JPH01277792A - Counting device of radiation pulse - Google Patents

Abstract

PURPOSE:To make the rate of omission in counting smaller than that by a conventional counting method, by obtaining a radiation pulse count value by dividing the total sum of time widths of a pulse signal subjected to pulse-height discrimination, by a non-sensing time of a detection system containing an amplifier circuit. CONSTITUTION:When a radiation falls on a detector 1, a signal pulse is transformed into a voltage pulse by a preamplifier 2, subjected to amplification and wave-shaping by a proportional amplifier 3 and transformed into a logic pulse by a pulse-height discriminator 4. An AND gate 5 which is put in an ON state while the logic pulse is outputted passes an output pulse from a high- frequency pulse generator 6 and counts as a count value P the total sum T of time widths of the output pulse of the pulse-height discriminator 4 which is inputted to a counter 7 and in the course of measurement therein. The upper (M-N) bits (non-sensing time tau=2N) of the count value P in said counter 7 are read out by a reading circuit 8 and taken as a radiation pulse count value.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application> The present invention relates to a device for counting constant energy radiation, pulses, for example for XvA imaging devices.

<Prior art> The basic circuit configuration for counting radiation pulses is described in the fifth section.
As shown in the figure. A signal pulse generated by radiation incident on the detector 1 is converted into a voltage pulse by a preamplifier 2 and amplified into a waveform by a proportional amplifier 3. The output of the proportional amplifier 3 is introduced into a pulse height discriminator 4 and converted into logic pulses in order to remove noise pulses and count only pulses having a predetermined pulse height value or higher. This pulse width differs when the energy of the radiation changes even if the measurement system is the same, but it remains constant when counting radiation of a constant energy emitted from a radioisotope or the like. The output pulses of the pulse height discriminator 4 are counted by a counter 10.

<Problem to be solved by the invention> By the way, all detection devices have two problems that occur within the detector.
There is a minimum amount of time, or dead time, required for two events to be separated from each other and recorded independently. This time limit is determined by the time required for signal generation within the detector, the signal processing time of the attached circuit, and the like. When events occur randomly, such as with radiation, signal pulses that are close in time are counted and dropped. This will be specifically explained below.

For example, consider the case where the output pulse width of the pulse height discriminator 4 is generated by a single event. In this case, τ becomes the dead time. Now, FIG. 6 illustrates the state of signals in each part when six events occur. In this example, counter 10 makes only three counts for six events. This counting loss is based on the true event count rate (
As the true number of events occurring per unit time (hereinafter referred to as the true counting rate) increases (. Let the true counting rate be n, the actual counting rate by the counter 10 (hereinafter referred to as the observed counting rate) Letting m be, it is expressed as m""ne-'τ. This relationship is illustrated in Figure 7. As is clear from this figure, as the true counting rate n increases, the number of missed counts increases. Not only that, n is 1/τ
After the observation count rate m reaches the maximum (=1/τe) at , it begins to decrease. For this reason, there is a possibility that the true counting rate may be erroneously estimated from the actually observed counting rate.

The present invention has been made in consideration of these points, and is a radiation pulse counting device that is capable of counting radiation with constant energy, has fewer missed counts, and does not reduce the observed counting rate at high counting rates. intended to provide.

Means for Solving the Problems> In order to achieve the above object, the present invention includes a timer for measuring the sum of time widths of pulse signals whose pulse heights have been discriminated, and a timer for measuring the time results by a detector and its output. It is provided with a division means for dividing by the dead time of the detection system including the amplifier circuit, and is configured to output the division result as the radiation pulse count result.

Effect> As is clear from the example shown in FIG. 6, when two events occur close to each other, the output pulse width of the pulse height discriminator 4 becomes longer than the output pulse width of a single event. If two events occur close together, the pulse width will be even longer. In other words, even if each pulse was counted as one in the conventional counting method, two or three pulses overlap and become one.
For those forming individual pulses, that amount will be reflected in the pulse width. Therefore, if we calculate the sum of the pulse widths and divide this by the dead time τ, that is, the output pulse width of a single event, the result of the division will be a truer count value than the count value based on the conventional counting method. Close to and true 5
The observed count rate does not decrease in areas where the count rate is high.

<Example> An example of the present invention will be described below based on the drawings. FIG. 1 is a circuit diagram of an embodiment of the present invention.

A detector 1 such as a semiconductor radiation detector, a preamplifier 2, a proportional amplifier 3, and a pulse height discriminator 4 are equivalent to conventional devices, and when radiation is incident on the detector 1, it is output from the detector 1. After the pulse signal is amplified and shaped into a waveform through a preamplifier 2 and a proportional amplifier 3, a pulse height discriminator 4 extracts only pulse signals having a predetermined pulse height value or higher.

