JP5995246B2 - X-ray or gamma ray detector - Google Patents

Description

  The present invention relates to a radiation detector used for radiation transmission imaging and the like. More specifically, the present invention relates to a novel radiation detector capable of simultaneously detecting radiation with a high count rate and collecting energy information.

  X-rays and gamma rays, which are electromagnetic radiation (hereinafter simply referred to as radiation), have a high transmission capability, so they are widely used for non-destructive inspection and crystal structure analysis in the industrial field, including diagnosis in the medical field. Yes. In recent years, as a radiation transmission imaging method using the radiation transmission capability, instead of X-ray imaging using a photosensitive action on a photographic plate, the radiation excitation action on a detection medium is electrically detected, and the digital result is obtained based on the detection result. What gets images has become mainstream. When radiation energy is applied to the inside of the radiation detector, depending on the type of detection medium, for example, an electron / hole pair for a semiconductor, an electron / ion pair for a gas, and a scintillator In the case of fluorescence and superconductors, excitons such as quasi-electrons are generated. In the radiation detector, these excitons are converted into a voltage corresponding to the applied energy in the detection unit, whereby the energy of the radiation transmitted through the human body or the like can be measured.

  However, when the radiation detector described above is used, the induced voltage is measured every time the radiation is incident. Therefore, when the next radiation is incident while the excitons are moving to the electrode, There is a risk of recognizing it as a single radiation. As a result, the dose of radiation incident per unit time (hereinafter also simply referred to as “dose”) is limited, and it is extremely high in a short time as in conventional radiation transmission imaging methods (such as X-ray CT scan). When measuring a dose of radiation, there is a problem that the energy information of the radiation cannot be used. On the other hand, when the dose is limited to a low dose, a radiographic image cannot be obtained in a short time. It was extremely difficult to obtain.

  Therefore, in the conventional radiographic imaging apparatus, the amount of X-rays transmitted per unit time is measured by detecting the current flowing through the electrodes, rather than measuring the energy of individual X-rays. The method was taken (refer patent document 1).

  On the other hand, when an image for determining the presence or absence of cancer tissue is obtained using an iodine contrast agent, only a small part of the irradiated X-rays are absorbed by iodine, so it is necessary to irradiate a large amount of X-rays on the human body. There was a problem of increasing the risk of radiation exposure. In view of this, a low-exposure type X-ray transmission imaging apparatus has been developed that employs an energy difference method that utilizes the difference in X-ray absorption between the human body and iodine. In this type of X-ray transmission imaging apparatus, those using white X-rays and those using two types of monochromatic X-rays are generally used. However, in the past proposed by the present inventors, a lanthanum filter (La filter) is used. ) Irradiate the human body with filter X-rays from which the high-energy portion has been cut, and measure the filter X-rays transmitted through the human body in the upper and lower energy ranges with the K shell absorption edge of iodine in between. By taking the difference between the two types of energy information obtained, an image in which only the contrast agent is emphasized is obtained (see Patent Document 2).

  However, the radiation detector of Patent Document 1 can cope with a high dose, but information on energy imparted from individual radiation is lost, so there is, for example, conventional X-ray transmission imaging. Only information about whether the amount of X-rays that have passed through the tissue is large or small can be obtained. Therefore, it has been difficult to apply such a radiation detector to the energy difference method of Patent Document 2.

  In view of such a problem, the present inventor has detected a detection medium that generates a charge by energy applied from incident radiation and a position at which the distance from the radiation incident end of the detection medium is different from each other at the detection medium. The radiation detector provided with the several electrode installed is disclosed (refer patent document 3). In addition, a radiation detector is disclosed in which a large number of radiation detection elements including CsI (Tl) scintillators and photodiodes are arranged along the radiation incident direction (see Non-Patent Document 1).

JP-A-2005-77152 JP 2004-223158 A JP 2007-71602 A

I. Kanno, et al. "A Current-Mode Detector for Unfolding X-ray Energy Distribution", Journal of NUCLEAR SCIENCE and TECHNOLOGY, Vol. 45, no. 11, p. 1165-1170 (2008).

Although the radiation detector described in Non-Patent Document 1 has an excellent feature that it can detect a high dose of radiation and collect energy information at the same time, the CsI (Tl) scintillator is an index of radiation stopping power. × Z _eff ⁴ (where d represents the density (g / cm ³ ) of CsI (Tl), Z _eff represents the effective atomic number of CsI (Tl)) is as high as 38 × 10 ⁶ , and the radiation incident end Since the scintillator near the position absorbs most of the radiation, the radiation incident on the scintillator located far from the incident end becomes extremely weak and the accuracy of the energy information is lowered. Further, the radiation detector described in Non-Patent Document 1 includes detection elements made of the same scintillator, and the response characteristics of each detection element to the energy of incident radiation are similar. In order to obtain with high accuracy, there is a problem that a great deal of calculation is required.

