JPH0557549B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The output of a radiation detector, such as a proportional counter or a photomultiplier with a scintillator, which sends out pulses with a wave height corresponding to the energy of the radiation, is generally amplified by a preamplifier and then passed through a delay line and The waveform is shaped by a differentiating circuit, etc., and after being amplified, the wave height is discriminated and the signal is added to a counting circuit. In other words, the output of the preamplifier is an integral waveform due to stray capacitance, etc., so by differentiating this, it is converted into a narrow pulse without losing the proportionality with the peak value, and the signal is overlapped. This shortens the dead time. However, with conventional devices, it was impossible to make the time width of the shaped pulse less than the output pulse width of the detector, and the limit was about 1 microsecond.
Another problem was that the peak value of the output pulse varied depending on the time constant of the differentiating circuit. The present invention aims to eliminate these drawbacks, shorten the width of the pulse applied to the pulse height discrimination circuit to about one tenth of the conventional width, and eliminate the dead time by superimposing the signals. This will be explained in detail below.

The present invention utilizes the fact that the output of the preamplifier, which has a waveform obtained by substantially integrating the output signal of the detector, contains various rising components, and extracts each component. By adjusting the amplitudes of the components and recombining them, the waveform is converted into a step-like waveform with a steep rise at the tip, and this is applied to a shaping circuit to obtain a pulse with a sufficiently narrow width. That is, as shown in FIG. 1, for example, the core wire of the proportional counter PC is connected to the preamplifier AO, and a high voltage is applied from the terminal TO, and the output pulse is amplified. In the present invention, the output of the preamplifier AO is applied to the differentiating circuit group DC, and then the output is sequentially applied to the shaping circuit FC, the main amplifier Al, the pulse height analysis circuit PHA, and the scaler S. Figure 2 is an example of the current waveform obtained from the core wire of the proportional counter PC, where the vertical axis is the amplitude y and the horizontal axis is the time t.When such a pulse is applied to the preamplifier AO, its integral effect It is converted into a voltage waveform as shown by the solid line in FIG. 3 and sent out. The above current waveform y is generally expressed as y = _l 〓 ^{h = 1} a _h exp (-t/b _h ... (1), where a _h and b _h are constants and t is time. Assuming that the output waveform Y of the amplifier has a proportionality constant of 1 and a sufficiently large discharge time constant, then Y= _l 〓 ^h=1 {−a _h・b _h exp (−t/b _h )} ……( 2) Therefore, the curve in Figure 2 seems to include a rising component from 80 x sec to /μsec, but this component is also included in the integral curve shown by the solid line L in Figure 3a. It is.

By sequentially differentiating the above equation (2), the differential function is Y′,
Y″……Y ^m , then Y′= _l 〓 ^h=1 {a _h exp(−t/b _h )} Y″= _l 〓 ^h=1 {a _h (−1/b _h ) exp(− t/b _h )} 〓 Y ^m = _l 〓 ^h=1 {a _h (-1/b _h ^m-1 exp(-t/b _h )}.The waveforms of the above differential functions Y, Y, and Y are They are shown in Figures 3b to 3d.As is clear from the figure, as the differential order increases, the waveform contains faster rising components at the tip.These Y, Y', Y''...
...Y ^m is multiplied by appropriate coefficients a ₀ , a ₁ ...a _n to synthesize the function Z = m 〓 ⁱ⁼⁰ d _i Y _i = {a _1n 〓 i=0d _i (-1/ b _h ) ^2-1 }exp(-t/b _h )} +{a _2n 〓 ⁱ⁼⁰ α _i (-1/b _h ) ^i-1 }exp(-t/b _h )〓 +{a _l m 〓 ⁱ⁼⁰ α _i (-1/b _h ) ^i-1 } exp (-t/b _h ) = _l 〓 ^h=1 [a _h { _n 〓 ⁱ⁼⁰ α _i (-1/b _h ) ^i-1 } exp (-t/b _h )] ...(3). Therefore, select an appropriate differential function Y ^m and
By appropriately adjusting a _i , we set _n 〓 ⁱ⁼⁰ α _i (-1/b _h ) ^-1 to 0 for some of the waveform components b _h , and can make this the desired value. This is the third
Of the rising components of the curve shown by the solid line in the figure,
By attenuating components greater than 0.1 μsec and emphasizing components less than that and performing additive synthesis, a staircase waveform signal with a steep rise at the tip as shown by the broken line M in Figure 3a can be obtained. It shows. This staircase waveform M is a waveform whose tip rises steeply, so the chain double-dashed line N in FIG.
There is no undershoot as shown in . Such undershoot N is determined by the above-mentioned predetermined coefficient coefficient αi
This may occur depending on the selection method (see Utility Model Application Publication No. 135136/1983). In this way, even if the undershoot N exists and the waveform slightly overlaps with the adjacent staircase waveform, there is no risk of reducing the wave height value of the adjacent staircase waveform. Therefore, the proportionality between the peak value of the staircase waveform and the peak value of the output of the comparison counter (radiation detector) PC is not impaired.

