Gallium-68
Ga-68 · Gallium, Z = 31, A = 68
Gallium-68 (Ga-68) carries 1.51e+18 Bq/g, or 4.09e+7 Ci/g: a gigabecquerel is 661 pg, no weighable quantity at all. It decays by electron capture with beta-plus with a half-life of 67.71 minutes, which leaves 4.0e-5% of today's activity after a day and 1.5e-43% after a week.
1 GBq at 1 m reads 0.129 mGy/h, and 1 Ci at the same distance 4.76 mGy/h, from an air kerma rate constant of 0.129 mGy·m²/(GBq·h) — 1.7× Cs-137 and 2.4× less than Co-60, and 16 of 96 among the photon emitters carried here.
8 lines clear the 20 keV cutoff, but one of them carries 90% of the dose rate. The leading one is 511.0 keV at 96.2% of the total — its emission probability is 177.82052%, which is also the highest.
Halving the air kerma rate calls for 4.01 mm of lead, or 10.7 mm of steel where lead is unwelcome, and a factor of ten calls for 13.6 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 10.9 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 67.71 minutes (4063 s) |
| Decay mode | electron capture with beta-plus |
| Specific activity | 1.51e+18 Bq/g (4.09e+7 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.129 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.129 mGy/h |
| Dose rate, 1 Ci at 1 m | 4.76 mGy/h |
| Kerma-weighted mean photon energy | 534.6 keV |
511 keV carries 96% of the dose rate
4 further lines below the 20 keV cutoff, the highest at 9.61 keV and 4.92% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 511.00 | 177.82052 | 96.15 |
| 1077.34 | 3.22 | 3.40 |
| 1883.16 | 0.13653 | 0.22 |
| 1261.08 | 0.09435 | 0.11 |
| 805.83 | 0.09402 | 0.08 |
| 578.52 | 0.03349 | 0.02 |
| 1744.42 | 0.0095 | 0.01 |
| 2338.44 | 0.00113 | 0.00 |
4.01 mm of lead halves this spectrum
Solved numerically across all 8 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 4.01 | 13.6 | 3.39 |
| tungsten | 2.75 | 9.28 | 3.38 |
| iron | 10.7 | 35.8 | 3.34 |
| copper | 9.49 | 31.7 | 3.34 |
| concrete | 34.6 | 115 | 3.33 |
| water | 73.2 | 244 | 3.33 |
| aluminum | 31.1 | 104 | 3.33 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over the first hours
Ten half-lives is 11.3 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 1.63 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 | 3.75 hours |
| Time to fall to 1 % | 7.50 hours |
| Time to fall to 0.1 % | 11.2 hours |
Limits of these dose rates
- 0.129 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 4.01 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 4 lines under 20 keV, carrying 4.92% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Ga-68
Other Gallium nuclides: Ga-67
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.