Angular correlation of the two gamma rays produced in the thermal neutron capture on gadolinium-155 and gadolinium-157

The ANNRI-Gd collaboration studied in detail the single γ -ray spectrum produced from the thermal neutron capture on ¹⁵⁵Gd and ¹⁵⁷Gd in our previous publications. Gadolinium targets were exposed to a neutron beam provided by the Japan Spallation Neutron Source (JSNS) in J-PARC, Japan. In the present analysis, one new additional coaxial germanium crystal was used in combination with the 14 germanium crystals in the cluster detectors to study the angular correlation of the two γ rays emitted in the same neutron capture. We present for the first time angular correlation functions for two γ rays produced during the electromagnetic cascade transitions in the (n, γ ) reactions on ¹⁵⁵Gd and ¹⁵⁷Gd. As expected, we observe mild angular correlations for the strong, but rare transitions from the resonance state to the two energy levels of known spin-parities. Contrariwise, we observe negligibly small angular correlations for arbitrary pairs of two γ rays produced in the majority of cascade transitions from the resonance state to the dense continuum states.