Iodine-131
I-131 · Iodine, Z = 53, A = 131
At 4.60e+15 Bq/g — 1.24e+5 Ci/g — a gigabecquerel of I-131 amounts to 217 ng, which is why activity rather than mass is how anyone states the quantity. Iodine-131 decays by beta-minus decay with a half-life of 8.0252 days, falling to 54.63% of today's activity in a week and 0.04% in three months.
1 GBq at 1 m reads 0.0522 mGy/h, and 1 Ci at the same distance 1.93 mGy/h, from an air kerma rate constant of 0.0522 mGy·m²/(GBq·h) — 1.5× less than Cs-137 and 5.9× less than Co-60, and 40 of 96 among the photon emitters carried here.
24 lines clear the 20 keV cutoff, but 3 of them carry 90% of the dose rate. The leading one is 364.5 keV at 76.4% of the total — its emission probability is 81.5%, which is also the highest.
Halving the air kerma rate calls for 2.32 mm of lead, or 8.87 mm of steel where lead is unwelcome, and a factor of ten calls for 8.78 mm of lead — sheet thicknesses that a glovebox or a transport container can carry. Reaching 20 µSv/h from 1 GBq at a metre takes 3.30 mm of lead.
The beta endpoint is 0.807 MeV, mean 0.182 MeV over 6 branches; that endpoint comes from a branch of only 0.39%, while 89.6% of decays take the 0.606 MeV branch. Shielding is sized on the endpoint, dose on the mean. That endpoint stops in 2.65 mm of acrylic or 1.25 mm of glass. Of the beta energy, 0.17% turns into X-rays in acrylic and 2.32% in lead.
Half-life, specific activity, dose rate and beta energies
| Half-life | 8.0252 days (6.934e+5 s) |
| Decay mode | beta-minus decay |
| Specific activity | 4.60e+15 Bq/g (1.24e+5 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0522 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0522 mGy/h |
| Dose rate, 1 Ci at 1 m | 1.93 mGy/h |
| Kerma-weighted mean photon energy | 397.1 keV |
| Beta endpoint / mean | 0.807 MeV / 0.182 MeV |
364 keV leads, and 3 lines make 90%
1 further line below the 20 keV cutoff, the highest at 4.54 keV and 0.604% emission probability in all, is excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 364.49 | 81.5 | 76.43 |
| 636.99 | 7.1557 | 11.78 |
| 284.31 | 6.12065 | 4.36 |
| 722.91 | 1.76855 | 3.27 |
| 29.78 | 2.67884 | 1.10 |
| 29.46 | 1.44604 | 0.61 |
| 503.00 | 0.35941 | 0.47 |
| 80.19 | 2.61615 | 0.44 |
| 642.72 | 0.21679 | 0.36 |
| 34.03 | 0.95986 | 0.31 |
| 33.73 | 0.77658 | 0.25 |
| 325.79 | 0.27303 | 0.23 |
| 177.21 | 0.26895 | 0.11 |
| 318.09 | 0.07742 | 0.06 |
Showing the 14 largest contributors of 24 lines above the cutoff; the remainder together carry 0.21% of the dose rate.
2.32 mm of lead halves this spectrum
Solved numerically across all 24 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 2.32 | 8.78 | 3.78 |
| tungsten | 1.65 | 6.15 | 3.72 |
| iron | 8.87 | 30.7 | 3.46 |
| copper | 7.75 | 26.8 | 3.46 |
| concrete | 29.6 | 101 | 3.43 |
| water | 63.2 | 215 | 3.41 |
| aluminum | 26.5 | 91.2 | 3.44 |
A single energy would give 3.32. What a spread of energies does instead.
2.65 mm of acrylic stops the 0.807 MeV endpoint
Katz–Penfold fit to the 0.807 MeV endpoint — ±10%, and a stopping thickness rather than an attenuation length. Why the material matters more than the thickness.
| Absorber | Range for the endpoint (mm) |
|---|---|
| acrylic | 2.65 |
| aluminum | 1.16 |
| water | 3.13 |
| glass | 1.25 |
| Shield material | Fraction of beta energy converted to X-rays |
|---|---|
| acrylic (Z ≈ 6) | 0.17% |
| lead (Z = 82) | 2.32% |
Activity over months and years
Ten half-lives is 80.3 days — a storage problem rather than a disposal one, with 2.0e-12% of today's activity still there after a year. The mean life 1/λ is 11.6 days.
| Elapsed | Fraction remaining |
|---|---|
| 1 half-life | 50.0 % |
| 2 half-lives | 25.0 % |
| 5 half-lives | 3.12 % |
| 10 half-lives | 0.0977 % |
| Time to fall to 10 % of today's activity | 26.7 days |
| Time to fall to 1 % | 53.3 days |
| Time to fall to 0.1 % | 80.0 days |
Limits of these dose rates
- 0.0522 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 2.32 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 1 line under 20 keV, carrying 0.604% of all emissions.
- 2.65 mm of acrylic for the 0.807 MeV endpoint: a ±10% fit, and not a skin dose.
- What every sheet leaves out, internal dose included.
Gamma, beta and decay calculators for I-131
- Gamma dose rate and shielding
- Beta dose rate and shielding
- Decay and half-life
- Mass and activity
- Detection limits (MDA / MDC)
Other Iodine nuclides: I-125, I-129
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.