A method and apparatus for monitoring a neutron source are provided, the method comprising: i) providing a plurality of neutron detectors (16) at known positions relative to one another; ii) monitoring the neutron detection rates for the detectors caused by the unknown source(s); iii) proposing a model source location(s) relative to the detectors and an activity level for the model source(s); iv) predicting neutron detection rates for the model source(s); v) comparing the predicted and actual detection rates; vi) adjusting the model source location(s) and/or activity(s) to reduce the difference between the predicted and actual detection rates. The reduced and preferably minimized, difference between the predicted and actual detection rates leads to a model source location(s) and activity(s) which represents an accurate equivalent to the unknown. The location information and/or activity information may be conveyed to the operator of the method.