JPS6171381A - Waveform discrimination circuit - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a waveform discrimination circuit that discriminates between neutrons and gamma rays, and more particularly to a zero-crossing method that discriminates between neutrons and gamma rays based on the difference in pulse width.

When neutrons or gamma rays are incident on an organic scintillator, the scintillator 10 emits light, but the decay time of the emitted light is several tens of nanoseconds in the case of gamma rays, while it takes several hundred nanoseconds in the case of neutrons.

The zero-cross method is a waveform discrimination circuit that distinguishes between neutrons and gamma rays based on the difference in decay time.

As shown in FIG. 1, the light emitted from the scintillator 10 is converted into an electric pulse by photoelectrons 7 times 'u2, amplified by the preamplifier 3, and input to the waveform shaping circuit 4.

As shown in FIG. 2(a), this input pulse has different pulse widths for neutrons and gamma rays due to the difference in attenuation time. This pulse is shaped by the waveform shaping circuit 4 into a bipolar pulse with positive and negative symmetry as shown in FIG. 2(b), and then by the amplitude limiting amplifier 5, as shown in FIG. It becomes a symmetrical square wave pulse with different positive and negative crossing times. This time has variations for neutrons and gamma rays, and their distribution is shown in Figure 2 (
As shown in d), gamma rays are sharp and neutrons are relatively spread out. Therefore, time gate 6 with a time width of
．． By setting 7 to neutrons and γ rays and detecting the timing of crossing the zero level, each count can be obtained. The problem with this method is that since the gain of the amplitude limiting amplifier 5 is very large, the DC level of the output fluctuates due to the drift of circuit elements and the influence of temperature, which causes changes in the counts of neutrons and gamma rays.

Ideally, if the gain of the amplitude limiting amplifier 5 were infinite, the fall time for crossing the zero level would be zero and would not be affected by fluctuations in the DC level, but in reality the fall time is only a few nanoseconds. Therefore, fluctuations in the DC level cause the distribution of neutrons and gamma rays to shift with respect to the set time gate, resulting in a change in the count. In particular, the direct current level of the waveform shaping circuit 4 is significantly affected by temperature, ±o, s m v
Even if it fluctuates, the gain of the amplitude limiting amplifier 5 increases by about 1000 times 6, so the soomv will fluctuate and it cannot be used up to 1.

For this reason, conventionally, the output of the amplitude limiting amplifier 5 is stabilized by applying direct current negative feedback to the waveform shaping circuit 4.

As shown in FIG. 1, the output DC level of the amplitude limiting circuit 5 is stabilized by removing the signal pulse, that is, the AC component, and giving negative feedback only to the DC component. However, it is impossible to completely remove the alternating current component with an RC circuit, and the extent to which it can be completely removed also depends on the counting rate. Moreover, in direct current proportional negative feedback, an average deviation from the set value always remains.

As a result, the output DC level of the amplitude limiting amplifier 5 fluctuates by 15 mV/C as a result of the temperature effect, and the neutron and gamma ray count values change by about 1%/C. For this reason, as shown in FIG. 1, a variable resistor 8 for adjusting the DC level must be provided to adjust the DC level y4 each time it is used. Furthermore, even temperature changes of the order of normal room temperature can cause an error of several percent, so if continuous measurements are to be made, the circuit must be placed in a thermostatic oven to control the temperature.

An object of the present invention is to provide a waveform discrimination circuit that is not affected by changes in circuit elements over time or ambient temperature.

The present invention completely removes the alternating current component and negatively feeds only the direct current component, stabilizes the direct current level, stabilizes the count of neutrons and gamma rays, and prevents circuit elements from changing over time and being affected by ambient temperature. This is what we are trying to do. More specifically, in the present invention, if the waveform shaping circuit 4 is adjusted so that the output signal pulse of the amplitude limiting amplifier is symmetrical in positive and negative, and the output is integrated, the signal pulse cancels out the positive and negative signals and becomes zero, and the DC component By introducing the multiplier, ;, DC negative feedback is made possible. Furthermore, since it is a negative feedback based on an integral operation, it works so that deviation from the set value of the DC level becomes zero.

FIG. 3 shows an embodiment of the present invention. The operational amplifier 9 and the integrating capacitor 10 perform integral operation 1'.1. A bias current is applied to one input of the operational amplifier, and the operational amplifier 9
By adjusting the output of the amplifier I, the DC level of the amplitude limiting amplifier I unit 5 can be varied.

Therefore, according to this example, the fluctuation in the DC level becomes ±Imy for a temperature change of θ to 40C, the change in the counts of neutrons and gamma rays becomes negligibly small, and the dependence on the count rate is y, x'i: It doesn't show up until Tkcps.

Furthermore, this embodiment is effective against changes over time in the circuit elements used, and enables a waveform discrimination circuit that can be used continuously over a long period of time without adjustment.

As explained above, according to the present invention, the circuit elements are not affected by aging or ambient temperature.

[Brief explanation of the drawing]

FIG. 1 is a conventional waveform discrimination circuit diagram, FIG. 2 is a converted pulse waveform diagram, and FIG. 3 is a circuit diagram showing an embodiment of the present invention. 4... Waveform shaping circuit, 5... Amplitude limiting amplifier, 9...
...Operation amplifier.

Claims (1)

[Claims] 1. A waveform shaping circuit that shapes an input pulse into a bipolar pulse, an amplitude limiting amplifier that amplifies the bipolar pulse into a square wave with positive and negative symmetry, and a negative feedback circuit that negatively feeds the output of the amplitude limiting amplifier to the waveform shaping circuit. A waveform discrimination circuit comprising a feedback loop, wherein an integrator for integrating the output of the amplitude limiting amplifier and extracting a DC component is inserted and connected between the amplitude limiting amplifier and the negative feedback loop. circuit.