Rhenium-188m
Re-188m · Rhenium, Z = 75, A = 188 · isomeric state m
Rhenium-188m (Re-188m) carries 1.99e+18 Bq/g, or 5.38e+7 Ci/g: a gigabecquerel is 502 pg, no weighable quantity at all. It decays by isomeric transition with a half-life of 18.59 minutes, which leaves 4.8e-22% of today's activity after a day and 5.9e-162% after a week.
At 0.00987 mGy·m²/(GBq·h) the air kerma rate constant is 7.8× less than Cs-137 and 31× less than Co-60, placing it 72 of 96 photon emitters in this dataset. 1 GBq at 1 m reads 0.00987 mGy/h, and 1 Ci at the same distance 0.365 mGy/h.
10 lines clear the 20 keV cutoff, but 6 of them carry 90% of the dose rate. The leading one is 61.1 keV at 24.8% of the total — its emission probability is 29.0797%, which is also the highest.
Shielding barely arises: 0.138 mm of lead halves the air kerma rate and 0.474 mm takes it to a tenth, thicknesses a source capsule is likely to exceed on its own. Steel does the halving in 1.02 mm. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way.
Half-life, specific activity and dose rate
| Half-life | 18.59 minutes (1115 s) |
| Decay mode | isomeric transition |
| Specific activity | 1.99e+18 Bq/g (5.38e+7 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.00987 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.00987 mGy/h |
| Dose rate, 1 Ci at 1 m | 0.365 mGy/h |
| Kerma-weighted mean photon energy | 72.68 keV |
10 lines above the cutoff, and what each contributes
1 further line below the 20 keV cutoff, the highest at 10.06 keV and 45.0% emission probability in all, is excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 61.14 | 29.0797 | 24.75 |
| 63.58 | 22.3 | 19.12 |
| 59.72 | 16.80807 | 14.30 |
| 105.85 | 11.5 | 13.29 |
| 70.21 | 12.34381 | 10.78 |
| 69.39 | 9.66626 | 8.43 |
| 92.46 | 5.2 | 5.26 |
| 71.19 | 2.67755 | 2.34 |
| 169.44 | 0.51 | 1.03 |
| 156.05 | 0.38 | 0.69 |
0.138 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 | 0.138 | 0.474 | 3.44 |
| tungsten | 0.0884 | 0.332 | 3.76 |
| iron | 1.02 | 4.03 | 3.97 |
| copper | 0.686 | 2.79 | 4.07 |
| concrete | 12.9 | 44.0 | 3.40 |
| water | 35.7 | 119 | 3.34 |
| aluminum | 10.8 | 36.8 | 3.42 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over the first hours
Ten half-lives is 3.10 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 26.8 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 | 1.03 hours |
| Time to fall to 1 % | 2.06 hours |
| Time to fall to 0.1 % | 3.09 hours |
Limits of these dose rates
- 0.00987 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 0.138 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 1 line under 20 keV, carrying 45.0% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Re-188m
Other Rhenium nuclides: Re-188
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.