JPH02236191A - Method for measuring dosage - Google Patents

Abstract

PURPOSE:To easily realize dosage detection free from influence of fluctuation in gain of a light detector by a method wherein light intensity of scintillation generated in a scintillator by irradiation of radioactive rays is attenuated with various optical attenuation ratios to have the number of output pulses in the respective optic attenuation ratios measured. CONSTITUTION:When radioactive rays are irradiated by a scintillator 1 in a form of a plastic scintillator or the like, scintillation (illumination having very short duration) having light intensity according to the amount of energy applied by the radioactive rays is generated sequentially with random time relation. The scintillations is led to a variable light attenuator 3 through an optic fiber light guide 2 as a light path to be attenuated by the attenuator 3 with various pre-specified light attenuation ratios. The attenuated light inputs to a light detector 4 and its light pulses are detected. The light pulses are amplified to convenient size by a proportional amplifier 5 to be input to a wave height discriminator 6 so that the number of output pulses is counted by a counter 7.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention provides a method for measuring the dose received by a scintillator due to radiation irradiation relatively easily and with high reliability by utilizing the scintillation phenomenon in the scintillator. Regarding possible dosimetry methods.

(Conventional technology) Technology to accurately measure radiation doses is essential in fields such as the nuclear industry, in order to improve the efficiency and efficiency of radiation utilization technology, and from the standpoint of radiation protection. be.

Dosimetry methods that utilize various physicochemical phenomena have been devised and used since ancient times. Among these, methods that utilize the scintillation phenomenon in scintillators such as fluorescent materials have long been attracting attention as useful methods for dose measurement, along with methods that utilize ionization phenomena, and improvements and practical applications of these methods have been developed. Numerous research and development efforts have been carried out on a global scale.

In general, dosimetry methods based on scintillation have lower reproducibility and reliability of measured values than methods based on ionization, and have not yet achieved the status of a high-precision measurement method.

The scintillation-based dosimetry method is based on the experimental fact that there is an approximate proportional relationship between the dose received by the scintillator (absorbed dose) and the total amount of scintillation light emission. It is up to you to decide.

Here, the total amount of scintillation light emission is generally measured by a direct current measurement method or a pulse height spectrum measurement method using a photomultiplier tube as a photodetector.

These measurements require highly sensitive photomultiplier tubes because scintillation is extremely weak, but generally high-sensitivity photomultiplier tubes are subject to fluctuations in gain (multiplication factor) due to various factors. Therefore, to ensure high reliability in these measurements, a gain stabilization device using advanced technology is required.

On the other hand, among the above two measurement methods for measuring the total amount of scintillation, the DC current measurement method can be constructed at a relatively low cost, but is inferior to the pulse height spectrum measurement method in terms of measurement sensitivity. . However, the measurement system using the pulse height spectrum measurement method requires relatively expensive measuring instruments including a multi-channel analyzer, which causes problems such as increased costs.

[Problems to be Solved by the Invention] As mentioned above, existing dosimetry methods based on scintillation have a problem in ensuring high reliability of measurement results due to gain fluctuations of the photomultiplier tube, which is a photodetector. However, in order to overcome this problem, the structure of the measuring device would become complicated and the cost would increase. In order to solve these problems, the inventors of the present invention
The total amount of scintillation light emitted in the scintillator,
In other words, we developed a dosimetry method that can measure the dose received by a scintillator using a relatively simple measurement system and with high reliability (Japanese Patent Application No. 63-72,
No. 903). The measurement method is different from conventional measurement methods.

The measurement method first optically attenuates the scintillation that occurs when the scintillator receives radiation. The optical pulse obtained by attenuation is converted into an electric pulse, and the number of converted electric pulses is measured. and,
The total amount of scintillation light emission is determined from the number of electrical pulses measured. In this measurement method, the scintillation is optically sufficiently attenuated, so it is extracted as a light pulse of extremely low light intensity, which is then incident on a photodetector, where it is
Electric pulses are generated with a probability of occurrence proportional to the light intensity of the incident light pulse. The number of electrical pulses generated is less than lθ% of the number of scintillation events occurring in the scintillator (compared to this, in measurements using the conventional pulse height spectrum measurement method, all scintillation events are usually equivalent to one scintillation event). (detected as an electrical puzzle), the sensitivity of dosimetry is inevitably low.

An object of the present invention is to solve the above-mentioned problems and to provide a relatively simple measurement system for measuring the total amount of scintillation light emission, that is, the dose received by the scintillator, without being affected by the gain fluctuation of the photomultiplier tube. The object of the present invention is to provide a dosimetry method that can be used to measure doses with high reliability and has relatively low measurement sensitivity.

