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Isotopes of strontium
Isotopes of strontium
Isotopes of strontium
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Isotopes of strontium (38Sr)
Main isotopes[1] Decay
Isotope abun­dance half-life (t1/2) mode pro­duct
82Sr synth 25.35 d ε 82Rb
83Sr synth 32.41 h β+ 83Rb
84Sr 0.56% stable
85Sr synth 64.846 d ε 85Rb
86Sr 9.86% stable
87Sr 7% stable
88Sr 82.6% stable
89Sr synth 50.56 d β− 89Y
90Sr trace 28.91 y β− 90Y
Standard atomic weight Ar°(Sr)

The alkaline earth metal strontium (38Sr) has four stable, naturally occurring isotopes: 84Sr (0.56%), 86Sr (9.86%), 87Sr (7.0%) and 88Sr (82.58%), giving it a standard atomic weight of 87.62.

Only 87Sr is radiogenic; it is produced by decay from the radioactive alkali metal 87Rb, which has a half-life of 4.97 × 1010 years (i.e. more than three times longer than the current age of the universe). Thus, there are two sources of 87Sr in any material: primordial, formed during nucleosynthesis along with 84Sr, 86Sr and 88Sr; and that formed by radioactive decay of 87Rb. The ratio 87Sr/86Sr is the parameter typically reported in geologic investigations;[4] ratios in minerals and rocks have values ranging from about 0.7 to greater than 4.0 (see rubidium–strontium dating). Because strontium has an electron configuration similar to that of calcium, it readily substitutes for calcium in minerals.

In addition to the four stable isotopes, thirty-two unstable isotopes of strontium are known to exist, ranging from 73Sr to 108Sr. Radioactive isotopes of strontium primarily decay into the neighbouring elements yttrium (89Sr and heavier isotopes, via beta minus decay) and rubidium (85Sr, 83Sr and lighter isotopes, via positron emission or electron capture). The longest-lived of these isotopes, are 90Sr with a half-life of 28.91 years, 85Sr at 64.846 days, 89Sr at 50.56 days, and 82Sr at 25.35 days. All other strontium isotopes have half-lives shorter than 10 hours, most under 10 minutes.

Strontium-89 is an artificial radioisotope used in treatment of bone cancer;[5] this application utilizes its chemical similarity to calcium, which allows it to substitute calcium in bone structures. In circumstances where cancer patients have widespread and painful bony metastases, the administration of 89Sr results in the delivery of beta particles directly to the cancerous portions of the bone, where calcium turnover is greatest.

Strontium-90 is a by-product of nuclear fission, present in nuclear fallout. The 1986 Chernobyl nuclear accident contaminated a vast area with 90Sr.[6] It causes health problems, as it substitutes for calcium in bone, giving it a long lifetime in the body. Because it is a long-lived high-energy beta emitter, it is used in SNAP (Systems for Nuclear Auxiliary Power) devices. These devices hold promise for use in spacecraft, remote weather stations, navigational buoys, etc., where a lightweight, long-lived, nuclear-electric power source is required.

In 2020, researchers have found that mirror nuclides 73Sr and 73Br were found to not behave identically to each other as expected.[7]

