Sodium-24m
Na-24m · Sodium, Z = 11, A = 24 · isomeric state m
Sodium-24m (Na-24m) carries 8.62e+23 Bq/g, or 2.33e+13 Ci/g: a gigabecquerel is 1.16 fg, no weighable quantity at all. It decays by isomeric transition with a half-life of 20.18 milliseconds, which leaves 0% of today's activity after a day and 0% after a week.
1 GBq at 1 m reads 0.0641 mGy/h, and 1 Ci at the same distance 2.37 mGy/h, from an air kerma rate constant of 0.0641 mGy·m²/(GBq·h) — 1.2× less than Cs-137 and 4.8× less than Co-60, and 36 of 96 among the photon emitters carried here.
One line at 472.2 keV carries all of that.
Halving the air kerma rate calls for 3.45 mm of lead, or 10.2 mm of steel where lead is unwelcome, and a factor of ten calls for 11.4 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 5.79 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 20.18 milliseconds (0.02018 s) |
| Decay mode | isomeric transition |
| Specific activity | 8.62e+23 Bq/g (2.33e+13 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0641 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0641 mGy/h |
| Dose rate, 1 Ci at 1 m | 2.37 mGy/h |
| Kerma-weighted mean photon energy | 472.2 keV |
A single photon line at 472 keV
No recorded line for Na-24m 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 (%) |
|---|---|---|
| 472.20 | 99.9 | 100.00 |
3.45 mm of lead halves this spectrum
Solved numerically across all 1 line, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 3.45 | 11.4 | 3.32 |
| tungsten | 2.39 | 7.95 | 3.32 |
| iron | 10.2 | 33.8 | 3.32 |
| copper | 8.97 | 29.8 | 3.32 |
| concrete | 33.0 | 110 | 3.32 |
| water | 69.9 | 232 | 3.32 |
| aluminum | 29.7 | 98.6 | 3.32 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over the first hours
Ten half-lives is 0.202 seconds, so the activity moves measurably while a count is running: every figure has to carry the time it was referred to. The mean life 1/λ is 0.0291 seconds.
| 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 | 0.0670 seconds |
| Time to fall to 1 % | 0.134 seconds |
| Time to fall to 0.1 % | 0.201 seconds |
Limits of these dose rates
- 0.0641 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 3.45 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-24m
Other Sodium nuclides: Na-22, Na-24
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.