[Means for Solving the Problems] In order to achieve such an object, the present invention extracts scintillation generated when a scintillator receives radiation, optically attenuates it at various optical attenuation ratios, and obtains the scintillation obtained by the attenuation. The scintillator is characterized by converting the optical pulses into electrical pulses, measuring the number of pulses, and determining the dose received by the scintillator from the radiation from the measured number of electrical pulses and the relative magnitude of the optical attenuation ratio. do.

[Function] In tree inventions, seeds. By optically attenuating scintillation with various optical attenuation ratios and determining the light intensity distribution of optical pulses from the number of electrical pulses per unit time and the relative magnitude of each optical attenuation ratio in each case, scintillation can be The total amount of light emitted by the scintillator, that is, the dose received by the scintillator, can be determined by calculation. [Example] The principle of the present invention is that when a light pulse is detected using a photomultiplier tube as a photodetector, when the absolute value of the light intensity of the incident light pulse is within a limited range of low intensity region, is based on the fact that the probability of generating one or more photoelectrons at the photocathode of a photomultiplier tube for each incident light pulse is determined according to a nonlinear function with the light intensity of the incident light pulse as a variable.

More specifically, if one or more photoelectrons are generated at the photocathode, a light pulse of sufficiently low light intensity is generated by optically attenuating the scintillation generated in the scintillator by various optical attenuation ratios within a certain range. incident on a photomultiplier tube having the property of producing an output pulse,
It is based on the fact that information on the light intensity distribution of light pulses can be obtained from a series of measurements obtained by measuring the number of pulses output from a photomultiplier tube per unit time. The total amount of scintillation light emission, ie the dose received by the scintillator, can be immediately calculated from the light intensity distribution of the obtained light pulses.

Hereinafter, the present invention will be explained in detail based on examples.

FIG. 1 is a block diagram showing an embodiment of the dosimetry method of the present invention. For example, when a scintillator 1 in the form of a plastic scintillator is irradiated with radiation, the scintillator 1 performs scintillation (i.e., light emission for a very short duration) with a light intensity that corresponds to the magnitude of the energy imparted by the radiation for a random period of time. Occur one after another at intervals.

These scintillations are guided to a variable optical attenuator 3 through an optical fiber light guide 2 as an optical path. The intensity of the light guided to the variable optical attenuator 3 is reduced by various predetermined optical attenuation ratios. The light whose light intensity has been reduced is input to the photodetector 4, and a light pulse is detected.

A so-called high-resolution photomultiplier tube with a high-gain first dynode used as the photodetector 4 actually produces one photoelectron per incident light pulse at its cathode.
It has a characteristic that it can be considered that an output pulse is always generated when more than one pulse occurs, and plays an important role in implementing the present invention.

The electric pulse output from the photodetector 4 is proportional. amplifier 5
After being amplified to an appropriate size by the wave height discriminator 6
is input into . The electric pulses output from the pulse height discriminator 6 are input to a counter 7, and the number thereof is measured.

FIG. 2 shows the relationship between the intensity of an incident light pulse input to the photodetector 4 and the probability of occurrence p of a pulse output from the photodetector 4. The intensity of the incident light pulse is indicated by the average number m of photoelectrons generated from the photocathode when one incident light pulse enters the photodetector 4.

Here, the pulse occurrence probability p is p = 1 -exp (-m) ・
As expressed by (1), when m is larger than about 5, p actually becomes a constant value 1, and when m is smaller than about 0.2, p becomes approximately equal to m.

In other words, the graph shown in FIG. 2 shows that only when the optical pulse intensity m is greater than about 0.2 and less than about 5, the pulse generation probability p is actually expressed by a nonlinear function of m. are not doing so.

FIG. 3 conceptually shows the pulse height distribution of output pulses when one photoelectron is generated for each incident light pulse in the photocathode of a high-resolution photomultiplier tube. Here, a high-resolution photomultiplier tube is used as the photodetector 4. Curve A shown by a solid line shows the case where the gain of the high-resolution photomultiplier tube is stable, and curve B shown by the broken line shows the case where the gain of the photodetector 4 drifts and increases.

When one photoelectron is generated at the photocathode for each optical pulse incident on a high-resolution photomultiplier tube, the pulse height distribution of the output pulses takes a characteristic shape with a prominent peak P1. This means that when one photoelectron is generated at the photocathode for each large pulse of light, the peak P. The total area S0 of the area surrounded by the vertical and horizontal axes of the pulse height distribution curve is the event in which one photoelectron is generated at the photocathode for each incident light pulse. Therefore, the photodetector 4 can be treated as corresponding to the number of times of the incident light pulse.
This shows that when more than one photoelectron is generated at the photocathode, it can be considered to have the characteristic that an output pulse is always generated.

