JPH0512673B2 - - Google Patents

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) The present invention is a device in which three or more glass elements for a fluorescent dosimeter are each covered with a predetermined filter to reduce the amount of fluorescence after exposure to radiation. The present invention relates to a composite element for a fluorescent glass dosimeter that enables highly accurate separate measurement of gamma rays and neutron rays by sequential measurement.

(Prior Art) Fluorescent glass dosimeters generally use a glass element made of phosphate glass containing silver ions (hereinafter referred to as silver-activated phosphate glass).
This glass element emits fluorescence when it is activated by exposing it to radiation and then excited by ultraviolet light with a wavelength of 300 to 400 nm.The fluorescence intensity is measured by taking advantage of the fact that the fluorescence intensity at this time is proportional to the exposure radiation dose. By doing this, the exposure radiation dose is determined.

In measuring such radiation dose, the light emitted from the excitation ultraviolet light source is passed through an optical filter to selectively extract ultraviolet rays of a predetermined wavelength, and the ultraviolet rays are incident almost perpendicularly onto one surface of a rectangular parallelepiped glass element. Then, the silver-activated phosphate glass of the glass element emits fluorescence due to this ultraviolet rays, and this fluorescence is extracted in a direction perpendicular to the incident ultraviolet rays, and light in a predetermined wavelength range is selectively extracted through an optical filter for photoelectron multiplication. Fluorescence intensity is measured from an output signal obtained by photoelectric conversion using a photoelectric conversion element such as a tube.

However, among various types of radiation, thermal neutron rays have a strong interaction with matter, and gamma rays are emitted as a result of this interaction, so it is rare for thermal neutron rays to exist alone, and they are usually mixed with gamma rays. There is.
Since thermal neutron rays and gamma rays have different effects on the human body, it is necessary to separate and measure thermal neutron rays and gamma rays at the same time with high precision in managing the personal exposure of radiation workers. .

The following two methods are used to measure thermal neutron beams using conventional fluorescent glass dosimeters.

(1) Prepare two glass elements as a set, cover one element with a cadmium (Cd) filter, and the other element with a tin (Sn) filter, so that the cadmium absorbs the neutron beam. A method that uses the so-called n, gamma reaction, which emits gamma rays, to measure the difference in sensitivity between two glass elements.

(2) A method in which two glass elements with different sensitivities to thermal neutrons are prepared as a set and measurements are made based on the difference in sensitivity between them.

(Problems to be Solved by the Invention) Among the conventional methods described above, the filter coating method (1) is based on the method in which gamma rays emitted by cadmium due to n, gamma reactions are incident on a glass element partially coated with tin. This has the disadvantage of causing errors.

In addition, since the above method (2) using two elements with different sensitivities measures two types of glass made of different materials, in order to read the fluorescence of glass,
When using a nitrogen gas laser reader like the one shown in Publication No. 38352, if the two glasses have different fluorescence attenuation characteristics, it is necessary to set the fluorescence sampling time for each, which is a drawback that can easily cause errors. There is.

In addition, although both of the above methods can separate thermal neutrons and gamma rays, it is not possible to evaluate the energy of gamma rays, and there are large errors associated with the separation of gamma rays, which have different sensitivities depending on energy. .

[Structure of the invention]

(Means for Solving the Problems) The present invention provides, in a composite element used in a fluorescent glass dosimeter, three or more glass elements that are activated by radiation exposure and excited by ultraviolet light to emit fluorescence. At least one glass element is covered with an n/gamma conversion filter that absorbs thermal neutron beams and emits gamma rays, and at least two of the other glass elements transmit thermal neutron beams and have a transmittance for low-energy gamma rays of 200 KeV or less. By coating each of two types of n and gamma transmission filters with different values, it is possible to separate and measure thermal neutron beams and gamma rays with high precision.

(Function) The glass element for a fluorescent dosimeter is activated when exposed to radiation, and emits fluorescence when excited by ultraviolet light. At this time, the luminous efficiency of fluorescence is directly proportional to the amount of radiation exposed. As mentioned above, three or more glass elements of the same type are prepared and each n and γ
Conversion filter, type 1 n with high gamma ray transmittance,
γ transmission filter and second filter with low gamma ray transmittance
Since the glass element is coated with either a type n or gamma transmission filter, if the fluorescence emission efficiency of the entire glass element is measured after exposure to radiation, the respective exposure doses of neutron rays and gamma rays can be calculated mathematically from the obtained values. can.

