JP2939992B2 - Radiation detection device and method for exploring underground cavity - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a radiation detection device, and more specifically, to a radiation detection device having directivity. The invention also relates to a method for exploring an underground cavity.

[Prior Art] Since a radiation detecting device, for example, a gamma ray detecting device generally has no directivity and is often used exclusively in a laboratory, such a necessity has not been recognized.

Further, as a method for obtaining distributions of various radiation sources, a method is known in which a detection device for radiation or thermal infrared is mounted on an observation satellite, and observation values at different time points are compared and calculated.

[Problems to be Solved by the Invention] In the method using the observation satellite, the gamma ray detecting device is at a far distance from the measurement target area, and the gamma ray detecting device itself has to move. Therefore, even at the same position, it is impossible to observe from which direction the radiation is coming.

The inventor of the present application has been engaged in the development of geodetic and surveying techniques using radiation for many years, and has discovered that geological conditions have a certain influence on terrestrial radiation dose. In order to non-destructively investigate underground or underground conditions, there are radar methods and thermal infrared methods other than gamma ray methods, but these methods can only detect extremely shallow cavities up to about 10 m . On the other hand, gamma rays can detect up to about 50m underground.

As described above, the usefulness of the gamma ray detection device as a means of remote sensing is being recognized, and the first aspect of the present invention is described.
The purpose of the present invention is to present a radiation detecting device having directivity.

A second object of the present invention is to provide a radiation detection device capable of electrically controlling the directivity.

It is a third object of the present invention to provide an exploration method for nondestructively exploring an underground cavity.

[Means for Solving the Problems] A radiation detecting apparatus according to the present invention is disposed on a main radiation detecting means and a circumference centered on a main axis of the main radiation detecting means, and individually controls operation / non-operation by external control. A plurality of free auxiliary radiation detecting means, an inverse simultaneous means for generating a control signal for capturing the output of the main radiation detecting means which is not simultaneous with the output of the auxiliary radiation detecting means, and the main radiation according to the output of the inverse simultaneous means Analyzing means for analyzing the output of the detecting means.

The radiation detection apparatus according to the present invention also includes a main radiation detection unit, a plurality of sub-radiation detection units arranged on a circumference around a main axis of the main radiation detection unit, and a second analysis unit that analyzes an output of the main radiation detection unit. One analysis means, a second analysis means for analyzing the output of the auxiliary radiation detection means, and one of the main radiation detection means and the plurality of auxiliary radiation detection means, relative to the other, And a moving means for moving the main radiation detecting means in the main axis direction.

The radiation detection apparatus according to the present invention also includes a main radiation detection unit, a plurality of sub-radiation detection units arranged on a circumference around a main axis of the main radiation detection unit, and a second analysis unit that analyzes an output of the main radiation detection unit. One analysis means, a second analysis means for analyzing the output of the auxiliary radiation detection means, and one of the main radiation detection means and the plurality of auxiliary radiation detection means, relative to the other, Rotating means for rotating the main radiation detecting means in a circumferential direction about the main axis direction.

The method for exploring an underground cavity according to the present invention is characterized in that a gamma ray from the ground is detected by a directional gamma ray detecting device, and the presence of the underground cavity is determined from changes in potassium and bismuth of the gamma ray. .

[Operation] By setting one or more inactive states of the auxiliary radiation detecting means, a so-called observation window of the main radiation detecting means is opened at that part, and the radiation incident on the observation window is inversely counted by the coincidence method. Can be detected. In addition, the observation window can be electrically rotated by cyclically deactivating one of the plurality of auxiliary radiation detection units.

For example, by moving the main radiation detecting means up and down with respect to the sub-radiation detecting means, the viewing angle in the main axis direction of the main radiation detecting means changes, and detection values at different viewing angles can be obtained.

The gamma ray detection device using the above-described directional radiation detection device can detect gamma rays from different or narrow directions on the spot, and can clearly detect a change in a relatively small portion such as an underground cavity. Therefore, the underground cavity can be more easily explored.

