JPH0567917B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION "Field of Industrial Application" The present invention relates to a dose measuring device that measures radiation using a scintillator and a photodiode.

``Prior art'' When radiation hits a phosphor, it emits a flash of light. This phenomenon is called scintillation, and the fluorescent substance used for scintillation is called a scintillator.
In this specification, a device that measures radiation dose and dose rate using a scintillator will be referred to as a dosimetry device.

FIG. 4 shows a conventionally used dose measuring device. When the radiation emitted from the radiation source 1 passes through the scintillator 2, it emits light with an intensity corresponding to the radiation dose. Photomultiplier tube (PMT) 3
converts this into electricity, and sequentially multiplies the secondary electrons using a plurality of dynodes (not shown). The current 4 thus obtained is supplied to a microammeter 5, and the current value is integrated over the measurement time.
In this way, the radiation dose is measured. The dose per unit time is called the dose rate.

However, since such a conventional device uses a photomultiplier tube 3, the following problems occur.

(i) It has been difficult to miniaturize the detector itself due to the size of the photomultiplier tube itself and the bias power supply used for it.

(ii) The bias power source consumes a large amount of power and is uneconomical.

In order to solve these problems, it has been proposed to perform photoelectric conversion using a photodiode (Japanese Patent Application No. 59-212726).

FIG. 5 shows this basic configuration. Light emitted from the scintillator 11 is photoelectrically converted by a photodiode (PD) 12.
The converted electrical signal is amplified by a preamplifier 13 and a linear amplifier 14, and a microammeter 15 measures the radiation dose.

However, in such a dosimeter, the photodiode 12 is directly sensitive not only to the light from the scintillator 11 but also to radiation. As a result, the photodiode outputs both an electrical signal after photoelectric conversion and a radiation detection signal.

FIG. 6 is for explaining this and shows an example in which cobalt 60 is used as a radiation source. In this figure, a solid line 17 represents the relationship between the energy of pulsed light (pulse height) emitted from the scintillator 11 and the number of counts within a predetermined time. Dashed line 18 is photodiode 12
These solid lines 17
The sum of the counts shown by the broken line 18 is actually obtained as the measurement result.

``Problem to be Solved by the Invention'' As described above, the dose measuring device using a photodiode has a problem in that the photodiode is also sensitive to radiation, making it impossible to measure the dose with high accuracy.

In view of these circumstances, an object of the present invention is to provide a dose measuring device that uses a photodiode as a photoelectric conversion element and can eliminate the direct effects of radiation.

``Means for Solving the Problems'' The present invention includes a scintillator, a photodiode that photoelectrically converts the output, an amplifier that amplifies the output of the photodiode, and when radiation is incident on the scintillator, the amplifier responds to the radiation. A waveform discrimination circuit that discriminates between a first pulse signal to be output and a second pulse signal that is output from the amplifier in response to radiation incident on the photodiode, and a discrimination result of the waveform discrimination circuit. The dose measuring device is equipped with a gate that allows only the first pulse signal to pass through and blocks the second pulse signal, and a measuring means that measures the dose by integrating the electrical signal that has passed through the gate. .

According to the present invention, by focusing on the difference between the output waveform of a photodiode due to photoelectric conversion and the waveform output from the photodiode as a result of direct sensitivity to X-rays or γ-rays, a waveform discrimination circuit detects only the output waveform of the former. Since the dose is measured selectively, highly accurate measurement is possible. Here, the waveform discrimination circuit can discriminate between the first pulse signal and the second pulse signal using various methods. For example, these signals may be identified by the length of the rise time of the pulse signal input to the waveform discrimination circuit, or the pulse signal may be first shaped into a bipolar waveform by a waveform shaping means, and the zero cross of the shaped waveform may be identified. The points may be detected by a zero-crossing point detection means, and the determination may be made based on the length of time from the rise of the waveform to the zero-crossing point.

"Example" The present invention will be explained in detail with reference to Examples below.

FIG. 1 shows a dose measuring device according to an embodiment of the present invention. In this device, the photodiode 21 not only photoelectrically converts the optical pulse output from the scintillator 22, but also is sensitive to X-rays and γ-rays directly incident on the photodiode 21, as described above. As the scintillator 22, for example, NaI (sodium iodide) or CsI (cesium iodide) crystals are used.

