Sodium-22
Na-22 · Sodium, Z = 11, A = 22
At 2.31e+14 Bq/g — 6.25e+3 Ci/g — a gigabecquerel of Na-22 amounts to 4.33 µg, which is why activity rather than mass is how anyone states the quantity. Sodium-22 decays by electron capture with beta-plus with a half-life of 2.6018 years, falling to 76.61% of today's activity in a year and 2.4e-3% in forty years.
Among the strong external emitters here: Γ of 0.280 mGy·m²/(GBq·h) ranks 5 of 96, 3.6× Cs-137 and 1.1× less than Co-60. 1 GBq at 1 m reads 0.280 mGy/h, and 1 Ci at the same distance 10.4 mGy/h — a metre-scale hazard at gigabecquerel activities, and 71.4 MBq is already 20 µSv/h at that distance.
2 lines clear the 20 keV cutoff, but 2 of them carry 90% of the dose rate. The leading one is 1274.5 keV at 55.4% of the total — its emission probability is 99.94%, which is lower than the 179.91% of the 511.0 keV line it outranks.
This is a shield that has to be designed: 6.53 mm of lead for a factor of two and 26.6 mm for a factor of ten, or 13.5 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 31.5 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 2.6018 years (8.210e+7 s) |
| Decay mode | electron capture with beta-plus |
| Specific activity | 2.31e+14 Bq/g (6.25e+3 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.280 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.280 mGy/h |
| Dose rate, 1 Ci at 1 m | 10.4 mGy/h |
| Kerma-weighted mean photon energy | 933.7 keV |
1275 keV leads, and 2 lines make 90%
No recorded line for Na-22 falls below the 20 keV cutoff, so this table is the whole photon spectrum. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 1274.54 | 99.94 | 55.36 |
| 511.00 | 179.91 | 44.64 |
6.53 mm of lead halves this spectrum
Solved numerically across all 2 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 6.53 | 26.6 | 4.07 |
| tungsten | 4.26 | 16.7 | 3.91 |
| iron | 13.5 | 46.7 | 3.46 |
| copper | 12.0 | 41.6 | 3.47 |
| concrete | 43.0 | 148 | 3.45 |
| water | 90.9 | 313 | 3.45 |
| aluminum | 38.7 | 133 | 3.45 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over decades and centuries
Ten half-lives is 26.0 years, which puts decay storage out of reach: 2.4e-3% survives forty years. The mean life 1/λ, the quantity that enters an integrated dose, is 3.75 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 | 8.64 years |
| Time to fall to 1 % | 17.3 years |
| Time to fall to 0.1 % | 25.9 years |
Limits of these dose rates
- 0.280 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 6.53 mm of lead halves this spectrum, narrow beam, scatter not added back.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Na-22
Other Sodium nuclides: Na-24, Na-24m
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.