CIRCUITRY AND CIRCUIT ELEMENTS Radiation DetectorsLogarithmicMetersRadiation detection circuit currents or pulse rates vary over a wide range of values. Thecurrent output of an ionization chamber may vary by 8 orders of magnitude. Forexample, the range may be from 10^{-13} amps to 10^{-5} amps. The most accurate method todisplay this range would be to utilize a linear current meter with several scales, and thecapability to switch those scales. This is not practical. A single scale which covers theentire range of values is used. This scale is referred to as logarithmic.The logarithmic output meter must be provided with a signal which is proportional to thelogarithm of the input signal. This is easily done by using a diode when the input signalis from an ionization chamber. The voltage across the diode equals the logarithm of thecurrent through the diode. Using this principle, the simplified circuit, shown in Figure33, is used to convert ionization chamber current to a voltage proportional to thelogarithm of this current.Figure 33 Log Count Rate MeterIC-06 Page 60 Rev. 0