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

Abstract

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 tube counter. The sensitivity S may be increased further (b) by careful selection of the operating voltage and the pressure of the filling gas. For every counter an optimum pressure and voltage may be found at which the counter is many times more sensitive than it would be if no care were given to the selection of these operating conditions. For a tube counter 12 cm long and 2 cm in diameter maximum sensitivity is obtained with a 100-mesh copper gauze cathode when the counter contains air at 6.5 cm Hg pressure and is operated 170 volts above threshold. The elevated sensitivity of these gamma-ray counters permits the direct measurement of the feeble gamma-radiation from the radioactive impurities in ordinary granitic rocks, and, when combined with independent measurements of the uranium content by the radon emanation method, leads to thorium measurements having a probable statistical error of about 12×10−7 g Th per g rock when only two hours of measurements are made on 1400 g of rock.