List of isotopes

[edit]
Nuclide
[n 1] 		Z N Isotopic mass (Da)[8]
[n 2][n 3] 		Half-life[1]
[n 4] 		Decay
mode[1]
[n 5] 		Daughter
isotope
[n 6][n 7] 		Spin and
parity[1]
[n 8][n 4] 		Natural abundance (mole fraction)
Excitation energy Normal proportion[1] Range of variation
73Sr 38 35 72.96570(43)# 25.3(14) ms β+, p (63%) 72Kr (5/2−)
β+ (37%) 73Rb
74Sr 38 36 73.95617(11)# 27.6(26) ms β+ 74Rb 0+
75Sr 38 37 74.94995(24) 85.2(23) ms β+ (94.8%) 75Rb (3/2−)
β+, p (5.2%) 74Kr
76Sr 38 38 75.941763(37) 7.89(7) s β+ 76Rb 0+
β+, p (0.0034%) 75Kr
77Sr 38 39 76.9379455(85) 9.0(2) s β+ (99.92%) 77Rb 5/2+
β+, p (0.08%) 76Kr
78Sr 38 40 77.9321800(80) 156.1(27) s β+ 78Rb 0+
79Sr 38 41 78.9297047(80) 2.25(10) min β+ 79Rb 3/2−
80Sr 38 42 79.9245175(37) 106.3(15) min β+ 80Rb 0+
81Sr 38 43 80.9232114(34) 22.3(4) min β+ 81Rb 1/2−
81m1Sr 79.23(4) keV 390(50) ns IT 81Sr (5/2)−
81m2Sr 89.05(7) keV 6.4(5) μs (7/2+)
82Sr 38 44 81.9183998(64) 25.35(3) d EC 82Rb 0+
83Sr 38 45 82.9175544(73) 32.41(3) h β+ 83Rb 7/2+
83mSr 259.15(9) keV 4.95(12) s IT 83Sr 1/2−
84Sr 38 46 83.9134191(13) Observationally Stable[n 9] 0+ 0.0056(2)
85Sr 38 47 84.9129320(30) 64.846(6) d EC 85Rb 9/2+
85mSr 238.79(5) keV 67.63(4) min IT (86.6%) 85Sr 1/2−
β+ (13.4%) 85Rb
86Sr 38 48 85.9092607247(56) Stable 0+ 0.0986(20)
86mSr 2956.09(12) keV 455(7) ns IT 86Sr 8+
87Sr[n 10] 38 49 86.9088774945(55) Stable 9/2+ 0.0700(20)
87mSr 388.5287(23) keV 2.805(9) h IT (99.70%) 87Sr 1/2−
EC (0.30%) 87Rb
88Sr[n 11] 38 50 87.905612253(6) Stable 0+ 0.8258(35)
89Sr[n 11] 38 51 88.907450808(98) 50.563(25) d β− 89Y 5/2+
90Sr[n 11] 38 52 89.9077279(16) 28.91(3) y β− 90Y 0+
91Sr 38 53 90.9101959(59) 9.65(6) h β− 91Y 5/2+
92Sr 38 54 91.9110382(37) 2.611(17) h β− 92Y 0+
93Sr 38 55 92.9140243(81) 7.43(3) min β− 93Y 5/2+
94Sr 38 56 93.9153556(18) 75.3(2) s β− 94Y 0+
95Sr 38 57 94.9193583(62) 23.90(14) s β− 95Y 1/2+
96Sr 38 58 95.9217190(91) 1.059(8) s β− 96Y 0+
97Sr 38 59 96.9263756(36) 432(4) ms β− (99.98%) 97Y 1/2+
β−, n (0.02%) 96Y
97m1Sr 308.13(11) keV 175.2(21) ns IT 97Sr 7/2+
97m2Sr 830.83(23) keV 513(5) ns IT 97Sr (9/2+)
98Sr 38 60 97.9286926(35) 653(2) ms β− (99.77%) 98Y 0+
β−, n (0.23%) 97Y
99Sr 38 61 98.9328836(51) 269.2(10) ms β− (99.90%) 99Y 3/2+
β−, n (0.100%) 98Y
100Sr 38 62 99.9357833(74) 202.1(17) ms β− (98.89%) 100Y 0+
β−, n (1.11%) 99Y
100mSr 1618.72(20) keV 122(9) ns IT 100Sr (4−)
101Sr 38 63 100.9406063(91) 113.7(17) ms β− (97.25%) 101Y (5/2−)
β−, n (2.75%) 100Y
102Sr 38 64 101.944005(72) 69(6) ms β− (94.5%) 102Y 0+
β−, n (5.5%) 101Y
103Sr 38 65 102.94924(22)# 53(10) ms β− 103Y 5/2+#
104Sr 38 66 103.95302(32)# 50.6(42) ms β− 104Y 0+
105Sr 38 67 104.95900(54)# 39(5) ms β− 105Y 5/2+#
106Sr 38 68 105.96318(64)# 21(8) ms β− 106Y 0+
107Sr 38 69 106.96967(75)# 25# ms
[>400 ns] 		1/2+#
108Sr[9] 38 70
This table header & footer:
  1. ^ mSr – 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. ^ a b # – Values marked # are not purely derived from experimental data, but at least partly from trends of neighboring nuclides (TNN).
  5. ^ Modes of decay:
    EC: Electron capture


