Increased Gamma‐Ray Sensitivity of Tube Counters and the Measurement of the Thorium Content of Ordinary Materials

The useful sensitivity S of a tube counter exposed to feeble gamma‐radiation is proportional to (N−B)/(N+B)½, where B is the background counting rate and N is the counting rate when the gamma‐ray source is present. S depends upon the product of (a) the efficiency of production of secondary electrons in the counter by the incident radiation, and (b) the efficiency of the tube counter in discharging once for each such secondary electron formed within its sensitive volume. Systematic comparison of many forms of tube counters reveals that the sensitivity to gamma‐radiation from a source of given strength and geometry, may be markedly increased (a) by increasing the effective area of the cathode by employing screen‐wire cathodes or grooved tube cathodes in place of solid smooth cathodes, and by using cathodes of high atomic number. If the cathode is not radioactive, various cleaning treatments do not increase its sensitivity. Temperature changes between 0° and 45°C do not appreciably affect the copper cathode ...