Schematic drawing of PFNS-measurement types. The blue rectangle in the orange fission chamber is the isotope investigated; the blue circles indicate neutrons, including the incident neutron, and scattered or background neutrons; the green arrow denotes fission neutrons headed to the neutron detector; other fission neutrons are given by the black arrows. Figure (a) is for shape measurements. Figures (b) show the main measurement (top) and the monitor measurement (bottom) for a clean-ratio shape measurements. Figures (c) show the main measurement (top) and an associated ratio-calibration measurement (bottom) made in a different experimental environment to quantify the neutron detector efficiency. Figure 3 (c-bottom) can also illustrate calibration with a spontaneous fission source such as ²⁵²Cf.