Embodiment An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention, and FIG. 2 is a sectional view of the embodiment shown in FIG. The gamma ray detector 10 of the present embodiment has a gamma detector 12 that can move up and down in the center, and six gamma ray detectors arranged in a hexahedron around it.
12,13,14,15,16,17,18 are closely arranged. Hereinafter, the central gamma ray detector 12 is called a main detector, and the surrounding gamma ray detectors 13 to 18 are called sub detectors. The main detector 12 is, for example,
It consists of a 3 inch germanium semiconductor or premature sodium scintillator. The secondary detectors 13-18 are, for example, 5 inch premature sodium scintillators.

Although the details will be described later, the operating voltage of the sub-detectors 12 to 18 (the driving voltage of the photo-electron tube when the photo-electron tube is provided, and the voltage generated by radiation in the case of the semiconductor detector) The application / non-application of voltage for extracting a pair of electrons can be individually controlled. This control removes radiation from the background and enables observation windows in the circumferential direction, in other words, directivity. Control.

The sub-detectors 13 to 18 are rotating disks having a gear 20 on the peripheral surface.
The rotating disk 22 is fixed on the base 22, and is disposed on the base 24. The gear 28 of the rotating shaft of the motor 26 meshes with the gear 20 of the rotating disk 22, and the rotating disk 22 rotates around the main gamma ray detector 12 by the rotation of the motor 26.

The motor 26, the gear 28 and the rotating disk 22 are provided with auxiliary detectors 13 to 18.
The purpose of the present invention is to make the directivity obtained by the operation voltage control more precise by signal processing. Specifically, after shutting off the operating voltage to one of the sub-detectors 13 to 18 to provide an observation window on the peripheral surface to detect gamma rays, the motor 26 controls the entire sub-detectors 13 to 18 as a whole. By slightly rotating the detector 12 around the center, the circumferential angle of the observation window is slightly shifted, and the observation is performed again. In this way, by obtaining the difference between two detection values having different angle positions of the observation window, it is possible to obtain observation values with higher resolution. When such resolution is not required, the configuration of the motor 26, the gear 28, and the rotating disk 22 may be omitted.

There is a through hole 25 in the center of the base 24 and the rotating disk 22,
A post 32 held by a bearing 30 of the base 24 penetrates. The main detector 12 is supported at the upper end of the post 32. A rack 34 is fixed to the lower side surface of the post 32, and a gear 38 attached to the rotating shaft of a motor 36 is
34 is engaged. By controlling the rotation of the motor 36, the main detector 12 can be moved up and down, that is, in the axial direction of the post 32.

FIG. 3 is a block diagram showing a configuration of an electric circuit system of the gamma ray detecting device shown in FIGS. 1 and 2. 1 and 2 are denoted by the same reference numerals. Reference numeral 40 denotes a power supply circuit for supplying the operating voltages of the gamma ray detectors 12 to 18. However, when it is desired to set the operating voltages to be supplied to the detectors 12 to 18 to different values, a transformer circuit may be provided in the middle or a separate power supply circuit may be provided. The output of the power supply circuit 40 is directly applied to the operating voltage input terminal of the main detector 12, and the switches 41, 42, 43, 44, 45, 46 are connected to the operating voltage input terminals of the sub-detectors 13 to 18, respectively. The output of the power supply circuit 40 is applied via the. The control circuit 48 controls opening and closing of the switches 41 to 46. The control circuit 48 also controls the rotation of the motors 26, 36 via the motor drive circuits 50, 52.

The detection outputs of detectors 12 to 18 are preamplifiers 54 and 5, respectively.
It is amplified by 5,56,57,58,59,60. Preamplifier 54-60
Is applied to an inverse simultaneous circuit 62, and the output of the preamplifier 54 is passed through a variable delay circuit 64 for adjusting timing with the output of the inverse simultaneous circuit 62 to a known multi-channel analyzer 66.
Is applied to The reverse coincidence circuit 62 compares the detection output of the main detector 12 with the detection output of any of the sub-detectors 13 to 18 on the time axis based on a known reverse coincidence counting method, and And outputs a gate signal for extracting the output of the main detector 12 at a timing substantially different from the detection timing. This gate signal is applied to the control terminal of the multi-channel analyzer 66, and the multi-channel analyzer 66 analyzes the output unique to the central gamma ray detector 12 according to the gate signal by a known method. The output of each channel of the multi-channel analyzer 66 is output to an output terminal 68.

