The present invention relates to a material density measurement system using radioactive rays, particularly for providing a substantially linear output signal.
The relationship between the ionization current y and the density x of a material interrupting the radioactive rays is nonlinear and expressed in the following equation: EQU y=ke.sup.-.mu.x
where k and .mu. are constants.
In order to avoid the dificculty in handling the nonlinearity, there have been proposed various devices for converting an ionization current signal into a linear signal.
For instance, U.S. Pat. No. 3,529,162 discloses a device which approximates, making use of diode characteristics, the input/output characteristics with a plurality of polygonal lines. Also, the magazine TOBACCO INTERNATIONAL (Apr. 30, 1976) discloses a system which converts a nonlinear output signal into digital signals for processing. Further, a device referred to as a "log amplifier" is adapted to produce an output signal proportional to the logarithm of input voltage using analog elements.
The device making use of diode characteristics for approximating the output with a plurality of polygonal lines, however, has a drawback in that it is difficult to obtain an accurate approximation. The system incorporating a computer for processing digital signals requires a complicated circuit which considerably increases the system cost. Further, the device incorporating the log amplifier inconveniently exhibits a large fluctuation in output caused by temperature drift. The influence of the temperature drift is serious particularly when the changes in the input and output signals are small.