Iron-59
Fe-59 · Iron, Z = 26, A = 59
At 1.84e+15 Bq/g — 4.98e+4 Ci/g — a gigabecquerel of Fe-59 amounts to 543 ng, which is why activity rather than mass is how anyone states the quantity. Iron-59 decays by beta-minus decay with a half-life of 44.490 days, falling to 62.66% of today's activity in a month and 0.34% in a year.
1 GBq at 1 m reads 0.147 mGy/h, and 1 Ci at the same distance 5.43 mGy/h, from an air kerma rate constant of 0.147 mGy·m²/(GBq·h) — 1.9× Cs-137 and 2.1× less than Co-60, and 13 of 96 among the photon emitters carried here.
8 lines clear the 20 keV cutoff, but 2 of them carry 90% of the dose rate. The leading one is 1099.2 keV at 53.1% of the total — its emission probability is 56.5%, which is also the highest.
This is a shield that has to be designed: 9.80 mm of lead for a factor of two and 32.9 mm for a factor of ten, or 15.9 mm of steel to halve it, at which point the mass of the shield is part of the problem. Reaching 20 µSv/h from 1 GBq at a metre takes 28.4 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 44.490 days (3.844e+6 s) |
| Decay mode | beta-minus decay |
| Specific activity | 1.84e+15 Bq/g (4.98e+4 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.147 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.147 mGy/h |
| Dose rate, 1 Ci at 1 m | 5.43 mGy/h |
| Kerma-weighted mean photon energy | 1182 keV |
1099 keV leads, and 2 lines make 90%
3 further lines below the 20 keV cutoff, the highest at 7.68 keV and 0.0212% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 1099.24 | 56.5 | 53.12 |
| 1291.59 | 43.2 | 46.12 |
| 192.34 | 3.08 | 0.49 |
| 142.65 | 1.02 | 0.11 |
| 334.80 | 0.27 | 0.08 |
| 1481.70 | 0.059 | 0.07 |
| 382.00 | 0.018 | 0.01 |
| 189.00 | 0.0011 | 0.00 |
9.80 mm of lead halves this spectrum
Solved numerically across all 8 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 9.80 | 32.9 | 3.36 |
| tungsten | 6.10 | 20.5 | 3.35 |
| iron | 15.9 | 53.1 | 3.34 |
| copper | 14.2 | 47.4 | 3.34 |
| concrete | 50.2 | 168 | 3.34 |
| water | 106 | 354 | 3.33 |
| aluminum | 45.2 | 151 | 3.34 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over months and years
Ten half-lives is 1.22 years — a storage problem rather than a disposal one, with 0.34% of today's activity still there after a year. The mean life 1/λ is 64.2 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 | 148 days |
| Time to fall to 1 % | 296 days |
| Time to fall to 0.1 % | 1.21 years |
Limits of these dose rates
- 0.147 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 9.80 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 3 lines under 20 keV, carrying 0.0212% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Fe-59
Other Iron nuclides: Fe-55
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.