JP2722672B2 - Radiation measurement equipment - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial application field> The present invention relates to a radiation detector of a radiation application measuring apparatus for measuring physical quantities (basis weight, moisture, etc.) of paper, plastic, rubber, etc. using a semiconductor radiation detector. And radiation sources.

<Prior art> When radiation (for example, β-rays) passes through a material layer, it gradually loses energy due to ionization and excitation, attenuates, and undergoes such inelastic scattering many times to change its traveling direction. . Therefore, as the physical quantity (eg, thickness) of the measurement object increases, the number of transmitted β-rays decreases. An apparatus that measures the physical quantity of various sheet-like substances by applying such a principle is known.

In such a radiation applied measuring apparatus, a radiation source (hereinafter simply referred to as a radiation source) 1 and a radiation detector (hereinafter simply referred to as a detector) 2 are arranged facing each other as shown in FIG. It is configured to measure with the body 3 interposed.
As shown in FIG. 6, the spatial intensity distribution of the radiation from this source is the strongest at the front, and becomes weaker as the distance from the front increases. Accordingly, when the radiation source 1 and the detector 2 move relatively in the X, Y or Z directions, there is a problem that the radiation dose incident on the detector 2 changes and the output fluctuates.

Hitherto, as shown in FIGS. 7 (a), 7 (b) and 7 (c), there have been proposed devices for removing this kind of output fluctuation. That is, an absorption plate 6 is arranged at a portion 2a (hereinafter simply referred to as a light receiving portion) of the detector which receives the radiation at right angles to the radiation irradiation direction and the X direction, and the positional relationship between the radiation source 1 and the light receiving portion 2a. This reduces output fluctuations caused by changes in.

FIG. 7 (a) is a plan view showing the relationship between the radiation source 1, the light receiving portion 2a, and the absorbing plate 6, and the absorbing plate 6 is located at the center of the light receiving portion of the detector at right angles to the X direction. The source is located at the center of the light receiving section. The absorption plate 6 has a length l longer than the diameter of the light receiving portion, and a width W larger than that of the radiation source and smaller than the diameter of the light receiving window.
It is attached to the center of the front of the light receiving section 2a,
A part of the strongest part of the radiation beam of the radiation source 1 is blocked to reduce the amount of radiation incident on the light receiving part 2a. The source 1 is usually wrapped in a metal box or the like as a safety measure, and the exit of the source box is covered with a thin metal plate or the like. Become. For this reason, the intensity of the radiation beam is strongest in front of the radiation source 1 and becomes weaker as the distance from the front increases.

FIG. 7 (b) shows the detector 2 in the X direction (left side as viewed).
A side view of the X ₁ shift state, R represents an equivalent dose of radiation. When such a shift occurs, the output becomes weaker because the left side moves away from the radiation source, but the strongest part of the radiation blocked by the absorption plate 6 irradiates the light receiving surface on the right side. So the output will be stronger. Therefore, the total amount of radiation received by the light receiving unit does not change, and no output fluctuation due to the deviation occurs.

FIG. 7 (c) shows that the detector is z _{1 in the} Z direction (upward in the figure).
This is a side view showing a shifted state. In this example, since the light receiving surface is close to the radiation source, the portion not covered by the absorbing plate 6 acts to increase the output, and at the same time, the absorbing plate 6 is wider than the strong radiation portion. Since it is covered, the total amount of radiation does not change, and the output does not fluctuate due to the deviation.

According to the above configuration, the total amount of radiation can be made substantially the same even if the relationship between the radiation source and the detector moves in the X and Z directions. Note that the illustrated absorbing plate cannot cope with the displacement in the Y direction.

<Problems to be solved by the invention> However, in the above-mentioned conventional radiation applied measurement device, the sensitivity of the detector is reduced to 1/2 to 1 / since the radiation sensitivity is adjusted by using an absorbing plate in front of the detector. There was a problem that it would drop to 5. When an absorbing plate is used, its location must be determined by trial and error according to the properties of the measured object.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the related art, and has a structure in which a detection element is formed of a plurality of semiconductors, and the detection element is arranged so that a portion located at the center of the radiation source becomes a dead zone. It is an object of the present invention to realize a radiation applied measurement device in which the influence of output fluctuation is reduced by providing a plurality of the light sources and flattening the intensity of radiation irradiating the light receiving unit.

