Rhodium-106m
Rh-106m · Rhodium, Z = 45, A = 106 · isomeric state m
Rhodium-106m (Rh-106m) carries 5.01e+17 Bq/g, or 1.36e+7 Ci/g: a gigabecquerel is 1.99 ng, no weighable quantity at all. It decays by beta-minus decay with a half-life of 131 minutes, which leaves 0.05% of today's activity after a day and 6.9e-22% after a week.
Among the strong external emitters here: Γ of 0.367 mGy·m²/(GBq·h) ranks 2 of 96, 4.8× Cs-137 and 1.2× Co-60. 1 GBq at 1 m reads 0.367 mGy/h, and 1 Ci at the same distance 13.6 mGy/h — a metre-scale hazard at gigabecquerel activities, and 54.4 MBq is already 20 µSv/h at that distance.
47 lines clear the 20 keV cutoff, but 18 of them carry 90% of the dose rate. The leading one is 511.7 keV at 16.2% of the total — its emission probability is 85.5%, which is also the highest.
This is a shield that has to be designed: 6.39 mm of lead for a factor of two and 25.0 mm for a factor of ten, or 13.2 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 33.1 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 131 minutes (7860 s) |
| Decay mode | beta-minus decay |
| Specific activity | 5.01e+17 Bq/g (1.36e+7 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.367 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.367 mGy/h |
| Dose rate, 1 Ci at 1 m | 13.6 mGy/h |
| Kerma-weighted mean photon energy | 894.1 keV |
47 lines above the cutoff, and what each contributes
1 further line below the 20 keV cutoff, the highest at 3.05 keV and 0.0754% emission probability in all, is excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 511.70 | 85.5 | 16.20 |
| 1046.70 | 30.3525 | 10.97 |
| 1529.40 | 17.5275 | 8.48 |
| 717.20 | 28.899 | 7.53 |
| 1127.70 | 13.68 | 5.25 |
| 748.50 | 19.323 | 5.24 |
| 1200.50 | 11.3715 | 4.58 |
| 616.10 | 20.178 | 4.58 |
| 450.80 | 24.1965 | 4.03 |
| 825.00 | 13.5945 | 4.02 |
| 804.60 | 12.996 | 3.76 |
| 1224.20 | 8.1225 | 3.33 |
| 1573.90 | 6.669 | 3.29 |
| 808.40 | 7.4385 | 2.16 |
Showing the 14 largest contributors of 47 lines above the cutoff; the remainder together carry 16.58% of the dose rate.
6.39 mm of lead halves this spectrum
Solved numerically across all 47 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 6.39 | 25.0 | 3.91 |
| tungsten | 4.17 | 15.8 | 3.80 |
| iron | 13.2 | 45.6 | 3.45 |
| copper | 11.7 | 40.6 | 3.46 |
| concrete | 42.2 | 145 | 3.44 |
| water | 89.2 | 306 | 3.43 |
| aluminum | 38.0 | 130 | 3.44 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over the first hours
Ten half-lives is 21.8 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 3.15 hours.
| 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 | 7.25 hours |
| Time to fall to 1 % | 14.5 hours |
| Time to fall to 0.1 % | 21.8 hours |
Limits of these dose rates
- 0.367 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 6.39 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 1 line under 20 keV, carrying 0.0754% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Rh-106m
Other Rhodium nuclides: Rh-106
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.