Sigma/porosity formation logging tool with neutron monitor

A tool 30 for formation logging includes a support configured for movement in a borehole; a neutron source 31 disposed on the support; a neutron monitor 32 disposed on the support and configured to monitor an output of the neutron source; a gamma-ray detector 34 disposed on the support and spaced apart from the neutron source; and a shielding material 33 disposed between the gamma-ray detector and the neutron source. The neutron monitor is arranged to give a measure of the output of the neutron source, enabling normalisation of the gamma-ray detector output. The tool may additionally include one or more epithermal or thermal neutron detectors (figure 5) and may be configured to determine the thermal neutron capture cross-section or the porosity of the formation.