The effective pulses discriminated by the pulse height discriminator 4 are AND
It is input to gate 5. This AND gate 5 has
A clock pulse from a high frequency pulse generator 6 is introduced as the other input, and the AND output thereof is counted by a counter 7.

Here, the cycle of the clock pulse from the high frequency pulse generator 6 is made sufficiently smaller than the pulse width τ of a single event from the pulse height discriminator 4. Since semiconductor radiation detectors usually have a pulse width on the order of microseconds, the frequency of the clock pulse is selected to be approximately 10 MHz to GHz.

The counter 7 is, for example, an M-bit binary counter, and its counting result is read out by the reading circuit 8. The readout circuit 8 increases the count value of the counter 7 by 2N by reading the upper (M−N) bits of the counter 7.
It is configured to obtain the value divided by . This 2N is
It is set equal to the number p of clock pulses generated in the time width τ of the output pulse of the pulse height discriminator 4 corresponding to the flower event.

Next, we will discuss the effect. FIG. 2 is a time chart showing examples of signal waveforms at each part. The AND gate 5 allows the output pulse from the high frequency pulse generator 6 to pass only when the output pulse from the pulse height discriminator 4 is in the ON state. This, AN
The count value P of the counter 7 that counts the output of the D gate 5 is
Therefore, it represents the total time width T of the output pulses of the pulse height discriminator 4 during measurement. The output of the readout circuit 8 which reads out the upper (M−N) bits of the count value of the counter 7, that is, P/p, therefore represents T/τ.

As shown in FIG. 6 mentioned above, the width of the output pulse of the pulse height discriminator 4 when two or more events occur close to each other is longer than the output pulse width τ when a single event occurs. This reflects the overlap of pulses. Therefore, the value obtained by dividing the sum of the widths of the output pulses of the pulse height discriminator 4 during a certain measurement period by τ is closer to the true value than the count value obtained by the conventional counting method, and the number of missed counts is reduced. Moreover,
Even if the counting rate becomes extremely high, the count value T/τ may saturate to a certain value, but will not decrease.

FIG. 3 is a graph showing the relationship between the true counting rate and the observed counting rate when the dead time τ=1 μsec, where the solid line shows the embodiment of the present invention and the broken line shows the conventional example.

In the conventional counting method, the observed counting rate decreases at 106 counts/sec or more, but in the embodiment of the present invention, it increases.

Figure 4 also shows the ratio of the observed count rate to the true count rate (
Similarly, the solid line indicates the example of the present invention,
The broken line shows the conventional example. As is clear from this graph, in the embodiment of the present invention, the counting efficiency is improved at high counting rates.

Note that in the above embodiment, the sum of the time widths of the output pulses of the pulse height discriminator 4 was measured by the high frequency pulse generator 6, the AND gate 5, and the counter 7, but any other arbitrary time measurement means may be adopted. Of course.

In addition, as a method for dividing the high-frequency pulse count by the counter 7 by a value corresponding to the dead time τ, in addition to reading out the upper (M - N) bits of the count by the counter 7, a division method using ordinary digital calculations is adopted. It goes without saying that it can be done.

<Effects of the Invention> As explained above, according to the present invention, the radiation pulse count value is obtained by dividing the sum of the time widths of pulses after pulse height discrimination by the dead time of the detection system. In comparison, the percentage of missed counts decreases. This means that when creating an image using this count value, the image quality will improve. Also,
Since the count value does not decrease even when the count rate is extremely high, there is no possibility of incorrect concentration display, and reliability is improved.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing the configuration of an embodiment of the present invention, Fig. 2 is a time chart showing examples of signal waveforms of each part, and Fig. 3 shows the relationship between the observed counting rate and the true counting rate according to the embodiment of the present invention. FIG. 4 is a graph showing the relationship between the counting efficiency and the true counting rate between the embodiment of the present invention and the conventional example. FIG. 5 is the configuration of a conventional radiation pulse counting device. FIG. 6 is an explanatory diagram of counting loss by the conventional device, and FIG. 7 is a graph showing the relationship between the observed counting rate and the true counting rate by the conventional device. 1... Detector 2... Preamplifier 3... Proportional amplifier 4... Pulse height discriminator 5... AND gate 6... High frequency pulse generator 7... Counter 8... Readout circuit

Claims (1)

[Claims] A detector that outputs a pulse-like signal when radiation is incident, an amplifier circuit that amplifies the output of the detector, and a pulse output from the amplifier circuit that inputs only the pulse signal that is higher than a preset peak value. In a device that has a pulse height discriminator for extraction and outputs the count value of the pulse signal after pulse height discrimination as the measurement result of the incident radiation dose to the detector, the time width of the pulse signal after the pulse height discrimination during measurement is It is characterized by comprising: a timer for measuring the total sum; and a dividing means for dividing the timer result by a dead time of a stage preceding the amplifier circuit, and configured to output the division result as the measurement result. Radiation pulse counting device.