As shown in FIG. The pulse height value H1, which is sufficiently lower than the pulse height value corresponding to , is set as the lower limit value of the pulse wave height value. In this case, the number of output pulses of the wave height distribution is set to 11,
The product S integrated over the entire interval above is the total area S mentioned above. There is no difference between them and they are approximately equal.

Furthermore, even if a gain change occurs in the photodetector 4, this area S is not affected by this change and remains substantially constant. child. This fact ensures the accuracy of dose measurement by the dosimetry method of the present invention, and the counting rate n of the output pulses of the pulse height discriminator 6 (i.e., the number of output pulses per unit time) is the same as that of the photodetector 4. becomes almost unaffected by gain changes.

Next, when the optical attenuation ratio in the variable optical attenuator 3 is changed, the optical intensity of scintillation is We will explain in detail how distribution information can be obtained. The light intensity of each scintillation (light pulse) is assumed to be distributed in the light intensity range from O to L0 with L0 as the upper limit of the light intensity, and the light intensity distribution is expressed in the form of a spectrum having r components. It can be expressed in histograms.

The width of each component of the histogram is equal, and the height of the i-th component is Ht (where i=1.2, 3..., r
) indicates the counting rate (number of optical pulses per unit time) of optical pulses having the optical intensity corresponding to the i-th component.

Figure 4 shows the relationship between output pulse generation probability and light intensity. More specifically, when scintillation is optically attenuated and extracted as a light pulse with low light intensity and incident on the photodetector 4, photoelectrons are generated on the photocathode when one light pulse is incident on the photodetector 4. The upper limit of light intensity is L when the average number of m is . The value shown is Jl-20, II1
-10, m”5, m-2, m-1, m-0.5
and l-02, the pulse generation probability of the pulse output from the photodetector 4 is expressed as a function of the light intensity by the equation (1
) shows the results calculated based on

From the output pulse generation probability shown in FIG. 4, when scintillation is optically attenuated by the variable optical attenuator 3,
When the light intensity is, for example, m-5, the output pulse generation probability of the photodetector 4 is easily determined for a light pulse having a light intensity corresponding to each component of the histogram.

Therefore, if the output pulse generation probability for the optical pulse of the i-th component of the histogram is 61▲, the counting rate of the output pulse from the photodetector 4 due to these optical pulses is ε-th H. , is expressed as .

Therefore, due to all the incident light pulses, the photodetector 4
The counting rate rli of the output pulses from the histodaram is the sum of the counting rates of the output pulses from the photodetector 4 b) due to the optical pulses of each component of the histodaram, and is expressed by the following equation (hereinafter referred to as the observation equation). expressed.

nl = ε ll'I'll'' ε lj・
H2" ε 13' n3+ .."61r
・■, ... (2) Similarly, scintillation is optically attenuated by the variable optical attenuator 3, and the light intensity corresponding to the upper limit value Lo of the light intensity is, for example, m-4, m=3. 5, m-3, m-
2.5, the counting rate of the output pulses from the photodetector 4 in each case is n2, n3, n4+, respectively.
ns, and in each case, the value calculated from equation (1) as the output pulse generation probability for a light pulse having a light intensity corresponding to each component of the histogram is ε21. ε3▲
．． ε41+ε51 (where i=1. 2, 3,...
, r), the following observation equation is obtained.

” n2=6・1・■1・ε2,・H2・ε23・■
3.. ．． ．． ◆ε2r・■, ・・・
(3) ns “tsI” Hl” 1!: 32° 82”! 33゜ H3+,,・+ 63r・ ■, ...(4) n4 = 641 II1" E42・ 112"
e43・H3"...÷ε4r・11,...(5) ns" 6s1 }I1" 652° 11,+
6 53”H3”. ．． ．． +65r・I
+, ...(6) By changing the optical attenuation ratio of the variable optical attenuator 3 to various values and measuring the count rate of the output pulses from the photodetector 4 each time, such an observation equation can be further modified. You can create many.

Equations (2) to (6) all have r unknown numbers Hl+H2
．． H3. ．． ．． ．． ．． I1, and these observation equations constitute a multidimensional simultaneous equation. If certain conditions are met, this multi-dimensional simultaneous equation can be solved using an appropriate mathematical solution method. Since the obtained solution gives the height of each component of the histogram, the scintillation light intensity distribution has been determined in the form of a histogram.

Furthermore, the total amount of scintillation light emission T is given as the sum of the products of the average light intensity of each component of the histogram and the counting rate, but the average value of the light intensity of the i-th component of a histogram consisting of r components is , Lo(2i-1)/2r, T is given by the following formula.

Note that in order for the simultaneous equations consisting of observation equations such as equations (2) to (6) to have good solutions, it is necessary that the observation equations are mutually independent.

In order to satisfy this condition, the main attenuation ratio of the variable optical attenuator 3 is adjusted so that the maximum optical intensity of the optical pulse incident on the photodetector 4 is in the range of m~0.2~5. In addition, it is necessary to set the values to be different from each other.