    IT: Isomeric transition
    n: Neutron emission
    p: Proton emission
  6. ^ Bold italics symbol as daughter – Daughter product is nearly stable.
  7. ^ Bold symbol as daughter – Daughter product is stable.
  8. ^ ( ) spin value – Indicates spin with weak assignment arguments.
  9. ^ Believed to decay by β+β+ to 84Kr
  10. ^ Used in rubidium–strontium dating
  11. ^ a b c Fission product

See also

[edit]

Daughter products other than strontium

References

[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Isotopes of strontium

View on Grokipedia
from Grokipedia
Strontium ( 38) has four stable —84Sr, 86Sr, 87Sr, and 88Sr—with natural abundances of 0.56%, 9.86%, 7.00%, and 82.58%, respectively. Sixteen radioactive isotopes are also known, ranging from 73Sr to 107Sr, many of which are short-lived beta emitters produced in nuclear reactions or fission. The most significant radioactive isotope is 90Sr, with a of approximately 28.8 years, which behaves chemically like calcium and accumulates in tissue, posing health risks from or reactor accidents.

Stable Isotopes

The stable isotopes of strontium dominate its natural occurrence and are key to various scientific applications. 88Sr is the most abundant, comprising over 82% of terrestrial strontium, while 84Sr is the rarest at under 1%. The 87Sr/86Sr ratio varies due to the radioactive decay of 87Rb (half-life 4.96 × 1010 years) to 87Sr, enabling the rubidium-strontium dating method for determining the age of rocks and meteorites older than about 10 million years. This isotopic signature also serves as a geochemical tracer for tracking flow, sediment provenance, and environmental processes, with global maps of 87Sr/86Sr ratios aiding in and studies.

Radioactive Isotopes

Radioactive strontium isotopes arise primarily from , where 90Sr is produced with a yield of approximately 5.8% alongside other fission products like 89Sr ( 50.5 days). 90Sr decays via beta emission to 90Y ( 64 hours), releasing significant and contributing to long-term ; it has been detected in fallout from atmospheric nuclear tests and accidents like Chernobyl. Other notable isotopes include 85Sr ( 64.8 days, used historically in research) and 82Sr ( 25.3 days), but most have half-lives under a day and limited practical use beyond experiments. Due to strontium's chemical similarity to calcium, these isotopes can enter the via soil and water, leading to in humans and animals, particularly in bones where they may induce or other cancers.

Applications and Significance

Strontium isotopes have diverse applications across geosciences, , and . In and forensics, variations in 87Sr/86Sr ratios in or reflect an individual's geographic origin, enabling migration tracking or identification of remains. Medically, 89Sr is administered intravenously to target metastatic cancer, delivering beta to alleviate in 60–80% of patients while minimizing damage to healthy tissue; it is produced via neutron irradiation of stable 88Sr. Stable isotopes like 84Sr and 86Sr are enriched for use as tracers in nutritional studies or , while 90Sr has powered remote lighthouses and space missions (e.g., Soviet satellites) due to its reliable heat. Overall, strontium isotopes provide critical tools for understanding Earth's history, , and nuclear safety.

General overview

Natural occurrence and abundance

Strontium occurs naturally in the Earth's crust at an average abundance of 370 parts per million (ppm). In standard terrestrial samples, its stable isotopes are present in the following proportions: ⁸⁴Sr at 0.56%, ⁸⁶Sr at 9.86%, ⁸⁷Sr at 7.00%, and ⁸⁸Sr at 82.58%. These abundances reflect the primordial composition of strontium, with minor variations arising from the radiogenic production of ⁸⁷Sr through the decay of ⁸⁷Rb in geological materials. Natural variations in strontium isotope abundances are primarily driven by geological processes such as continental weathering and riverine transport. Rivers deliver strontium from weathered continental rocks to marine environments, where inputs from silicate-rich sources with higher ⁸⁷Sr/⁸⁶Sr ratios can elevate the overall seawater strontium composition compared to more primitive, low-radiogenic sources like volcanic rocks. This riverine flux contributes to localized enrichment of strontium in coastal and marine sediments, influencing the global oceanic inventory and creating measurable isotopic gradients. Trace amounts of the radioactive isotope ⁹⁰Sr are present in the environment due to fallout from atmospheric nuclear weapons testing conducted primarily before the 1963 Partial Test Ban Treaty. The total global production of ⁹⁰Sr from these tests was approximately 622 petabecquerels (PBq). Post-treaty estimates indicate a declining atmospheric and global inventory, with the total strontium-90 burden (including deposited and stratospheric components) at about 15.1 megacuries (roughly 559 PBq) by 1966, continuing to decrease through natural decay and deposition.

