Source Range Channel:

Source range instrumentation also measures the rate of change of the count rate. The rate of change is displayed on meters in terms of the startup rate from -1 to +10 decades per minute. Protective functions are not generally related along with source range instrumentation since of inherent limitations in this range.  Therefore, interlocks might be incorporated.

Several reactor plants have found it must to place source range proportional counters in lead shielding to decrease gamma flux at the detectors. That serves two functions: (a) it increases the low end sensitivity of the detector, and (b) it adds to detector life.   Another means through that detector life is extended is to disable the high voltage power supply to the detector and short the signal lead while neutron flux has passed into the intermediate range. There are a few reactor plants which have made provisions for moving the source range detectors from their operating positions to a position of decreased neutron flux level, at one the flux level rise above the source range.

Above Figure displays a classical source range channel in functional form.

Figure: Source Range Channel

B¹⁰ lined or B^F gas-filled proportional counters are generally used as source range detectors. Proportional counter outcome is in the form of one pulse for each ionizing event; thus, there is a series of random pulses varying in magnitude representing neutron and gamma ionizing events.

A pulse height might only be a few millivolts that are too low to be straightly used without amplification. A linear amplifier amplifies the input signal through a factor of various thousand to increase the pulse height to various volts.