Iron-55
Fe-55 · Iron, Z = 26, A = 55
Iron-55 decays by electron capture, half-life 2.744 years. Specific activity is 8.77e+13 Bq/g (2.37e+3 Ci/g), so a gigabecquerel comes to 11.4 µg — weighable, but on an analytical balance. Over a year the activity falls to 77.68%, and over forty years to 4.1e-3%.
All 3 recorded photon lines sit below the 20 keV cutoff, so Γ is zero by convention and no external dose rate is quoted. The strongest line is 6.51 keV. A monitor calibrated on Cs-137 under-responds at these energies, so a reading taken without an energy correction understates Fe-55 rather than missing it outright. The recorded spectrum runs 5.89 to 6.51 keV and carries 28.4% emission probability in all, which sets what a thin-window or proportional detector has to see.
Decay is the only handle: a year leaves 77.68% of today's activity and forty years leaves 4.1e-3%, with ten half-lives at 27.4 years and a mean life of 3.96 years per atom.
Half-life 2.744 years, 8.77e+13 Bq/g, and no line recorded
| Half-life | 2.744 years (8.659e+7 s) |
| Decay mode | electron capture |
| Specific activity | 8.77e+13 Bq/g (2.37e+3 Ci/g) |
Activity over decades and centuries
Ten half-lives is 27.4 years, which puts decay storage out of reach: 4.1e-3% survives forty years. The mean life 1/λ, the quantity that enters an integrated dose, is 3.96 years.
| 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 | 9.12 years |
| Time to fall to 1 % | 18.2 years |
| Time to fall to 0.1 % | 27.3 years |
Limits of these figures
- Γ excludes 3 lines under 20 keV, carrying 28.4% of all emissions.
- What every sheet leaves out, internal dose included.
Decay and mass calculators for Fe-55
Other Iron nuclides: Fe-59
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.