Zinc-65
Zn-65 · Zinc, Z = 30, A = 65
At 3.05e+14 Bq/g — 8.24e+3 Ci/g — a gigabecquerel of Zn-65 amounts to 3.28 µg, which is why activity rather than mass is how anyone states the quantity. Zinc-65 decays by electron capture with beta-plus with a half-life of 243.93 days, falling to 91.83% of today's activity in a month and 35.42% in a year.
1 GBq at 1 m reads 0.0719 mGy/h, and 1 Ci at the same distance 2.66 mGy/h, from an air kerma rate constant of 0.0719 mGy·m²/(GBq·h) — 1.1× less than Cs-137 and 4.3× less than Co-60, and 30 of 96 among the photon emitters carried here.
4 lines clear the 20 keV cutoff, but one of them carries 90% of the dose rate. The leading one is 1115.5 keV at 97.2% of the total — its emission probability is 50.04%, which is also the highest.
This is a shield that has to be designed: 9.20 mm of lead for a factor of two and 31.0 mm for a factor of ten, or 15.4 mm of steel to halve it, at which point the mass of the shield is part of the problem. Reaching 20 µSv/h from 1 GBq at a metre takes 17.1 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 243.93 days (2.108e+7 s) |
| Decay mode | electron capture with beta-plus |
| Specific activity | 3.05e+14 Bq/g (8.24e+3 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0719 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0719 mGy/h |
| Dose rate, 1 Ci at 1 m | 2.66 mGy/h |
| Kerma-weighted mean photon energy | 1099 keV |
1116 keV carries 97% of the dose rate
3 further lines below the 20 keV cutoff, the highest at 8.94 keV and 39.5% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 1115.54 | 50.04 | 97.25 |
| 511.00 | 2.842 | 2.75 |
| 770.60 | 0.00268 | 0.00 |
| 344.95 | 0.00253 | 0.00 |
9.20 mm of lead halves this spectrum
Solved numerically across all 4 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 9.20 | 31.0 | 3.37 |
| tungsten | 5.77 | 19.4 | 3.36 |
| iron | 15.4 | 51.2 | 3.33 |
| copper | 13.7 | 45.7 | 3.33 |
| concrete | 48.5 | 162 | 3.33 |
| water | 102 | 341 | 3.33 |
| aluminum | 43.7 | 146 | 3.33 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over months and years
Ten half-lives is 6.68 years — a storage problem rather than a disposal one, with 35.42% of today's activity still there after a year. The mean life 1/λ is 352 days.
| 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 | 2.22 years |
| Time to fall to 1 % | 4.44 years |
| Time to fall to 0.1 % | 6.66 years |
Limits of these dose rates
- 0.0719 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 9.20 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 3 lines under 20 keV, carrying 39.5% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Zn-65
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.