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 gamma-ray spectrum produced from the thermal neutron capture on Gd-155 and Gd-157 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 gamma rays emitted in the same neutron capture. We present for the first time angular correlation functions for two gamma rays produced during the electromagnetic cascade transitions in the (n, gamma) reactions on Gd-155 and Gd-157. 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 gamma rays produced in the majority of cascade transitions from the resonance state to the dense continuum states.