JP3008478B2 - Radiation measurement circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial applications]

The present invention relates to a radiation measurement device used in the field of science and engineering, a nuclear medicine diagnostic device such as an Anger-type gamma camera, a semiconductor gamma camera, and the like, and particularly to an improvement in a radiation measurement circuit.

[Prior art]

Radiation is detected by a combination of a scintillator and a photomultiplier tube, or by a semiconductor detector, but the pulse signal related to the radiation event obtained by them is very weak and has a poor S / N ratio, so the waveform is shaped. It is necessary to increase the S / N ratio. Therefore, in a usual radiation measurement circuit, the waveform of the analog pulse signal is analog-shaped, and the peak value after the shaping is measured. As the waveform shaping filter, a non-time-variable filter such as a quasi-Gaussian distribution filter or a delay line pulse-shaping filter, a time-variable filter such as a gate integrator, or a trapezoidal filter combining both are used (FSGoulding, IEEE Trans.Nucl.Sci., NS−
29 (1982) 1125 and V. Radeka, IEEE Trans. Nucl. Sci., NS
-15 (1968) 455). There is also known a radiation measuring circuit that digitally integrates (that is, integrates) instead of the gate integrator (Japanese Patent Laid-Open No. 61-3284).

[Problems to be solved by the invention]

However, conventionally, there is a problem that it is not possible to select an optimum type of waveform shaping filter and its time constant. That is, in the radiation measurement circuit, in order to minimize the circuit noise, to reduce the influence of the variation of the rise time, and to improve the count rate characteristics, depending on the conditions such as the radiation detector, preamplifier and count rate used. It is necessary to select the optimal type of waveform shaping filter and its time constant.
However, in the case of analog waveform shaping, although many filters have been devised, those that can be realized by actual circuits are limited, and a dedicated circuit is required for each type, and the time constant is also large. Only a few types can be selected. In addition, digital integration is essentially the same as a gate integrator for analog processing, and has the same problem as described above. SUMMARY OF THE INVENTION It is an object of the present invention to provide a radiation measuring circuit capable of obtaining a result equivalent to selecting various waveform shaping filters and their time constants.

[Means for Solving the Problems]

In order to achieve the above object, in the radiation measurement circuit according to the present invention, it is determined whether or not a pulse signal relating to a radiation event from a radiation detector has exceeded a predetermined threshold value, and the pulse signal has a threshold value. An output determining means for generating an output when the output signal exceeds the threshold, a timing signal generating means for generating a timing signal of a predetermined period for a predetermined time in accordance with the output of the output determining means, and Counting means for generating a count signal representing the number of times, delay means for delaying the pulse signal, and A / D conversion means for sequentially A / D converting the pulse signal passed through the delay means in accordance with the timing signals And a predetermined value is stored in an address designated by the output signal of the A / D conversion means and the number-of-times signal, and the A / D
A conversion table storage means for reading the stored value each time the conversion signal is accessed by the output signal of the conversion means and the number-of-times signal, and a means for repeatedly adding the output of the conversion table storage means are provided.

[Action]

When radiation enters the radiation detector, a pulse signal is generated from the radiation detector in response to the event. A pulse signal relating to a radiation event from the radiation detector is input to output determination means, and it is determined whether or not a predetermined threshold has been exceeded. When the pulse signal exceeds the threshold, the output determination means Output occurs. The timing signal generating means generates a timing signal having a predetermined period for a predetermined time according to the output of the output determining means. The number of occurrences of the timing signal is counted by the counting means, and a number signal indicating the number is generated. On the other hand, the above-mentioned pulse signal is delayed through delay means. The delayed pulse signal is sent to A / D conversion means, and is sequentially A / D converted in accordance with each of the timing signals. The output of the A / D conversion means is sent to the conversion table storage means together with the number of times signal, and a predetermined value stored at an address designated by the output of the A / D conversion means and the number of times signal is read out. It is. Therefore, it is possible to read digital values in which sequentially obtained outputs of the A / D conversion means are weighted differently in terms of time. Then, the digital values thus weighted are repeatedly added and integrated. The integrated value thus obtained corresponds to the peak value of a signal obtained by processing an analog pulse signal related to a radiation event by an analog filter. A predetermined value can be stored in each address of the conversion table storage means, and this value can be arbitrary. Therefore, the characteristics of the above-mentioned weighting coefficients with respect to time can be arbitrarily set.
That is, a result equivalent to changing the type of the waveform shaping filter in analog processing or selecting various time constants can be easily obtained without changing the circuit. Also,
It is possible to easily obtain a result corresponding to a filter that cannot be realized by an analog circuit or a filter that can be realized only by a rough approximation using a complicated circuit. Furthermore, since an arbitrary weighting function can be realized, it is easier to reduce circuit noise by using an optimum weighting function as compared with a conventional analog filter or digital integration.

