JPH04332895A - Radiation counting method - Google Patents

Abstract

PURPOSE:To improve the counting rate without addition of any system and without deterioration of measuring accuracy by sampling plural voltage values from the vicinity of a peak after shaping voltage pulse produced by radiation and by obtaining radiation energy through weight adding calculation of the values. CONSTITUTION:In order to shorten the time constant during measurement and to suppress deterioration of energy resolution as well, waveshaped pulse 1 which is input to a wave height discriminator 9 from a wave shaping amplifier 7 is sampled by a discriminator 9 at eight points t1 to t8 which are set forth by a sampling time setter 8, and then weighted adding calculation is also conducted so as to reduce noise thereabout. In this way, insufficient reduction of noise produced by integration during waveshaping is compensated during the sampling period of wave height values, and consequently the energy resolution is not deteriorated though pulse width lessens, and counting rate can be improved without addition of any new radiation detection system.

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 differentially measuring radiation energy detected by a radiation detector.

0002

2. Description of the Related Art Since the generation of photons or other radiation is a random phenomenon that follows a certain probability distribution, the counted values thereof vary. For this reason, in measuring the intensity of radiation, it is necessary to count more than a certain level in order to reduce the ratio of variation in the counted value to the counted value in accordance with the accuracy of determination.

[0003] Therefore, in order to measure the radiation intensity in a short time, it is necessary to increase the radiation count per unit time. Possible measures include reducing the distance between. However, when using a radiation detector to not only count the intensity of radiation but also to discriminate its energy, as the frequency of radiation entering the radiation detector increases,
In other words, as the intensity increases, the phenomenon of miscounting becomes a problem.

By the way, photons and other radiation particles detected by a radiation detector are converted into charge pulses and shaped into waveforms, and the energy can be determined from the peak value of the pulses, and the number of radiation can be determined from the number of pulses. It will be done. However, as the frequency of radiation incidence on the radiation detector increases, pulses are generated more frequently, and as shown in Figure 6, pulses 2 and 3 generated by multiple separate photons overlap to form one large pulse 4. It becomes like this. When pulses overlap in this way, the number of radiation particles is counted to be less than the actual number (missing), and the peak value also changes (pile-up), making it impossible to accurately measure the energy.

In order to prevent this overlapping of pulses, the time constant (hereinafter referred to as time constant) for waveform shaping can be selected to be small so that the pulse width becomes small during waveform shaping.
In this case, there is a problem in that noise removal through integration during waveform shaping is insufficient, and the error in the peak value, that is, the error in energy measurement becomes large (deterioration of energy resolution).

Conventionally, the following methods have been considered to solve these problems. (1) A measurement circuit that rapidly measures the timing and number of pulses entering the radiation detector is installed in parallel with the peak value measurement circuit into which the detection signal of the radiation detector is input, and this measurement circuit detects the overlap of pulses. A method of correcting the count value to reduce the apparent number of pulses being counted.

(2) By using a plurality of radiation detectors and their attached signal processing circuits, the counting rate of the entire system can be increased without increasing the number of radiation that enters each radiation detector per unit time. (Japanese Unexamined Patent Publication No. 62-121382). (3) When multiple pulses overlap, combine them into one
A method of correcting the counting rate based on the pulse width by taking advantage of the fact that the pulse width becomes longer when considered as one pulse, thereby reducing the apparent number of missing pulses. Public bulletin).

[0008]

[Problems to be Solved by the Invention] However, these methods have the following problems.

Although the method (1) above can count the total number of incident radiation with fewer omissions, it does not necessarily measure the energy of each radiation, so radiation of multiple energies can be counted. When measuring , the apparent count value increases, but there is a problem in that the dispersion of the count value is not improved at all.

[0010] Furthermore, the method (2) above does not involve changing the incident conditions to improve the counting rate, such as increasing the output of the radiation generator or reducing the distance between the measurement target 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 can be installed in a limited space. There are limits to how much space can be installed, and it is not necessarily practical.

Furthermore, in method (3) above, when measuring radiation having multiple energies as in case (1), even if it is possible to correct the count value for all radiation, the measurement of energy is Since it cannot be performed for every single radiation incident on the detector, the measurement accuracy of the counted values cannot be improved as a result.

An object of the present invention is to improve the counting rate by substantially reducing the number of missed pulses without adding a new radiation detector system or degrading the energy measurement accuracy. The object of the present invention is to provide a radiation counting method that can achieve the following.