Stable versus radioactive isotopes

Strontium (Z = 38) has approximately 36 known isotopes, four of which are stable: ^{84}Sr, ^{86}Sr, ^{87}Sr, and ^{88}Sr. These stable nuclides do not undergo radioactive decay on observable timescales, with half-lives effectively infinite compared to the age of the universe, as determined by their positions near the line of beta stability and lack of measurable decay branches. ^{87}Sr is stable but its natural abundance includes significant radiogenic contributions from the decay of ^{87}Rb. The remaining 32 isotopes are radioactive, spanning mass numbers from ^{73}Sr to ^{108}Sr, and their stability is assessed based on nuclear binding energies and decay pathways rather than natural occurrence. Radioactive strontium isotopes display characteristic decay trends influenced by their neutron-to-proton ratios. Neutron-rich isotopes, typically those with mass numbers greater than 88, predominantly undergo beta minus (β⁻) decay, in which a is converted to a proton, emitting an and an antineutrino, leading to daughters. In contrast, neutron-deficient isotopes, with mass numbers below 84, favor (EC) or beta plus (β⁺) decay, where a proton transforms into a , often resulting in daughters; β⁺ decay is less common due to energy constraints in heavier nuclei. These modes reflect the odd-even pairing effects and the Coulomb barrier's influence on proton-rich decay. The relative stability among strontium isotopes is further enhanced by nuclear shell effects near Z = 38. Even-even configurations, such as ^{88}Sr (with N = 50), benefit from closed proton and shells, approximating a doubly magic nucleus that increases and reduces decay probability compared to neighboring odd-A or odd-odd isotopes. This shell closure contributes to the observed robustness of stable strontium nuclides against or .

Stable isotopes

Properties and characteristics

Strontium has four stable isotopes: ⁸⁴Sr, ⁸⁶Sr, ⁸⁷Sr, and ⁸⁸Sr. These isotopes have atomic masses of 83.913425 u, 85.909260 u, 86.908877 u, and 87.905612 u, respectively. Their nuclear spins are 0 ħ for the even-even isotopes ⁸⁴Sr, ⁸⁶Sr, and ⁸⁸Sr, while ⁸⁷Sr, with an odd number of neutrons, has a spin of 9/2 ħ. In natural terrestrial , the relative abundances are 0.56% for ⁸⁴Sr, 9.86% for ⁸⁶Sr, 7.00% for ⁸⁷Sr, and 82.58% for ⁸⁸Sr, reflecting the greater nuclear stability of the heavier even-even isotope. The nuclear binding energies per for these isotopes, derived from evaluations, are 8.681 MeV for ⁸⁴Sr, 8.710 MeV for ⁸⁶Sr, 8.706 MeV for ⁸⁷Sr, and 8.732 MeV for ⁸⁸Sr. These values indicate a slight increase in stability with , with ⁸⁸Sr having the highest binding energy per among stable Sr isotopes due to even-even effects, consistent with its highest natural abundance. separation energies, which measure the energy required to remove a and thus indicate stability against , are approximately 11.9 MeV for ⁸⁴Sr, 12.3 MeV for ⁸⁶Sr, 8.43 MeV for ⁸⁷Sr (reflecting odd- ), and 11.11 MeV for ⁸⁸Sr. The higher separation energies for even-mass isotopes underscore their enhanced stability.
IsotopeAtomic Mass (u)Spin (ħ)Abundance (%)Binding Energy per Nucleon (MeV)Neutron Separation Energy (MeV)
⁸⁴Sr83.91342500.568.68111.9
⁸⁶Sr85.90926009.868.71012.3
⁸⁷Sr86.9088779/27.008.7068.43
⁸⁸Sr87.905612082.588.73211.11
Due to their similar chemical properties as isotopes of the same element, the stable strontium isotopes exhibit only minor mass-dependent in chemical processes, such as or in strontium compounds, typically resulting in δ⁸⁸/⁸⁶Sr variations of less than 0.5‰. This arises from subtle differences in isotopic masses affecting bond strengths and reaction rates, but it is small compared to lighter elements owing to strontium's high .

