Neptunium-237
Np-237 · Neptunium, Z = 93, A = 237
Neptunium-237 (Np-237) is slow enough to be handled as a material rather than as a trace: 2.60e+7 Bq/g, or 0.000703 Ci/g, puts a gigabecquerel at 38.4 g. Decay is by alpha decay, half-life 2.144 × 10⁶ years, and shedding even one per cent of the activity takes 31,086 years.
Alpha emission is led by 4.788 MeV at 47.64%, one of 12 recorded lines. None of it reaches through skin, so the limit here is intake, not external dose.
Decay is the only handle: forty years leaves 100.00% of today's activity and ten thousand years leaves 99.68%, with ten half-lives at 21.4 million years and a mean life of 3.09 million years per atom.
Half-life 2.144 × 10⁶ years, 2.60e+7 Bq/g, and no line recorded
| Half-life | 2.144 × 10⁶ years (6.766e+13 s) |
| Decay mode | alpha decay |
| Specific activity | 2.60e+7 Bq/g (0.000703 Ci/g) |
12 alpha lines, strongest 4.788 MeV
12 recorded lines from 4.640 to 4.873 MeV, the strongest 4.788 MeV at 47.64% of 100.3% total alpha emission.
| Energy (MeV) | Emission probability (%) |
|---|---|
| 4.7880 | 47.64 |
| 4.7714 | 23.2 |
| 4.7665 | 9.3 |
| 4.6400 | 6.43 |
| 4.6650 | 3.478 |
| 4.8168 | 2.43 |
| 4.8727 | 2.39 |
| 4.8035 | 2.014 |
| 4.7083 | 1.174 |
| 4.7123 | 1.174 |
Activity over geological time
Ten half-lives is 21.4 million years. On any timescale a facility can be planned over the activity is constant — 100.00% is left after forty years — and the mean life 1/λ is 3.09 million 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 | 7.12 million years |
| Time to fall to 1 % | 14.2 million years |
| Time to fall to 0.1 % | 21.4 million years |
Limits — intake, not external dose
- Alpha reaches nothing through skin — Np-237 is an intake problem, not an external one.
- What every sheet leaves out, internal dose included.
Decay and mass calculators for Np-237
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.