A gamma -ray energy filter for improving the uniformity of the response of a Geiger-Müller tube (1) comprises two and only two spaced bodies (6, 7). To improve the uniformity of the energy response, the bodies (6, 7) consist of a lead/tin alloy containing substantially less than 95% but not substantially less than 40%, and suitably 50-60%, of lead. To improve the polar response, particularly in directions well away from the normal to the longitudinal axis and at quite low energies, adjacent edges (26, 27) of the bodies (6, 7) are inclined to the longitudinal axis over a majority of their radial thickness at less than 45 DEG , and circumferentially-spaced apertures (19) with axes inclined to the longitudinal axis are provided in one of the bodies (6).
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