JPH0827356B2 - Radiation counting method - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radiation counting method for discriminating and measuring radiation energy detected by a radiation detector.

[0002]

2. Description of the Related Art The generation of photons or other radiation is a random phenomenon that follows a certain probability distribution, and therefore its count value has variations. Therefore, in the radiation intensity measurement, it is necessary to count a certain value or more in order to reduce the ratio of the variation of the count value to the count value according to the accuracy of the calculation.

Therefore, in order to measure the intensity of the radiation in a short time, it is necessary to increase the count value of the radiation per unit time. For that purpose, the output of the radiation generator is increased, and the measurement target and the radiation detector are increased. It is conceivable to reduce the distance between them. However, when not only counting the intensity of radiation by the radiation detector but also discriminating energy, if the frequency of radiation entering the radiation detector increases,
In other words, if the strength increases, the phenomenon of counting down the count value becomes a problem.

By the way, the photons and other radiation particles detected by the radiation detector are converted into charge pulses and waveform-shaped, the energy is obtained from the peak value of the charge pulse, and the number of radiation rays is obtained from the number of the pulses. To be However, when the frequency of incidence of radiation on the radiation detector increases, pulses are also generated more frequently, and as shown in FIG. 6, pulses 2 and 3 generated by a plurality of separate photons overlap each other and one large pulse 4 is generated. It becomes like this.
When the pulses are overlapped in this way, the number of radiations is counted less than the real number (counting), and the crest value also changes (pile-up), so that the energy cannot be accurately measured.

In order to prevent the overlapping of the pulses, it is sufficient to select a small time constant for waveform shaping (hereinafter referred to as a time constant) so that the pulse width becomes small at the time of waveform shaping.
In this case, there is a problem that the noise removal by integration at the time of waveform shaping becomes insufficient and the error of the crest value, that is, the error of energy measurement becomes large (deterioration of energy resolution).

Conventionally, the following method has been considered as a method for solving these problems. (1) A measurement circuit that measures the timing and number of incidences on the radiation detector at high speed is provided in parallel with the crest value measurement circuit to which the detection signal of the radiation detector is input, and this measurement circuit detects overlapping pulses. A method that corrects the count value to reduce the apparent counting of pulses.

(2) By using a plurality of radiation detectors and a plurality of signal processing circuits attached thereto, the counting rate of the entire system can be increased without increasing the number of radiations entering one radiation detector per unit time. Method of raising (JP-A-62-121382). (3) If multiple pulses overlap, add 1
By utilizing the fact that the pulse width becomes longer when it is regarded as one pulse, the counting rate is corrected from the pulse width to apparently reduce the number of pulses to be missed (Japanese Patent Laid-Open No. 63-108302). Gazette).

[0008]

However, these methods have the following problems.

In the above method (1), the total number of incident radiations can be counted with little counting, but since the energy of each radiation is not necessarily measured, radiations of a plurality of energies are not necessarily measured. When measuring, the apparent count value increases, but there is a problem that variations in the count value are not improved at all.

Further, the above method (2) does not change the incident conditions for improving the count rate, such as increasing the output of the radiation generator or reducing the distance between the object to be measured and the radiation detector. This is the simplest and most convenient method because it can improve the counting rate, but it requires the addition of an expensive radiation detector (semiconductor detector) system and multiple radiation detectors in a limited space. There is a limit to how much it can be installed in, and it is not always realistic.

Further, in the method (3), when the radiation having a plurality of energies is measured as in the case (1), even if the count value of all the radiations can be corrected, the radiation is measured in the energy. Since it is not possible to carry out all of the individual radiation incident on the detector, the measurement accuracy of the count value is not improved as a result.

The object of the present invention is to improve the counting rate without adding a new radiation detector system, without degrading the accuracy of energy measurement, and substantially reducing the number of counting pulses. It is to provide a radiation counting method capable of achieving the above.

[0013]

In order to achieve the above-mentioned object, the present invention shapes a voltage pulse waveform generated by radiation in a radiation counting method for counting radiation and discriminating energy using a radiation detector. The energy of radiation is obtained by sampling a plurality of voltage values from the vicinity of the peak of the voltage pulse after the shaping and adding the respective sampled values by weighting.

Further, the timing for sampling a plurality of voltage values from the vicinity of the peak of the shaped voltage pulse and the coefficient for adding each sampled value by weighting are
The setting is made based on a pulse waveform defined in advance so that the variation of the weighted addition value is reduced even when the sampling timing is totally deviated.

