Barium-137m
Ba-137m · Barium, Z = 56, A = 137 · isomeric state m
Barium-137m (Ba-137m) carries 1.99e+19 Bq/g, or 5.38e+8 Ci/g: a gigabecquerel is 50.2 pg, no weighable quantity at all. It decays by isomeric transition with a half-life of 2.552 minutes, which leaves 1.4e-168% of today's activity after a day and 0% after a week.
1 GBq at 1 m reads 0.0815 mGy/h, and 1 Ci at the same distance 3.02 mGy/h, from an air kerma rate constant of 0.0815 mGy·m²/(GBq·h) — 1.1× Cs-137 and 3.8× less than Co-60, and 26 of 96 among the photon emitters carried here.
6 lines clear the 20 keV cutoff, but one of them carries 90% of the dose rate. The leading one is 661.7 keV at 98.1% of the total — its emission probability is 89.9%, which is also the highest.
This is a shield that has to be designed: 5.34 mm of lead for a factor of two and 18.1 mm for a factor of ten, or 11.7 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 11.0 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 2.552 minutes (153.1 s) |
| Decay mode | isomeric transition |
| Specific activity | 1.99e+19 Bq/g (5.38e+8 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0815 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0815 mGy/h |
| Dose rate, 1 Ci at 1 m | 3.02 mGy/h |
| Kerma-weighted mean photon energy | 649.8 keV |
662 keV carries 98% of the dose rate
1 further line below the 20 keV cutoff, the highest at 4.97 keV and 0.966% emission probability in all, is excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 661.66 | 89.9 | 98.11 |
| 32.19 | 3.87114 | 0.88 |
| 31.82 | 2.10125 | 0.49 |
| 36.83 | 1.42561 | 0.26 |
| 36.48 | 1.13867 | 0.21 |
| 37.26 | 0.28694 | 0.05 |
5.34 mm of lead halves this spectrum
Solved numerically across all 6 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 5.34 | 18.1 | 3.39 |
| tungsten | 3.54 | 12.0 | 3.39 |
| iron | 11.7 | 39.5 | 3.39 |
| copper | 10.4 | 35.1 | 3.39 |
| concrete | 37.2 | 126 | 3.39 |
| water | 79.0 | 266 | 3.37 |
| aluminum | 33.5 | 113 | 3.39 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over the first hours
Ten half-lives is 25.5 minutes, 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 3.68 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 | 8.48 minutes |
| Time to fall to 1 % | 17.0 minutes |
| Time to fall to 0.1 % | 25.4 minutes |
Limits of these dose rates
- 0.0815 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 5.34 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 1 line under 20 keV, carrying 0.966% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Ba-137m
Other Barium nuclides: Ba-133, Ba-133m
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.