[0013]

[Means for Solving the Problems] In order to achieve the above object, the present invention shapes the voltage pulse waveform generated by radiation in a radiation counting method that uses a radiation detector to count radiation and discriminate energy. The radiation energy is determined by sampling a plurality of voltage values near the peak of the shaped voltage pulse and adding each sampling value with weights.

[0014] Also, the timing of sampling multiple voltage values from near the peak of the voltage pulse after shaping and the coefficients when adding each sampling value with weights are as follows:
The setting is made based on a predefined pulse waveform so that the fluctuation of the weighted addition value is reduced even if the sampling timing is shifted overall.

[0015]

[Operation] In the method of the present invention, instead of sampling only one point at the peak as in the conventional method, multiple points around the peak are sampled at a certain timing and the weighted sum of those values is calculated. By extracting and outputting the signal, it is possible to compensate for the lack of noise removal due to integration during waveform shaping at the sampling stage of the peak value, and it is possible to reduce the pulse width without deteriorating the energy resolution.

Furthermore, by sampling multiple points on both sides of the peak near the peak of the pulse after waveform shaping, weighting and adding them, the sampling start time of each sampling point is slightly shifted overall due to jitter. too,
Since the increase or decrease in the value obtained by multiplying each sampling value by the weighting coefficient is canceled out by adding each sampling value, the effect on the weighted addition value can be reduced, resulting in the effect of further improving energy resolution. It will be done.

[0017]

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 that uses a semiconductor detector as a radiation detector to perform fluorescent X-ray analysis on a galvanized steel sheet and feed it back to a production line. As shown in Figure 1,
When the X-rays are incident on the semiconductor detector 5, the X-rays are converted into electrical pulses by the semiconductor detector 5, and are inputted to the waveform shaping amplifier 7 and the sampling timing setter 8 via the preamplifier 6. The waveform shaping amplifier 7 shapes and amplifies the output signal of the preamplifier 6 and sends it to the pulse height discriminator 9, the details of which will be described later.The sampling timing setter 8 is a waveform shaping amplifier that is input to the pulse height discriminator 9. This is used to set the timing for sampling the signal waveform from 7. The output signal subjected to pulse height discrimination by the pulse height discriminator 9 is counted by a counter 10.

Next, the operation of this embodiment will be explained. First, the X-rays incident on the radiation detector 5 contain Zn and F.
These are kα rays, kβ rays, etc. of e. Among these, if it is the Znkα ray that we want to measure, then the Znka ray, which has the closest energy,
An energy resolution that can be counted energetically separated from kβ is required. Furthermore, in order to shorten the measurement time, measurement at a high counting rate is required. In this case, simply increasing the intensity of the X-ray tube to increase the number of X-rays that enter the semiconductor detector 5 will not only cause miscounts and not be effective enough, but if the intensity is too strong, it will not be effective. This may also lead to a decrease in the counting rate. Furthermore, simply reducing the time constant to avoid this will result in a decrease in energy resolution, resulting in insufficient separation of Znkα rays and kβ.

Therefore, in order to shorten the time constant and at the same time suppress the deterioration of energy resolution, a sampling timing setter 8 is set for the waveform shaping pulse 1 as shown in FIG. Waveform discriminator 9 at 8 points (t=ti, i=1 to 8) set by
Sampling is performed using , weighting is performed to reduce noise, and addition is performed. Here, the weighting coefficient is ai(
i=1 to 8), the following calculations are performed in the pulse height discriminator 9. S=Σai・V(ti)

[0021] S is a value proportional to the energy of incident X-rays. Then, the pulse height discriminator 9 outputs a pulse to the counter when S corresponds to a value corresponding to a preset energy range.

In this case, in order to prevent the S/N ratio at each sampling point from becoming too bad, sampling is performed at a value larger than half of the peak, and the weighting coefficient is calculated from the constant of the waveform shaping circuit for each sampling. The value is proportional to the relative theoretical strength at a point. As a specific example, in a waveform shaping circuit as shown in FIG. 3, C×R=
T T: Time constant C2 = 2C1 C1 (R1 + R2) = T R1 = R2 If we use a circuit with time constant C2 = 2C1, the response Fs to a step input of this circuit is, where t is time, Fs = A0
[t/T+1+21/2 sin(t-0.75π)
]exp(t/T). However, A0 is a constant.