【Example】

Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings. In FIG. 1, a radiation detector 1 is composed of, for example, a combination of a scintillator and a photomultiplier tube, and generates a pulse-like current output in response to the incidence of radiation. This current output is integrated by the charge-sensitive preamplifier 2 and converted into a voltage output proportional to the energy of the incident radiation. Further, the waveform is shaped short by the pre-filter 3 to form, for example, a waveform as shown in FIG.
Delayed by TD, as shown in Fig. 2B, high-speed A / D
Input to the converter 5. The high-speed A / D converter 5 comprises, for example, a flash A / D converter, and its digital output is sent to a conversion table memory 6. The A / D converter 5 performs an A / D conversion operation using a signal from a down counter 11 described later as a timing signal. The signal relating to a radiation event is usually a signal of a single positive polarity as shown in FIGS. 2A and 2B, but since noise exists in both positive and negative, the input range of the A / D converter 5 is bipolar. It is desirable that On the other hand, the output of the prefilter 3 is a discriminator 9
To determine whether a predetermined threshold value has been exceeded. When the output of the prefilter 3 exceeds a predetermined threshold, a signal is generated from the discriminator 9 to trigger the down counter 11. When triggered in this manner, the down-counter 11 counts for a certain time (for example, 2.8
μsec). Since a signal of, for example, 80 MHz from the transmission circuit 10 is input to the down counter 11, this signal is frequency-divided for the above-mentioned period to become a signal of, for example, 10 MHz. This 10MHz signal is A /
The signal is sent to the D converter 5 and used as a timing signal. The signal is also sent to the sample counter 12 and counted to obtain the number n of times of sampling. Since n is 2.8 μsec / 100 ns = 28 in this example, n = 1, 2, 3,.
It will count up sequentially with 28. The output n of the sample counter 12 is sent to the conversion table memory 6. In the conversion table memory 6, the output of the A / D converter 5 and the output n of the sample counter 12 are input in parallel and addressed by these. The data read from this address is sent to the adder 7, added to the data read from the register 8, and the result is stored in the register 8 again. In this example, the A / D converter 5 performs A / D conversion 28 times, and each time the sample counter 1
Since the output n is generated from 2, the output of the conversion table memory 6 occurs 28 times and the content of the register 8 is initially 0,
The outputs of the conversion table memory 6 are integrated by repeating the addition. The conversion table memory 6 is composed of a rewritable memory such as a RAM. If the content is determined linearly with respect to the A / D conversion output and non-linearly with respect to the number of samplings n, the weighting (amplification) as a function of time can be arbitrarily determined. This weighting function w
(T) can be as shown in FIG. 2C. In the conversion table memory 6, the output of the A / D converter 5 is multiplied by the weighting function w (t). After multiplying in this manner, the result is added by the adder 7 and integrated by the register 8. . Therefore, the contents of the register 8 are as shown in FIG. 2D. After the integrated value signal is fetched at the time TG, the register 8 is reset to 0. FIG. 3 is a block diagram for explaining the filtering process.
Figure A shows a time-change filter, wherein the input signal x (t), when the response function of the filter h (t) the output signal ^{y (t) = ▲ ∫ t} -∞ ▼ x (u) h ( tu) du is obtained. Figure 3 B shows the case of a time-change filter, the input signal x (t) is integrated after being multiplied by the weighting function w (t), the output signal y (Tm) is y ^(Tm) = ▲ ∫ Tm _-∞ ▼ w (t) x (t) dt Therefore, if w (t) = h (Tm−t),
At the time t = Tm, the output in FIG. 3B has the same result as FIG. 3A (the output waveform is different but the S / N ratio is the same). In the case of the above example, the weighting function w in FIG.
This corresponds to arbitrarily changing (t). In the above embodiment, the pre-filter 3 and the delay circuit 4
However, the circuit shown in FIG. 1 can be modified in various ways, for example, it can be constructed without using these, or two or more registers 8 can be provided in parallel and used alternately.

【The invention's effect】

According to the radiation measurement circuit of the present invention, a result equivalent to changing the type of the waveform shaping filter in analog processing or selecting various time constants can be easily obtained without changing the circuit. In addition, filters that are difficult to realize with analog circuits or filters that can be realized only with coarse approximation using complex circuits (for example, a caps type filter,
Note that this caps type filter theoretically minimizes circuit noise, but it is impossible to realize strictly, and it is difficult to measure the peak value.) (Ie, the same result as the peak value obtained when the filter is used can be obtained as a digital integrated value). Furthermore, since an arbitrary weighting function can be realized, it is easier to reduce circuit noise by using an optimum weighting function as compared with a conventional analog filter or digital integration.

[Brief description of the drawings]

FIG. 1 is a block diagram of one embodiment of the present invention, FIG. 2 is a waveform diagram, and FIG. 3 is a block diagram for explaining filter processing. DESCRIPTION OF SYMBOLS 1 ... Radiation detector, 2 ... Preamplifier, 3 ... Prefilter, 4 ... Delay circuit, 5 ... A / D converter, 6 ... Conversion table memory, 7 ... Adder, 8 ... Register , 9
… Discriminator, 10 oscillating circuit, 11… down counter, 12… sample counter.

Continued on the front page (58) Fields surveyed (Int.Cl. ⁷ , DB name) G01T 1/17 G01T 1/164 G01R 19/25 H03H 17/02

Claims (1)

(57) [Claims] An output determining means for determining whether or not a pulse signal relating to a radiation event from a radiation detector has exceeded a predetermined threshold, and generating an output when the pulse signal has exceeded the threshold; A timing signal generating means for generating a timing signal of a predetermined period for a predetermined period in accordance with an output of the output determining means, a counting means for counting the number of times the timing signal is generated and generating a number signal representing the number of times; Delay means for delaying the pulse signal, and the pulse signal passing through the delay means is sequentially transmitted in accordance with each of the timing signals.
A / D conversion means for performing A / D conversion, and a predetermined value is stored in an address designated by the output signal of the A / D conversion means and the number of times signal, and the A / D conversion means A conversion table storage means for reading out the stored value each time the access signal is accessed by the output signal and the number-of-times signal, and a means for repeatedly adding the output of the conversion table storage means. Measurement circuit.