JPH0553394B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention utilizes the scintillation phenomenon in the scintillator to measure the dose received by the scintillator due to radiation irradiation, relatively easily and with high reliability. This invention relates to a dose measuring device and a measuring method capable of measuring dose.

[Prior Art] Today, ionizing radiation is used in various fields of society, such as the nuclear industry, engineering, and medicine. Under these circumstances, technology to accurately measure radiation doses is essential in order to improve the efficiency and efficiency of its use and from the standpoint of radiation protection.

BACKGROUND ART Dosimetry devices 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 phosphors 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 are in order. A large amount of research and development has been carried out on a global scale.

In scintillation-based dosimetry devices, the optical coupling between the scintillator and the photodetector is
It is necessary to make the light transmission efficiency as high as possible, and the scintillator and photodetector are usually integrated into the detection section. For this reason, there are various restrictions on the use of the photodetector due to the effects of temperature, magnetic field, etc. on the photodetector, or due to constraints due to the size of the photodetector.

In general, dosimetry methods based on scintillation have lower reproducibility and reliability of measured values than methods based on ionization, and the current situation is that they have not yet reached the position of being a high-precision measurement method. The dosimetry method based on scintillation 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 determines the dose. 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. In order to ensure high reliability in these measurements, highly sophisticated gain stabilization devices are 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. There is. However, the measurement system of the pulse height spectrum measurement method requires relatively expensive measuring instruments including a multi-channel analyzer, resulting in high 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.

The purpose of the present invention is to measure the total amount of scintillation light emission, that is, the dose received by the scintillator, without being affected by gain fluctuations of the photomultiplier tube, using a relatively simple measurement system and with high reliability. It is an object of the present invention to provide a dose measuring device and a measuring method that can perform dose measurement.

[Means for Solving the Problem] In order to achieve such an object, the present invention has the following features:
A scintillator, an optical attenuation means for optically attenuating scintillation generated when the scintillator receives radiation, a means for converting the optical pulse obtained by the attenuation into an electric pulse, and a measurement for measuring the number of electric pulses. and means.

In addition, the present invention optically attenuates scintillation generated when a scintillator receives radiation, converts the optical pulse obtained by the attenuation into an electric pulse, measures the number of electric pulses, and From the number of electrical pulses, the dose received by the scintillator by the radiation is determined.

[Operation] In the present invention, scintillation generated in the scintillator by radiation is optically sufficiently attenuated and extracted as a light pulse of extremely low light intensity, and then incident on a photodetector where it is incident on the scintillator. Electric pulses are generated with a probability of occurrence proportional to the light intensity of the light pulse. Under these conditions, the number of electrical pulses from the photodetector per unit time is equal to the total amount of scintillation light emitted per unit time in the scintillator, i.e.
It is proportional to the dose received by the scintillator per unit time. Therefore, by measuring the number of such electrical pulses, the dose of radiation received by the scintillator can be determined.

[Example] The principle of the present invention is that when a light pulse is detected using a photomultiplier tube as a photodetector, if the light intensity of the incident light pulse is sufficiently low, the photoelectron multiplication is performed for each incident light pulse. It is based on the fact that the probability of generating one or more photoelectrons at the photocathode of the tube is proportional to the light intensity of the incident light pulse. Therefore, by using a photomultiplier tube that has the characteristic that an output pulse is generated whenever one or more photoelectrons are generated at the photocathode, the scintillation generated in the scintillator is optically attenuated, and the light intensity is sufficiently low. When this light pulse is input to the above-mentioned photomultiplier tube, the number of output pulses per unit time from the photomultiplier tube is proportional to the amount of scintillation light emitted by the scintillator per unit time. It depends on the fact that

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. Here, 1 is, for example, NaI
It is a scintillator such as, and emits light when irradiated with radiation. Reference numeral 2 denotes an optical fiber light guide, which guides the light generated by the scintillator 1 to the optical attenuator 3. The optical attenuator 3 reduces the intensity of the light guided by the fiber optic light guide 2. Although any type of commercially available variable attenuator can be used as the optical attenuator 3, here, the one proposed by the present inventor and published in JP-A-60-237328 was used. 4 is a photodetector in the form of a photomultiplier tube or the like. 5 is a proportional amplifier. 6 is a pulse height analyzer. 7 is a counter (scaler).

When the scintillator 1 is irradiated with radiation,
Scintillation (that is, light emission of very short duration) of light intensity corresponding to the amount of energy imparted by the radiation is generated one after another at random time intervals. These scintillations are sent to an optical attenuator 3 through an optical fiber light guide 2, and after their light intensity is reduced, they are incident on a photodetector 4. The electric pulse output from the photodetector 4 is amplified to an appropriate magnitude by a proportional amplifier 5 and then input to a pulse height analyzer 6. The electric pulses output from the pulse height analyzer 6 are input to a counter 7, and the number of the electric pulses is measured.

