JP2004170122A - Method for discriminating gamma ray prevalent incidence direction - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for determining a prevalent incidence direction (incidence direction from a horizontal direction when an apparatus is installed perpendicularly) of gamma rays on a plane. <P>SOLUTION: In the method for discriminating a prevalent incidence direction of gamma rays, a shielding body for allowing angular response of a cylindrical gamma ray detection section to have directional dependence and to rotate around the detection section for changing the position is covered on the detection section, and then is rotated by a fixed angle, thus determining the prevailing incidence direction according to the difference in a plurality of measurement values being measured each time. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a method for discriminating the predominant incidence direction of gamma rays, and more particularly to a method for determining the predominant incidence direction in one plane of a gamma ray (the incidence direction from the horizontal direction when the apparatus is installed vertically).
[0002]
[Prior art]
Conventionally, a method of discriminating an incident direction in a narrow visual field has been developed. In the medical field, a large number of detectors are arranged around a subject, and these detectors are used to discriminate the incident direction of radiation, and are used for grasping the shape of internal organs or cancer tissues.
[0003]
However, in the incident direction discriminator in the narrow field of view, it is necessary to repeat a large number of measurements or use a large number of measuring instruments in order to obtain information from all directions. Requires funding. Further, a measuring instrument system used in the medical field cannot target externally incident radiation.
[0004]
On the other hand, conventional environmental radiation measuring instruments are considered to be ideal measuring instruments that have no direction dependence, and have been developed to correctly calculate the dose rate at the measurement point. No method has been developed to efficiently obtain information about
[0005]
[Problems to be solved by the invention]
The present invention, omitting such a current situation, in pursuing a more detailed study of the environmental radiation level, it has been studied based on the idea that it is very important to obtain information on the incident direction of radiation, It is an object of the present invention to provide a method capable of easily obtaining the gamma ray incident direction information in a relatively short time.
[0006]
[Means for Solving the Problems]
The present invention covers a cylindrical detection unit with a shield of a specific shape so as to obtain a prominent direction of gamma rays incident from the outside, and gives direction dependence to the angular response of the detection unit, so that the shield An excellent gamma ray incident direction is obtained by multiple measurements at different positions. According to the gamma ray predominant incidence direction discrimination method of the present invention, the gamma ray predominant incidence direction can be determined by short-time measurement.
[0007]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail.
[0008]
The present invention is a method for discriminating a predominant gamma ray incident from the outside by using a cylindrical gamma ray detection unit to determine a predominant gamma ray incidence direction, giving direction dependency to the angular response of the detection unit, and A cover configured to be able to change its position by rotating around the detector is placed on the detector, and then the shield is rotated by a fixed angle, and the difference between a plurality of measured values measured each time This is to obtain a more prominent incident direction. This rotation may be performed manually or automatically measured while being slowly rotated by a motor by remote control. This measurement is preferably performed on the count rate, or on both the count rate and the energy spectrum. Based on this measurement, the dominant incidence direction can be determined using the difference in count rates or the difference in count rates in a particular energy region.
[0009]
As described above, when the shield is rotated at a fixed angle and measurement is performed each time, for example, when the shield is rotated every 90 degrees, a total of four measurement times are performed, and the count rates are A, B, C, and C, respectively. Let D be (D) and (AC) / ((BD), (BD) / (AC) to calculate the dominant incident direction.
[0010]
The discriminator preferably used in the gamma ray dominant incident direction discriminating method of the present invention is a gamma ray dominant incident direction discriminator for finding a dominant direction of gamma rays incident from the outside, a cylindrical gamma ray detecting unit, and the detection unit. A shield that can be placed over the unit to make the angular response of the detection unit have direction dependence, and the shield is configured to rotate around the detection unit to change the position. For the cylindrical gamma ray detection unit, it is preferable to use a normal commercially available NaI (Tl) scintillator. As the size, for example, a system for measuring a counting rate using a scintillator of 1 inch (2.5 cm) × 1 inch (2.5 cm) φ, or 3 inches (7.6 cm) × 3 inches (7.6 cm) A system that uses a φ scintillator and measures a counting rate and an energy spectrum is used.
