Yttrium-90m
Y-90m · Yttrium, Z = 39, A = 90 · isomeric state m
Yttrium-90m (Y-90m) carries 4.04e+17 Bq/g, or 1.09e+7 Ci/g: a gigabecquerel is 2.47 ng, no weighable quantity at all. It decays by isomeric transition with a half-life of 3.19 hours, which leaves 0.54% of today's activity after a day and 1.4e-14% after a week.
1 GBq at 1 m reads 0.0835 mGy/h, and 1 Ci at the same distance 3.09 mGy/h, from an air kerma rate constant of 0.0835 mGy·m²/(GBq·h) — 1.1× Cs-137 and 3.7× less than Co-60, and 25 of 96 among the photon emitters carried here.
4 lines clear the 20 keV cutoff, but 2 of them carry 90% of the dose rate. The leading one is 479.5 keV at 70.7% of the total — its emission probability is 90.5%, which is lower than the 97% of the 202.5 keV line it outranks.
Halving the air kerma rate calls for 2.10 mm of lead, or 8.75 mm of steel where lead is unwelcome, and a factor of ten calls for 10.0 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 5.57 mm of lead. The tenth-value layer runs 4.8 times the half-value layer, not the 3.32 a single energy would give.
Half-life, specific activity and dose rate
| Half-life | 3.19 hours (1.148e+4 s) |
| Decay mode | isomeric transition |
| Specific activity | 4.04e+17 Bq/g (1.09e+7 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0835 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0835 mGy/h |
| Dose rate, 1 Ci at 1 m | 3.09 mGy/h |
| Kerma-weighted mean photon energy | 400.2 keV |
480 keV leads, and 2 lines make 90%
6 further lines below the 20 keV cutoff, the highest at 17.01 keV and 8.47% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 479.51 | 90.5 | 70.71 |
| 202.53 | 97 | 28.93 |
| 681.80 | 0.32 | 0.35 |
| 2318.97 | 0.0018 | 0.01 |
2.10 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 | 2.10 | 10.0 | 4.78 |
| tungsten | 1.48 | 6.94 | 4.68 |
| iron | 8.75 | 30.4 | 3.48 |
| copper | 7.56 | 26.6 | 3.52 |
| concrete | 30.0 | 102 | 3.39 |
| water | 63.9 | 216 | 3.38 |
| aluminum | 26.9 | 91.3 | 3.39 |
A single energy would give 3.32. What a spread of energies does instead.
Activity over days and weeks
Ten half-lives is 1.33 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 4.60 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 | 10.6 hours |
| Time to fall to 1 % | 21.2 hours |
| Time to fall to 0.1 % | 1.32 days |
Limits of these dose rates
- 0.0835 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 2.10 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 6 lines under 20 keV, carrying 8.47% of all emissions.
- What every sheet leaves out, internal dose included.
Gamma and decay calculators for Y-90m
Other Yttrium nuclides: Y-88, Y-88m1, Y-88m2, Y-90, Y-91
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.