The output of the gamma ray detector 12 at a timing other than the gamma ray detection timing by the sub gamma ray detectors 13 to 18 is obtained at the output terminal 68. Therefore, one of the sub detectors 13 to 18
For example, when the switch 43 is opened and the application of the power supply voltage to the gamma ray detector 15 is cut off, gamma rays from the circumferential angle where the gamma ray detector 15 exists can be selectively detected.

The outputs of the preamplifiers 55 to 60 are also applied to a multi-channel analyzer 70 similar to the multi-channel analyzer 66. The multi-channel analyzer 70 is
1818 are used to process the detected outputs as background. Although the details will be described later, the reverse simultaneous circuit 62 is inactivated, and the delay amount of the variable delay circuit 64 is set to zero,
The detection values of the main detector 12 and the detection values of the sub detectors 13 to 18 are separately integrated for a predetermined period. Then, by subtracting all the detection values of the sub-detectors 13 to 18 from the detection value of the main detector 12, the gamma dose of the main-axis (Z) direction viewing angle φ (see FIG. 2) of the main detector 12 is known. be able to.

Various operations can be instructed to the control circuit 48 by the operation switch 74. For example, of the sub-detectors 13 to 18, one that cuts off the operating voltage is specified, or the motor 26 is rotated by a predetermined amount to slightly change the rotational position of the sub-detector to which the operating voltage is not applied. An operation instruction such as an instruction is input to the control circuit 48. In the present embodiment, the directivity in the circumferential direction is electrically controlled basically by controlling the application / non-application of the operation voltage of the sub-detectors 13 to 18, that is, the operation / non-operation, and the motor 26 is exclusively used. Are used as supplementary means for this electrical directivity control.

Next, the directivity in the circumferential direction of the illustrated embodiment will be described. FIG. 4 is a plan view of the gamma ray detectors 12 to 18 of FIG. 1 as viewed from above. For example, the existence range θ of the sub detector 15 is considered. Then, gamma rays from the circumferential direction within the range of the angle θ are detected substantially simultaneously by the sub-detector 15 and the main detector 12, and are removed from the output of the main detector 12 by the inverse simultaneous circuit 62. However, when the switch 43 is opened to cut off the operating voltage of the sub-detector 15, the output of the sub-detector 15
Do not enter in. Therefore, the circumferential angle θ by the sub-detector 15
Gamma rays in the range of
The analysis is performed at 66, and the analysis result is output to the output terminal 68. In other words, this corresponds to the observation window being opened in the portion of the sub-detector 15 where the operating voltage is cut off.

Therefore, by selectively and cyclically cutting off the operating voltage of one of the sub-detectors 13 to 18, the circumferential position of the observation window can be electrically changed or rotated. The radius of the sub detectors 13 to 18 is r ₀ , and the radius of the main detector 12 is r
Then, the viewing angle θ by the sub-detector 15 that cuts off the operating voltage is Becomes At this time, for example, assuming that the inner surface of the tunnel is being observed, the natural gamma dose N (count / second m ² ) is
It is obtained from the following equation.

Here, n is a gamma ray observation value (0.5 second observation value), H is the height of the tunnel, d is the distance from the central gamma ray detector 12 to the side of the tunnel, and θ is the viewing angle obtained by equation (1).

