Krypton-85m
Kr-85m · Krypton, Z = 36, A = 85 · isomeric state m
Krypton-85m (Kr-85m) carries 3.05e+17 Bq/g, or 8.24e+6 Ci/g: a gigabecquerel is 3.28 ng, no weighable quantity at all. It decays by beta-minus decay with a half-life of 4.480 hours, which leaves 2.44% of today's activity after a day and 5.1e-10% after a week.
At 0.0187 mGy·m²/(GBq·h) the air kerma rate constant is 4.1× less than Cs-137 and 16× less than Co-60, placing it 56 of 96 photon emitters in this dataset. 1 GBq at 1 m reads 0.0187 mGy/h, and 1 Ci at the same distance 0.693 mGy/h.
5 lines clear the 20 keV cutoff, but 2 of them carry 90% of the dose rate. The leading one is 151.2 keV at 69.6% of the total — its emission probability is 75.2%, which is also the highest.
A half-value layer of 0.452 mm in lead puts this in foil and thin sheet, with 5.32 mm needed if the material is steel; ten-fold attenuation comes at 2.55 mm of lead. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way. The tenth-value layer runs 5.6 times the half-value layer, not the 3.32 a single energy would give.
Half-life, specific activity and dose rate
| Half-life | 4.480 hours (1.613e+4 s) |
| Decay mode | beta-minus decay |
| Specific activity | 3.05e+17 Bq/g (8.24e+6 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0187 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0187 mGy/h |
| Dose rate, 1 Ci at 1 m | 0.693 mGy/h |
| Kerma-weighted mean photon energy | 197.7 keV |
151 keV leads, and 2 lines make 90%
12 further lines below the 20 keV cutoff, the highest at 15.19 keV and 7.13% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 151.19 | 75.2 | 69.63 |
| 304.87 | 14 | 30.06 |
| 129.81 | 0.301 | 0.23 |
| 731.60 | 0.008 | 0.04 |
| 451.00 | 0.011 | 0.04 |
0.452 mm of lead halves this spectrum
Solved numerically across all 5 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 0.452 | 2.55 | 5.63 |
| tungsten | 0.337 | 1.87 | 5.53 |
| iron | 5.32 | 18.7 | 3.52 |
| copper | 4.25 | 15.3 | 3.61 |
| concrete | 22.8 | 76.7 | 3.36 |
| water | 49.6 | 166 | 3.36 |
| aluminum | 20.3 | 68.4 | 3.37 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over days and weeks
Ten half-lives is 1.87 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 6.46 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 | 14.9 hours |
| Time to fall to 1 % | 1.24 days |
| Time to fall to 0.1 % | 1.86 days |
Limits of these dose rates
- 0.0187 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 0.452 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 12 lines under 20 keV, carrying 7.13% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Kr-85m
Other Krypton nuclides: Kr-85
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.