Abstract
A quick response ionization survey-meter and some problems concerned with a-contamination are described. To measure small currents a high resistor and a D. C. amplifier are used. The time response of such a device has usually been improved by the negative feed back. But there is a limitation because of the capacity parallel to the high resistor. This capacity can be reduced by means of a new device shown in a figure, and the use of a high gain D. C. amplifier at the same time makes the response extremely quick. By this method, the time constant less than 0.5 second was accomplished with a resistor of 1013 ohms, and 1 mr/h of full scale was realized with an air ionization chamber of the volume about 400 cc. The response of the device is actually limited by the statistical fluctuations of the ionization current due to the measuring γ-radiations. The theo-retical considerations about these problems are discussed and confirmed by the measurement using an experimental instrument.
Another problem for quickening the response and raising the sensitivity of the survey-meter is the effect of α-contamination of the air ionization chamber. Random pulses or spikes are pro-duced by stray α-emitters found in the chamber. By the analysis of their pulse height distribution, five causes of these random pulses are traced, among which the emitters in the wall material, on the walls and in the air are in the proportion of 55: 27:18. These spikes were reduced to 60% after the chamber was left sealed for one year, and this reduced value is nearly equal to the first figure of the above mentioned ratio. When the response is sufficiently quick, the presense of α-contamination has no influence on the measurement, because the recovery to the normal state is quick and the discrimination of these pulses is very easy.
