An apparatus comprises a neutron detector. The neutron detector comprises a conversion layer comprising a mixture of a neutron absorbing material and a scintillation material; and a photodetector optically coupled to the conversion layer and arranged to detect photons generated as a result of neutron absorption events in the conversion layer; wherein the apparatus is adapted to be carried by a user and the conversion layer is positioned within the neutron detector such that when the apparatus is being carried by a user in normal use neutrons are absorbed in the conversion layer after passing through the user such that the user's body provides a neutron moderating effect. In some cases the apparatus may be carried in association with a backpack or clothing worn by a user, for example, the neutron detector may be sized to fit in a pocket. In other cases the apparatus may be a hand-held device with the conversion layer arranged within a handle of the device to be gripped by a user when being carried.