<Means for Solving the Problems> The configuration of the present invention for solving the above problems is described in claim 1.
With respect to the above, in a radiation applied measurement apparatus which detects radiation emitted from a radiation source and penetrates an object to be measured by a semiconductor radiation detector and measures a physical quantity of the object to be measured, the detection of the semiconductor radiation detector A plurality of rectangular detection elements having similar in-plane sensitivity are arranged on the same plane so as to form a square, and a dead zone is formed at a butt portion located at the center of the radiation source. A radiation applied measurement apparatus characterized by being arranged in parallel to a moving direction of a measurement object. According to claim 2, the radiation object is irradiated from a radiation source in which a spatial intensity distribution of the radiation has a Gaussian distribution. In a radiation applied measurement device which detects transmitted radiation with a semiconductor radiation detector and measures the physical quantity of the object to be measured, the radiation intensity on the detection surface of the radiation detector is measured. With degrees to several equally spaced said radiation source so as to be substantially uniform, the expected X of the radiation source and the radiation detector, when the maximum deviation of the Y direction is x _2, the radiation detecting The range of the radiation intensity on the detection surface of the vessel is at least the maximum deviation width x ₂
Is larger than the range including.

<Operation> Regarding claim 1, the shape of the semiconductor radiation detector is rectangular, and the dead zone is arranged in the strongest part of the radiation.
Even if the output is reduced due to the displacement between the radiation source and the detector, the strongest part of the radiation located in the dead zone irradiates the detector, so that the fluctuation of the output can be prevented.

According to the second aspect, since the intensity distribution of the radiation source causing the output fluctuation is made flat, the output does not fluctuate even if the radiation source and the detector are shifted.

<Embodiment> FIG. 1 is a plan view showing a detection element according to an embodiment of the present invention, which is a radiation detector such as Si or CdTe. In the figure, reference numerals 10 and 11 denote first and second rectangular photodiodes, which are arranged in parallel with a gap (dead zone) 13 therebetween. The X direction is arranged at right angles to the paper movement direction, and the Y direction is arranged at right angles to the paper movement direction. Also, the detection element has a uniform detection sensitivity.

First, the detector is a square with a side length of 2a, and the relationship between detector size and output when there is no dead zone is examined.

If the sensitivity distribution of the detector is uniform, the output can be represented by the volume of the radiation intensity distribution on the detector surface.

Gaussian distribution is generally Is represented by

Also, the cut of a Gaussian rotator cut by a plane parallel to the Z axis has a Gaussian distribution, and its curve is Is represented by

Then the volume of the case where the center and the center of the intensity distribution of the radiation detector are matched to V ₁ So, substitute the expression into this expression Becomes here If you put Becomes Becomes

In general Since dt cannot be calculated, the following approximate expression is used.

However, μ = 1 / (1 + px) and | ε (x) | ≦ 2.5 × 10 ^-5 Where p; 0.47047, a ₁ ; 0.3480242, a ₂ ; −0.0958798, a ₃ ; 0.7478556 where Then, V ₁ = 2π {f (a)} ² ...

Next, let V ₂ be the volume when the center of the detector and the center of the source are shifted by Δa in the y-axis direction. Becomes When this equation is expanded in the same manner as the above equation, V ₂ = π · f (a) {f (a−Δa) + f (a + Δa)}...

Therefore, the ratio of the output depending on the presence or absence of the displacement between the detector and the radiation source can be expressed by the following equation.

V ₂ / V ₁ × 100 (%)... FIG. 2 shows the result of calculating an equation by setting one side of the detector to 2a and changing the length of a. As can be seen from the figure, the output fluctuation decreases as a increases.

Further, for example, when the length of the side of the detector is 33 mm and the allowable value of the variation based on the shift between the radiation source and the detector is 0.04%, the allowable value of the shift is about 1.5 mm.

In FIG. 1, it is assumed that the radiation source is located above the intersection of the X and Y coordinates, and the width of the dead zone is b in the + direction and -b in one direction.

An output when there is no relative displacement of the detector and the radiation source in the above configuration and V _3, the output when shifted by d in the Y-axis direction
Assuming that V ₄ , the output ratio can be expressed as follows from the equation.

Fig. 3 shows the case where β-rays are used as radiation, the distance between the source and the detector is 12.5mm, and two 16.4mm × 33mm Si photodiodes are used as the detector and the dead zone width is 0.2mm (ie,
The detector has a square of 33 mm on each side (2a) and a dead zone of 0.2 mm in the center) and the output ratio (V ₄ / V ₃ ) with respect to the displacement of the radiation source.

