Molybdenum-99
Mo-99 · Molybdenum, Z = 42, A = 99
Molybdenum-99 (Mo-99) carries 1.78e+16 Bq/g, or 4.81e+5 Ci/g: a gigabecquerel is 56.2 ng, no weighable quantity at all. It decays by beta-minus decay with a half-life of 65.924 hours, which leaves 17.09% of today's activity after a week and 1.1e-8% after three months.
At 0.0194 mGy·m²/(GBq·h) the air kerma rate constant is 4.0× less than Cs-137 and 16× less than Co-60, placing it 53 of 96 photon emitters in this dataset. 1 GBq at 1 m reads 0.0194 mGy/h, and 1 Ci at the same distance 0.717 mGy/h.
32 lines clear the 20 keV cutoff, but 3 of them carry 90% of the dose rate. The leading one is 739.5 keV at 62.0% of the total — its emission probability is 12.2%, which is also the highest.
This is a shield that has to be designed: 5.19 mm of lead for a factor of two and 19.9 mm for a factor of ten, or 11.3 mm of steel to halve it, at which point the mass of the shield is part of the problem. At 1 GBq and a metre it is already under 20 µSv/h with nothing in the way.
The beta endpoint is 1.22 MeV, mean 0.389 MeV over 9 branches. That endpoint stops in 4.45 mm of acrylic or 2.10 mm of glass. Of the beta energy, 0.26% turns into X-rays in acrylic and 3.49% in lead.
Half-life, specific activity, dose rate and beta energies
| Half-life | 65.924 hours (2.373e+5 s) |
| Decay mode | beta-minus decay |
| Specific activity | 1.78e+16 Bq/g (4.81e+5 Ci/g) |
| Air kerma rate constant Γ (δ = 20 keV) | 0.0194 mGy·m²/(GBq·h) |
| Dose rate, 1 GBq at 1 m | 0.0194 mGy/h |
| Dose rate, 1 Ci at 1 m | 0.717 mGy/h |
| Kerma-weighted mean photon energy | 676.8 keV |
| Beta endpoint / mean | 1.22 MeV / 0.389 MeV |
740 keV leads, and 3 lines make 90%
3 further lines below the 20 keV cutoff, the highest at 18.37 keV and 2.90% emission probability in all, are excluded here and from Γ. Why the two columns rank differently.
| Energy (keV) | Emission probability (%) | Share of dose rate (%) |
|---|---|---|
| 739.50 | 12.2 | 62.01 |
| 777.92 | 4.3066 | 22.93 |
| 181.07 | 6.0512 | 6.77 |
| 366.42 | 1.20048 | 3.05 |
| 20.82 | 0.54496 | 1.28 |
| 20.70 | 0.47061 | 1.12 |
| 822.97 | 0.1342 | 0.75 |
| 40.58 | 1.03822 | 0.66 |
| 960.75 | 0.09516 | 0.61 |
| 528.79 | 0.05319 | 0.20 |
| 21.00 | 0.07436 | 0.17 |
| 620.03 | 0.02794 | 0.12 |
| 621.77 | 0.0183 | 0.08 |
| 411.49 | 0.01501 | 0.04 |
Showing the 14 largest contributors of 32 lines above the cutoff; the remainder together carry 0.21% of the dose rate.
5.19 mm of lead halves this spectrum
Solved numerically across all 32 lines, narrow beam. Why not one representative energy.
| Material | HVL (mm) | TVL (mm) | TVL / HVL |
|---|---|---|---|
| lead | 5.19 | 19.9 | 3.84 |
| tungsten | 3.40 | 13.0 | 3.82 |
| iron | 11.3 | 40.1 | 3.56 |
| copper | 9.95 | 35.6 | 3.58 |
| concrete | 36.6 | 128 | 3.50 |
| water | 77.7 | 271 | 3.49 |
| aluminum | 32.9 | 115 | 3.50 |
A single energy would give 3.32. What a spread of energies does instead.
4.45 mm of acrylic stops the 1.22 MeV endpoint
Katz–Penfold fit to the 1.22 MeV endpoint — ±10%, and a stopping thickness rather than an attenuation length. Why the material matters more than the thickness.
| Absorber | Range for the endpoint (mm) |
|---|---|
| acrylic | 4.45 |
| aluminum | 1.95 |
| water | 5.25 |
| glass | 2.10 |
| Shield material | Fraction of beta energy converted to X-rays |
|---|---|
| acrylic (Z ≈ 6) | 0.26% |
| lead (Z = 82) | 3.49% |
Activity over days and weeks
Ten half-lives is 27.5 days — a storage problem rather than a disposal one, with 9.4e-39% of today's activity still there after a year. The mean life 1/λ is 3.96 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 | 9.12 days |
| Time to fall to 1 % | 18.2 days |
| Time to fall to 0.1 % | 27.4 days |
Limits of these dose rates
- 0.0194 mGy/h at a metre — bare point source, no capsule, no self-absorption.
- 5.19 mm of lead halves this spectrum, narrow beam, scatter not added back.
- Γ excludes 3 lines under 20 keV, carrying 2.90% of all emissions.
- 4.45 mm of acrylic for the 1.22 MeV endpoint: a ±10% fit, and not a skin dose.
- What every sheet leaves out, internal dose included.
Gamma, beta and decay calculators for Mo-99
- Gamma dose rate and shielding
- Beta dose rate and shielding
- Decay and half-life
- Mass and activity
- Detection limits (MDA / MDC)
Computed from the IAEA Nuclear Data Section — Livechart API (ENSDF) and the NIST X-Ray Mass Attenuation Coefficients. Derivations and citations.