The present inventors have made various studies to solve the above problem. As a result, a scintillator that generates light by energy applied from incident radiation is used as a detection medium, and a plurality of light beams detected by the scintillator are detected at different positions from the radiation incident end of the scintillator. D × Z _eff ⁴ (where d represents the density of the scintillator (g / cm ³ ), where Z _eff is the index of the attenuation coefficient of radiation in the scintillator) By setting the scintillator's effective atomic number to 30 × 10 ⁶ or less in at least a part of the scintillator, it is possible to suppress the absorption of radiation by the scintillator near the incident end of the radiation, and at a position far from the incident end. The amount of radiation incident on the scintillator increases and the energy information becomes more accurate. There can be improved.

Further, by changing the d × Z _eff ⁴ of the scintillator at positions where the distances from the radiation incident end in the scintillator are different from each other, the response characteristic with respect to the energy of the incident radiation is changed according to each d × Z _eff ⁴ . It has been found that energy information can be obtained accurately and easily, and the present invention has been completed.

That is, according to the present invention, the light generated by the scintillator is detected at a position where the distance from the radiation incident end of the scintillator differs from that of the scintillator that generates light by energy applied from the incident radiation. A radiation detector comprising a plurality of photodetectors, wherein d × Z _eff ⁴ (where d is the density of the scintillator (g / cm ³ )), and Z _eff is the scintillator effective A radiation detector is provided in which the atomic number is 30 × 10 ⁶ or less. In the radiation detector, it is preferable that d × Z _eff ^{4 of the} scintillator is different at positions where the distances from the radiation incident end of the scintillator are different from each other.

According to the present invention, there is provided a scintillator comprising an alkaline earth metal halide, containing at least one rare earth element selected from Sm, Gd, Tb, Dy and Ho, and There is provided a scintillator characterized in that d × Z _eff ⁴ (where d represents the density (g / cm ³ ) of the scintillator and Z _eff represents the effective atomic number of the scintillator) is 30 × 10 ⁶ or less. The The scintillator can be suitably used as a detection medium for the radiation detector.

  ADVANTAGE OF THE INVENTION According to this invention, the radiation detector which can detect a high dose of radiation and collect energy information simultaneously, and can acquire energy information accurately can be provided. Such a radiation detector can be suitably used for a medical X-ray inspection apparatus using an energy difference method, an industrial X-ray inspection apparatus, a gamma ray inspection apparatus, or the like.

This figure is a schematic block diagram of an X-ray inspection apparatus using the radiation detector of the present invention. This figure is the schematic of the radiation detector of this invention. This figure is the schematic of the radiation detector of this invention. This figure is the schematic of the radiation detector of this invention. This figure is a graph showing an X-ray energy spectrum. This figure is a schematic diagram showing an embodiment of Example 1. FIG. This figure is a cross-sectional profile of the subject obtained in Example 1 and Comparative Example 1. This figure is a cross-sectional profile of the subject obtained in Example 1 and Example 2.

  Hereinafter, the radiation detector of the present invention will be described in detail with reference to the drawings. In the following description, the case where a transmission X-ray image is obtained using an iodine contrast agent will be specifically described. However, the radiation detector of the present invention is not limited to such applications, and various types using γ rays or the like. The same can be applied to other radiation inspection apparatuses.

  FIG. 1 is a schematic configuration diagram of an X-ray inspection apparatus using the radiation detector of the present invention, and FIGS. 2 to 4 are schematic diagrams of the radiation detector of the present invention. FIG. 5 is a graph showing an X-ray energy spectrum after the filter X-ray passes through iodine.

  As shown in FIG. 1, an X-ray inspection apparatus 1 according to the first embodiment includes an X-ray tube 2 and an X-ray detector array 4 in which X-ray detectors (radiation detectors) 3 are arranged vertically and horizontally. It comprises a preamplifier 5, a main amplifier 6, two types of integrators 7, 8, a contrast agent thickness calculator 9, an imaging device 10, and the like.

<Radiation detector>
Hereinafter, the radiation detector of the present invention will be described in detail. In the following description, referring to FIG. 2, a description will be given of an embodiment in which a plurality of scintillators are used and a plurality of photodetectors respectively corresponding to the plurality of scintillators are used. As shown in FIG. 3, a single scintillator is used. (SC) may be used, or a scintillator (SC) that is a single scintillator and is provided with cuts so as to separate portions corresponding to the respective photodetectors as shown in FIG. 4 may be used.

  Further, the number of photodetectors is not particularly limited as long as it is plural. However, since the energy information can be obtained with higher accuracy as the number of such photodetectors is larger, it is preferable that the number is three or more. On the other hand, if the number of photodetectors is excessively large, the cost for manufacturing the radiation detector increases, and therefore it is preferable that the number is six or less.