A first high gain detector used as a photodetector 4.
A so-called high-resolution photomultiplier tube with a dynode has a characteristic that can be considered to generate an output pulse whenever one or more photoelectrons are generated per one incident light pulse at its photocathode. It plays an important role in implementing the present invention.

FIG. 2 shows the relationship between the pulse generation probability p at the output of the photodetector and the intensity of the incident light pulse expressed as the average number m of photoelectrons generated from the photocathode per pulse of the incident light pulse. This graph shows the fact that a linear relationship holds in a region where the incident light pulse intensity is sufficiently low. Here, the pulse generation probability p is expressed as p=1-exp(-m)...(1), but when m is sufficiently smaller than 1, p
is approximately equal to m. That is, it is expressed as pm.

Figure 3 shows that the intensity of the incident light pulse is sufficiently low;
Photodetector 4 when the average number of photoelectrons per pulse at the photocathode of photodetector 4 is about 0.2
The figure conceptually shows the pulse height distribution of the output pulse. Curve A shown by a solid line shows a case where the gain of the photodetector is stable, and curve B shown by a broken line shows a case where the gain of the photodetector drifts and increases.

When the intensity of the optical pulse incident on a high-resolution photomultiplier tube is sufficiently low, about m0.2, the pulse height distribution of the output pulse will have a pronounced peak p ₁ corresponding to a single photoelectron and a peak corresponding to two photoelectrons. (the height of _the peak p ₂ becomes relatively lower as the light pulse intensity becomes lower than m0.2). Therefore, we now set a value H ₁ that is sufficiently lower than the pulse height corresponding to peak p ₁ as the lower limit of the pulse height, and set a value H ₂ that is sufficiently higher than the pulse height corresponding to peak p ₂ as the upper limit. Then, even if the gain of the photodetector 4 changes, the area obtained by integrating the number of output pulses in the interval from H ₁ to H ₂ is kept almost constant without being affected by this. This fact is an extremely important finding in ensuring the accuracy of dose measurement by the dosimetry method of the present invention.

That is, the output pulse of the comparator amplifier 5 is applied to the pulse height analyzer 6, but by setting the two discrimination levels of the pulse height analyzer 6 to correspond to the above-mentioned H ₁ and H ₂ , the output pulse of the pulse height analyzer 6 is applied. The counting rate (that is, the number of output pulses per unit time) n of output pulses is almost unaffected by the gain change of the photodetector 4.

Next, the counting rate n of the electric pulses output from the pulse height analyzer 6 is determined by the unit time in the scintillator 1 under the condition that the intensity of scintillation light incident on the photodetector 4 is sufficiently low. The total amount of scintillation light emitted per hit (i.e.,
It will be explained in detail that the size is proportional to the dose (dose per unit time).

When the scintillator 1 is being irradiated with radiation, each time radiation is incident, _{the energy E i ( with} the subscript i (i = 1, 2, 3, . . . representing individual radiation incident events) are applied to the scintillator 1, and the scintillator 1 emits scintillation with a light intensity proportional to the energy E _i . In other words, the scintillator 1 calculates the light intensity L i in the light intensity range from 0 to L _n (where L _n is the scintillation light intensity corresponding to the energy imparted by energy E _n ) each time an individual radiation enters the scintillator ₁ . emits scintillation. If N radiations are incident on the scintillator 1 per unit time, the energy (i.e. absorbed dose) E received by the scintillator 1 per unit time is given as the sum of the energy E _i given in each event. Therefore, E= _N 〓 ⁱ⁼¹ ...(2) However, on the other hand, since the light intensity L _i of each scintillation is proportional to the applied energy E _i
It is expressed as follows, where k ₁ is a constant of proportionality.

L _i =k ₁ E _i ...(3) Since the total amount of scintillation light emission L is equal to the sum of the light intensities of individual scintillations, Equation (3)
Using the relationship of equation (2), it is expressed as follows.

L= _N 〓 ⁱ⁼¹ =k _1N 〓 ⁱ⁼¹ =k ₁ E...(4) Equation (4) expresses that the total amount of scintillation L is proportional to the total amount of energy E given to the scintillator 1. ing.