Geochemical and cosmochemical significance

Stable strontium isotopes, particularly the 87Sr/86Sr^{87}\mathrm{Sr}/^{86}\mathrm{Sr} ratio, serve as powerful geochemical tracers due to the radiogenic enrichment of 87Sr^{87}\mathrm{Sr} from the decay of 87Rb^{87}\mathrm{Rb}, which varies systematically across Earth's reservoirs. In mantle-derived rocks, this ratio typically ranges from 0.704 to 0.730, reflecting minimal crustal influence, whereas higher values (up to 0.750 or more) indicate interaction with during ascent. This isotopic signature is widely used in igneous to identify crustal contamination processes, such as assimilation or fractional , in volcanic and plutonic systems. In marine , seawater 87Sr/86Sr^{87}\mathrm{Sr}/^{86}\mathrm{Sr} ratios (historically around 0.709, with variations over geological time) track global fluxes and circulation patterns, linking continental to deep-sea records. The rubidium-strontium (Rb-Sr) dating method exploits the of 87Rb^{87}\mathrm{Rb} to stable 87Sr^{87}\mathrm{Sr}, with a of 4.88×10104.88 \times 10^{10} years, precise age determinations for geological materials. This isochron technique plots 87Sr/86Sr^{87}\mathrm{Sr}/^{86}\mathrm{Sr} against 87Rb/86Sr^{87}\mathrm{Rb}/^{86}\mathrm{Sr} for multiple samples from a , yielding both age and initial isotopic composition; it has dated ancient rocks and meteorites up to 4.5 billion years old, such as the Allende . The method's robustness stems from strontium's moderate incompatibility during and its immobility in most metamorphic events, making it ideal for whole-rock and mineral isochrons in terrains. In cosmochemistry, variations in strontium isotope ratios among reveal processes of early solar system differentiation and . Lunar basalts exhibit 87Sr/86Sr^{87}\mathrm{Sr}/^{86}\mathrm{Sr} ratios of approximately 0.700 to 0.702, lower than , suggesting a homogenized reservoir from ocean without significant Rb fractionation. Similarly, carbonaceous chondrites show ratios around 0.698 to 0.710, while ordinary chondrites are slightly higher, indicating heterogeneous accretion and thermal processing in the that influenced planetary compositions. These signatures, preserved in meteorites, provide constraints on the timing and extent of volatile loss and core formation in planetesimals.

Radioactive isotopes

Long-lived isotopes

Long-lived radioactive isotopes of strontium are those with half-lives exceeding approximately 50 days, enabling significant environmental persistence compared to shorter-lived counterparts. These isotopes, primarily produced through or natural decay processes, exhibit or modes and share strontium's chemical similarity to calcium, facilitating their uptake in biological systems and mobility in soils and water. This behavior enhances their potential for long-term ecological and radiological impact, particularly in contaminated environments. Strontium-90 (⁹⁰Sr) is the most prominent long-lived isotope, with a half-life of 28.8 years, decaying via beta emission to yttrium-90 (⁹⁰Y). The beta particle has a maximum energy of 0.546 MeV, while the daughter ⁹⁰Y, with a half-life of 64 hours, further decays by beta emission with a higher energy of up to 2.28 MeV, contributing to the overall radiological hazard. Due to its moderate half-life and fission yield of about 5.7% in uranium-235 reactors, ⁹⁰Sr is a key component of nuclear fallout, where it persists in the environment and bioaccumulates in bone tissue, mimicking calcium's metabolic pathways. Studies of atmospheric nuclear testing and reactor releases have utilized ⁹⁰Sr as a tracer for global dispersion and soil contamination patterns. Strontium-89 (⁸⁹Sr), with a of 50.5 days, is a pure beta emitter decaying to yttrium-89 (⁸⁹Y), releasing a with a maximum energy of 1.49 MeV. It arises primarily from , with a yield of around 5.5% in thermal neutron fission of ²³⁵U, making it a significant in operations and weapons testing. Although its shorter limits long-term persistence relative to ⁹⁰Sr, ⁸⁹Sr contributes to acute radiological doses in fresh fission products and exhibits similar environmental mobility to calcium, aiding its through aquatic and terrestrial systems. Other notable long-lived isotopes include strontium-85 (⁸⁵Sr), which decays by to stable rubidium-85 (⁸⁵Rb) with a of 64.8 days, emitting characteristic X-rays and gamma rays up to 514 keV. This is produced in low yields during fission and has been used in tracer studies for industrial and medical applications, though its environmental role is minor due to limited natural occurrence. Additionally, stable ⁸⁷Sr contains primordial radiogenic contributions from the beta decay of rubidium-87 (⁸⁷Rb), which has an extremely long of 4.88 × 10¹⁰ years, influencing strontium ratios in geological samples and enabling via the rubidium-strontium method. The chemical analogy between and calcium underlies the environmental mobility of all these isotopes, allowing them to substitute for calcium in structures and facilitating their cycling in food webs and .