  In the present invention, the photodetector is not particularly limited, and a conventionally known photodiode, avalanche photodiode, photomultiplier tube, or the like can be appropriately used.

As shown in FIG. 2, the radiation detector 3 of the present invention includes a first scintillator SC1 to a fourth scintillator SC4 that are detection media, and a first photodetector 11 to detect light emitted from the scintillator. And a fourth photodetector 14. The first scintillator SC1 to the fourth scintillator SC4 are arranged in order along the X-ray incident direction, and each scintillator is covered with a reflector 15. The first photodetector 11 to the fourth photodetector 14 are arranged in order in the X-ray incident direction on the end face of the scintillator, and the currents I _{1 to} I ₄ output from the respective detectors are arranged. Each is sent to the preamplifier 5 described above.

  In the X-ray inspection apparatus 1 of FIG. 1, when X-rays are irradiated from the X-ray tube 2 to the subject S, the X-rays transmitted through the subject S are X-ray detectors 3 in the X-ray detector array 4. Is incident on. The X-ray irradiated from the X-ray tube 2 is not particularly limited. For example, an electron accelerated to 50 kV collides with a tungsten target, and a high-energy portion (38.9 keV or more) is emitted from the emitted white X-ray by a La filter. Filter X-rays obtained by removing () are preferably used.

When X-rays enter the X-ray detector 3, the first scintillator SC <b> 1 to the fourth scintillator SC <b> 4 generate light by the energy applied from the incident X-rays, and the first photodetector 11 to the fourth photodetector 14. Currents I _{1 to} I ₄ are respectively output from. The currents I _{1 to} I ₄ are amplified by the preamplifier 5 and the main amplifier 6, integrated by the integrators 7 and 8 for each energy region, and output to the contrast agent thickness calculator. In the contrast agent thickness calculator 9, the thickness of the iodine contrast agent in the subject S is calculated based on the integration results of the integrators 7 and 8, and the imaging device 10 generates a transmission X-ray image based on the calculation result. .

When X-rays are irradiated to a subject S in which an iodine contrast medium is injected into a lesion or blood vessel such as cancer, as shown in FIG. 5, the energy level (33.2 keV) near the K-shell absorption edge of iodine is discontinuous. The obtained X-ray energy spectrum is obtained. FIG. 5 is a graph of an X-ray energy spectrum obtained by irradiating water having a thickness of 20 cm imitating a human body with iodine imitating an iodine contrast medium and irradiating with filter X-rays. The spectrum is divided into four according to the X-ray energy, and the respective energy ranges are represented by e ₁ to e ₄ . Y _{1 to} Y ₄ represent the number of X-rays included in the energy range e _{1 to} e ₄ . In addition, in the above-mentioned Patent Document 2, the applicant of the present invention is based on the ratio of the number of X-rays Y ₂ and Y ₃ included in the energy ranges e ₂ and e ₃ in FIG. It has been disclosed that it is possible to determine the thickness.

The radiation detector 3 cannot correctly measure the energy of incident X-rays by the current output from a single photodetector. That is, in FIG. 2, since the X-rays of the entire energy range e _{1 to} e ₄ contribute to the current I ₁ output from the first photodetector 11, the current is measured only by the first photodetector 11. In this case, it is naturally impossible to measure how many X-rays in which energy range exist (Equation 1).

式１において、ｗはシンチレーターにおいて一つの光子を生成するために必要なエネルギであり、Ｔは測定時間であるが、以降の説明においては、説明が煩雑になることを防ぐため、ｗおよびＴはともに１とする（考慮しない）。また、Ｅ_ｊ（ｊ＝１〜４）は各エネルギ範囲の平均エネルギであり、Ｃ_ｉｊ（ｉ＝１〜４，ｊ＝１〜４）は平均エネルギＥ_ｊのＸ線がシンチレーターの第ｉ光検出器（１１〜１４）に対応する部位で吸収される相対確率を示す。

In Equation 1, w is the energy required to generate one photon in the scintillator, and T is the measurement time. In the following description, in order to prevent the explanation from becoming complicated, w and T are Both are set to 1 (not considered). E _j (j = 1 to 4) is the average energy in each energy range, and C _ij (i = 1 to 4, j = 1 to 4) is the i-th light of the scintillator where the X-ray of the average energy E _j is The relative probability absorbed by the site | part corresponding to a detector (11-14) is shown.

Equation 2 shows currents I _{1 to} I ₄ output from the first photodetector 11 to the fourth photodetector 14.

