a process of two dimensional interpolation of the simultaneous signals arising from all the photomultipliers which view the scintillation the ability of the present invention to concentrate light flashes from the scintillator eliminates loss of scintillation light and prevents the spreading of light over a large number of photomultiplier tubes dilution or spreading of light increases the statistical uncertainty of the neutron interaction centroid signals particularly at the scintillator edge referring now to fig3 the index of refraction of air gap 16 the polished face 12 and index of refraction of scintillator 10 and the adjacent face and index of refraction of light coupler 18 together comprise a light concentrator which limits transmission of light flashes 34 f 34 from scintillator 10 to predeterminently dimensioned cones 60 which open toward light coupler 18 a high voltage from supply 27 is applied to photomultipliers 22 to provide for secondary emmission therein the position of the neutron interaction in scintillator 10 is determined by a position encoding circuit which calculates the centroid of a scintillation as viewed by the photomultiplier tubes referring now to fig2 operation of the position encoding circuit will be explained the outputs of the photomultipliers are encoded by weighting resistors 28 29 into x and y signals dividers 30 31 divide the x and y signals by the unweighted sum e signal of all the photomultiplier signals produced by summer 32 and provide the normalized coordinate signals x e and y e the normalization results in coordinate signals which are independent of fluctuations in e by virtue of the light coupler 18 each scintillation is seen by more than one photomultiplier this permits centroid determination by interpolation which is the underlying basis for obtaining high spatial resolution with this type of detector system the.