Abstract: Measurements for prompt neutron decay constant α were conducted with a highly enriched uranium spherical shell using ²⁵²Cf source driven noise analysis method, which could interpret data in the frequency domain. In these measurements carried out at a slightly subcritical state, two methods of interpreting data were compared. The value for a reactivity of -0.1 subcritical obtained by the ratio of the real to the imaginary part of the cross power spectral density with a resulting value 0.58 μs^-1 agrees with the Rossi-α result.