式２から判るように、電流Ｉ_１〜Ｉ_４は、シンチレーターの第ｉ光検出器（１１〜１４）に対応する部位で吸収されるＸ線の数に依存している。ここで、Ｅ_ｊ（ｊ＝１〜４）は既知の値であるが、Ｙ_ｉ（ｉ＝１〜４）およびＣ_ｉｊ（ｉ＝１〜４，ｊ＝１〜４）は未知数であり、式が４つで未知数が合計２０個となるため、そのままでは式２の連立方程式を解くことができない。

As can be seen from Equation 2, the currents I _{1 to} I ₄ depend on the number of X-rays absorbed at the site corresponding to the i-th photodetector (11 to 14) of the scintillator. Here, E _j (j = 1 to 4) is a known value, but Y _i (i = 1 to 4) and C _ij (i = 1 to 4, j = 1 to 4) are unknowns, Since there are four equations and a total of 20 unknowns, the simultaneous equations of Equation 2 cannot be solved as they are.

  By the way, the intensity of X-rays decreases exponentially when passing through a substance (Equation 3).

ここで、μ（Ｅ_ｊ）は放射線のエネルギに依存する減衰係数である。また、Ｘ_１−２は放射線がシンチレーター中を第１光検出器１１に対応する位置から第２光検出器１２に対応する位置まで通過した距離、Ｘ_２−３は放射線がシンチレーター中を第２光検出器１２に対応する位置から第３光検出器１３に対応する位置まで通過した距離、Ｘ_３−４は放射線がシンチレーター中を第３光検出器１３に対応する位置から第４光検出器１４に対応する位置まで通過した距離である。

Here, μ (E _j ) is an attenuation coefficient depending on the energy of radiation. Further, X _1-2 is the distance the radiation passes from the position corresponding to in the scintillator to the first optical detector 11 to a position corresponding to the second optical detector 12, X _2-3 is radiation through the scintillator second The distance that X _3-4 has passed from the position corresponding to the light detector 12 to the position corresponding to the third light detector 13, X _3-4 is the fourth light detector from the position corresponding to the third light detector 13 through the scintillator. 14 is a distance that has passed to a position corresponding to 14.

Therefore, C _ij can be obtained by obtaining in advance the absorption probability in the scintillator when the radiation of energy E _j is incident from the simulation code of the interaction between the radiation and the substance. As a result, the simultaneous equations of Expression 2 include _four unknowns Y ₁ to Y 4 and can be solved (Expression 4). As the simulation code, a code known in the art such as SANDII can be used as appropriate.

かかる計算によって得られた各エネルギ範囲における放射線の個数（Ｙ_１〜Ｙ_４）を用いて、エネルギ差分法を適用することにより、入射放射線のエネルギや被検体の厚さに関わらず、着目する成分を強調した画像を得ることができる。すなわち、例えばヨウ素造影剤を用いたＸ線ＣＴスキャンによる医療診断において、ヨウ素のＫ殻吸収端のエネルギ準位（３３．２ｋｅＶ）を挟むようにｅ_２とｅ_３を設定し、かかるエネルギ準位を挟んで高エネルギ側のＸ線の個数（Ｙ_３）を低エネルギ側のＸ線の個数（Ｙ_２）で除した値（Ｙ_３／Ｙ_２）を用いて画像化することにより、被検体の厚さ（被検者の脂肪量）に影響されることなく、病巣に蓄積したヨウ素造影剤を強調した画像を得ることができる。

By applying the energy difference method using the number of radiations (Y _{1 to} Y ₄ ) in each energy range obtained by such calculation, the component of interest can be obtained regardless of the energy of the incident radiation and the thickness of the subject. Can be obtained. That is, for example, in medical diagnosis by X-ray CT scan using an iodine contrast agent, e ₂ and e ₃ are set so as to sandwich the energy level (33.2 keV) of the K shell absorption edge of iodine, and the energy level is set. By imaging using the value (Y ₃ / Y ₂ ) obtained by dividing the number of high-energy X-rays (Y ₃ ) by the number of low-energy X-rays (Y ₂ ) An image in which the iodine contrast agent accumulated in the lesion is emphasized can be obtained without being affected by the thickness (the amount of fat of the subject).

Here, when the attenuation coefficient μ (E _j ) depending on the energy of the radiation is too large, a signal from a photodetector installed at a position far from the incident end of the radiation becomes very weak to obtain the energy information. The accuracy of the calculation is significantly deteriorated. More specifically, for example, in the above description, the signal I ₄ from the fourth photodetector installed at the position farthest from the radiation incident end is weak, and the fourth expression in the expression 2 is substantially Therefore, it is difficult to accurately obtain the solutions (Y _{1 to} Y ₄ ). The attenuation coefficient μ (E _j ) depends on d × Z _eff ⁴ of the scintillator, and μ (E _j ) decreases as d × Z _eff ⁴ decreases. Therefore, in the present invention, in order to obtain energy information with high accuracy, it is essential that d × Z _eff ⁴ of at least a part of the scintillator is 30 × 10 ⁶ or less. For example, in a mode in which a plurality of scintillators are used and a plurality of photodetectors respectively corresponding to the plurality of scintillators are used, d × Z _eff ⁴ of at least one scintillator among the plurality of scintillators is 30 × 10 ⁶ or less. It is essential. As a result, a sufficiently strong signal can be obtained from the photodetector installed at a position far from the radiation incident end, and the energy information of the radiation can be detected with high accuracy.

