Tantalum-182m2
Ta-182m2 · Tantalum, Z = 73, A = 182 · isomeric state m2
Tantalum-182m2 (Ta-182m2) carries 2.41e+18 Bq/g, or 6.52e+7 Ci/g: a gigabecquerel is 414 pg, no weighable quantity at all. It decays by isomeric transition with a half-life of 15.84 minutes, which leaves 4.3e-26% of today's activity after a day and 2.7e-190% after a week.
At 0.0339 mGy·m²/(GBq·h) the air kerma rate constant is 2.3× less than Cs-137 and 9.0× less than Co-60, placing it 47 of 96 photon emitters in this dataset. 1 GBq at 1 m reads 0.0339 mGy/h, and 1 Ci at the same distance 1.25 mGy/h.
10 lines clear the 20 keV cutoff, but 7 of them carry 90% of the dose rate. The leading one is 171.6 keV at 28.7% of the total — its emission probability is 48%, which is also the highest.
A half-value layer of 0.297 mm in lead puts this in foil and thin sheet, with 2.97 mm needed if the material is steel; ten-fold attenuation comes at 1.47 mm of lead. Reaching 20 µSv/h from 1 GBq at a metre takes 0.216 mm of lead. The tenth-value layer runs 5.0 times the half-value layer, not the 3.32 a single energy would give.
Half-life, specific activity and dose rate
| Half-life | 15.84 minutes (950.4 s) |
| Decay mode | isomeric transition |
| Specific activity | 2.41e+18 Bq/g (6.52e+7 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0339 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0339 mGy/h |
| Dose rate, 1 Ci at 1 m | 1.25 mGy/h |
| Kerma-weighted mean photon energy | 150.2 keV |
10 lines above the cutoff, and what each contributes
2 further lines below the 20 keV cutoff, the highest at 16.27 keV and 58.0% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 171.59 | 48 | 28.73 |
| 146.78 | 36.48 | 18.02 |
| 184.95 | 24 | 15.76 |
| 57.53 | 46.8883 | 11.89 |
| 318.40 | 6.72 | 8.36 |
| 56.28 | 26.89513 | 6.92 |
| 66.07 | 19.70017 | 4.95 |
| 65.31 | 15.51195 | 3.89 |
| 66.98 | 4.18823 | 1.06 |
| 356.47 | 0.288 | 0.41 |
0.297 mm of lead halves this spectrum
Solved numerically across all 10 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 0.297 | 1.47 | 4.95 |
| tungsten | 0.227 | 1.10 | 4.84 |
| iron | 2.97 | 15.3 | 5.15 |
| copper | 2.30 | 12.4 | 5.38 |
| concrete | 18.2 | 65.9 | 3.62 |
| water | 43.8 | 149 | 3.42 |
| aluminum | 15.8 | 58.3 | 3.70 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over the first hours
Ten half-lives is 2.64 hours, 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 22.9 minutes.
| 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 | 52.6 minutes |
| Time to fall to 1 % | 1.75 hours |
| Time to fall to 0.1 % | 2.63 hours |
Limits of these dose rates
- 0.0339 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 0.297 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 2 lines under 20 keV, carrying 58.0% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Ta-182m2
Other Tantalum nuclides: Ta-182, Ta-182m1
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.