Thermal-neutron doses can be simply and inexpensively measured over many orders of magnitude of integrated flux by a count of induced-fission-damage tracks in a solid with uranium impurities. Examples are given of the use of a single ordinary glass to measure neutron flux from 3 × 1014 to 4 × 1018nvt and of the use of glass to measure the spatial variation of neutron flux. Other materials, either glassy or crystalline, allow a wide range of fluxes to be measured.