Technetium-99m
Tc-99m · Technetium, Z = 43, A = 99 · isomeric state m
Technetium-99m (Tc-99m) carries 1.95e+17 Bq/g, or 5.27e+6 Ci/g: a gigabecquerel is 5.12 ng, no weighable quantity at all. It decays by isomeric transition with a half-life of 6.0072 hours, which leaves 6.27% of today's activity after a day and 3.8e-7% after a week.
At 0.0153 mGy·m²/(GBq·h) the air kerma rate constant is 5.0× less than Cs-137 and 20× less than Co-60, placing it 61 of 96 photon emitters in this dataset. 1 GBq at 1 m reads 0.0153 mGy/h, and 1 Ci at the same distance 0.566 mGy/h.
6 lines clear the 20 keV cutoff, but one of them carries 90% of the dose rate. The leading one is 140.5 keV at 92.6% of the total — its emission probability is 89%, which is also the highest.
A half-value layer of 0.229 mm in lead puts this in foil and thin sheet, with 3.60 mm needed if the material is steel; ten-fold attenuation comes at 0.827 mm of lead. 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 | 6.0072 hours (2.163e+4 s) |
| Decay mode | isomeric transition |
| Specific activity | 1.95e+17 Bq/g (5.27e+6 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0153 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0153 mGy/h |
| Dose rate, 1 Ci at 1 m | 0.566 mGy/h |
| Kerma-weighted mean photon energy | 131.7 keV |
141 keV carries 93% of the dose rate
3 further lines below the 20 keV cutoff, the highest at 18.37 keV and 6.59% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 140.51 | 89 | 92.58 |
| 20.82 | 1.23924 | 3.68 |
| 20.70 | 1.07015 | 3.22 |
| 21.00 | 0.16908 | 0.49 |
| 142.63 | 0.0222 | 0.02 |
| 89.60 | 0.00104 | 0.00 |
0.229 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 | 0.229 | 0.827 | 3.61 |
| tungsten | 0.171 | 0.618 | 3.61 |
| iron | 3.60 | 13.0 | 3.61 |
| copper | 2.76 | 9.97 | 3.61 |
| concrete | 18.1 | 65.3 | 3.61 |
| water | 40.6 | 145 | 3.57 |
| aluminum | 16.0 | 57.8 | 3.61 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over days and weeks
Ten half-lives is 2.50 days, 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 8.67 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 | 20.0 hours |
| Time to fall to 1 % | 1.66 days |
| Time to fall to 0.1 % | 2.49 days |
Limits of these dose rates
- 0.0153 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 0.229 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 3 lines under 20 keV, carrying 6.59% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Tc-99m
Other Technetium nuclides: Tc-99
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.