Short-lived isotopes

Short-lived isotopes of strontium, defined here as those with half-lives less than one year, are primarily of interest in for studying decay processes near the proton and drip lines through accelerator-based experiments. These isotopes exhibit a range of decay modes influenced by their neutron-to-proton ratio: proton-rich variants (lower mass numbers) predominantly undergo (EC) or (β⁺), while neutron-rich ones (higher mass numbers) favor β⁻ decay. Q-values for these decays typically range from 0.1 to several MeV, reflecting the energy available for the transition to daughter nuclei, often with branching ratios dominated by a single primary mode but occasionally including minor or isomeric transitions. Proton-rich examples include ⁸¹Sr and ⁸³Sr. The isotope ⁸¹Sr decays primarily via EC/β⁺ (branching ratio ≈100%) to ⁸¹Rb, with a of 22.3(4) minutes and a Q-value of approximately 5.2 MeV for the β⁺ branch. Similarly, ⁸³Sr has a of 32.41(3) hours and decays by EC (≈86%) and β⁺ (≈14%) to ⁸³Rb, with a total Q-value of 2.276 MeV; the decay populates excited states in the , leading to γ emissions such as 0.356 MeV (≈50% intensity). The borderline case of ⁸²Sr, with a of 25.36(2) days, undergoes pure EC (100%) to ⁸²Rb (Q-value 3.364 MeV), with no significant γ rays due to ground-state feeding. Neutron-rich short-lived isotopes, often produced in fission or fragmentation reactions, demonstrate β⁻ decay schemes that probe shell structures in daughters. For instance, ⁹¹Sr decays by β⁻ (100%) to ⁹¹Y with a of 9.63(5) hours and Q-value of 2.699 MeV, featuring high-energy β particles (E_max ≈ 2.7 MeV) and associated γ rays like 1.024 MeV (≈15%). The ⁹²Sr isotope has a of 2.71(2) hours and β⁻ decays (100%) to ⁹²Y (Q-value 1.911 MeV), with branching to excited levels yielding γ transitions such as 0.935 MeV (≈95%). Further neutron excess is seen in ⁹³Sr, which has an extremely short of 7.423(7) minutes and β⁻ decays (100%) to ⁹³Y (Q-value 4.137 MeV), often with delayed neutron emission (branching ≈20-30%) due to population of high-lying states. At the neutron-rich extreme, isotopes like ¹⁰⁰Sr represent limits of stability, with a half-life of 202(20) ms and β⁻ decay (100%, Q-value ≈7.08 MeV) to ¹⁰⁰Y, including a minor β⁻n channel (≈0.78%) that aids in mapping the drip line. These rapid decays highlight the role of short-lived Sr isotopes in advancing models of nuclear deformation and β-delayed processes in astrophysical r-process simulations.
IsotopeHalf-lifePrimary Decay Mode (Branching)Q-value (MeV)Key Features
⁸¹Sr22.3 minEC/β⁺ (100%)5.2Proton-rich, high Q for imaging studies proxy
⁸²Sr25.36 daysEC (100%)3.364Borderline long, ground-state to ⁸²Rb
⁸³Sr32.41 hEC (86%), β⁺ (14%)2.276γ cascade in ⁸³Rb
⁹¹Sr9.63 hβ⁻ (100%)2.699Fission product analog
⁹²Sr2.71 hβ⁻ (100%)1.911Strong γ from daughter
⁹³Sr7.423 minβ⁻ (100%), β⁻n (~25%)4.137Delayed neutron emission
¹⁰⁰Sr202 msβ⁻ (99.22%), β⁻n (0.78%)7.08Drip-line probe

