Abstract
A design consideration for a fast response radiation thickness gauge which uses a cylindrical ionization chamber as the detector is descrived. It is pointed out that, in realizing a fast response, the time lag in the response of the ionization chamber must be taken into account as well as that in the amplifier stage.
According to the expressions derived for the transient responses of the overall system, fundamental requirements are considered for attaining a fast response or a short rise time (0.1∼0.9) in the indicial response.
Besides the importance of using an amplifier with small time lag, it is shown that the ionization chamber has an optimum ratio of the diameter of the outer electrode to that of the inner electrode, depending upon the time lag in the amplifier. When the amplifier has a fairly large time lag, a small value of the ratio is preferable in the design of the ionization chamber. This will reduce the time required for positive ions to reach the outer electrode. On the contrary, in case that the time lag is relatively small, a large value of the ratio is recommendable so as to increase the output voltage due to electrons.
It is also shown that the response time can be improved more or less by either raising the voltage between the electrodes (V0) or lowering the filling gas pressure (p) or employing a filling gas with high mobility (μβ), but the these procedures are effective only within a limited range of the value of V0μβ/p.
The above considerations have proved to be in good agreement with the experimental results of the indicial and frequency responses.