(Example) The details of the present invention will be explained by referring to the illustrated example. Figure 1 shows a cross section in the assembled state, 1
2 is a box-shaped case made of synthetic resin; 2 is a holder housed in the case 1; 3a, 3b, and 3c are three glass elements arranged in parallel on this support frame 2; 4 is a second and third glass element; A gamma ray shielding body disposed between the glass elements 3b and 3c, 5 and 5 are first type n and γ transmission filters with high gamma ray transmittance that cover both the front and back surfaces of the first glass element 3a; A second type n, γ transmission filter with a low gamma ray transmittance covers both the front and back surfaces of the glass element 3b of No. 2, and n, γ conversion filters 7, 7 cover both the front and back surfaces of the third glass element 3c.

The case 1 has a bottom body 11 and a lid body 12 integrally molded from synthetic resin, as shown in FIGS. 2 and 5.
The two bodies 11 and 12 are substantially symmetrical, and three filters 5, 6, and 7 are attached to their inner surfaces as will be described later.

The holder 2 is made of synthetic resin, etc., and has external dimensions to be accommodated in the case 1, as shown in FIG. 4, and has three glass elements 3a, 3b, 3c along one long side. The three accommodation holes 21a, 21b, 21c are bored so that their long sides are parallel, and the middle of the second and third accommodation holes 21b, 21c is particularly widened to form a gamma ray shield. A shielding hole 22 for accommodating 4 is bored. Moreover, it has a display surface 23 along the other long side, and here the types of glass elements 3a, 3b, 3c,
An optical display 24 is provided to indicate the number, worker ID number, etc.

The above three glass elements 3a, 3b, and 3c are all made of 60% by weight of aluminum metaphosphate, 20% by weight of sodium metaphosphate, and 20% by weight of sodium orthophosphate as seen in Japanese Patent Publication No. 10333/1983.
As shown in Figure 3, a glass having a composition of 0.3% by weight of silver metaphosphate is added to
It is cut into a rectangular parallelepiped shape of ×7 × 3 mm and polished.It has almost no sensitivity to thermal neutron beams, and if activated by exposure to gamma rays,
It emits fluorescence when excited by ultraviolet light of m. Then, each accommodation hole 21a, 21 of the holder 2
b and 21c, respectively. Note that each storage hole 21a, 21b, 21c of the holder 2 is
The edges may be lowered so that the glass elements 3a, 3b, 3c can be easily attached and detached.

The gamma ray shield 4 is made of lead, for example, and has a size of 10×2×5 mm, for example, as shown in FIG.
It is cast in the shape of a rectangular parallelepiped and has the property of blocking gamma rays well. In this way, it is tightly fitted and held in the shielding hole 22 described above.

The first type n and gamma transmission filters 5, 5 are, for example, aluminum plates with a thickness of 1 mm, and are
It has the property of transmitting low-energy gamma rays of 200 KeV or less well. The second type of n, gamma transmission filters 6, 6 are, for example, tin plates with a thickness of 1 mm, which transmit neutron beams well, and the transmittance of low energy gamma rays of 200 KeV or less is much lower than that of aluminum plates. Furthermore, the n and gamma conversion filters 7, 7 are, for example, cadmium plates with a thickness of 1 mm, and have the property of absorbing neutron beams well and emitting gamma rays. On the inner surfaces of the bottom body 11 and the lid body 12 of the case 1, the first type n, γ transmission filters 5, 5 are connected to the first glass element 3a, and the second type n, γ transmission filters 6, 6 are connected to the first glass element 3a. N and γ conversion filters 7, 7 are fixed to the second glass element 3b with their sizes and positions determined so as to face each of the third glass elements 3c and sufficiently cover the front and back surfaces thereof. Note that the size and position of the gamma ray shielding body 4 are determined so that the gamma rays emitted from the n, γ conversion filter 7 do not enter the second glass element 3b.

Since the composite element of this embodiment is constructed as described above, when exposed to radiation, this radiation passes through each of the filters 5, 6, and 7, and the glass element 3
a, 3b, and 3c, and show different sensitivities depending on the characteristics of the respective filters 5, 6, and 7.
FIG. 6 shows the energy dependence of the sensitivity of each glass element 3a, 3b, 3c to gamma rays at this time. The figure shows gamma ray energy (KeV) on the horizontal axis.
The curve A is the sensitivity curve of the first type n, the first glass element 3a covered with the gamma transmission filter 5, that is, the aluminum plate, and the curve B is the sensitivity curve of the second glass element 3a covered with an aluminum plate. The sensitivity curve of the second glass element 3b covered with the type n, γ transmission filter 6, i.e., a tin plate, and the curve C, the sensitivity curve of the third glass element 3c, covered with the n, γ conversion filter 7, i.e., a cadmium plate. Each is shown below.

Also, a third filter covered with n, γ conversion filter 7
The sensitivity of the glass element 3c to thermal neutron beams is 1.6 times when the sensitivity to gamma rays is 1.0. On the other hand, the sensitivity of the first and second glass elements 3a and 3b covered with the first type n, gamma transmission filter 5 and the second type n, gamma transmission filter 6 to thermal neutron beams is almost zero.

