Patent Number: 
Section: description

The embodiments of the present invention will now be explained with reference to the drawings. FIG. 1 shows one embodiment of the invention. In FIG. 1, the current pulse signals output from a neutron detector 1 are converted and amplified into voltage pulse signals at a pre-amplifier 2 before being input to a pulse count rate measurement unit 3. The current pulse signals output from the neutron detector 1 are negative. The voltage pulse signals (negative neutron signals) output from the pre-amplifier 2 is input to a negative pulse height discriminator 8 that constitutes the pulse count rate measurement unit 3. The negative pulse height discriminator 8 is provided with a negative discrimination level Ln for measuring the negative pulse count of the neutron signals. When the voltage pulse signal is smaller (greater in absolute value) than the negative discrimination level Ln, the negative pulse height discriminator 8 outputs a pulse signal to add to a negative pulse counter 9. In a positive pulse height discriminator 18, the positive discrimination level Lp is set by a positive discrimination level setting circuit 5 for measuring the positive pulses caused by excessive extrinsic electric noises. The discriminator 18 outputs a pulse signal when the noise pulse signal exceeds the positive discrimination level Lp to add to a positive pulse counter 19. The discrimination level setting circuits 4 and 5 for negative pulse count measurement and positive pulse count measurement each sets the negative or positive pulse height discrimination level Ln or Lp as shown in FIG. 3(b). The pulse count of the negative pulse counter 9 is provided to a display device 11, a trip circuit 12a, and a pulse count rate correction circuit 22. Further, the output of the pulse count rate correction circuit 22 is provided to the display device 11 and a trip circuit 12b.  The pulse count of the positive pulse counter 19 is provided to the display device 11, the pulse count rate correction circuit 22 and a count rate anomaly detection circuit 24, and the output of the count rate anomaly detection circuit 24 is provided to the display device 11. The output of the trip circuits 12a and 12b are provided to the display device 11. Next, the operation mentioned above is explained with reference to the flowchart of FIG. 2. The current pulse signal shown in FIG. 3(a) output from the neutron detector 1 for detecting the neutron flux density is converted and amplified into a voltage pulse signal by the pre-amplifier 2 before being input to a negative pulse height discriminator 8 constituting the pulse count unit 3. The negative pulse height discriminator 8 outputs a pulse signal when the voltage pulse signal is smaller (greater in absolute value) than the negative discrimination level Ln to add to the negative pulse counter 9. On the other hand, the positive pulse height discriminator 18 provides a pulse signal to the positive pulse counter 19 that counts the pulses when the noise pulse signal exceeds the positive discrimination level Lp. In step S1 of the pulse count unit 3, the negative pulse counter 9 counts the number of negative pulses, and in step S2, the positive pulse counter 19 carries out the count-rate process of the positive pulses. In step S3, the count rate anomaly detection circuit 24 compares the counted value of the positive pulses counted by the positive pulse counter 19 with a preset value. If the counted value does not reach the preset value, the procedure advances to step 5, and if the counted value is equal to or greater than the preset value, the procedure advances to step 4. In step S4, the count rate anomaly detection circuit 24 displays the count rate anomaly on the display device. Normally, when no electric noise exists, there is no count rate anomaly displayed on the display device 11, but when electric noise pulses exceeding a certain level is mixed in, the count rate anomaly is displayed on the display device 11. In step S5, the pulse count rate correction circuit 22 inputs the negative pulse count rate of the negative pulse counter 9 and the positive pulse count rate of the positive pulse counter 19, and computes xe2x80x9cnegative pulse countxe2x80x94positive pulse countxe2x80x9d to correct the measured value, and thereafter in step S6, the corrected measured value (count rate) is displayed as the measured value on the display device 11. In step S7, the trip circuit 12 compares the corrected measured value with the trip preset value, and if the corrected measured value has not reached the trip preset value the procedure is terminated, and if the corrected measured value is equal to or greater than the trip preset value, the procedure advances to step 8. Instep 8, the trip circuit 12 displays on the display device 11 that trip output exists. This is how the neutron measurement is performed, but normally, there exists no excessive extrinsic electric noise, so only the negative neutron pulses are counted by the negative pulse counter 9. If excessive extrinsic electric noise is generated and the negative pulse counter 9 counts the neutron pulses and the electric noise pulses, not only the negative pulse count rate but also the positive pulse count rate measured by the positive pulse counter 19 is displayed simultaneously on the display device 11. Further, the count rate anomaly