Production and synthesis

Natural production processes

Strontium isotopes are primarily produced through stellar nucleosynthesis processes in the universe. The stable isotopes ^{86}\text{Sr}, ^{87}\text{Sr}, and ^{88}\text{Sr} are predominantly synthesized via the slow neutron capture process (s-process) occurring in asymptotic giant branch (AGB) stars, where neutron captures on lighter seed nuclei build up heavier elements over extended periods. Peaks in the s-process yield are observed at ^{87}\text{Sr} and ^{88}\text{Sr}, making these isotopes key calibration points for models of nucleosynthesis in low-mass AGB stars. In contrast, ^{84}\text{Sr} is synthesized via the p-process in explosive astrophysical sites such as core-collapse supernovae. The r-process contributes a minor fraction (approximately 15%) to ^{88}\text{Sr}, while ^{86}\text{Sr}, ^{87}\text{Sr}, and the majority of ^{88}\text{Sr} originate from the s-process. In the early solar system, additional ^{87}\text{Sr} was generated through the radioactive decay of its parent isotope ^{87}\text{Rb}, which has a decay constant of \lambda = 1.42 \times 10^{-11} , \text{year}^{-1}. This primordial production occurred as rubidium incorporated into condensing materials decayed over billions of years, contributing to the initial ^{87}\text{Sr}/^{86}\text{Sr} ratios observed in meteorites and differentiating planetary reservoirs. On Earth, geological production of strontium isotopes is minor compared to cosmic origins. Cosmogenic ^{85}\text{Sr} forms in trace amounts via spallation reactions, where high-energy cosmic rays fragment heavier atmospheric nuclei such as to produce this unstable isotope. Similarly, negligible quantities of ^{90}\text{Sr} arise from of natural ^{238}\text{U} in ores, with yields too low to significantly impact environmental inventories.

Artificial production methods

Artificial production of strontium isotopes primarily occurs through nuclear reactor-based fission and processes, as well as accelerator-driven and heavy-ion reactions, enabling the synthesis of both stable and radioactive variants for research and applications. In nuclear reactors, the radioactive isotopes ^{89}Sr and ^{90}Sr are generated as fission products from the neutron-induced fission of ^{235}U, with cumulative fission yields of approximately 4.9% for the 89 (leading to ^{89}Sr) and 5.7% for ^{90}Sr. These yields reflect the distribution of fission fragments around 90, where sits in the light peak of the fission yield curve. The production is efficient in high-flux reactors, such as those used for isotope generation, due to the significant branching ratio in uranium fission. Neutron activation in reactors provides an alternative route for ^{89}Sr via the reaction ^{88}Sr(n,\gamma)^{89}Sr, utilizing enriched ^{88}Sr targets to minimize competing isotopes. The thermal cross-section for this reaction is 0.058 barns, which, while low compared to fission yields, allows appreciable production in high-neutron-flux environments like solution reactors. This method is particularly useful for medical-grade ^{89}Sr, as it avoids the complex separation from fission product mixtures. Accelerator-based methods target neutron-deficient isotopes, such as the short-lived ^{82}Sr (half-life 25.3 days), through proton-induced on thick or targets with beams of 100-800 MeV protons. Facilities like the Los Alamos Meson Physics Facility have routinely produced up to 20-30 Ci of ^{82}Sr per irradiation cycle by bombarding natural , followed by chemical separation using cation exchange . For more exotic neutron-rich isotopes beyond A=98, such as ^{100}Sr and heavier, heavy-ion including multi-nucleon transfer and projectile fragmentation are employed, using beams of heavy projectiles like ^{238}U on light targets to populate the neutron-rich side of the isotope chart. These approaches, conducted at facilities like GSI or , yield cross-sections on the order of microbarns for such rare isotopes, enabling their study in nuclear structure experiments.