On the other hand, since each scintillation is optically attenuated as described above and is incident on the photodetector 4 as a light pulse with sufficiently low light intensity, the electric power output from the photodetector 4 for each scintillation event is The pulse generation probability p _i is proportional to the scintillation light intensity L _i and is expressed as follows, where k ₂ is a proportionality constant.

p _i =k ₂ L _i ...(5) The value obtained by summing the output pulse generation probability p _i described above for each scintillation event occurring during a unit time for all scintillation events is The counting rate n of the output pulses from the pulse height analyzer 6 is equal to the average value of the number of pulses per unit time output from the photodetector 4, and as mentioned above, the counting rate n of the output pulses from the photodetector 4 is equal to the average value of the number of pulses per unit time output from the photodetector 4. As a result, the output pulse count rate n described above can be expressed as follows using the relationship of equations (5) and (4).

n= _N 〓 ⁱ⁼¹ = k _2N 〓 ⁱ⁼¹ = k ₂ L=k ₂ k ₁ E...(6) In other words, equation (6) shows that the above output pulse counting rate n is the scintillation per unit time. This shows that it is proportional to the total amount of light emitted L and the absorbed energy E, that is, to the dose.

The proportionality constant k ₂ in Equation (5) varies depending on how the light intensity of the scintillation pulse incident on the photodetector 4 is expressed, but as described above, the light intensity is calculated as follows: When expressed as the average number m of photoelectrons generated at the photocathode No. 4, k ₂ =1. That is, when the light intensity L _i of each scintillation pulse is expressed by the average number of photoelectrons m _i described above, the output pulse generation probability p _i is expressed as follows instead of equation (5).

p _i = m _i ...(5') As mentioned above, equation (5') and equation (5) approximately hold true when the light intensity is low, but the lower the light intensity, the more the approximation becomes The degree of this will improve.

For example, when m _i =0.1 and m _i =0.02, the output pulse occurrence probability p _i given from equation (5') is
The values are about 5% and about 1% higher, respectively, than the true value given by equation (1). Therefore, when it is necessary to perform dose measurement with a good measurement accuracy of 5% or more, for example, the light intensity m _i of the scintillation pulse with the maximum light intensity at the stage of incidence on the photodetector 4 is m By setting the optical attenuation rate of the optical attenuator 3 so that _i = 0.1, a predetermined measurement accuracy is guaranteed in dose measurement.

As described above, the dosimetry method of this example makes it possible to perform scintillation-based dose measurements using a relatively simple measurement system without being affected by gain fluctuations of the photodetector. becomes.

One of the features of the dosimetry method of this embodiment is that the method of optical coupling between the scintillator and the photodetector is in sharp contrast to conventional methods.

In the measurement method of this example, the optical coupling between the scintillator and the photodetector can be performed with extremely low optical transmission efficiency, so it can be carried out using optical fiber technology, which has undergone remarkable technological progress in recent years. can. In other words, since the scintillator and the photodetector can be connected with an optical fiber and placed several meters apart from each other, the detector can be easily miniaturized and there are no restrictions on use due to temperature, magnetic fields, etc. Excellent effects can be obtained, such as a significant reduction in

In addition, the dose measurement method of this embodiment uses a variable optical attenuator with a high attenuation ratio as the optical attenuator 3 shown in the block diagram of the embodiment in FIG. It can also be easily applied to the measurement of extremely high doses.

Furthermore, in this embodiment, an optical fiber can be used for optical coupling between the scintillator and the photodetector, making it possible to realize a highly flexible dosimeter unlike the conventional one.

[Effects of the Invention] As explained above, in the present invention, the light intensity of scintillation generated in a scintillator by radiation irradiation is sufficiently attenuated and then incident on a photodetector to determine the total amount of scintillation, That is, since the dose is measured, it is possible to easily realize dose measurement that is not affected by gain fluctuations of a photodetector, which has been extremely difficult to achieve in the past.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the dosimetry method of the present invention, and FIG. 2 is a diagram showing the relationship between the output pulse generation probability p of the photodetector and the light intensity of the incident light pulse.
FIG. 3 is a diagram showing the pulse height distribution of the output pulse of the photodetector when the light intensity of the incident light pulse is sufficiently low. 1... scintillator, 2... optical fiber light guide, 3... optical attenuator, 4... photodetector, 5...
...proportional amplifier, 6... pulse height analyzer, 7... counter.

Claims (1)

[Claims] 1. A scintillator, an optical attenuator for optically attenuating scintillation generated when the scintillator receives radiation, and a means for converting the optical pulse obtained by the attenuation into an electric pulse. , a measuring means for measuring the number of electric pulses. 2. Optically attenuates the scintillation generated when the scintillator receives radiation, converts the optical pulse obtained by the attenuation into an electrical pulse, measures the number of electrical pulses, and converts the measured electrical pulse into an electrical pulse. A dosimetry method characterized in that the dose received by the scintillator by radiation is determined from the number of scintillators.