Strontium-85m
Sr-85m · Strontium, Z = 38, A = 85 · isomeric state m
Strontium-85m (Sr-85m) carries 1.21e+18 Bq/g, or 3.27e+7 Ci/g: a gigabecquerel is 825 pg, no weighable quantity at all. It decays by isomeric transition with a half-life of 67.63 minutes, which leaves 3.9e-5% of today's activity after a day and 1.4e-43% after a week.
At 0.0268 mGy·m²/(GBq·h) the air kerma rate constant is 2.9× less than Cs-137 and 11× less than Co-60, placing it 51 of 96 photon emitters in this dataset. 1 GBq at 1 m reads 0.0268 mGy/h, and 1 Ci at the same distance 0.993 mGy/h.
6 lines clear the 20 keV cutoff, but one of them carries 90% of the dose rate. The leading one is 231.9 keV at 91.3% of the total — its emission probability is 83.9%, which is also the highest.
A half-value layer of 0.781 mm in lead puts this in foil and thin sheet, with 6.46 mm needed if the material is steel; ten-fold attenuation comes at 2.73 mm of lead. Reaching 20 µSv/h from 1 GBq at a metre takes 0.323 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 67.63 minutes (4058 s) |
| Decay mode | isomeric transition |
| Specific activity | 1.21e+18 Bq/g (3.27e+7 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0268 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0268 mGy/h |
| Dose rate, 1 Ci at 1 m | 0.993 mGy/h |
| Kerma-weighted mean photon energy | 225.4 keV |
232 keV carries 91% of the dose rate
12 further lines below the 20 keV cutoff, the highest at 16.09 keV and 13.5% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 231.86 | 83.9 | 91.26 |
| 151.19 | 12.8 | 8.28 |
| 238.78 | 0.275 | 0.31 |
| 129.82 | 0.15 | 0.08 |
| 731.80 | 0.0146 | 0.05 |
| 450.79 | 0.0107 | 0.02 |
0.781 mm of lead halves this spectrum
Solved numerically across all 6 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 0.781 | 2.73 | 3.49 |
| tungsten | 0.580 | 2.03 | 3.49 |
| iron | 6.46 | 21.7 | 3.35 |
| copper | 5.38 | 18.1 | 3.37 |
| concrete | 24.5 | 81.7 | 3.33 |
| water | 52.7 | 175 | 3.33 |
| aluminum | 21.9 | 73.0 | 3.33 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over the first hours
Ten half-lives is 11.3 hours, so the activity moves measurably while a count is running: every figure has to carry the time it was referred to. The mean life 1/λ is 1.63 hours.
| Elapsed | Fraction remaining |
|---|---|
| 1 half-life | 50.0 % |
| 2 half-lives | 25.0 % |
| 5 half-lives | 3.13 % |
| 10 half-lives | 0.0977 % |
| Time to fall to 10 % of today's activity | 3.74 hours |
| Time to fall to 1 % | 7.49 hours |
| Time to fall to 0.1 % | 11.2 hours |
Limits of these dose rates
- 0.0268 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 0.781 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 12 lines under 20 keV, carrying 13.5% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Sr-85m
Other Strontium nuclides: Sr-85, Sr-89, Sr-90
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.