Note that it is not necessary that d × Z _eff ⁴ of all scintillators included in the X-ray detector 3 (see FIG. 1) be 30 × 10 ⁶ or less, but scintillators located on the radiation incident end side (in FIG. 2) Any or all of d × Z _eff ⁴ of scintillators other than SC4 are preferably 30 × 10 ⁶ or less.

Further, a variable set of _C i1 _{-C i4} corresponding to the i-th photodetector, and the second i 'variable _{C i'1 ~C i'4} corresponding to the light detector pairs, mutually independent Preferably there is. That is, when the variable sets are not independent from each other, the four expressions listed in Expression 2 are substantially three, and the solution (Y _{1 to} Y ₄ ) cannot be uniquely determined. , previously Y _i is to previously obtain a I _i using known calibrated X-ray source, to be carried out a lot of computation, such as using a combination of the known I _i and Y _i, solutions (Y _{1 to} Y ₄ ) cannot be obtained with high accuracy. On the other hand, when the sets of variables are independent from each other and show a tendency to deviate significantly, the solution (Y _{1 to} Y ₄ ) can be accurately obtained by simple calculation. In view of this point, it is preferable that the radiation detector of the present invention has different d × Z _eff ⁴ of the scintillator at positions where the distances from the radiation incident end are different from each other. Since the part corresponding to the i-th photodetector and the part corresponding to the _{i'th photodetector} differ in d × Z _eff ⁴ of the scintillator, a set of variables C _{i1 to} C _i4 and C _i′1 to C _{i It} shows a tendency that the set of variables of _i′4 is significantly different from each other. Further, in the case of the X-ray detector 3 used for the energy difference method, a scintillator on the side closer to the radiation incident end in order to obtain a sufficiently strong signal from the photodetector installed at a position far from the radiation incident end. embodiments of the d × Z _eff ^4, and a d × Z _eff ⁴ smaller value on the far side of the scintillator from the incident end of the radiation, as employed in example 2 below, is a preferred embodiment.

<Scintillator>
Hereinafter, the scintillator of the present invention will be described in detail.

The scintillator of the present invention comprises an alkaline earth metal halide, contains at least one rare earth element selected from Sm, Gd, Tb, Dy and Ho, and d × Z _eff ⁴ (where d is a scintillator) represents a density (g / cm ³⁾ of the, _{Z eff} represents an effective atomic number of the scintillator) is characterized in that it is 30 × 10 ⁶ or less.

  Such a scintillator can be suitably used in the radiation detector of the present invention because it has a high light emission intensity upon incidence of radiation and has an appropriate transparency to the radiation.

  Specific examples of the alkaline earth metal halides include fluorides, chlorides, bromides and iodides of Mg, Ca, Sr and Ba, and solid solutions thereof. Among the alkaline earth metal halides, fluoride is preferable because it is difficult to deliquescent and has excellent chemical stability.

Furthermore, Among the _{fluoride, CaF 2, (Ca x Sr} 1-x) F 2 ( provided that 0 <x <1.), SrF 2 and _{(Sr x Ba 1-x)} F 2 ( where Fluoride represented by 0.3 ≦ x <1) is particularly preferable. Such fluorides have good chemical stability, and in normal use, no performance deterioration is observed in a short period of time. Furthermore, mechanical strength and workability are also good, and it is easy to process and use it in a desired shape. In processing, a known cutting machine such as a blade saw or wire saw, a grinding machine, or a polishing machine can be used without any limitation.

The scintillator permeability to radiation is expressed by d × Z _eff ⁴ (where d represents the density of the scintillator (g / cm ³ ), and Z _eff represents the effective atomic number of the scintillator), A scintillator having permeability to radiation can be appropriately selected and used. In the present invention, the effective atomic number is an index defined by the following formula.

Effective atomic number = (ΣW _i Z _i ⁴ ) ^1/4
In the formula, W _i and Z _i are the mass fraction and atomic number of the i-th element among the elements constituting the scintillator, respectively.