[0015]

In such a method of the present invention, instead of sampling only one point of the peak as in the conventional case, a plurality of points around the peak are sampled at a certain timing and the weighted sum of these values is calculated. By outputting the output, the lack of noise removal due to integration at the time of waveform shaping can be compensated for in the sampling stage of the crest value, and the energy resolution can be prevented from deteriorating while reducing the pulse width.

Further, by sampling a plurality of points on both sides of the peak near the peak of the pulse after waveform shaping, weighting and adding, the sampling start time of each sampling point is slightly shifted due to jitter. Also,
Since the increase or decrease in the value obtained by multiplying each sampling value by the weighting coefficient is canceled by adding each sampling value, it is possible to reduce the influence on the weighted addition value and to obtain the effect that the energy resolution can be further improved. To be

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows an example of a configuration in which the method of the present invention is applied to a system in which a semiconductor detector is used as a radiation detector to perform a fluorescent X-ray analysis of a galvanized steel sheet and feed it back to a production line. As shown in Figure 1,
When the X-ray enters the semiconductor detector 5, the X-ray is converted into an electric pulse by the semiconductor detector 5, and is input to the waveform shaping amplifier 7 and the sampling timing setting device 8 via the preamplifier 6. The waveform shaping amplifier 7 shapes and amplifies the output signal of the preamplifier 6 and outputs it to the wave height discriminator 9 whose details will be described later. The sampling timing setting device 8 is also input to the wave height discriminator 9. The timing for sampling the signal waveform from 7 is set. The output signal subjected to the wave height discrimination by the wave height discriminator 9 is counted by the counter 10.

Next, the operation of this embodiment will be described. First, the X-rays incident on the radiation detector 5 are Zn and F.
These are the kα line and the kβ line of e. If the Znkα ray is to be measured, the Zn with the closest energy is used.
It is necessary to have an energy resolution that can be counted separately from kβ in terms of energy. Moreover, in order to shorten the measurement time, measurement at a high count rate is required. In this case, simply increasing the intensity of the X-ray tube to increase the number of X-rays incident on the semiconductor detector 5 not only results in counting down, but a sufficient effect cannot be obtained, but rather it is too strong. It may lead to a decrease in the counting rate. Further, in order to avoid this, if the time constant is simply reduced, the energy resolution will be deteriorated, and the separation between the Znkα ray and kβ will be insufficient.

Therefore, in order to shorten the time constant and at the same time suppress the deterioration of the energy resolution, the sampling timing setting unit 8 is applied to the waveform shaping pulse 1 as shown in FIG. The waveform discriminator 9 at the eight points (t = ti, i = 1 to 8) set by
Sampling is performed by using, weighting is performed so that noise is reduced, and addition is performed. Here, the weighting coefficient is ai
When (i = 1 to 8), the wave height discriminator 9 performs the following calculation. S = Σai · V (ti)

S is a value proportional to the energy of the incident X-ray. Then, the wave height discriminator 9 outputs a pulse to the counter 10 when S corresponds to a value corresponding to a preset energy range.

In this case, the S / N ratio at each sampling point
Sampling is half the peak to prevent it from getting too bad
The weighting coefficient is
At each sampling point calculated from the constants of the shape shaping circuit
The value is proportional to the relative theoretical strength. As a specific example
In the waveform shaping circuit as shown in FIG. 3, C × R = T T: time constant C₂= 2C₁  C₁(R₁+ R₂) = TR₁= R₂  Is used, the step input of this circuit
The response Fs to is Fs = A, where t is time.₀[T / T + 1 + 2^1/2 sin (t-0.75π)] exp (t / T). However, A₀Is a constant.

From this equation, the magnitude of the pulse corresponding to the sampling timing can be theoretically obtained. Then, the measured pulse size at each sampling point is multiplied as a weighting coefficient by the size theoretically obtained from the pulse at each sampling timing, and the weighting factor is added.

The sampling start timing (the time of the first sampling point) is set by the sampling timing setting device 8 which is a high-speed pulse processing circuit provided in parallel with the waveform shaping amplifier 7 and the wave height discriminator 8. In this way, the signal output from the wave height discriminator 9 is sent to the counter 10 and counted.

Here, the time constant of the waveform shaping amplifier 7 is set to 0.
When the present invention is applied with 8 μsec, the time constant is 2.
Fig. 4 shows a comparison of the counting efficiency when the conventional method is adopted for 2 µsec. In this case, the energy resolution is Znk
Both are about 315 eV with respect to α.

