A neutron spectrometer comprising: a first conversion screen comprising a neutron absorbing material and a phosphor material; a first wavelength-shifting light-guide arranged to receive photons from the phosphor material of the first conversion screen and generate wavelength-shifted photons therefrom; a first photodetector optically coupled to the first wavelength-shifting light-guide and arranged to detect the wavelength-shifted photons from the first wavelength-shifting light-guide; a second conversion screen comprising a neutron absorbing material and a phosphor material; a second wavelength-shifting light-guide arranged to receive photons from the phosphor material of the second conversion screen and generate wavelength-shifted photons therefrom; and a second photodetector optically coupled to the second wavelength-shifting light-guide and arranged to detect the wavelength-shifted photons from the second wavelength-shifting light-guide; wherein the neutron spectrometer further comprises a neutron moderator material arranged between the first and second conversion screens to moderate neutrons; and wherein the neutron spectrometer further comprises a processor arranged to receive signals output from each of the photodetectors, and to determine respective numbers of neutron events that occur in each of the conversion screens, and to determine a ratio between the number of neutron events that occur in the first conversion screen and the number of neutron events that occur in the second conversion screen to obtain an indication of the neutron source from the relative number of neutron events detected in the first and second conversion screens. a first conversion screen comprising a neutron absorbing material and a phosphor material; a first wavelength-shifting light-guide arranged to receive photons from the phosphor material of the first conversion screen and generate wavelength-shifted photons therefrom; a first photodetector optically coupled to the first wavelength-shifting light-guide and arranged to detect the wavelength-shifted photons from the first wavelength-shifting light-guide; a second conversion screen comprising a neutron absorbing material and a phosphor material; a second wavelength-shifting light-guide arranged to receive photons from the phosphor material of the second conversion screen and generate wavelength-shifted photons therefrom; and a second photodetector optically coupled to the second wavelength-shifting light-guide and arranged to detect the wavelength-shifted photons from the second wavelength-shifting light-guide; wherein the neutron spectrometer further comprises a neutron moderator material arranged between the first and second conversion screens to moderate neutrons; and wherein the neutron spectrometer further comprises a processor arranged to receive signals output from each of the photodetectors, and to determine respective numbers of neutron events that occur in each of the conversion screens, and to determine a ratio between the number of neutron events that occur in the first conversion screen and the number of neutron events that occur in the second conversion screen to obtain an indication of the neutron source from the relative number of neutron events detected in the first and second conversion screens.