Detailed Investigation of Neutron–Neutron Angular Correlations in the Spontaneous Fission of $${}^{{252}}$$Cf

New precise measurements of angular neutron–neutron ( $$nn$$ ) correlations in the spontaneous fission of $${}^{252}$$ Cf were performed with the aim of studying the mechanism of prompt neutron emission in the nuclear fission process. In contrast to earlier studies, particular attention is given in the present experiment to exploring experimental systematic effects that owe their existence to neutron rescattering on the structural elements of the setup used and which distort angular distributions. The experimentally determined angular dependence of the number of $$nn$$ coincidences for seven neutron-detection thresholds in the range between 490 and 2080 keV is compared with the results of refined model calculations in which the anisotropy of neutron emission in the fragment center-of-mass frame is taken into account. It is shown that these experimental data can be described successfully if, in addition to the bulk of neutrons from fully accelerated fission fragments, 6 to 10 $${\%}$$ of neutrons emitted isotropically in the laboratory frame are included in the total number of prompt fission neutrons. The energy distribution of this component is also determined. These extra neutrons may be interpreted as neutrons emitted at the instant of disintegration of the nucleus being considered (so-called scission neutrons).