Zirconium-95
Zr-95 · Zirconium, Z = 40, A = 95
At 7.95e+14 Bq/g — 2.15e+4 Ci/g — a gigabecquerel of Zr-95 amounts to 1.26 µg, which is why activity rather than mass is how anyone states the quantity. Zirconium-95 decays by beta-minus decay with a half-life of 64.032 days, falling to 72.27% of today's activity in a month and 1.92% in a year.
1 GBq at 1 m reads 0.0975 mGy/h, and 1 Ci at the same distance 3.61 mGy/h, from an air kerma rate constant of 0.0975 mGy·m²/(GBq·h) — 1.3× Cs-137 and 3.1× less than Co-60, and 22 of 96 among the photon emitters carried here.
3 lines clear the 20 keV cutoff, but 2 of them carry 90% of the dose rate. The leading one is 756.7 keV at 56.1% of the total — its emission probability is 54.38%, which is also the highest.
This is a shield that has to be designed: 6.29 mm of lead for a factor of two and 20.9 mm for a factor of ten, or 12.7 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 14.4 mm of lead.
Half-life, specific activity and dose rate
| Half-life | 64.032 days (5.532e+6 s) |
| Decay mode | beta-minus decay |
| Specific activity | 7.95e+14 Bq/g (2.15e+4 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0975 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0975 mGy/h |
| Dose rate, 1 Ci at 1 m | 3.61 mGy/h |
| Kerma-weighted mean photon energy | 742.0 keV |
757 keV leads, and 2 lines make 90%
6 further lines below the 20 keV cutoff, the highest at 18.95 keV and 0.672% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 756.73 | 54.38 | 56.07 |
| 724.19 | 44.27 | 43.85 |
| 235.69 | 0.27 | 0.08 |
6.29 mm of lead halves this spectrum
Solved numerically across all 3 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 6.29 | 20.9 | 3.33 |
| tungsten | 4.11 | 13.7 | 3.33 |
| iron | 12.7 | 42.1 | 3.32 |
| copper | 11.3 | 37.5 | 3.32 |
| concrete | 40.3 | 134 | 3.32 |
| water | 85.2 | 283 | 3.32 |
| aluminum | 36.3 | 120 | 3.32 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over months and years
Ten half-lives is 1.75 years — a storage problem rather than a disposal one, with 1.92% of today's activity still there after a year. The mean life 1/λ is 92.4 days.
| Elapsed | Fraction remaining |
|---|---|
| 1 half-life | 50.0 % |
| 2 half-lives | 25.0 % |
| 5 half-lives | 3.12 % |
| 10 half-lives | 0.0977 % |
| Time to fall to 10 % of today's activity | 213 days |
| Time to fall to 1 % | 1.16 years |
| Time to fall to 0.1 % | 1.75 years |
Limits of these dose rates
- 0.0975 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 6.29 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 6 lines under 20 keV, carrying 0.672% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Zr-95
Other Zirconium nuclides: Zr-93
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.