Thallium-201m
Tl-201m · Thallium, Z = 81, A = 201 · isomeric state m
Thallium-201m (Tl-201m) carries 9.84e+23 Bq/g, or 2.66e+13 Ci/g: a gigabecquerel is 1.02 fg, no weighable quantity at all. It decays by isomeric transition with a half-life of 2.11 milliseconds, which leaves 0% of today's activity after a day and 0% after a week.
1 GBq at 1 m reads 0.115 mGy/h, and 1 Ci at the same distance 4.25 mGy/h, from an air kerma rate constant of 0.115 mGy·m²/(GBq·h) — 1.5× Cs-137 and 2.7× less than Co-60, and 17 of 96 among the photon emitters carried here.
7 lines clear the 20 keV cutoff, but 2 of them carry 90% of the dose rate. The leading one is 588.3 keV at 65.7% of the total — its emission probability is 94.63%, which is also the highest.
Halving the air kerma rate calls for 3.31 mm of lead, or 10.1 mm of steel where lead is unwelcome, and a factor of ten calls for 13.1 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 9.49 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 2.11 milliseconds (0.002110 s) |
| Decay mode | isomeric transition |
| Specific activity | 9.84e+23 Bq/g (2.66e+13 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.115 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.115 mGy/h |
| Dose rate, 1 Ci at 1 m | 4.25 mGy/h |
| Kerma-weighted mean photon energy | 497.0 keV |
588 keV leads, and 2 lines make 90%
1 further line below the 20 keV cutoff, the highest at 12.15 keV and 5.62% emission probability in all, is excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 588.30 | 94.63 | 65.74 |
| 331.10 | 86.1 | 33.02 |
| 72.87 | 6.47977 | 0.49 |
| 70.83 | 3.83861 | 0.29 |
| 83.63 | 2.85819 | 0.23 |
| 82.63 | 2.20369 | 0.17 |
| 84.87 | 0.6545 | 0.05 |
3.31 mm of lead halves this spectrum
Solved numerically across all 7 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 3.31 | 13.1 | 3.95 |
| tungsten | 2.29 | 8.86 | 3.86 |
| iron | 10.1 | 34.3 | 3.42 |
| copper | 8.85 | 30.3 | 3.42 |
| concrete | 33.1 | 112 | 3.38 |
| water | 70.2 | 236 | 3.37 |
| aluminum | 29.7 | 100 | 3.38 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over the first hours
Ten half-lives is 0.0211 seconds, 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 0.00304 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 | 0.00701 seconds |
| Time to fall to 1 % | 0.0140 seconds |
| Time to fall to 0.1 % | 0.0210 seconds |
Limits of these dose rates
- 0.115 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 3.31 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 1 line under 20 keV, carrying 5.62% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Tl-201m
Other Thallium nuclides: Tl-201, Tl-204
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.