In the scintillator of the present invention, the d × Z _eff ⁴ is 30 × 10 ⁶ or less. When the d × Z _eff ⁴ exceeds 30 × 10 ⁶ , when applied to the radiation detector of the present invention, the radiation incident on the scintillator at a position far from the incident end becomes extremely weak, and the accuracy of energy information is reduced. Although lowered occurs, at least a portion of the radiation detector, by d × Z _eff ⁴ to adopt scintillator of the present invention which is a 30 × 10 ⁶ or less, it is possible to reduce this problem . The lower limit of d × Z _eff ⁴ is not particularly limited, but is preferably 1 × 10 ⁶ or more. By setting d × Z _eff ⁴ to 1 × 10 ⁶ or more, the scintillator can sufficiently absorb the radiation, so that the emission intensity of the scintillator is increased, and as a result, the intensity of the signal output from the photodetector is increased. be able to.

  The scintillator of the present invention emits light when radiation is incident by the action of at least one rare earth element selected from Sm, Gd, Tb, Dy and Ho. Among the rare earth elements, Tb is preferable because Tb has a particularly high emission intensity and has an emission wavelength of about 450 to 700 nm and can be detected with high sensitivity by a photodiode generally used as a photodetector.

  The content when the rare earth element is contained in the alkaline earth metal halide is not particularly limited, but is preferably in the range of 0.1 to 10 wt%, particularly in the range of 1 to 5 wt%. preferable. By setting the content of the luminescent center element to 0.1 wt% or more, more preferably 1 wt% or more, the probability of light emission through the rare earth element is increased, and high emission intensity can be obtained. In addition, by reducing the content of the luminescent center element to 10 wt% or less, more preferably 5 wt% or less, it is possible to avoid a decrease in light emission due to concentration quenching.

  The scintillator of the present invention may use either a polycrystal or a single crystal, but by using a single crystal, loss due to light dissipation and non-radiative transition at the grain boundary can be suppressed, and high light emission can be achieved. It is preferable because strength can be obtained.

  In the present invention, the shape of the scintillator is not particularly limited, but is generally used in a prismatic shape. The scintillator preferably has a light exit surface (hereinafter also simply referred to as a light exit surface) facing the photodetector in the radiation detector, and the light exit surface is preferably optically polished. By having such a light emitting surface, the light generated by the scintillator can be efficiently incident on the subsequent photodetector.

  The shape of the light emitting surface is not limited, but it is preferable that the light emitting surface has a shape equivalent to the shape of the light receiving surface of the photodetector and is joined to the photodetector as shown in FIG.

  In addition, it is preferable to apply a light reflecting film made of aluminum, barium sulfate, Teflon (registered trademark), or the like to the surface of the scintillator that does not face the photodetector, because it can prevent the dissipation of light generated by the scintillator. It is an aspect.

  In the present invention, the production method of the scintillator is not particularly limited, but in general, a raw material mixture is prepared by mixing a powder of an alkaline earth metal halide and a rare earth element halide, and the raw material mixture is heated and melted. After caulking, a method of cooling and solidifying can be employed. When the scintillator is manufactured as a single crystal, the Bridgeman method, the temperature gradient solidification method (Gradient Freeze method), the Czochralski method, or the micro-pulling down method, which are conventionally known single crystal manufacturing methods, are applied as appropriate. be able to.

  Hereinafter, in the present invention, a method for producing a scintillator single crystal will be described in detail by taking the Bridgeman method as an example.

First, a raw material mixture is prepared by mixing powders of raw materials of alkaline earth metal halide and rare earth element halide. Next, the raw material mixture is filled in a crucible and set in a chamber equipped with a heater, a heat insulating material, and a vacuum exhaust device. After evacuating the interior of the chamber to 1.0 × 10 ⁻³ Pa or less using a vacuum exhaust apparatus, an inert gas such as high-purity argon is introduced into the chamber to perform a gas replacement operation. The pressure in the chamber after the gas replacement operation is not particularly limited, but atmospheric pressure is common. By such a gas replacement operation, moisture adhering to the raw material or the chamber can be removed, and problems such as a decrease in emission intensity of the scintillator derived from such moisture can be avoided.

  In order to avoid adverse effects due to moisture that cannot be removed even by the gas replacement operation, it is preferable to remove moisture using a scavenger that is highly reactive with moisture. As such a scavenger, a gas scavenger such as tetrafluoromethane can be suitably used. In addition, when using a gas scavenger, the method of mixing in the said inert gas and introducing in a chamber is suitable.

  After performing the gas replacement operation, the raw material mixture is heated and melted by a heater. The heating method of the heater is not particularly limited, and for example, a high frequency induction heating method, a resistance heating method, or the like can be appropriately used.

  Next, the melted raw material mixture is lowered together with the crucible. The heater and the heat insulating material are arranged so that the upper part is at a high temperature and the lower part is at a low temperature, and the melt is solidified from below as it descends. Further, by continuously lowering the melt, the melt is solidified in one direction from the bottom to the top, and the crystal generated at the bottom of the crucible grows upward, whereby a scintillator single crystal can be produced. .