Next, gamma ray detection of the viewing angle φ centered on the principal axis direction (that is, the Z-line direction in FIG. 1) of the gamma ray detection device 10 will be described. For example, when detecting the gamma dose from the roadbed surface, the gamma ray detector 10 shown in FIG. 1 is inverted (that is, upside down) so that the gamma ray detectors 12 to 18 are located below the base 24. FIG. 5 is a schematic side sectional view of the gamma ray detecting device 10 in that state. In this case, the secondary detector 13
Apply operating voltage to all of ~ 18. That is, switches 41 to 46
Are all closed. Thereby, the sub detectors 13 to 18 surround the main detector 12 in a cylindrical shape and function as background detection. The sub-detectors 13 to 18 detect the background gamma ray, while the main detector 12 detects the gamma ray of the viewing angle φ and the background gamma ray. Therefore, by subtracting the detection outputs of the sub detectors 13 to 18 from the detection output of the main detector 12, the gamma dose of the viewing angle φ can be obtained. In this case, the output of the main detector 12 at a timing different from the detection timing of the sub-detectors 13 to 18 may be used as the final detection value by using the inverse simultaneous circuit 62 of FIG. Also, the main detector is simply used without using the inverse simultaneous circuit 62.
The detection outputs of the sub detectors 13 to 18 may be subtracted from the twelve detection outputs.

By moving the main detector 12 up and down with respect to the sub-detectors 13 to 18, the detection range unique to the main detector 12, that is, the viewing angle φ
(FIG. 5) can be changed. The viewing angle φ becomes φ ≒ 2tan ^-1 (h1 / h2) (3). However, h1 is the horizontal length of the roadbed from the side of the end face of the main detector 12 to the inside of the end faces of the sub-detectors 13 to 18, and h2
Is the vertical length from the tip surface of the main detector 12 to the roadbed.
Gamma rays from a direction deviating from the viewing angle φ are detected by both the main detector 12 and the sub detectors 13 to 18, but are finally removed as a background.

In this way, the natural gamma dose N (count / second m ² ) can be calculated from the line observation value by the following equation.

Here, n is the gamma ray observation value from which the background has been removed, φ is the viewing angle according to equation (3), h3 is the distance (m) from the tip surface of the sub-detectors 13 to 18 to the roadbed, and D is the sub-detectors 13 to 18
Is the diameter (m).

In principle, radiation is also incident from the lower side of the detection device 10, that is, from the −Z direction, but this may be shielded by a shielding material such as lead, or the lower side of the main detector 12 − A sub-detector that removes radiation from the Z direction as a background may be arranged and removed by the inverse coincidence method.
Since the lead plate is not effective unless it has a considerable thickness, the latter configuration is effective.

With the above configuration, according to the present embodiment, directivity can be provided for gamma ray detection in the circumferential direction of the side surface of the gamma ray detection device 10, and the angle position can be electrically changed and controlled. Also, in the main axis direction of the gamma ray detection device 10, the detection viewing angle can be changed by moving the main detector 12 up and down. By subtracting the two detection values having different viewing angles, the radiation dose at the angle of the difference between those viewing angles can be calculated, so that the detection is performed while gradually moving the main detector 12 upward or downward, If the difference between the detected values is obtained, the angular distribution of the radiation dose can be obtained.

In the above embodiment, the sub detectors 13 to 18 are stopped,
The main detector 12 has been moved up and down.
Of course, and the sub-detectors 13 to 18 may be moved up and down.

Next, a method and an apparatus for exploring a large-scale underground cavity will be described, but first, the basic principle will be briefly described. If the radiation source is underground, the radiation dose on the ground will be a function of the distance from the source. That is, the radiation source is point-like,
The radioactivity intensity is N ₀ , the distance is d, and the attenuation coefficient in the formation is μ
Then, the radioactivity N on the ground is N = N ₀ exp (−μd) (5) Cavities can be mathematically considered negative sources, with larger cavities being treated as larger negative sources. Thus, the natural radioactivity above ground of potassium 40 is minimized directly above the cavity. In addition, a crack has developed in the vicinity due to the formation of a cavity, and radon rises to the ground through the crack. Therefore, the radon daughter element bismuth 214 is maximized immediately above the cavity.

