Subcritical Measurements of a Plutonium Sphere Reflected by Polyethylene and Acrylic

Subcritical measurements were conducted with an alpha-phase plutonium sphere using the ²⁵²Cf source-driven noise analysis method. Measurements were performed with both polyethylene and acrylic reflectors. For each reflector type, five different reflector thicknesses were investigated: 0 (bare), 1.27, 2.54, 3.81, and 7.62 cm. A certain ratio of spectral quantities that depends on the fluctuations in the fission chain multiplication process was measured for each configuration. In addition, two types of Monte Carlo calculations were employed to estimate the k_eff and spectral ratio values of each configuration. From the measured and computed quantities, the multiplication and uncertainty of the system can be inferred. The polyethylene measurements compared well to previous measurements conducted with the same plutonium sphere and polyethylene reflector thicknesses. The acrylic measurements provide benchmark data of an alpha-phase plutonium sphere reflected by acrylic.