DISCHARGE DELAY IN LOW-VOLTAGE HALOGEN COUNTERS
L.S. Eyg · OSTI OAI (U.S. Department of Energy Office of Scientific and Technical Information) · 1957
It is usually considered that the formation of negative ions is responsible for discharge delay and the appearance of an appreciable number of spurious counts which tend to increase the plateau slope. The primary electron appearing within the volume of the counter may stick'' to an electronegative atom thus forming a negative ion. The mobility of such an ion is smaller by two or three orders than the mobility of an electron. For this reason the ion will take a much longer time to reach the filament. Near the filament, the negative ion dissociates and a free electron appears again and initiates a discharge. The duration of the delay was measured by two methods. In the first method a triple telescope was used consisting of two high-voltage counters and a low-voltage counter between them. The delay time in the high-voltage counters was estimated to be 0.1 mu sec and hence one can assume that, compared with low-voltage counters, high-voltage counters have negligible delay. The high-voltage counters were connected to a coincidence circuit and the output from this circuit was used to produce the horizontal deflection of an oscilloscope. The pulse from the low- voltage counter was used to produce themore » vertical deflection of the oscilloscope. In this way, the delay could be determined visually. This method is not very accurate and introduces subjective errors. The second method is more accurate and is basod on the measurement of double coincidences with change in the resolving time of the coincidence scheme. The pulses from a high-voltage counter were fed into one of the channels of the coincidence scheme, aad the pulse from the low-voltage counter was fod into the second channel. The pulse length in the high-voltage channel could be varied between 2 and 22 mu sec. At a given voltage, the number of coincidences was measured as a function of the pulse length in the high-voltage channel. As the pulse length in the high-voltage channel is increased the number of coincidences also increases. At the pulse length comparable with the delay time of the low-voltage counter, the slope of the curve suddenly changes. It is clear that in counters having cathodes 18 mm in diameter, the delay is greater than in counters with cathedes 10 mm in diameter. This is explained by the fact that in the bigger counter the negative ion has to cover a larger distance between the moment of its formation and the moment when it loses the electron. At a nominal voltage of 400 v, low-voltage counters can be used in coincidence schemes having resolving times not less than 7 to 10 mu sec. (TCO)« less