Tube Counters With Improved Gamma Ray Sensitivity In Ordinary Materials
The usable sensitivity S of a tube counter exposed to weak gamma radiation is proportional to (NB)/(N+B)12, where N is the counting rate while the gamma ray source is present and B is the background counting rate. S is equal to the product of (a) the efficiency with which incoming radiation produces secondary electrons in the counter, and (b) the efficiency with which the tube counter discharges once for each secondary electron created inside its sensitive volume. The sensitivity to gamma radiation from a source of given intensity and shape may be significantly enhanced (a) by increasing the effective area of the cathode by utilizing screenwire cathodes or grooved tube cathodes instead of solid smooth cathodes, and (b) by using cathodes with high atomic number.
Various cleaning methods do not improve the sensitivity of the cathode if it is not radioactive. The copper cathode tube counter is unaffected by temperature variations between 0° and 45°C. Gamma sensitivity S may be improved even further (b) by carefully selecting the operating voltage and filling gas pressure. For each counter, an optimal pressure and voltage may be determined at which the counter is many times more sensitive than it would be if these working parameters were not carefully chosen. Maximum sensitivity is achieved with a 100mesh copper gauze cathode in a tube counter 12 cm long and 2 cm in diameter when the counter is filled with air at 6.5 cm Hg pressure and operated 170 volts over threshold. When combined with independent measurements of uranium content using the radon emanation method, thorium measurements have a probable statistical error of around 12107 g Th per g rock when just two hours of measurements are made on 1400 g of rock.