In summary, immediately above the cavity, the larger the cavity, the smaller the potassium 40 and the larger the bismuth 214.
Then, a value obtained by dividing the measured value of bismuth 214 by the measured value of potassium 40, the value of bismuth 214 / potassium 40 becomes maximum immediately above the cavity. Also, the shallower the cavity, the wider the area of influence of potassium 40, and the greater the amount of change immediately above. This relationship is schematically illustrated in FIGS. 6A and 6B. FIG. 6A shows the case where the cavity is at a shallow position, and FIG. 6B shows the case where the cavity is at a deep position and radon is rising through a crack.

In this way, the presence or absence and degree of underground cavities can be determined, which can also be used to determine the effect of repair after injecting repair material for such cavities. That is, it is possible to perform a nondestructive inspection on whether or not the cavity remains even after the repair.

The same applies to the roof of the tunnel, and the changes in potassium 40 and bismuth 214 / potassium 40 are shown in FIGS. 7A and 7B.
Shown in the figure. FIG. 7A shows a case where there is a cavity very close to the back of the roof, and FIG. 7B shows a case where there is a large cavity where a crack occurs. Potassium 40, bismuth 214,
Bismuth 214 / potassium 40 has the same relationship as in FIGS. 6A and 6B.

Next, a method and an apparatus for detecting an underground cavity will be specifically described. In this case, the multi-channel analyzers 66, 70
Analyze and observe natural gamma ray potassium 40 and bismuth 214. Then, a trolley, a car, or a pericopter that moves manually is used depending on the observation target range or size.
The observation equipment is a video camera for photographing the observation target area in addition to the gamma ray detection device 10 shown in FIGS.
100, a radiation thermometer 102 for observing the radiation temperature of the target ground, and a position detection device 104 (for a helicopter, for example, a GPS navigation system or a distance meter) for knowing the observation position are mounted on a bogie or the like. The image captured by the video camera 100 is recorded on the video tape recorder 106. The detection signals at the outputs 68 and 72 of the gamma ray detection device 10 are converted into digital signals by A / D converters 108 and 110, respectively, and supplied to a data processing device 112 such as a personal computer. The output of the radiation thermometer 102 and the output of the position detection device 104 are also supplied to the data processing device 112 at the same time. These observation data are subjected to predetermined processing by the data processing device 112 or are graphed by the pen recorder 116 as it is. These observation data are recorded by the digital data recorder 116 as necessary for the subsequent examination.

The synchronizing circuit 118 synchronizes the whole, and the gamma ray detection value (potassium 40 and bismuth 214) by the gamma ray detector 10 and the detection data of the radiation thermometer 102 and the position detector 104 are data at 0.5 second intervals. It is supplied to the processing device 112. Note that the value of 5 seconds is used for the determination of the presence or absence of a cavity.

The apparatus shown in FIG. 8 is mounted on a bogie or the like, and natural gamma rays are measured while running. Observation resolution is 1m per second (manpower)
2.8m at 0.5m, 5.6m / sec (20km / h running), 90km / h
(Helicopter) is 12.5m.

When measuring below the roadbed or in the roof of a tunnel, the first
Invert or erect the gamma ray detection device 10 in the figure,
The main detector 12 is moved up and down to detect gamma rays from a roadbed within a predetermined viewing angle. As described above, at this time, all of the sub detectors 13 to 18 are in the operating voltage application state,
Detect background gamma rays. The amount obtained by removing the detection values (background) of the sub detectors 13 to 18 from the detection value of the main detector 12 is the detection amount of the target viewing angle. Regarding the removal of the background, even if the background is removed by a method of subtracting the output of the output terminal 72 from the output of the output terminal 68 when the inverse simultaneous circuit 62 is not used, the inverse using the inverse simultaneous circuit 62 The detection value from which the background has been removed may be directly obtained from the output terminal 68 by the coincidence method.

When measuring the roadbed on the ground, radiation from the sky can be ignored. When measuring the roadbed and the roof in the tunnel, it is necessary to separate or shield gamma rays from the roof and the roadbed, respectively, which can be dealt with by the configuration described above.

