typically 90-95% ^{74}Se). The of ^{75}Se is precisely measured as 119.78 \pm 0.05 days through gamma-ray spectroscopy of its characteristic emissions at 136 keV and 265 keV, with uncertainties derived from high-resolution detector calibrations and . For the longer-lived ^{79}Se, the is 327,000 \pm 20,000 years, determined via beta-particle counting in scintillation detectors combined with for absolute activity normalization; the error reflects statistical counting uncertainties and chemical yield variations in sample preparation. These measurements supersede earlier estimates and are critical for and assessments. The decay kinetics of these isotopes follow the standard radioactive decay law, where the decay constant is given by λ=ln⁡2T1/2\lambda = \frac{\ln 2}{T_{1/2}} and the activity AA is A=λNA = \lambda N with NN representing the number of radioactive atoms. For ^{75}Se, this yields λ≈6.71×10−8\lambda \approx 6.71 \times 10^{-8} s^{-1}, while for ^{79}Se, λ≈6.71×10−14\lambda \approx 6.71 \times 10^{-14} s^{-1}, highlighting the vast difference in decay rates. ^{79}Se arises mainly as a fission product in nuclear reactors, with a cumulative fission yield of approximately 0.05% in the thermal neutron-induced fission of ^{235}U, contributing to long-term radioactive inventory in spent fuel. This yield is evaluated from post-irradiation radiochemical separations and beta-counting of accumulated activity relative to monitored fission monitors like ^{144}Ce.

Applications

Industrial uses

Selenium-75 is extensively utilized in gamma for non-destructive testing in industrial settings, particularly for inspecting welds and evaluating integrity. Its gamma emissions, with principal energies at 136 keV and 265 keV, allow effective penetration of materials up to 40 mm thick, making it suitable for mid-range applications where high image quality is required. Sources are typically double-encapsulated for safety and can achieve activities up to 100 GBq, enabling efficient exposure times while minimizing the size of controlled areas at sites like offshore oil rigs and power plants. In logging, selenium-75 serves as a gamma source to determine formation and through measurements, aiding in the assessment of subsurface reservoirs without physical sampling. This application leverages the isotope's moderate energy spectrum to provide accurate profiles in boreholes, contributing to and production optimization. Selenium-75 is also employed as a tracer in for industrial systems, where small quantities are introduced to identify pathways of fluid escape in pipelines, vessels, and process equipment. The gamma emissions facilitate non-invasive tracking with portable detectors, enhancing maintenance efficiency in sectors like petrochemical processing. The adoption of selenium-75 in these industrial roles expanded in the late , particularly from the 1980s onward, due to advancements in source design that improved thermal stability—up to 1200°C in some alloys—allowing it to replace in high-temperature environments without frequent source changes. Its longer of 120 days compared to iridium-192's 74 days further supports sustained operational use.

Scientific and medical roles

Stable isotopes of selenium, such as ^{77}Se and ^{82}Se, serve as safe and nutritionally relevant tracers in studies assessing selenium bioavailability and metabolism in humans and animals. These enriched isotopes allow researchers to track absorption, retention, and incorporation into biological tissues without the risks associated with radioactive alternatives, providing insights into how different chemical forms of selenium—such as selenite, selenate, or yeast-bound selenium—affect nutritional uptake. For instance, in human nutrition research, stable isotope tracers have been used to evaluate the bioavailability of selenium from various dietary sources, demonstrating differences in absorption rates between inorganic and organic forms. In animal models, including ruminants, ^{77}Se-enriched yeast and ^{82}Se-selenite have been administered to quantify rumen metabolism and overall retention, highlighting species-specific variations in selenium utilization. The radioactive isotope ^{75}Se, particularly in the form of , has been employed in for to evaluate pancreatic function, including the detection of pancreatic lesions that may indicate tumors or other abnormalities. This application leverages the uptake of by pancreatic tissue, allowing visualization of functional abnormalities through . Although less common today due to advancements in other modalities like computed tomography (CT) and , historical studies confirmed its utility in detecting pancreatic disorders with administered activities around 150 MBq, balancing diagnostic yield against radiation exposure. estimates for ^{75}Se-selenomethionine indicate effective doses of approximately 0.006 mSv/MBq, with primary organ doses to the and other tissues informing safe clinical protocols. While direct applications to cancer are limited, its role in pancreatic evaluation has contributed to early detection strategies in . ^{79}Se, a long-lived fission product with a half-life of about 3.27 × 10^5 years, is a focus of research for long-term environmental monitoring in the context of nuclear waste management. As a key radionuclide in safety assessments for geological repositories, ^{79}Se migration is tracked through radiochemical separation techniques like ion exchange or coprecipitation, enabling detection in effluents, low- and intermediate-level wastes, and surrounding groundwater. Studies have developed methods for its quantification via inductively coupled plasma mass spectrometry (ICP-MS), supporting models of radionuclide release and transport to predict environmental impacts over millennia. Continuous monitoring approaches, including gaseous emission detection from reprocessing plants, underscore its role in ensuring compliance with regulatory limits for radioactive waste disposal. Emerging research highlights isotope effects in selenoprotein synthesis, where stable isotopes are used to probe the incorporation of into residues essential for enzymes like peroxidases. Non-radioactive techniques, such as those employing enriched ^{77}Se or ^{82}Se, facilitate targeted profiling of the selenoproteome, revealing how availability influences protein expression and post-translational modifications in cellular . Post-2015 studies have advanced these methods with high-resolution , enabling precise measurement of isotopic signatures in biological samples to study selenoprotein dynamics under varying nutritional conditions. Additionally, investigations into natural variations in dietary isotope ratios have linked them to differences, with preliminary associations to health outcomes like oxidative stress-related diseases, though clinical trials specifically targeting isotope ratios remain limited.

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