Rhodium-106
Rh-106 · Rhodium, Z = 45, A = 106
Rhodium-106 (Rh-106) carries 1.31e+20 Bq/g, or 3.54e+9 Ci/g: a gigabecquerel is 7.63 pg, no weighable quantity at all. It decays by beta-minus decay with a half-life of 30.07 seconds, which leaves 0% of today's activity after a day and 0% after a week.
At 0.0275 mGy·m²/(GBq·h) the air kerma rate constant is 2.8× less than Cs-137 and 11× less than Co-60, placing it 50 of 96 photon emitters in this dataset. 1 GBq at 1 m reads 0.0275 mGy/h, and 1 Ci at the same distance 1.02 mGy/h.
59 lines clear the 20 keV cutoff, but 4 of them carry 90% of the dose rate. The leading one is 511.9 keV at 51.6% of the total — its emission probability is 20.4%, which is also the highest.
Halving the air kerma rate calls for 4.79 mm of lead, or 11.5 mm of steel where lead is unwelcome, and a factor of ten calls for 17.2 mm of lead — sheet thicknesses that a glovebox or a transport container can carry. Reaching 20 µSv/h from 1 GBq at a metre takes 2.17 mm of lead.
The beta endpoint is 3.54 MeV, mean 1.41 MeV over 12 branches. That endpoint stops in 15.0 mm of acrylic or 7.08 mm of glass. Of the beta energy, 0.74% turns into X-rays in acrylic and 10.16% in lead.
Half-life, specific activity, dose rate and beta energies
| Half-life | 30.07 seconds (30.07 s) |
| Decay mode | beta-minus decay |
| Specific activity | 1.31e+20 Bq/g (3.54e+9 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0275 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0275 mGy/h |
| Dose rate, 1 Ci at 1 m | 1.02 mGy/h |
| Kerma-weighted mean photon energy | 653.7 keV |
| Beta endpoint / mean | 3.54 MeV / 1.41 MeV |
59 lines above the cutoff, and what each contributes
1 further line below the 20 keV cutoff, the highest at 3.05 keV and 0.00711% emission probability in all, is excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 511.86 | 20.4 | 51.59 |
| 621.93 | 9.9348 | 30.32 |
| 1050.41 | 1.55856 | 7.53 |
| 616.22 | 0.7548 | 2.28 |
| 1128.07 | 0.40392 | 2.07 |
| 873.49 | 0.4386 | 1.82 |
| 1562.25 | 0.1632 | 1.07 |
| 1194.54 | 0.05732 | 0.31 |
| 2112.54 | 0.03448 | 0.28 |
| 1766.25 | 0.03427 | 0.25 |
| 2366.04 | 0.02326 | 0.20 |
| 1988.44 | 0.02611 | 0.20 |
| 1796.94 | 0.02774 | 0.20 |
| 1062.14 | 0.03203 | 0.16 |
Showing the 14 largest contributors of 59 lines above the cutoff; the remainder together carry 1.73% of the dose rate.
4.79 mm of lead halves this spectrum
Solved numerically across all 59 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 4.79 | 17.2 | 3.59 |
| tungsten | 3.22 | 11.4 | 3.54 |
| iron | 11.5 | 39.0 | 3.39 |
| copper | 10.2 | 34.6 | 3.39 |
| concrete | 37.0 | 125 | 3.38 |
| water | 78.3 | 265 | 3.38 |
| aluminum | 33.3 | 113 | 3.38 |
A single energy would give 3.32. What a spread of energies does instead.
15.0 mm of acrylic stops the 3.54 MeV endpoint
Katz–Penfold fit to the 3.54 MeV endpoint — ±10%, and a stopping thickness rather than an attenuation length. Why the material matters more than the thickness.
| Absorber | Range for the endpoint (mm) |
|---|---|
| acrylic | 15.0 |
| aluminum | 6.56 |
| water | 17.7 |
| glass | 7.08 |
| Shield material | Fraction of beta energy converted to X-rays |
|---|---|
| acrylic (Z ≈ 6) | 0.74% |
| lead (Z = 82) | 10.16% |
Activity over the first hours
Ten half-lives is 5.01 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 43.4 seconds.
| 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 | 1.66 minutes |
| Time to fall to 1 % | 3.33 minutes |
| Time to fall to 0.1 % | 4.99 minutes |
Limits of these dose rates
- 0.0275 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 4.79 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 1 line under 20 keV, carrying 0.00711% of all emissions.
- 15.0 mm of acrylic for the 3.54 MeV endpoint: a ±10% fit, and not a skin dose.
- What every sheet leaves out, internal dose included.
Gamma, beta and decay calculators for Rh-106
- Gamma dose rate and shielding
- Beta dose rate and shielding
- Decay and half-life
- Mass and activity
- Detection limits (MDA / MDC)
Other Rhodium nuclides: Rh-106m
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.