When observing the inner surface of the tunnel, the gamma ray detecting device 10 shown in FIG.
Cut off the operating voltage to the one facing the inside of the tunnel and open the observation window. Then, the gamma dose entering the main detector 12 from the observation window is measured and analyzed. That is, the reverse simultaneous circuit 62
Thus, the output of the main detector 12 at a different timing from the detection of the sub-detectors 13 to 18 to which the operating voltage is applied is extracted.

From the above observations, the change in the observed value of bismuth 214, and the change in the observed value of potassium 40, and their ratio,
The presence or absence of a cavity is determined.

In the above embodiment, six sub-detectors 13 to 18 are arranged around the main detector 12 for controlling the observation window in the circumferential direction.
The present invention is not limited to this, and five or less or seven or more sub-detectors may be arranged around the periphery. In addition, the operation / non-operation of the sub-detectors 13 to 18 is controlled by applying / cutting the operation voltage. However, the present invention is not limited as long as the output of the designated sub-detector can be substantially invalidated. At this stage, the output of the designated sub-detector may be cut off.

In the present embodiment, the sub-detectors 13 to 18 are arranged two-dimensionally around the main detector 12. However, these sub-detectors are three-dimensionally arranged, that is, the sub-detectors are arranged so as to cover the entire main detector 12. A detector may be provided. In that case, the observation window can be electrically changed and controlled in any of the three-dimensional directions.

Further, in the above-described embodiment, the observation window in the circumferential direction due to the cutoff of the operating voltage of the sub-detectors 13 to 18 is configured to be freely changeable to any rotational position around the main detector 12 by the motor 26 and the gear 28. A mechanism for rotating the sub detectors 13 to 18 by a specific angle (for example, θ / 2) by another mechanical transmission mechanism such as a crank mechanism may be provided.

Further, in the present embodiment, the sub detector 13 is provided around the main detector 12.
~ 18 are arranged closely so as not to overlap, but a gap is provided between adjacent sub-detectors, and a plurality of likewise are arranged outside them so that the sub-detectors can face the gap when viewed from the main detector. A secondary detector may be provided. That is, a number of sub-detectors are arranged at a slight distance from each other so as to double surround the main detector. In this way, the observation window due to the inactivation of one sub-detector can be more finely controlled.

In the above embodiments, gamma rays have been described as an example. However, it is needless to say that the present invention can be applied to detection devices for radiation other than gamma rays, for example, alpha rays, beta rays, and other cosmic rays.

The radiation detection apparatus according to the present invention is also effective for radiation measurement in outer space. For example, when observing gamma rays, the entire radiation detection apparatus 10 shown in FIG. 1 is covered with a plastic scintillator, and α Remove lines and particle beams.

[Effects of the Invention] As can be easily understood from the above description, according to the present invention, directivity can be provided in the circumferential direction and the main axis direction, and the directivity can be electrically controlled. it can. Therefore, the radiation dose from a specific narrow direction can be observed on the spot, which has a remarkable effect in various measurement fields using radiation. In addition, since the observation window in the circumferential direction can be electrically changed and controlled, and there is no movable part at the time of the change, for example, if the observation window is used in zero-gravity or low-gravity space such as outer space, stress is applied to the surroundings when the observation window is changed. It has the advantage of not having any effect.

[Brief description of the drawings]

FIG. 1 is an external perspective view of one embodiment of the present invention, and FIG.
FIG. 3 is a block diagram of an electric circuit according to the embodiment of FIG. 1, FIG. 4 is an explanatory diagram of directivity in the circumferential direction according to the embodiment, and FIG. 5 is a shaft of the embodiment. Illustration of directionality,
Figures 6A and 6B show the schematic characteristics of gamma-ray measurements from underground cavities, Figures 7A and 7B show the schematic characteristics of gamma-ray measurements from cavities in tunnel roofs, and Figure 8 shows underground cavities FIG. 3 is a block diagram of the device configuration. 10: gamma ray detector 10 (this embodiment), 12: main detector, 13
~ 18: Secondary detector, 22: Rotating disk, 24: Base, 25: Through hole,
26: motor, 28: gear, 30: bearing, 32: post, 34: rack, 36: motor, 40: power supply circuit, 41 to 46: switch, 48: control circuit, 50, 52: motor drive circuit, 54 to 60: Preamplifier, 6
4: Variable delay circuit, 66, 70: Multi-channel analyzer,
68, 72: Output terminal, 74: Operation switch

