Fluorine-18
F-18 · Fluorine, Z = 9, A = 18
Fluorine-18 (F-18) carries 3.52e+18 Bq/g, or 9.52e+7 Ci/g: a gigabecquerel is 284 pg, no weighable quantity at all. It decays by electron capture with beta-plus with a half-life of 109.77 minutes, which leaves 0.01% of today's activity after a day and 2.3e-26% after a week.
1 GBq at 1 m reads 0.135 mGy/h, and 1 Ci at the same distance 4.98 mGy/h, from an air kerma rate constant of 0.135 mGy·m²/(GBq·h) — 1.7× Cs-137 and 2.3× less than Co-60, and 14 of 96 among the photon emitters carried here.
One line at 511.0 keV carries all of that.
Halving the air kerma rate calls for 3.90 mm of lead, or 10.6 mm of steel where lead is unwelcome, and a factor of ten calls for 13.0 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 10.7 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 109.77 minutes (6586 s) |
| Decay mode | electron capture with beta-plus |
| Specific activity | 3.52e+18 Bq/g (9.52e+7 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.135 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.135 mGy/h |
| Dose rate, 1 Ci at 1 m | 4.98 mGy/h |
| Kerma-weighted mean photon energy | 511.0 keV |
A single photon line at 511 keV
No recorded line for F-18 falls below the 20 keV cutoff, so this table is the whole photon spectrum. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 511.00 | 193.46 | 100.00 |
3.90 mm of lead halves this spectrum
Solved numerically across all 1 line, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 3.90 | 13.0 | 3.32 |
| tungsten | 2.68 | 8.90 | 3.32 |
| iron | 10.6 | 35.1 | 3.32 |
| copper | 9.35 | 31.1 | 3.32 |
| concrete | 34.1 | 113 | 3.32 |
| water | 72.2 | 240 | 3.32 |
| aluminum | 30.7 | 102 | 3.32 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over the first hours
Ten half-lives is 18.3 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 2.64 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 | 6.08 hours |
| Time to fall to 1 % | 12.2 hours |
| Time to fall to 0.1 % | 18.2 hours |
Limits of these dose rates
- 0.135 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 3.90 mm of lead halves this spectrum, narrow beam, scatter not added back.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for F-18
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.