Applications and implications

Medical and biological uses

Strontium isotopes exhibit bone-seeking properties due to their to calcium, allowing them to be preferentially incorporated into crystals in tissue during therapeutic applications. This calcium-mimetic behavior facilitates targeted delivery of to osseous sites, particularly in treatments for metastatic . Dosimetry models for these isotopes account for their biodistribution, with effective dose factor for ingested ^{90}Sr estimated at 2.3 \times 10^{-8} Sv/ to the bone surface, guiding safe administration levels in clinical settings. Strontium-89 chloride, a pure beta-emitter, is administered intravenously for palliative relief of in patients with osteoblastic metastases from or . The standard dose is 148 MBq (4 mCi), which localizes primarily to sites of increased bone turnover, providing pain reduction in up to 80% of selected cases. Strontium-90, another beta-emitter, is utilized in high-dose-rate applicators, such as eye plaques, to treat ocular tumors including choroidal melanomas, achieving tumor control rates of approximately 90% with minimal impact on surrounding tissues. Additionally, ^{90}Sr serves as the parent in generators that produce carrier-free ^{90}Y for radioembolization, where ^{90}Y-loaded microspheres are delivered via hepatic arteries to treat unresectable liver tumors.

Scientific and environmental applications

Strontium-90 (⁹⁰Sr) serves as a key tracer for monitoring the dispersion of nuclear fallout in the environment due to its production in fission reactions and its chemical similarity to calcium, which facilitates its incorporation into soils, water, and biota. Following the 1986 Chernobyl accident, approximately 10 PBq of ⁹⁰Sr was released, primarily depositing within 100 km of the reactor site, with long-term mobility occurring through particle dissolution in neutral soils over 10–20 years and secondary contamination via riverine transport in systems like the Pripyat-Dnieper. This isotope's persistence, with an ecological half-life of about 20 years in aquatic environments, enables tracking of fallout plumes, as evidenced by elevated concentrations in the Pripyat River reaching 8 Bq/L during 1986 spring floods and declining to 0.15 Bq/L by 2003. Bioaccumulation of ⁹⁰Sr in food chains, particularly in dairy products, has been a focal point of post-Chernobyl environmental assessments, highlighting risks to human exposure through contaminated agriculture. Due to its affinity for bone tissue, ⁹⁰Sr concentrates in livestock bones after uptake from forage, with transfer to milk showing an ecological half-life of 3–4 years in affected regions; concentrations in milk were typically below 1 Bq/L 15 years after the accident, though countermeasures like clean feeding reduced activity by factors of 2–5. In the broader food web, ⁹⁰Sr bioaccumulates in fish, with concentrations of 1–2 Bq/kg in Dnieper reservoir species by 2002–2003, compared to ~2 kBq/kg in the Chernobyl cooling pond shortly after release, underscoring its role in tracing radionuclide transfer from sediments to higher trophic levels. Stable strontium isotope ratios, particularly ⁸⁷Sr/⁸⁶Sr, are employed in hydrology to delineate water sources and in ecology to reconstruct animal migration patterns, leveraging spatially distinct signatures derived from bedrock geology. In river systems, these ratios vary predictably with lithology, allowing mapping of watershed contributions; for instance, continental-scale isoscapes reveal patterned variations that remain stable over time, aiding in tracing groundwater-surface water interactions. For migratory species like salmon, ⁸⁷Sr/⁸⁶Sr analysis of otoliths provides a record of natal habitat use and movement, as seen in Central Valley Chinook salmon where ratios distinguish rearing in the Stanislaus River (higher values) from the San Joaquin River, revealing that 38–60% of surviving adults were parr migrants rather than fry or smolts. Similarly, in Sacramento River winter-run Chinook, otolith data show 44–65% of adults utilized non-natal habitats like the American River or Delta for juvenile growth, exiting freshwater at comparable sizes despite smaller natal departures. In nuclear forensics, isotopic signatures involving ⁹⁰Sr and ¹³⁷Cs ratios enable attribution of origins, including types and sources, by exploiting differences in fission yields and decay histories. The ⁹⁰Sr/¹³⁷Cs activity ratio, influenced by production rates and half-lives (28.8 years for ⁹⁰Sr versus 30.2 years for ¹³⁷Cs), serves as an age indicator for spent , with measurements via and modeling (e.g., ) achieving ~10% accuracy in estimating discharge dates from . These ratios vary by and , allowing differentiation of sources; for example, post-detonation debris or environmental samples can be matched to specific cycles using fission product fingerprints that include ⁹⁰Sr alongside ¹³⁷Cs. In scenarios like Chernobyl, plume-specific ⁹⁰Sr/¹³⁷Cs ratios (e.g., ~8 in affected regions) aid in source reconstruction when combined with other signatures.

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