Claims (13)

(57) [Claims] 1. A main radiation detecting means, a plurality of sub-radiation detecting means which are arranged on a circumference around a main axis of the main radiation detecting means and whose operation / non-operation can be individually controlled externally, and The output of the detecting means includes an inverse simultaneous means for generating a control signal for capturing the output of the main radiation detecting means which is not simultaneous, and an analyzing means for analyzing the output of the main radiation detecting means in accordance with the output of the inverse simultaneous means. A radiation detection device characterized by the above-mentioned. 2. A moving means for moving one of the main radiation detecting means and the plurality of sub-radiation detecting means relative to the other in the direction of the main axis of the main radiation detecting means. The radiation detection apparatus according to claim 1, wherein 3. The method according to claim 1, wherein one of said main radiation detecting means and said plurality of sub-radiation detecting means is moved relative to the other in a circumferential direction centered on said main axis direction of said main radiation detecting means. The radiation detecting device according to claim 1 or 2, further comprising a rotating unit that rotates the radiation detecting device. 4. A moving means for moving one of the main radiation detecting means and the plurality of sub-radiation detecting means relative to the other in the main axis direction of the main radiation detecting means, Rotating means for rotating any one of the main radiation detecting means and the plurality of sub-radiation detecting means relative to the other in a circumferential direction around the main axis direction of the main radiation detecting means; The radiation detection apparatus according to claim 1, wherein 5. The apparatus according to claim 1, further comprising a second analyzing means for analyzing an output of said auxiliary radiation detecting means. Radiation detection device. 6. The radiation detecting apparatus according to claim 1, wherein said main radiation detecting means and said auxiliary radiation detecting means are gamma ray detecting means. 7. A main radiation detecting means, a plurality of sub-radiation detecting means arranged on a circumference around a main axis of the main radiation detecting means, and a first analyzing means for analyzing an output of the main radiation detecting means. A second analyzing means for analyzing an output of the auxiliary radiation detecting means; and one of the main radiation detecting means and the plurality of auxiliary radiation detecting means, relative to the other, the main radiation detecting means. And a moving means for moving in the main axis direction. 8. The apparatus according to claim 1, wherein one of said main radiation detecting means and said plurality of sub-radiation detecting means is moved relative to the other in a circumferential direction around said main axis direction of said main radiation detecting means. The radiation detecting apparatus according to claim 7, further comprising rotating means for rotating. 9. A radiation detecting apparatus according to claim 7, wherein said main radiation detecting means and said auxiliary radiation detecting means are gamma ray detecting means. 10. A main radiation detecting means, a plurality of sub-radiation detecting means arranged on a circumference around a main axis of the main radiation detecting means, and a first analyzing means for analyzing an output of the main radiation detecting means. A second analyzing means for analyzing an output of the auxiliary radiation detecting means; and one of the main radiation detecting means and the plurality of auxiliary radiation detecting means, relative to the other, the main radiation detecting means. A rotation means for rotating in a circumferential direction around the main axis direction. 11. A radiation detecting apparatus according to claim 10, wherein said main radiation detecting means and said auxiliary radiation detecting means are gamma ray detecting means. 12. A method for exploring an underground cavity, wherein a gamma ray from the ground is detected by a directional gamma ray detecting device, and the presence of an underground cavity is determined from changes in potassium and bismuth of the gamma ray. 13. The underground cavity exploration method according to claim 12, wherein it is determined that a cavity exists at a point where potassium decreases and bismuth increases.