Neutron-gamma noise measurements in a zero-power reactor using organic scintillators

Noise measurements in light water reactor systems aid in generating validation data for integral point kinetic parameter predictions and generating monitoring parameters for reactor safety and safeguards. The CROCUS zero-power reactor has been used to support both efforts via measurements with thermal neutron detectors to observe neutron noise and inorganic scintillators to observe gamma noise. The cross-correlation of gamma and neutron noise has been investigated at CROCUS with separate gamma-only and neutron-only detectors. Organic scintillators, sensitive to both neutrons and gamma rays, can be used to cross-correlate gamma-ray and neutron noise with a single detector type. Herein, we present noise measurements using organic scintillators in a light-water, zero-power research reactor for the first time. We discuss data obtained with two 5.08 cm-length by 5.08 cm-diameter cylindrical trans-stilbene detectors and two 6-mm cubic scintillators – one trans-stilbene and one organic glass – set in the water reflector of CROCUS and outside the reactor vessel. The prompt neutron decay constant was estimated to be α _CPSD = (154 ± 1) s ^-1 at 3 mW critical from the (γ,γ) signal, which agrees within one standard deviation overlap with previous measurements with CeBr ₃ and simulation in Serpent 2. The (n,n) estimate of α _CPSD = (145 ± 23) s ^-1 at 3 mW critical lacked precision but agreed within one standard deviation overlap with previous ²³⁵ U fission chamber estimates and simulation in Serpent 2. The 6-mm cubic scintillators were too inefficient to estimate α, but informed possible small-cell, high-volume detection systems to improve the (n,n) estimate capabilities. Based on the high precision estimate of α from the (γ,γ) CPSD signal we suggest the further development of theory to calculate β _eff and Λ from gamma-ray noise measurements and recommend gamma noise measurements for future simulation validation and as a method of reactor monitoring.