Abstract: A radiation detector is assembled using spacers which contact one another through apertures in the electrode plates of the detector, so that the relevant thickness dimension of the dimensionally accurate spacers determines the spacing of the plates. Thickness variations of the plates can be compensated for at a supporting surface of the spacers being provided with depressable raised portions. The plates may be constructed as laminations for collimator beyond which a radiation-sensitive element is arranged, and can also form electrode plates for a gas ionization detector.