We measured the prompt neutron decay constant $$\alpha $$alpha 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 ($$\rho = - 1.4$$rho=-1.4 $ 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 $$\alpha $$alpha as reactor reactivity decreased, distinguishing critical from subcritical conditions. At the 960-mm critical level, $$\alpha $$alpha was estimated at 155.9 $$ \pm $$+/- 0.7 s-1, and for the 800-mm subcritical level, $$\alpha $$alpha increased significantly to 367.3 $$ \pm $$+/- 6.9 s-1. A linear regression of subcritical states yielded a critical estimate of 154.0 $$ \pm $$+/- 3.1 s-1, aligning with the static $$\alpha $$alpha estimate at critical. The bootstrapping method produced normally distributed $$\alpha $$alpha estimates, confirming data consistency. The gamma CPSD $$\alpha $$alpha 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.