In the above embodiment, one photodetector is used, and scintillation is optically attenuated at various optical attenuation ratios and made incident on the photodetector 4, and the light from the photodetector 4 at each optical attenuation ratio is Although the observation equation was created by measuring the number of pulses, it is not limited to this.

For example, if a measurement system is used in which scintillation is extracted through multiple optical paths and the number of electrical pulses is measured in each optical path by an optical attenuator, a photodetector, and each photodetector, one measurement can be performed. It is possible to create as many observation equations as there are optical paths. In this case, scintillation can be easily extracted through multiple optical paths by using a multi-branched optical fiber light guide or the like as the optical path.

Also, by extracting scintillation through multiple optical paths and performing double coincidence and multiple coincidence of output pulses between different photodetectors, the amount of information obtained from a single measurement is increased. Observation equations can also be created by performing such counts (
In this case, the probability of simultaneous pulse occurrence is given by the product of the output pulse occurrence probabilities of each photodetector), so a relatively large number of observation equations can be created at once using a small number of photodetectors.

When setting the optical attenuation ratio for scintillation passing through each optical path, there are two methods: a method of setting a constant optical attenuation ratio for each optical path, and a method of making the optical attenuation ratio variable for each optical path.

The former and the latter are used depending on the purpose. The former is used when trying to obtain partial information on an optical spectrum, and the latter is used when trying to obtain detailed information on an optical spectrum.

As described above, the dose measurement method of this embodiment allows dose determination based on scintillation to be performed using a relatively simple measurement system without being affected by gain fluctuations of the photodetector.

One of the characteristics of the dosimetry method of the third example is the dosimetry method of the related invention (patent application filed in 1983-
No. 072,903), it has made it possible to dramatically improve measurement sensitivity while retaining all the advantages.

That is, in the measurement method of this embodiment, the optical attenuation ratio of the variable optical attenuator 3 that optically attenuates scintillation is adjusted so that the maximum optical intensity of the optical pulse incident on the photodetector 4 is m 4 0 ．． The above-mentioned relationship is that the maximum light intensity of the light pulse incident on the photodetector is measured so that it is within the range of about 2 to 5, so the maximum light intensity of the light pulse entering the photodetector is about m-0.1 or less. Compared to the measurement using the PIIfx measurement method of the invention, a measurement sensitivity that is about 5 to 10 times higher can be obtained.

In addition, since the dose measurement method of this example is a method of determining the total amount of scintillation light emission based on the calculation of the light intensity distribution of scintillation generated in the scintillator, the amount of light given to the scintillator at the same time as the measurement of the dose is It has the characteristic of providing information about the distribution of energy.

Furthermore, in this example, the optical coupling between the scintillator and the photodetector is performed with extremely low optical transmission efficiency l1-"l-, so unlike conventional measurement methods, scintillators of various shapes can be used. In addition, an optical fiber can be used for optical coupling between the scintillator and the photodetector, making it possible to realize a highly flexible line sight measuring instrument.

[Effects of the Invention] As explained above, in the present invention, the light intensity of scintillation generated in the scintillator by radiation irradiation is attenuated by various optical attenuation ratios, and the number of output pulses at each optical attenuation ratio is Since the total amount of scintillation light emission, that is, the dose, is measured by the measurement, it is possible to easily realize dose measurement that is not affected by gain fluctuations of the photodetector.

2...optical fiber light guide, 3... variable optical attenuator, 4...photodetector, 5...proportional amplifier, 6... Wave height discriminator, 7...Counter.

[Brief explanation of drawings]

Figure 1 is a block diagram showing an apparatus for implementing the present invention; Figure 2 is a diagram showing the relationship between the output pulse generation probability of a photodetector and the light intensity of an incident light pulse; FIG. 4 is a diagram showing the pulse height distribution of output pulses, and is a diagram showing the relationship between output pulse generation probability and tip strength in an embodiment of the present invention. 1...Scintillator, Figure 3 shows the scintillator, Bukano ＼ 9 Lusu ρ 冫Friend Takagafu 2

Claims (1)

[Claims] 1) A scintillator extracts the scintillation generated when it receives radiation, optically attenuates it at various optical attenuation ratios, converts the optical pulse obtained by the attenuation into an electrical pulse, and converts the optical pulse obtained by the attenuation into an electrical pulse. A dosimetry method, characterized in that the number of electrical pulses is measured, and the dose received by the scintillator by the radiation is determined from the measured number of electrical pulses and the relative magnitude of the optical attenuation ratio. 2) The dosimetry method according to claim 1, wherein the scintillation is extracted through a plurality of optical paths, and the scintillation is optically attenuated at an attenuation ratio set for each optical path.