Critical Mass of a Water-Reflected Plutonium Sphere

We have determined the critical mass of a water-reflected plutonium sphere. The experimental data are:

1. Properties of metal: (a) all alpha-phase, (b) high purity, and (c) 94.5% ²³⁹Pu, 5.18% ²⁴⁰Pu
2. Measurements: (a) mass—5546 g, (b) diameter—8.126 em at 20°C, (c) density—19.74 g/cm³ at 20°C, and (d) neutron multiplication—71 (based on unreflected value of 5)
3. Reflector—effectively infinite water
4. Results: (a) critical mass, our experiment—5790 ± 25 g (extrapolated from appropriately adjusted data), (b) DTF calculation, our experiment—k_eff = 1.0037, and (c) applying above normalizing factor to DTF, critical mass of pure ²³⁹Pu—5430 (±25)g.