As described above, the time constant of the method of the present invention is about 1/3 of that of the conventional method (from 2.2 μsec to 0.2 μs) while keeping the energy resolution at the same level as that of the conventional method.
It can be shortened to 8 μsec), and as shown in FIG. 4, the counting efficiency, which is the ratio of the actual count value and the true count value without counting, can be greatly improved. When the incident X-ray is 30 Kcps, the counting efficiency is about 50% in the conventional method, while it is about 90% in the method of the present invention.
This indicates that it takes about half the time to count a certain number of X-ray photons as compared with the conventional method. Also,
It shows that the statistical error becomes about 0.7 times when the measurement time is fixed. Next, another embodiment according to the method of the present invention will be described.

In this embodiment, in addition to the above contents, a measure is taken so that the influence of the overall deviation of the sampling timing at all sampling points due to the jitter of the sampling timing setting unit 8 on the weighted sum of the sampling values is reduced. It is the one. Specifically, within a range in which the pulse shape is substantially symmetrical with respect to the peak point, the increase and decrease in the value obtained by multiplying the eight sampling values by the jitter by the weighting coefficient by four points before and after the peak are offset by adding. Thus, the time between each sampling point is set from the relative theoretical intensity of each pulse at each sampling point calculated from the constant of the waveform shaping circuit.

Specifically, eight points including the end points of the peak peak value of ½ or more are taken, and the weighting coefficient is obtained when a pulse is applied to the waveform shaping circuit as in the above embodiment. The peak value at the sampling point was used. As a result, in addition to the same effect as described above, the influence of the jitter on the energy measurement value can be reduced.

In this example, the sampling time is set as shown in Table 1 for the case A where the time constant is set to 1 μsec and the case A is devised as in the above embodiment and the case where it is not executed B.

[0030]

[Table 1] Further, FIG. 5 shows the effect reduction effect when the sampling points are set as shown in Table 1, and it is understood that the influence on the weighted sum of jitter is small.

On the other hand, in each of the above embodiments, a plurality of voltage values near the peak of the shaped voltage pulse are sampled and the respective sampled values are weighted and added. However, the sampling timing is set by the voltage pulse waveform before shaping. As in the previous case, the pulse after waveform shaping is sampled at the sampling timing, and the weighted addition is performed with a coefficient that becomes maximum at the peak position and becomes smaller as it moves away from the peak position for the plurality of sampled data. The effect of can be obtained.

Further, in each of the above embodiments, the case where the X-ray is used as the radiation has been described, but it goes without saying that the present invention can be applied to the measurement of other radiation. Further, the number of sampling points, the sampling position, the weighting coefficient, etc. may be set to various conditions different from the above without departing from the above-mentioned meaning.

[0033]

As described above, according to the present invention, it is possible to substantially count down the number of pulses without adding a new radiation detector system and without degrading the accuracy of energy measurement. It is possible to provide a radiation counting method that can reduce the number and improve the counting rate.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment for explaining a radiation counting method according to the present invention.

FIG. 2 is an explanatory diagram of a sampling method for a pulse after waveform shaping in the embodiment.

FIG. 3 is a configuration diagram showing a specific example of a waveform shaping circuit in the embodiment.

FIG. 4 is a graph showing a comparison of counting efficiencies between the method of the present invention and the conventional method.

FIG. 5 is a diagram showing the effect of reducing the influence of jitter by the radiation counting method according to another embodiment of the present invention.

FIG. 6 is a diagram showing an overlap of pulses generated by two X-ray photons successively incident on a detector by a conventional radiation counting method.

[Explanation of Codes] 1 ... Pulse after waveform shaping, 5 ... Semiconductor detector, 6 ...
... preamplifier, 7 ... waveform shaping amplifier, 8 ... sampling timing setting device, 9 ... wave height discriminator, 10 ... counter.

Claims (3)

[Claims] 1. In a radiation counting method for counting radiation and discriminating energy using a radiation detector, a voltage pulse waveform generated by radiation is shaped, and a plurality of voltage values are output from near the peak of the shaped voltage pulse. Is sampled and the respective sampling values are weighted and added to obtain the energy of the radiation. 2. The timing for sampling a plurality of voltage values from the vicinity of the peak of the shaped voltage pulse and the coefficient for adding the respective sampling values by weighting are weighted and added even when the sampling timing is totally deviated. The radiation counting method according to claim 1, wherein the radiation counting method is set on the basis of a pulse waveform defined in advance so that the variation of the value is reduced. 3. A radiation counting method for counting radiation and discriminating energy using a radiation detector, wherein sampling timing is set by a voltage pulse waveform generated by radiation, and the voltage pulse waveform is shaped,
Radiation characterized in that the pulse after this shaping is sampled at the sampling timing, and weighted addition is performed by a coefficient that becomes maximum at a peak position for a plurality of sampled data and becomes smaller as the distance from the peak position increases. Counting method.