  In the present invention, when the single crystal of the scintillator is manufactured, an annealing operation may be performed after the manufacture of the single crystal for the purpose of removing crystal defects caused by halogen atom loss or thermal strain.

  Hereinafter, the present invention will be specifically described with reference to examples, but the present invention is not limited to these examples. In addition, not all combinations of features described in the embodiments are essential to the solution means of the present invention.

Example 1
<Manufacture of scintillators>
First, 11 g of CaF ₂ and 0.6 g of TbF ₃ were mixed to prepare a raw material mixture. That is, in this example, a CaF ₂ single crystal containing 4 wt% Tb as a rare earth element was manufactured. In addition, the purity of each said raw material used the 99.99% or more raw material.
Next, the raw material mixture was filled in a carbon crucible and set in a chamber equipped with a resistance heating heater, a heat insulating material, and a vacuum exhaust device. Using a vacuum evacuation device, the inside of the chamber is evacuated to 2.0 × 10 ⁻⁴ Pa or less, and then a high-purity argon gas mixed with 5 vol% tetrafluoromethane is introduced into the chamber for gas replacement operation. Went. The pressure in the chamber after the gas replacement operation was atmospheric pressure.

  After performing the gas replacement operation, the raw material mixture was heated and melted by the heater. Next, the melt of the melted raw material mixture was continuously lowered together with the crucible, and the melt was solidified in one direction from below to above. In this example, the speed at which the melt was lowered was 1 mm / hr. By this operation, all of the melt was solidified and then gradually cooled to obtain a scintillator single crystal of the present invention.

The density of the single crystal was 3.2 g / cm ³ . The effective atomic number calculated from the chemical composition of the single crystal is 30. Therefore, d × Z _eff ⁴ of the scintillator made of CaF ₂ (Tb) of this example was 2.5 × 10 ⁶ .

  The obtained single crystal was cut into a rectangular parallelepiped scintillator having a size of 1.1 mm × 5.9 mm × 5 mm by cutting with a wire saw equipped with a diamond wire and optically polishing the cut surface. Among the optically polished surfaces, one surface having a size of 1.1 mm × 5.9 mm was used as a light emitting surface, and a light reflecting film made of Teflon (registered trademark) was applied to a surface other than the light emitting surface.

<Production and characterization of radiation detectors>
A radiation detector was manufactured using the manufactured scintillator made of CaF ₂ (Tb), and the characteristics of the radiation detector were evaluated.

  The radiation detector was manufactured by using four scintillators as shown in FIG. 2 and combining the photodetectors (11-14) with each of the scintillators (SC1-SC4). Note that a photodiode (S1337-16BR, manufactured by Hamamatsu Photonics), which is a silicon light receiving element, was used as the photodetector, and the light emitting surface of the scintillator was joined to the light receiving surface of the photodiode with transparent silicon grease. The size of the light receiving surface of the photodiode is 1.1 mm × 5.9 mm, which is the same as the size of the light emitting surface of the scintillator.

The characteristics of the radiation detector were evaluated by the following methods. First, as shown in FIG. 6, an X-ray source, a subject, and a radiation detector are installed to irradiate X-rays, and X-rays that have passed through the inside of the subject enter the radiation detector. Currents I _{1 to} I ₄ generated from 11 to the fourth photodetector 14 were measured. The subject is a cylinder made of acrylic resin with a diameter of 30 mm, and a hole with a diameter of 5 mm provided at the center of the cylinder is filled with an iodine solution. The iodine concentration of the iodine solution is adjusted so that the iodine thickness per 1 mm of the solution is 3 μm. Therefore, the iodine thickness corresponds to 15 μm in the diameter of 5 mm of the hole filled with the iodine solution.
The subject was moved every 0.4 mm in the direction indicated by the dotted line in FIG. 6, and currents I _{1 to} I ₄ at each position were measured. Next, the obtained currents I _{1 to} I ₄ were substituted into Equation 4, and the number of X-rays (Y _{1 to} Y ₄ ) in each energy range was obtained. The energy ranges are 15 to 33.2, 33.2 to 40, 40 to 80, and 80 to 120 keV, and E _{1 to} E ₄ that are average energies of the energy ranges are 24, 37, 60, and 100 keV, respectively. It was.

After scanning the entire width of the subject, the subject was rotated 30 degrees and scanned in the same manner as described above to obtain Y ₁ to Y ₄ at each position. Such scanning and rotation of the subject were repeated 6 times.