As is clear from the figure, when 2a = 34 mm and 2a = 32 mm, the output fluctuation increases as the deviation increases (for example, when 2a = 34, the output fluctuation exceeds 0.04% when the deviation becomes 2.0 mm). In the case of 2a = 32, the output fluctuation far exceeds 0.04% when the deviation becomes 3 mm).
On the other hand, in the case of 2a = 33, the maximum output fluctuation is about 0.024% even if the deviation becomes 3 mm.

The sensitivity drop of the detector due to the provision of the dead zone is 2
%, Which can be remarkably improved as compared with the case where a conventional absorption plate is used.

In this embodiment, two rectangular detection elements are used and formed into a square having a dead zone of 0.2 mm. However, the present invention is not limited to this example. For example, a detector having a cross-shaped dead zone by combining four square detection elements is used. It may be. In that case, the output fluctuation due to the shift in the X direction can be reduced.

Note that the width of the dead zone is selected to be optimal according to the intensity of the radiation source and the distance between the radiation source and the detector. In addition, it is difficult to manufacture a semiconductor device having a large area and uniform in-plane sensitivity. However, the split structure of the detector facilitates the manufacture of the device and improves the yield. is there.

In this case, an error with respect to the vertical (Z) displacement between the radiation source and the detector is not discussed, but generally, the vertical displacement is small.

4 (a) and 4 (b) are explanatory diagrams according to claim 2, wherein the number of radiation sources is n (16 in the figure). As the case β-ray source using a small radiation source, for example, of about 4mm in diameter, but the distance x ₀ of the inter-magnitude a and the source of the source is determined by the distance from the source to the detector. That is, the diameter (or one side) of the detector is d, and the maximum deviation width between the detector and the source is
flat portion D of the intensity of β rays When x ₂ may be determined so as to satisfy the D = na + (n-1 ) x 0 D> d + x 2.

<Effects of the Invention> As described above in detail with the embodiments, according to the present invention, since a dead zone is formed at the abutting portion located at the center of the radiation source, it is possible to prevent displacement in the X and Y directions. It is possible to realize a radiation application measuring apparatus with a small output fluctuation and a small rate of decrease in sensitivity. In claim 2, the intensity distribution from a source that causes an error is flattened, so that a radiation free from errors in principle is obtained. An applied measuring device can be realized.

[Brief description of the drawings]

FIG. 1 is a plan view showing an embodiment of a detecting element of a radiation detector according to claim 1 of the present invention, and FIG. 2 shows a relationship between a deviation between the radiation source and the detector and an output fluctuation when there is no dead zone. FIG. 3 is a diagram showing the relationship between the deviation between the radiation source and the detector and the output fluctuation when there is a dead zone. FIG. 4 is a plan view of the radiation source according to claim 2 of the present invention (a) and β-rays. Fig. 5 shows the relationship of the intensity (b), Fig. 5 illustrates the positional relationship between the radiation source and the detector, Fig. 6 shows the intensity distribution of the radiation,
FIG. 7 is an explanatory view of a conventional example. 1 ... radiation source, 2 ... detector, 3 ... object to be measured, 10 ... first radiation detector, 11 ... second radiation detector, 13 ... dead zone.

Claims (2)

(57) [Claims] 1. A radiation applied measurement apparatus for detecting radiation emitted from a radiation source and passing through a measured object by a semiconductor radiation detector and measuring a physical quantity of the measured object, wherein the semiconductor radiation detector In the detection element, a plurality of rectangular detection elements having the same in-plane sensitivity are arranged so as to form a square on the same plane, and a dead zone is formed at an abutting portion located at the center of the radiation source. A radiation application measuring apparatus, wherein the apparatus is arranged in parallel to a moving direction of the object to be measured. 2. A radiation which is irradiated from a radiation source having a spatial intensity distribution of Gaussian distribution of radiation and transmitted through a measured object by a semiconductor radiation detector and measures a physical quantity of the measured object. In the applied measurement device, the radiation sources are arranged at equal intervals so that the radiation intensity on the detection surface of the radiation detector is substantially uniform, and the expected X and Y directions of the radiation source and the radiation detector are set. maximum when the displacement width is x ₂ of the radiation detector in the range of the radiation intensity of the detection surface of the radiation application measuring apparatus, which is a larger than the range including at least the maximum deviation width x _2.