Iodine-125
I-125 · Iodine, Z = 53, A = 125
At 6.51e+14 Bq/g — 1.76e+4 Ci/g — a gigabecquerel of I-125 amounts to 1.54 µg, which is why activity rather than mass is how anyone states the quantity. Iodine-125 decays by electron capture with a half-life of 59.407 days, falling to 70.47% of today's activity in a month and 1.41% in a year.
At 0.0400 mGy·m²/(GBq·h) the air kerma rate constant is 1.9× less than Cs-137 and 7.6× less than Co-60, placing it 46 of 96 photon emitters in this dataset. 1 GBq at 1 m reads 0.0400 mGy/h, and 1 Ci at the same distance 1.48 mGy/h.
6 lines clear the 20 keV cutoff, but 4 of them carry 90% of the dose rate. The leading one is 27.5 keV at 46.7% of the total — its emission probability is 73.42821%, which is also the highest.
Shielding barely arises: 0.0176 mm of lead halves the air kerma rate and 0.0598 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 0.0926 mm. Reaching 20 µSv/h from 1 GBq at a metre takes 0.0176 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 59.407 days (5.133e+6 s) |
| Decay mode | electron capture |
| Specific activity | 6.51e+14 Bq/g (1.76e+4 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0400 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0400 mGy/h |
| Dose rate, 1 Ci at 1 m | 1.48 mGy/h |
| Kerma-weighted mean photon energy | 28.57 keV |
6 lines above the cutoff, and what each contributes
1 further line below the 20 keV cutoff, the highest at 4.13 keV and 14.8% emission probability in all, is excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 27.47 | 73.42821 | 46.71 |
| 27.20 | 39.43095 | 25.61 |
| 31.36 | 25.57389 | 12.48 |
| 31.09 | 21.01388 | 10.42 |
| 35.49 | 6.68 | 2.60 |
| 31.70 | 4.56001 | 2.18 |
0.0176 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 | 0.0176 | 0.0598 | 3.40 |
| tungsten | 0.0137 | 0.0468 | 3.41 |
| iron | 0.0926 | 0.317 | 3.42 |
| copper | 0.0610 | 0.209 | 3.42 |
| concrete | 2.71 | 9.21 | 3.40 |
| water | 16.5 | 55.4 | 3.35 |
| aluminum | 1.96 | 6.66 | 3.41 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over months and years
Ten half-lives is 1.63 years — a storage problem rather than a disposal one, with 1.41% of today's activity still there after a year. The mean life 1/λ is 85.7 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 | 197 days |
| Time to fall to 1 % | 1.08 years |
| Time to fall to 0.1 % | 1.62 years |
Limits of these dose rates
- 0.0400 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 0.0176 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 1 line under 20 keV, carrying 14.8% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for I-125
Other Iodine nuclides: I-129, I-131
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.