The output waveform a of the photodiode 21 is shown in Fig. 2a.
It will look like the one shown below. In the figure, a solid line 23 represents the pulse waveform of a signal photoelectrically converted by the photodiode 21. On the other hand, the broken line 24 is
It represents a pulse waveform output as a result of the photodiode 21 itself being sensitive to radiation.
For ease of comparison, these are shown as if both signals were output from the photodiode 21 at the same time, but in reality they are generated independently of each other with no relation to time, and both waveforms are It is unlikely that these two locations overlap in time in this way. Comparing these two waveforms, it can be seen that when X-rays or γ-rays are directly incident on the photodiode 21, a steep pulse waveform with a shorter time width is obtained.

The charge sensitive preamplifier 25 amplifies these input signals and outputs a signal b as shown in FIG. 2b. The linear amplifier 26 amplifies this signal b and outputs it as a signal c (FIG. 2c). The signal c is supplied to the gate 28 via the delay circuit 27, but is also supplied to the waveform discrimination circuit 29. In the waveform discrimination circuit 29, the signal c is output from the photodiode 2.
It is determined whether the method is based on photoelectric conversion (1) or the incidence of radiation. In the former case, a gate passage signal d (FIG. 2 d) for opening the gate 28
to occur. The gate passing signal d is supplied to the gate 28. The delay circuit 27 described above is a circuit for delaying the signal c by the processing time of the waveform discrimination circuit.

By the way, as shown in FIG. 2c, the signal c input to the waveform discrimination circuit 29 is based on the incidence of radiation (solid line 23), and the signal c input to the waveform discrimination circuit 29 is
The rise is steeper than that based on photoelectric conversion (broken line 24). Therefore, by measuring the rise time to a predetermined signal level and comparing it with a reference value, it is possible to discriminate between these two types of signals. When a light pulse is emitted from the scintillator 22, a gate passing signal d is generated at the timing when the signal c is delayed and reaches the gate 28. As a result, the delayed signal c
passes through the gate 28 as a signal e and is supplied to the pulse height analyzer 31. The pulse height analyzer 31 is composed of, for example, a microcurrent meter, and accurately measures the dose of radiation photoelectrically converted by the scintillator 22.

“Modified Example” The example above shows the discrimination between two types of signals based on the rising edge of the signal waveform, but the waveform discrimination circuit 29
It is possible to discriminate waveforms using various other methods. FIG. 3 shows an example of this. In this modification, the linear amplifier amplifies the signal b and also the bipolar waveform signal c' (the third
Format it as shown in Figure c′). The waveform discrimination circuit measures the time interval from the rising point of the waveform of the signal c' to the zero crossing point, and compares it with a reference value. As shown in c' in the same figure, the one based on the incidence of radiation (solid line 2
Since the time interval in case 3) is longer than that based on photoelectric conversion of the photodiode 21 (broken line 24), it becomes possible to discriminate between the two types of signals. FIG. 3 d' shows the timing of generation of the gate passing signal d' in this modification, and FIG. 3 e' shows the signal e' passing through the gate.

"Effects of the Invention" As explained above, according to the present invention, even if radiation is directly incident on the photodiode, the effect of radiation can be removed and the dose can be measured. No need to cover
It can contribute to miniaturization and weight reduction of the dosimetry device.

[Brief explanation of the drawing]

Fig. 1 is a block diagram showing an overview of a dosimetry device according to an embodiment of the present invention, Fig. 2 is various waveform diagrams for explaining the operation of this device, and Fig. 3 is another example of the operation of the waveform discrimination circuit. 4 is a configuration diagram showing an example of a conventional dosimetry device using a photomultiplier tube, and FIG. 5 is a configuration diagram of a conventionally proposed dosimetry device using a photodiode. , FIG. 6 is an explanatory diagram of wave height distribution for explaining measurement errors when measuring dose with this proposed device. 21...Photodiode, 22...Scintillator, 25...Charge sensitive preamplifier, 26...Linear amplifier, 28...Gate, 2
9... Waveform discrimination circuit, 31... Wave height analyzer.

Claims (1)

[Claims] 1. A scintillator, a photodiode that photoelectrically converts this output, an amplifier that amplifies the output of the photodiode, and a first pulse signal that is output from the amplifier in response to radiation incident on the scintillator, and the photodiode. a waveform discriminator circuit that identifies a second pulse signal output from the amplifier in response to radiation incident on the amplifier; and a waveform discriminator circuit that allows the first pulse signal to pass according to the discrimination result of the waveform discriminator circuit. A dose measuring device comprising: a gate that blocks passage of the second pulse signal; and a measuring device that measures the dose by integrating the electrical signal that has passed through the gate.