detection circuit 25 compares the positive pulse count rate with the preset value, and when the positive pulse count rate exceeds the preset value, it displays a count rate anomaly detection result on the display device 11. By looking at the display device 11, the operator can determine easily that the increase in negative pulse count rate is caused by the electric noise. Thereafter, when the electric noise disappears and the decrease in negative pulse count rate is displayed on the display device 11, the decrease in the positive pulse count rate measured by the positive pulse counter 19 is simultaneously displayed on the screen. Further, since the positive pulse count rate becomes smaller than the preset value, the count rate anomaly detection circuit 24 clears the count rate anomaly detection result and outputs the result to the display device 11, so it is shown on the display that the state is normal. According to these operations, it could easily be judged that the decrease in the negative pulse count rate is caused by the disappearance of the electric noise. As explained above, even if excessive extrinsic electric noise with shorter intervals than the signal pulse width is mixed into the pulse signals continuously, the present invention detects and displays the occurrence of the electric noise by counting the positive pulses, and moreover, detects and displays the variation of the positive pulse count rate simultaneously when detecting and displaying the variation of the negative pulse count rate accompanying the occurrence and disappearance of the extrinsic electric noise. Accordingly, the present invention enables the operator to confirm the status of the system easily and to determine that the fluctuation of the counted value is caused by electric noises. Next, the function of correcting the pulse count rate according to the present invention is explained. The negative pulse height discrimination level Ln and the positive pulse height discrimination level Lp are set in advance as shown in FIG. 3(b) so that the negative pulse count and the positive pulse count of the extrinsic electric noises are equal when no neutron signal exists. The pulse count rate correction circuit 22 carries out an operation to subtract the positive pulse count from the negative pulse count. Thereby, even when electric noise is superposed continuously over the neutron pulse signals with smaller intervals than the signal pulse width as shown in FIG. 3(c), the pulse count rate correction circuit 22 is capable of computing the corrected count rate excluding the fluctuation of the pulse count rate caused by the influence of the electric noises. This corrected count rate is displayed on the display device 11. Moreover, a trip circuit 12a and a trip circuit 12b are provided to correspond to the count rate before the correction and the count rate after the correction, and the trip output status is displayed on the display device 11, thereby enabling the operator to understand the status related to the electric noise more accurately. As explained above, even if excessive extrinsic electric noise having a shorter interval than the signal pulse width is mixed into the pulses, the present invention enables to measure the pulse count rate stably without being influenced by the noise. Accordingly, the present invention improves the reliability of the neutron flux measurement. FIGS. 4(a) through (c) show examples of the screen displaying the negative pulse count rate together with the positive pulse count rate and the corrected pulse count rate according to the present invention. FIG. 4(a) is a display screen example of the display device 11 showing the case where no noise exists in the input signal and no trip output of the xe2x80x9chighxe2x80x9d count rate exists. FIG. 4(b) is a display screen example of the display device 11 showing the case where the negative pulse count rate is increased since noise signals are mixed into the input signal, but the function to correct the pulse count rate enables to compute the correct count rate, so no high count rate trip output exists. FIG. 4(c) shows the displayed screen example of the display device 11 showing the case where no noise signal exists in the input signal, but the actual count rate exceeds the trip value, so therefore a high count rate trip is output. The measurement is performed as mentioned, and the counting of positive noise pulse signals are performed simultaneously when counting the negative pulse signals output from the neutron detector, and the positive pulse count per unit time is subtracted from the negative pulse count per unit time so as to measure the neutrons. In other words, the negative pulse count rate computed by subtracting the positive pulse count rate (noise pulses) from a negative pulse count rate that is the sum of the noise pulses and the neutron flux detection pulses counted within a measurement cycle of a unit time is set as the neutron measurement value. According to the present invention, the noise pulses detected in both positive and negative polarities are cancelled so that the count of the negative pulse signals corresponding to the detection pulse signals of the neutron flux is obtained for neutron measurement. Therefore, the neutrons can be measured with high accuracy without being influenced by the number of noise pulses that are mixed in per unit time.