Selenium-75
Se-75 · Selenium, Z = 34, A = 75
At 5.38e+14 Bq/g — 1.46e+4 Ci/g — a gigabecquerel of Se-75 amounts to 1.86 µg, which is why activity rather than mass is how anyone states the quantity. Selenium-75 decays by electron capture with a half-life of 119.78 days, falling to 84.06% of today's activity in a month and 12.08% in a year.
At 0.0481 mGy·m²/(GBq·h) the air kerma rate constant is 1.6× less than Cs-137 and 6.4× less than Co-60, placing it 43 of 96 photon emitters in this dataset. 1 GBq at 1 m reads 0.0481 mGy/h, and 1 Ci at the same distance 1.78 mGy/h.
15 lines clear the 20 keV cutoff, but 4 of them carry 90% of the dose rate. The leading one is 264.7 keV at 41.8% of the total — its emission probability is 58.9%, which is also the highest.
A half-value layer of 0.855 mm in lead puts this in foil and thin sheet, with 6.34 mm needed if the material is steel; ten-fold attenuation comes at 4.15 mm of lead. Reaching 20 µSv/h from 1 GBq at a metre takes 1.18 mm of lead. The tenth-value layer runs 4.8 times the half-value layer, not the 3.32 a single energy would give.
Half-life, specific activity and dose rate
| Half-life | 119.78 days (1.035e+7 s) |
| Decay mode | electron capture |
| Specific activity | 5.38e+14 Bq/g (1.46e+4 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0481 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0481 mGy/h |
| Dose rate, 1 Ci at 1 m | 1.78 mGy/h |
| Kerma-weighted mean photon energy | 252.6 keV |
15 lines above the cutoff, and what each contributes
7 further lines below the 20 keV cutoff, the highest at 14.89 keV and 65.9% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 264.66 | 58.9 | 41.82 |
| 279.54 | 25.02072 | 18.95 |
| 136.00 | 58.5466 | 18.66 |
| 400.66 | 11.41482 | 12.88 |
| 121.12 | 17.20469 | 4.79 |
| 303.92 | 1.31465 | 1.10 |
| 198.61 | 1.49606 | 0.76 |
| 96.73 | 3.4486 | 0.74 |
| 66.05 | 1.11144 | 0.20 |
| 572.40 | 0.03616 | 0.06 |
| 24.38 | 0.02533 | 0.02 |
| 419.08 | 0.01237 | 0.01 |
| 617.67 | 0.00451 | 0.01 |
| 373.86 | 0.00253 | 0.00 |
Showing the 14 largest contributors of 15 lines above the cutoff; the remainder together carry 0.00% of the dose rate.
0.855 mm of lead halves this spectrum
Solved numerically across all 15 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 0.855 | 4.15 | 4.85 |
| tungsten | 0.629 | 3.02 | 4.81 |
| iron | 6.34 | 23.0 | 3.63 |
| copper | 5.26 | 19.6 | 3.73 |
| concrete | 24.9 | 84.4 | 3.38 |
| water | 53.9 | 181 | 3.37 |
| aluminum | 22.3 | 75.5 | 3.39 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over months and years
Ten half-lives is 3.28 years — a storage problem rather than a disposal one, with 12.08% of today's activity still there after a year. The mean life 1/λ is 173 days.
| 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 | 1.09 years |
| Time to fall to 1 % | 2.18 years |
| Time to fall to 0.1 % | 3.27 years |
Limits of these dose rates
- 0.0481 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 0.855 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 7 lines under 20 keV, carrying 65.9% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Se-75
Other Selenium nuclides: Se-79
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.