Radiation Detectors
CIRCUITRY AND CIRCUIT ELEMENTS
Period Meters and Startup Rate
In many applications it is essential to know the rate of change of power. This rate
normally increases or decreases exponentially with time. The time constant for this
change is referred to as the period. A period of five seconds means that the value
changes by a factor of e (2.718) in five seconds. Figure 34 shows a basic period meter
circuit.
Figure 34 Period Meter Circuit
Placing the signal through an RC circuit causes a voltage that is proportional to the
reciprocal of the period. If the current output from the ionization chamber is constant,
no current flows through resistor R, and the output voltage is zero. This corresponds to
an infinite period. As the ion chamber output current changes, there is a voltage transient
across capacitor C, and current flows through resistor R. The more rapid the transient,
the greater the voltage drop across resistor R, and the shorter the period.
Rate information is displayed on a meter in decades per minute, and since it is used by
the operator to monitor the rate of change of power during startup, it is termed startup
rate. Startup rate (SUR) equates to reactor period using Equation 6-10.
(6-10)
SUR
26.06
t
Rev. 0
Page 61
IC-06
