The optimization of the neutron sensitivity of image plates

Image plates containing varying concentrations of the neutron converter Gd2O3 and the storage phosphor BaFBr:Eu2+ have been prepared. Their neutron sensitivity was measured for a neutron wavelength of 3 A and compared with a new model, which is in good agreement with the measured data, showing that the sensitivity, which is the photostimulated light output per irradiated neutron, of image plates is always higher if the neutron irradiation and the readout are done from the same side. In this case an optimum in sensitivity can be obtained for volume fractions of Gd2O3 between 20% and 30% of the total volume of storage phosphor and converter in the material. In the case of neutron irradiation and readout from opposite sides the volume fraction of Gd2O3 should be varied reciprocally with the thickness d of the phosphor layer in order to obtain a maximum of sensitivity. The experimental results and the model indicate that the maximum sensitivity is lower by a factor between 2 and 4 compared with the readout from the same side. The relation between sensitivity and detective quantum efficiency is discussed.