Patent Number: 044938110
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT In FIG. 7, a fission counter 20 as a neutron detector is installed in a pressure vessel (not shown) of a reactor. Neutron flux level signals detected by the fission counter 20 are transmitted to a signal input terminal 23 of a preamplifier 22, which is an embodiment of the present invention, through a coaxial cable 21. A high tension voltage HV is applied to the signal input terminal 23 through a resistor RH. A first amplifier 24 for high frequency band amplification includes a low input impedance circuit 24a comprised of an input resistor R6 grounded at one end and a coupling capacitor C6. Impedance of the input impedance circuit 24a is designated as Z6. An input terminal of the impedance circuit 24a is connected to the signal input terminal 23. If the capacitance of the capacitor C6 and resistance of the input resistor R6 are designated as C6' and R6' respectively, the following relation holds ##EQU6## where j.sup.2 =-1 .omega.=2.pi.f PA1 f=frequency of the input signal Capacitance C6' of the capacitor C6 is set so that the input impedance Z6 has a minimum value, Z6.apprxeq.R6', when the frequency of input signal applied to the input terminal of the input impedance circuit 24a is more than 1 MHz. The input impedance circuit 24a is impedance-matched to the coaxial cable 21. A second amplifier 25 for low frequency band amplification includes a high input impedance circuit 25a, in which an input resistor R7 grounded at one end and a coupling capacitor C7 are connected in series. The input terminal of the impedance circuit 25a is connected to the signal input terminal 23. Assume that resistance of the input resistor R7, capacitance of the capacitor C7, and impedance of the impedance circuit 25a are designated as R7', C7' and Z7, respectively. Capacitance C7' of the capacitor C7 is selected so that the impedance of the circuit 25a takes a minimum value, Z7.apprxeq.R7, when the impedance circuit 25a receives input signals of more than 1 kHz. Further, resistances R7' and R6' are selected so as to satisfy the relation R7'&gt;&gt;R6'. Impedances Z6 and Z7 are also selected so as to satisfy the relation Z6&lt;&lt;Z7 for input signals of 1 MHz or more, and Z6&gt;Z7 for input signals of 1 kHz to 10 kHz, respectively. A third amplifier 26 for pulse signal amplification (pulse signals in the start-up range of reactor operation) is connected at its input terminal to the output terminal of the first amplifier 24 and at its output terminal to an input terminal of a pulse counting system 27. A fourth amplifier 28 is a lower frequency band Campbell signal amplifier for amplifying lower frequency band Campbell signals included in the Campbell signals produced in the intermediate range of reactor operation. An input terminal of the fourth amplifier 28 is connected to the output terminal of the second amplifier 25. A fifth amplifier 29 is a higher frequency band Campbell signal amplifier for amplifying higher frequency band Campbell signals and connected at its input terminal to the output terminal of the first amplifier 24. Further, a switch 31 is provided for selectively connecting either of the output terminals of the fourth amplifier 28 and fifth amplifier 29 to the input terminal of a Campbelling system 30. The switch 31 is connected to the output terminal of the fourth amplifier 28 while the lower frequency band Campbell signals are measured by the Campbelling system 30. Upon finishing the measurement of the lower frequency band Campbell signals, the switch 31 is turned to the output terminal of the fifth amplifier 29 by a switching signal 32 from the Campbelling system 30. The operation of the preamplifier 22 will now be described. When the frequencies of the neutron flux level signals derived from the detector 20 are 1 MHz or more, the relationship Z6&lt;Z7 is established where Z6 is the input impedance of the first amplifier 24 and Z7 the input impedance of the second amplifier. Further, the impedance Z6 has a minimum value represented by R6'. Therefore, the signals of 1 MHz or more enter the first amplifier 24. Since the first amplifier 24 is impedance-matched with the coaxial cable 21, no reflection signal is produced in the signal path. Therefore, pulse signals with little waveform distortion are obtained from the output terminal of the first amplifier 24. The pulse signals are amplified by the third amplifier 26 and counted by the pulse counting system 27. When the frequencies of the output signals from the detector 20 range from approximately 1 kHz to 10 kHz, that is, when the output signals are the lower frequency Campbell signals, the impedance Z7 of the input impedance circuit 25a and the input impedance of the first amplifier 24 satisfy the relation Z6.gtoreq.Z7, and the impedance Z7 has a minimum value. Accordingly, the Campbell signals from the detector 20 smoothly enter the second amplifier 25 for low frequency band amplification. The resistance R7' of the input resistor R7 of the input impedance circuit 25a is set high, as mentioned above. Accordingly, the Campbell signals in the above frequency band can be produced from the second amplifier 25 with the improved S/N characteristic. The output signals (Campbell signals) from the second amplifier 25 are amplified by the fourth amplifier 28 for the lower band Campbell signal amplification and are then applied to the Campbelling system 30 through the selection switch 31. Even in the case where the level of the Campbell signal from the detector 20 is low, the Campbell signal can be measured with high accuracy. Further, the measuring range in the pulse signal counting system 27 can satisfactorily overlap that of the Campbelling system 30. This feature improves the reliability of the measurement. When the pulse rate of the detected signal from the neutron detector 20 increases and it is allowed to measure the neutron flux level by the high frequency Campbelling method, the switching signal 32 is applied from the Campbelling system 30 to the selection switch 31. Upon reception of the switching signal 32, the selection switch 31 connects the output terminal of the fifth amplifier 29 to the input terminal of the Campbelling system 30. In this case, the first amplifier 24 receives the Campbell signals from the detector 20 through the low resistance R6'. Therefore, the S/N of the first amplifier 24 is degraded. In this situation, however, the pulse rate of the input signals is satisfactorily high, so that the Campbell signal can be amplified. Further, the Campbell signal contains the high frequency component. This high frequency component enables the Campbelling system 30 to provide an output signal with a short response time and a small fluctuation factor I'. FIG. 8 shows a relationship of a fluctuation factor I' of the output signals from the Campbelling system 30 to the pulse rate (Nn) of the input signal to the preamplifier 22. In this graph, in the range from 10.sup.3 c.p.s to 10.sup.7 c.p.s of the pulse rate, the Campbell signal amplifier (4th amplifier) 28 connected to the Campbelling system 30 amplifies the output signals of the second amplifier 25, and, in the range of 10.sup.7 c.p.s or more, the Campbell signal amplifier (5th amplifier) 29 connected to the Campbelling system 30 amplifies the output signals of the first amplifier 24. When comparing this graph with that of FIG. 6, it will be seen that the measuring device of this embodiment remarkably improves the fluctuation factor and shortens the response time. Turning now to FIG. 9, a functional diagram of the measuring device is shown in FIG. 7. For simplicity, like reference numerals are applied to like or equivalent portions. In FIG. 9, the third amplifier 26 is made up of amplifiers 26A and 26B. An amplifier 33 is connected between the Campbelling system 30 and the selection switch 31. The pulse signal counting system 27 contains a discriminator 35, a diode pump 36 and an amplifier 37, which are connected in series. The amplifier 37 produces a logarithm count rate output signal. The Campbelling system 30 includes a buffer amplifier 39, a circuit 40 for obtaining a mean squared value of the effective value of the output signal from the amplifier 39, a logarithm converting circuit 41 and an amplifier 42, which are connected in series. Further, a circuit 44 for controlling the switch 31 and a level shift circuit 43 are contained. As a matter of course, the switch 31 is automatically operated by driving the circuit 44 with the output signal from the amplifier 42. The detailed description of the pulse signal counting system 27 and the Campbelling system 30 are omitted here, since these are not essential to this invention.