FIG. 4 is an example of the differential circuit group DC in FIG. 1 for obtaining a signal with a step-like steep rising waveform as described above. That is, the first
Adding the signal obtained from the preamplifier AO in the figure,
This is sequentially differentiated by a circuit in which a plurality of differentiating circuits each consisting of an input capacitor C1, an amplifier circuit A2, and its feedback resistor R1 are connected in series.
In addition, the amplifier circuit A3 and its plurality of input resistors R2,
R3 . . . and the feedback resistor R3 each constitute an adder circuit, and the input resistor R2 is connected to each stage of the above-mentioned differentiating circuit, and a plurality of such adder circuits are connected in series. Therefore, by appropriately setting the resistor R2 in each stage of the adder circuit, the signal shown by the solid line L in FIG. be able to.

FIG. 5 shows another example of a circuit that performs the same operation as in FIG.
The amplified signal is then amplified by the differential circuit in the next stage, and the differential circuits are connected in series.

As mentioned above, when the output signal of the preamplifier is differentiated by multiple differentiating circuits, the low-order differential output contains slow rising components, and as the differential order increases, the signal contains more fast rising components. is obtained. Therefore, the present invention combines these signals and the original signals in appropriate proportions,
By removing unnecessary rising components, we obtain a staircase waveform signal that has only extremely fast rising components, as shown by the broken line in Figure 3, and whose peak value is proportional to the peak value of the output of the proportional coefficient tube PC. It's something you get. Therefore, by applying this signal to the shaping circuit FC in Figure 1, the time width of the output pulse can be reduced to less than one-tenth of the conventional one, thus 0.1 μsec, as shown in Figure 3e.
The following shortened shaped signal U is obtained. As a result, the dead time of wave height analysis circuits, etc. is significantly shortened.
Accurate radiation intensity measurements are possible even at high count rates.

[Brief explanation of drawings]

Figure 1 is a diagram showing the configuration of an embodiment of the present invention, Figure 2 is a diagram showing the configuration of an embodiment of the present invention.
3 and 3 are waveform diagrams for explaining the principle of the present invention, and FIGS. 4 and 5 are examples of a part of the circuit shown in FIG. 1. In the figure, PC
is a proportional counter, AO is a preamplifier, DC is a differential circuit group, FC is a shaping circuit, Al is a main amplifier, PHA is a pulse height analysis circuit, and S is a scaler.

Claims (1)

[Claims] 1 A radiation detector that sends out a pulse with a wave height corresponding to the energy of the radiation, a preamplifier connected to the output terminal of this radiation detector, and a differentiation circuit that differentiates the output of this preamplifier are connected in series. A group of differentiating circuits that generate a staircase waveform signal with a steeply rising tip by combining the outputs of each differentiator circuit at an appropriate ratio, and a group of differentiating circuits that generate a signal with a staircase waveform whose tip rises steeply, and a group of differentiating circuits that generate a signal with a staircase waveform whose tip rises steeply. An output pulse shaping device for a radiation detector, comprising a shaping circuit for obtaining small output pulses.