JPH01201184A - Dose measuring instrument - Google Patents

Abstract

PURPOSE:To remove the influence of the direction incidence of radiation on a photodiode even in such a case and to take a measurement with high accuracy by selecting only the output waveform of the photodiode by a waveform discriminating circuit and measuring a dose. CONSTITUTION:The photodiode 21 of this device converts light pulses outputted by a scintillator 22 photoelectrically and also senses the X ray and gamma ray which are incident directly on the photodiode 21. The pulse waveform 23 of the signal which is converted photoelectrically by the photodiode 21 is compared with a pulse waveform 24 outputted when the photodiode 21 itself senses the radiation, and consequently when the X ray or gamma ray is incident directly on the photodiode 21, a steep pulse waveform with short time width is obtained. For the purpose, the time of a rise up to a specific signal is measured and compared with a reference value to discriminate two kinds of signals, and the dose of the photoelectrically converted radiation is accurately measured.

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 line l 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.

A photomultiplier tube (PMT) 3 photoelectrically converts this, and a plurality of dynodes (not shown) sequentially multiply the secondary electrons. 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, it has the following problems.

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

(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. The light emitted from the scintillator 11 is transmitted through a photodiode (P
D) photoelectrically converted by 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 dose measuring device, 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. The broken line 18 is a radiation detection signal from the photodiode 12, and the sum of the counts shown by the solid line 17 and the broken line 18 is actually obtained as the measurement result.

``Problems to be Solved by the Invention'' As described above, the line l measuring device using a photodiode has a problem in that the photodiode is also sensitive to radiation, making it impossible to accurately measure the dose.

In view of these circumstances, it is an object of the present invention to provide a line l measuring device that uses a photodiode as a photoelectric conversion element and can eliminate the direct influence 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 discriminator 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 discriminator 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 T-rays, a waveform discrimination circuit is used to distinguish only the former output waveform. 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-crossing point of the shaped waveform may be determined. These may be detected by a zero-crossing point detection means and determined based on the length of time from the rise of the waveform to the zero-crossing point.

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

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 converts the
Sensitive to lines and T lines.

As the scintillator 22, for example, a crystal of Na1 (sodium iodide) or Cs1 (cesium iodide) is used.

The output waveform a of the photodiode 21 is as shown in FIG. 2a. In the same figure, the solid line 23 is the photodiode 2.
1 represents the pulse waveform of the photoelectrically converted signal.

On the other hand, a broken line 24 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 exist in a time-continuous manner. 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 human input signals and outputs a signal as shown in FIG.

The linear amplifier 26 amplifies this signal C (second
Output as Figure C). The signal C is supplied to the gate 28 via the delay circuit 27, but is also supplied to the waveform discrimination circuit 29. The waveform discrimination circuit 29 discriminates whether the signal C is based on photoelectric conversion of the photodiode 21 or on the incidence of radiation. and in the former case gate 28
A gate passing signal d (FIG. 2d) is generated to open the gate. 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.
3) has a steeper rise than that based on photoelectric conversion of the photodiode 21 (broken line 24). Therefore, measure the rise time to a predetermined signal level,
By comparing this 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 the signal e and is supplied to the pulse height analyzer 31. The pulse height analyzer 31 is constituted by, for example, a microcurrent meter, and accurately measures the radiation line J photoelectrically converted by the scintillator 22.

"Modified Example" Although the example in which two types of signals are discriminated based on the rising edge of the signal waveform has been described above, the waveform discrimination circuit 29 can discriminate waveforms using various other methods. FIG. 3 shows an example of this.

In this modification, the linear amplifier amplifies the signal S and shapes it into a bipolar waveform signal c' (FIG. 3 c'). The waveform discrimination circuit measures the time interval from the rising point of the waveform of the signal C' to the zero cross point, and compares it with a reference value. As shown in C' in the figure, the time interval of the solid line 23) based on the incident radiation is longer than that based on the photoelectric conversion of the photodiode 21 (broken line 24), so that the two types of signals are It becomes possible to discriminate between FIG. 3 d' shows the gate passing signal d' in this modification.
, and e' in the same figure represents 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 radiation can be removed and the dose can be measured. There is no need to cover the dosimetry device, which can contribute to making the dosimeter smaller and lighter.

[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 operation of the waveform discrimination circuit. 4 is a configuration diagram showing an example of a conventional dose measuring device using a photomultiplier tube, and 5 is a diagram showing various waveforms for explaining the
This figure is a block diagram of a conventionally proposed dose measuring device using a photodiode, and FIG. 6 is an explanatory diagram of a pulse 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, 29 waveform discrimination circuit, 31 pulse height analyzer. Applicant Japan Atomic Energy Corporation Agent
Patent Attorney Umeo Yamauchi No. 1 Fig. 2 Fig. 3, 24 (e) --Har-

Claims (1)

[Claims] A scintillator, a photodiode that converts this output into electricity, and an amplifier that amplifies the output of the photodiode.
A first pulse signal output from the amplifier in response to radiation incident on the scintillator, and a second pulse signal output from the amplifier in response to radiation incident on the photodiode. A waveform discriminator circuit for identifying, a gate for passing the first pulse signal and blocking the passage of the second pulse signal according to the discrimination result of the waveform discriminator circuit, and integrating the electric signal passing through the gate. A dose measuring device comprising: a measuring means for measuring dose.