The gamma ray dose, gamma ray energy, and thermal neutron dose are determined as shown below from the sensitivity characteristics of each of the glass elements 3a, 3b, and 3c to gamma rays and thermal neutron rays as described above.

In FIG. 6, from the sensitivity curve A of the first glass element 3a and the sensitivity curve B of the second glass element 3b, a curve D showing the most uniform dose response for each gamma ray energy is determined using the following equation. put.

D=(aA+B)/b Here, a and b are coefficients.

Therefore, the reading of the first glass element 3a
The gamma ray dose value Rγ is determined from Ra and the read value Rb of the second glass element 3b using the following equation.

Rγ=(aRa+Rb)/b Also, as shown in FIG. 7, a curve A/ From B.
It is possible to evaluate the energy of low-energy gamma rays below 200KeV.

Furthermore, to determine the thermal neutron dose, it is necessary to subtract the mixed gamma ray dose. In the above method,
The gamma ray sensitivity c of the third glass element 3c at the determined gamma ray energy is expressed by curve C in FIG.
Then, the thermal neutron dose Rn can be determined by the following formula.

Rn=(Rc−cRγ)/1.6 Also, as described above, a gamma ray shield 4 is provided between the third glass element 3c and the second glass element 3b.
By arranging the n, gamma conversion filter 7, that is, the cadmium plate absorbs neutrons and emits gamma rays. The amount incident on 3b was made so small that it could be almost ignored.

In the present invention, the first type n, γ transmission filter and the second type n, γ transmission filter are not limited to the above-mentioned aluminum plates and tin plates, but may be made of other metals or nonmetals. It is sufficient to use a combination of two types of materials that transmit thermal neutron beams well and have different gamma ray transmittances. Similarly, the n and γ conversion filters may be made of a material that absorbs thermal neutron rays and emits gamma rays. Any number of combinations of these filters and glass elements may be used in parallel. Furthermore, the gamma ray shielding material is not limited to lead, and other materials may be used as long as they have the same effect.

Further, in the present invention, a case or a holder is not essential; for example, three glass elements may be covered with respective filters and housed in a synthetic resin case or bag. Furthermore, each glass element may be housed in a separate case. In addition, instead of the gamma ray shield, each glass element may be appropriately positioned or another object may be interposed.In short, the scattered gamma rays emitted from the n and γ conversion filters enter other glass elements. It is best not to deviate the measured value by doing so.

〔Effect of the invention〕

As described above, the composite element for a fluorescent glass dosimeter of the present invention includes three or more glass elements that are activated by radiation exposure and excited by ultraviolet light to emit fluorescence, and at least one of the glass elements. An n, gamma conversion filter covers the surface and absorbs thermal neutron beams and emits gamma, and the surfaces of two or more glass elements that are not covered by the n, gamma conversion filters are covered, and both of them transmit thermal neutron beams. and 200KeV
Two types of n and gamma transmission filters with different transmittances for low-energy gamma rays are provided, so they can also be used for personal exposure control dosimeters in controlled areas where thermal neutron beams are handled, and can transmit both gamma rays and thermal neutron beams. It has the advantage of being able to be measured separately with high precision.

[Brief explanation of drawings]

FIG. 1 is a sectional view of an embodiment of the composite element for a fluorescent glass dosimeter of the present invention, FIG. 2 is a perspective view of the lid of the case, and FIG. 3 is a perspective view of the glass element and shield. Figure 4 is a perspective view of the holder, Figure 5 is a perspective view of the bottom of the case, and Figure 6 is a perspective view of the bottom of the case.
The figure is a graph showing the sensitivity characteristics of each glass element to gamma ray energy, and FIG. 7 is a graph showing the sensitivity ratio between curve A and curve B in the graph of FIG. 1... Case, 11... Bottom body, 12... Lid body,
2... Holder, 3a, 3b, 3c... Glass element, 4... Shielding body, 5, 6...n, γ transmission filter, 7...n, γ conversion filter.

Claims (1)

[Claims] 1. Three or more glass elements for fluorescent dosimeters that are activated by radiation exposure and excited by ultraviolet light to emit fluorescence, and the surface of at least one of the glass elements for fluorescent dosimeters. an n- and gamma-conversion filter that absorbs thermal neutron beams and emits gamma rays; What is claimed is: 1. A composite element for a fluorescent glass dosimeter, comprising two types of n and gamma transmission filters having different transmittances for low-energy gamma rays of 200 KeV or less. 2 Three or more glass elements for fluorescent dosimeters are arranged in parallel, and a shielding body for shielding gamma rays is provided between the glass element for fluorescent dosimeters covered with n, gamma conversion filters and other glass elements for fluorescent dosimeters. A composite element for a fluorescent glass dosimeter according to claim 1, characterized in that it is provided with a fluorescent glass dosimeter.