We measured the prompt neutron decay constant of the CROCUS zero-power reactor at the Swiss Federal Institute of Technology Lausanne using cross-power spectral density (CPSD) analysis of gamma-gamma correlations from two trans-stilbene organic scintillators positioned near the reactor core. We measured critical and subcritical states, with water levels ranging from 960 mm (critical) to 800 mm ( $ subcritical). Our analysis used the Welch method, dividing signal segments for fast Fourier transform (FFT) frequency analysis and applying bootstrapping uncertainty quantification that uses Welch-defined segments. Results demonstrated a clear increase in the measured as reactor reactivity decreased, distinguishing critical from subcritical conditions. At the 960-mm critical level, was estimated at 155.9 0.7 s−1, and for the 800-mm subcritical level, increased significantly to 367.3 6.9 s–1. A linear regression of subcritical states yielded a critical estimate of 154.0 3.1 s–1, aligning with the static estimate at critical. The bootstrapping method produced normally distributed estimates, confirming data consistency. The gamma CPSD estimates clearly distinguish reactor states and improve monitoring of zero-power reactors. The future deployment of modular and microreactors as potential candidates for noise analysis is demonstrated in CROCUS, particularly zero-power mock-ups of new designs. The improvement of noise analysis in the subcritical domain from this work will support experimental data for reactor deployment and procedure.