JPH0453269B2 - - Google Patents

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, scintillation light emission occurs, but the decay time of the light emission is several tens of nanoseconds for gamma rays, while it takes several hundred nanoseconds for 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 1 is converted into an electric pulse by the photomultiplier tube 2.
The signal is amplified by the preamplifier 3 and enters the waveform shaping circuit 4.

As shown in FIG. 2a, this input pulse has different pulse widths for neutrons and gamma rays due to the difference in decay time. This pulse is shaped by the waveform shaping circuit 4 into a symmetrical bipolar pulse as shown in FIG.
As shown in Figure c, neutrons and gamma rays form symmetrical square wave pulses with different times to cross the zero level. This time varies for both neutrons and gamma rays, and as shown in Figure 2d, the distribution is sharp for gamma rays and relatively spread out for neutrons. Therefore, each count can be obtained by setting time gates 6 and 7 having a certain time width for neutrons and gamma rays, and detecting the timing at which they cross the zero level. The problem with this method is that since the gain of the amplitude limiting amplifier 5 is quite large, the DC level of the output fluctuates due to drift of circuit elements, temperature effects, etc., 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, even if the DC level of the waveform shaping circuit 4 fluctuates by ±0.5 mV, the influence of temperature is particularly significant, the gain of the amplitude limiting amplifier 5 is about 1000 times, so the 500 m
V will fluctuate and it cannot be used as is.

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 negative feedback of only the DC component using the RC circuit. However, it is impossible to completely remove the alternating current component with an RC circuit, and the extent to which it can be 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 fluctuation in the output DC level of the amplitude limiting amplifier 5 due to the temperature effect is 15 mV/℃,
The neutron and gamma ray counts change by about 1%/°C. Therefore, as shown in FIG. 1, a variable resistor 8 for adjusting the DC level must be provided to adjust the DC level 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 mixing.

The present invention completely removes the AC component and negatively feeds only the DC component, stabilizes the DC level, stabilizes the count of neutrons and gamma rays, and prevents circuit elements from changing over time or being affected by ambient temperature. This is what we are trying to do. More specifically, the present invention includes:
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 this output is integrated, the signal pulse will cancel out the positive and negative components and become zero, leaving only the DC component to be integrated. ,
By introducing an integrator, negative DC 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. operational amplifier 9
The integral capacitor 10 performs an integral operation. By flowing a bias current into one input of the operational amplifier and adjusting the output of the operational amplifier 9, the DC level of the amplitude limiting amplifier 5 can be varied.

Therefore, according to this embodiment, the temperature change is 0 to
The DC level fluctuation is ±1mV at 40℃.
Changes in neutron and gamma ray counts become negligible, and count rate dependence does not appear until several hundred kcps.

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 negative feedback that negatively feeds back the output of the amplitude limiting amplifier to the waveform shaping circuit. A waveform discriminator circuit comprising: an integrator that integrates the output of the amplitude-limiting amplifier and extracts a DC component is inserted between the amplitude-limiting amplifier and the negative feedback loop. .