[0011]
The above-mentioned shield is capable of covering the detection unit and giving direction dependency to the angular response of the detection unit, and is configured to be able to rotate around the detection unit to change the position. . It is preferable that the shield has a cylindrical shape having a cutout in a part of a side portion to have the above-described direction dependency. When viewed from one side, the shape of such a notch is such that the opening area gradually decreases from the maximum value to the minimum value during one rotation, and then gradually increases to return to the original maximum value. This is preferable in that the ability to discriminate the incident direction is improved. Alternatively, a shape in which many holes are formed so as to form an opening area such as a shape formed by diagonally cutting a cylinder, or a shape similar to these may be used. Further, the shape of the shield is a cylindrical shape that can be rotated over the scintillator, but a cylindrical shape having a constant thickness is more preferable because the data processing is easy. When adopting a cylindrical type with an inconsistent thickness, data of the input value change due to the rotational position is obtained in advance for a known radiation source, and the data obtained at the time of actual measurement is corrected based on it. .
[0012]
Here, one embodiment of the discriminator suitably used in the present invention will be described in more detail with reference to the drawings. FIG. 1 is a schematic diagram showing such a gamma ray dominant incident direction discriminator. FIG. 1 shows a state in which the detection unit 1 is covered with a shield 2 (having a triangular notch). As a normal use condition, the gamma-ray dominant incident direction discriminator is vertically attached to the detector support 4 by using the fixing bracket 6 of the detector fixing rod 5 as shown in FIG. Determine the dominant incidence direction from the fixed, horizontal direction. The output from the detector 1 is connected to the rate meter 9 when only the dose rate is obtained, and to the wave height discriminator 10 when the energy spectrum is also obtained.
[0013]
The detector uses a 1 inch (2.5 cm) × 1 inch (2.5 cm) φ NaI (Tl) scintillator or a 3 inch (7.6 cm) × 3 inch (7.6 cm) φ NaI (Tl) scintillator. However, detectors of other sizes can also be used. In order to make the installation direction of the detection unit reproducible, the magnet 11 is mounted on a support for determining the direction, and is used as a guide of the installation direction.
[0014]
As shown in FIG. 1, the fixing bar 5 of the detection unit is fixed by a fixing bracket 6 so as not to move when the upper shield 2 is rotated. Further, in order to accurately determine the rotational position of the shield 2, a mark 3 at a fixed position is provided, and the mark 3 is stabilized at the fixed position.
[0015]
FIG. 2 shows the structure of the detection unit 1 when the shield 2 is removed. A rotating shaft 12 for rotating the shield 2 and a ball bearing socket 13 are attached to the upper part of the detection unit 1. A detection unit fixing rod 5 for fixing to the support 4 is also attached.
[0016]
FIG. 3 shows the shape and development of the shield 2. The development view shows a case where the detection unit 1 is 3 inches (7.6 cm) × 3 inches (7.6 cm) φ, and the dimension described as “plus α” is the size in consideration of the thickness of the coating material of the scintillator. Indicates a slightly larger portion. Reference numeral 14 denotes a support for the shield rotating shaft 12, and reference numeral 15 denotes a socket for the shield rotating shaft 12.
[0017]
The triangular cutout portion of the shield 2 is set so that the vertical relationship exactly overlaps the position of the internal scintillator wrapped in the double layer. The angle discrimination resolution is improved by making the shape of the shield 2 a little curved instead of a triangle, but the effect is not so great, and a simple triangle may be used. The thickness of the shield 2 varies depending on the energy of the target gamma ray, but for normal use, a lead thickness of 2 cm is appropriate. Uses 1 inch (2.5 cm) x 1 inch (2.5 cm) φ or 3 inch (7.6 cm) x 3 inch (7.6 cm) φ NaI (Tl) coated with aluminum as a standard scintillator I do.
[0018]
The shield 2 is rotated by a fixed angle, and the method of obtaining the dominant incident direction from the difference between the results of multiple measurements measured each time is adopted. Therefore, if the rotation angle is reduced and the number of measurements is increased, the incident direction will be more accurate. Although it can be obtained at a high level, considering the practicality, a method of measuring at every 90-degree rotation will be described here.
[0019]
FIG. 4 shows the response of four measurements for each gamma ray incidence direction by the discriminator in the method of the present invention. The four measurements show a state where the phases are shifted by 90 degrees. Here, as a response to a low-energy gamma ray of several hundred keV or less, a cross-sectional area of a portion without the shield 2 as viewed from the gamma ray incident direction is taken as a response to an energy gamma ray of several MeV or more as viewed from the gamma ray incident direction. The integrated value of the portion without the shield 2 weighted by the length that can pass through the inside of the detection unit is used. Both values are shown as ratios when normalized to the values without the shield 2. The high-energy response shows a tendency to oscillate slightly larger.
