CIRCUITRY AND CIRCUIT ELEMENTS
Radiation detection circuit currents or pulse rates vary over a wide range of values. The
current output of an ionization chamber may vary by 8 orders of magnitude. For
example, the range may be from 10-13 amps to 10-5 amps. The most accurate method to
display this range would be to utilize a linear current meter with several scales, and the
capability to switch those scales. This is not practical. A single scale which covers the
entire 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 the
logarithm of the input signal. This is easily done by using a diode when the input signal
is from an ionization chamber. The voltage across the diode equals the logarithm of the
current through the diode. Using this principle, the simplified circuit, shown in Figure
33, is used to convert ionization chamber current to a voltage proportional to the
logarithm of this current.
Figure 33 Log Count Rate Meter