FIG. 7 shows a cross-sectional profile when the energy difference method is applied using Y _{1 to} Y ₄ at each obtained position and angular rotation angle. The horizontal axis in FIG. 7 shows the position where the center of the subject is 0 mm and transmits X-rays, and the vertical axis is the X-axis on the high energy side across the energy level (33.2 keV) of the K shell absorption edge of iodine. the number of lines to _{(Y 3)} shows a divided by the number of low energy X-rays side _{(Y 2) (Y 3 /} Y 2). The vertical axis indicates a value corrected so that Y ₃ / Y _{2 in} the air (when there is no subject) is zero. From FIG. 7, it can be seen that by using the radiation detector of the present invention, a result in which the thickness of iodine is emphasized can be obtained regardless of the thickness of the acrylic as the subject.

Further, using the Y _3, it was determined CT value of the subject. The CT value is a value representing the X-ray absorption coefficient of the subject. Here, the X-ray absorption coefficient with respect to the average value E _{3 in} the energy range of 40 to 80 keV, that is, μ (E ₃ ) in the equation 3 above. Represents. FIG. 8 shows a cross-sectional profile of the obtained CT value. The CT value at an iodine thickness of 15 μm is 0.7 (dashed line in FIG. 8).

Comparative Example 1
<Production and characterization of radiation detectors>
A radiation detector was produced in the same manner as in Example 1 except that a commercially available scintillator made of CsI (Tl) was used.

The characteristics of the radiation detector were evaluated by the same method as in Example 1. FIG. 7 shows a cross-sectional profile when the energy difference method is applied using the obtained Y ₁ to Y ₄ . In this comparative example, CsI (Tl) d × Z _eff ⁴ is as high as 38 × 10 ^6, and the radiation incident on the scintillator at a position far from the incident end becomes extremely weak and the accuracy of the energy information decreases. It can be seen that the obtained Y ₃ / Y ₂ varies greatly.

On the other hand, in Example 1 of the present invention, since d × Z _eff ⁴ of CaF ₂ (Tb) is as low as 2.5 × 10 ⁶ , a sufficient number of radiations are also emitted to the scintillator at a position far from the incident end. Since it is incident, the accuracy of energy information is improved, and it can be seen that the resulting variation in Y ₃ / Y ₂ is small.

Example 2
<Manufacture of scintillators>
2% by weight of Tb is contained in the same manner as in Example 1 except that 0.8 g of SrF ₂ , 10 g of BaF ₂ and 0.3 g of TbF ₃ are mixed to prepare a raw material mixture (Sr _0.1 Ba _0.9 ). It was produced F ₂ single crystal.

Further, a BaF ₂ single crystal containing 2 wt% Tb was produced in the same manner as in Example 1 except that 11 g of BaF ₂ and 0.3 g of TbF ₃ were mixed to prepare a raw material mixture.

The d × Z _eff ⁴ of the scintillator composed of (Sr _0.1 Ba _0.9 ) F ₂ (Tb) and BaF ₂ (Tb) was 36 × 10 ⁶ and 38 × 10 ⁶ , respectively.

<Production and characterization of radiation detectors>
The scintillator made of CaF ₂ (Tb) produced in Example 1 was used as SC1 and SC2, the produced (Sr _0.1 Ba _0.9 ) F ₂ (Tb) was used as SC3, and the produced BaF _{2 A} radiation detector was produced in the same manner as in Example 1 except that (Tb) was used as SC4.

The characteristics of the radiation detector were evaluated by the same method as in Example 1. FIG. 8 shows a cross-sectional profile of the obtained CT value. From FIG. 8, it can be seen that the iodine thickness can be correctly obtained by setting the scintillator d × Z _eff ⁴ to be different at positions where the distances from the radiation incident ends are different from each other as in this embodiment. In Example 1 as well, it was possible to obtain the same result as Example 2 by increasing the accuracy of calculation when obtaining the iodine thickness from Y ₃ / Y ₂ , but such calculation is extremely complicated. There was a huge number of calculations and time.

1 X-ray inspection device 2 X-ray tube 3 X-ray detector (radiation detector)
4 X-ray detector array 5 Preamplifier 6 Main amplifier 7 Integrator 8 Integrator 9 Contrast agent thickness calculation device 10 Imaging device 11 First photodetector 12 Second photodetector 13 Third photodetector 14 First 4 light detectors 15 reflector SC scintillator SC1 scintillator 1
SC2 scintillator 2
SC3 scintillator 3
SC4 scintillator 4

Claims (2)

A scintillator that generates light by energy applied from incident X-rays or gamma rays, and a plurality of lights that detect light generated by the scintillators at different positions from the X-ray or gamma ray incident end of the scintillator An x-ray or gamma ray detector comprising a detector, wherein d × Z _eff ⁴ (where d represents the density of the scintillator (g / cm ³ )), and Z _eff represents the scintillator An X-ray or gamma ray detector characterized by having an effective atomic number of 30 × 10 ⁶ or less. 2. The X-ray or gamma ray detector according to claim 1, wherein d × Z _eff ^{4 of the} scintillator is different at a position where distances from the incident end of the X-ray or gamma ray are different from each other.

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