[0020]
The response to gamma rays of other intermediate energies encountered in the environment is expected to take a value intermediate between the two curves shown in FIG. As shown in FIG. 4, the fact that there is no significant difference in the dependence of the low energy and the high energy on the incident direction indicates that the incident direction can be obtained regardless of the energy.
[0021]
FIG. 5 shows the ratio of the difference when A, B, C, and D are measured, respectively, that is, (AC) / (DB) and (DB) / (AC). Of the gamma ray with respect to the incident direction. By calculating the difference between the two measurements in this way, the commonly measured background value can be canceled. Also in this case, the case of a low energy gamma ray and the case of a high energy gamma ray are shown, but the difference between them is not so large, and it is shown that there is no large difference in the determination of the incident direction.
4 and 5 show that the same result can be obtained with a cylindrical scintillator regardless of the size of the detection unit.
[0022]
The ratio is determined based on the four measurement results, and the dominant gamma ray incident direction is determined from FIG. At this time, if the value of the ratio is close to zero, the angle discrimination ability deteriorates. Therefore, when the absolute value of the ratio is smaller than 1, the reciprocal thereof is taken, and the angle is read from the reciprocal curve. Further, since two incident directions are obtained only by the value of the ratio, the difference between the magnitudes of D and B is determined as one of the two incident directions as an auxiliary parameter.
[0023]
In the above case, four count rates are used. However, when spectrum measurement is performed, the incident angle can be determined by the same calculation using the difference in the number of measurements in the specific energy region. When such an energy spectrum is used, it is possible to identify the energy or nuclide of gamma rays from the dominant incident direction.
[0024]
In the method for distinguishing the gamma ray dominant incident direction according to the present invention, if there are two dominant gamma ray incident directions, the present apparatus cannot discriminate each of them, and recognizes the intermediate direction between the two as the dominant direction. For example, if the contamination source is present in two directions at the same time and shows the same count rate at the measurement point, it is determined that the contamination source exists in the intermediate direction. In order to avoid such a determination, when it is suspected that two or more contamination sources exist, it is preferable to measure at a plurality of points and specify a direction in which the counting rate becomes higher.
[0025]
【The invention's effect】
According to the method of the present invention, a site contaminated with a radionuclide can be identified more quickly. In addition, it is very effective for searching for a lost radiation source and determining a high-dose location.
[Brief description of the drawings]
FIG. 1 is a schematic diagram of a gamma ray dominant incidence direction discriminator suitably used in the method of the present invention.
FIG. 2 shows a structure of a detection unit of a gamma ray dominant incident direction discriminator.
FIG. 3 shows the structure of the shield of the gamma ray dominant incidence direction discriminator.
FIG. 4 shows the response when low-energy and high-energy measurements are performed in four directions using a gamma-ray dominant incident direction discriminator.
FIG. 5 is a graph for determining a dominant incidence angle using a gamma ray dominant incidence direction discriminator.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Detection part 2 ... Shield 3 ... Shield position marker 4 ... Detector support 5 ... Detector fixing bar 6 ... Fixture 7 ... Level meter 9 ... Rate meter 12 ... Shield rotation axis

Claims (7)

A gamma ray dominant incident direction discrimination method for determining a dominant direction of a gamma ray incident from the outside by using a cylindrical gamma ray detecting unit, wherein the angular response of the detecting unit has direction dependency, and A shield configured to be able to change the position by rotating is placed on the detection unit, and then the dominant incident direction is determined from a difference between each measurement value of data obtained by measuring the shield at a certain angle. A method for discriminating the predominant gamma-ray incident direction, which is characterized by being obtained. The method for discriminating a gamma ray dominant incident direction according to claim 1, wherein the measurement measures a counting rate. The method of claim 1, wherein the measurement measures a counting rate and an energy spectrum. 4. The method according to claim 3, wherein the dominant incidence direction is determined by using a difference in the count rate of a specific energy region. 2. The method for distinguishing a gamma ray dominant incident direction according to claim 1, wherein the shield has a cylindrical shape having a notch on a part of a side portion. 6. The method according to claim 5, wherein the notch is substantially triangular. The measurement is performed four times by rotating each 90 degrees, and the respective count rates are A, B, C, and D, and (AC) / ((BD), (BD) / (A The method of claim 2 or 3, wherein -C) is calculated to determine the dominant incident direction.

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