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

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

Here, the time constant of the waveform shaping amplifier 7 is set to 0.
FIG. 4 shows a comparison of the counting efficiency between the case where the present invention is applied with a time constant of 8 μsec and the case where the conventional method is adopted with a time constant of 2.2 μsec. In this case, the energy resolution for Znkα is approximately 315 eV in both cases.

As described above, while keeping the energy resolution at the same level as the conventional method, in the method of the present invention, the time constant is shortened to about 1/3 of the conventional method (from 2.2 μsec to 0.8 μsec). As shown in FIG. 4, the counting efficiency, which is the ratio of the actual count value to the true count value when no counts are omitted, was greatly improved. Furthermore, 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.
It is. This indicates that the time required to count a certain number of X-ray photons is about half that of the conventional method. Also,
It is shown that when the measurement time is constant, the statistical error increases by about 0.7 times. Next, other embodiments according to the method of the present invention will be described.

In this embodiment, in addition to the above, measures are taken to reduce the influence of the overall deviation in sampling timing of all sampling points due to jitter of the sampling timing setter 8 on the weighted sum of sampling values. This is what I did. Specifically, within the range where the pulse shape is almost symmetrical with respect to the peak point, the increase or decrease in the value obtained by multiplying the 8 sampled values by the weighting coefficient due to jitter is canceled out by adding 4 points before and after the peak. The time between each sampling point is set from the relative theoretical intensity of the pulse at each sampling point calculated from the constants of the waveform shaping circuit.

Specifically, 8 points including the end points are taken from a portion equal to or more than 1/2 of the peak wave height value, and weighting coefficients are set for each point when a pulse enters the waveform shaping circuit, as in the case of the above embodiment. It was taken as the wave height value at the sampling point. As a result, in addition to the same effect as described above, it is possible to reduce the influence of jitter on the energy measurement value.

In this example, the time constant was set to 1 μsec, and the sampling timings were set as shown in Table 1 for case A in which the measures as in the above embodiment were implemented and case B in which they were not implemented.

[0030]

[Table 1] Furthermore, the influence reduction effect when the sampling points are set as shown in Table 1 is shown in FIG. 5, and it can be seen 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 voltage pulse after shaping were sampled and the respective sampling values were added together with a weight, but the timing of sampling was set based on the voltage pulse waveform before shaping. , the pulse after waveform shaping is sampled at the sampling timing, and the sampled data is maximized at the peak position, and weighted addition is performed using a coefficient that decreases as the distance from the peak position increases. It is possible to obtain the following effects.

Furthermore, in each of the above embodiments, the case where X-rays are used as the radiation has been described, but it goes without saying that the present invention can also be applied to measurement of other radiation. Furthermore, the number of sampling points, sampling positions, weighting coefficients, etc. may also be set to various conditions different from those described above without departing from the spirit described above.

[0033]

[Effects of the Invention] As described above, according to the present invention, the number of pulses can be substantially omitted without adding a new radiation detector system or deteriorating the energy measurement accuracy. It is possible to provide a radiation counting method that can improve the counting rate by reducing the amount of radiation.

[Brief explanation 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 pulses after waveform shaping in the same embodiment.

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

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

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

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

[Explanation of symbols]

1...Pulse after waveform shaping, 5...Semiconductor detector, 6...
...Preamplifier, 7... Waveform shaping amplifier, 8... Sampling timing setter, 9... Wave height discriminator, 10... Counter.

Claims (3)

[Claims] Claim 1: In a radiation counting method that uses a radiation detector to count radiation and discriminate energy, a voltage pulse waveform generated by radiation is shaped, and a plurality of voltage values are calculated from the vicinity of the peak of the shaped voltage pulse. A radiation counting method characterized by determining the energy of radiation by sampling and adding each sampling value with weights. [Claim 2] The timing of sampling multiple voltage values from near the peak of the voltage pulse after shaping, and the coefficients used when adding each sampling value with weights, can be weighted and added even when the sampling timings are shifted overall. 2. The radiation counting method according to claim 1, wherein the radiation counting method is set based on a predefined pulse waveform so as to reduce variation in value. 3. A radiation counting method for counting radiation and discriminating energy using a radiation detector, the method comprising: setting sampling timing based on a voltage pulse waveform generated by the radiation; and shaping the voltage pulse waveform. The pulse after the shaping is sampled at the sampling timing, and the plurality of sampled data are subjected to weighted addition using a coefficient that is maximum at a peak position and becomes smaller as the distance from this peak position increases. Counting method.