JPS6327588A - Radiation image conversion process and fluorescent material used in said process - Google Patents

Abstract

PURPOSE:To obtain a clear radiation image with reduced X-ray exposure, by absorbing radiation in a specific alkaline earth metal halide doped with bivalent Eu, irradiating the activated halide with electromagnetic radiation and detecting the emitted fluorescent light. CONSTITUTION:Radiation passed through or emitted from a specimen is absorbed in a fluorescent material consisting of an Eu<++>-doped alkaline earth metal halide of formula (M is Ca, Sr or Ba; X and X' are Cl, Br or I; 0.5<=a<=1.8; 10<-4=b<=10<-2>) and having an absorption band of complex color center of 700-1,500nm. The excited fluorescent material is irradiated with electromagnetic radiation of a wavelength range of 700-1,500nm. The excited fluorescent material is irradiated with electromagnetic radiation of a wavelength range of 700-1,500nm to release the accumulated radiation energy from the fluorescent material in the form of fluorescent light, which is detected to effect the conversion of radiation image.

Description

[Detailed Description of the Invention] [Summary] In order to solve the large amount of X-ray exposure and blurred images in conventional radiography methods, a stimulated luminescent material made using a new process is used, and the oscillation wavelength is By using electromagnetic waves, such as semiconductor laser (LD) light, in the range of 700 to 1500 rv+, clear radiation images can be obtained with reduced X-ray exposure.

[Industrial application field]

The present invention relates to a radiation image conversion method, and particularly to a radiation conversion method using a new stimulated luminescence and LD excitation.

[Conventional technology]

Traditionally, the so-called radiographic method, which uses a combination of a radiographic film having an emulsion layer made of a silver salt photosensitive material and an intensifying screen, has been used as a method for obtaining radiographic images. Is there any power in the above radiographic method? For example, as one method of
A radiation image conversion method using a photostimulable photoreceptor as described in Japanese Patent No. 5-12145 is known and attracting attention. This method uses radiation that has passed through the subject,
Alternatively, radiation emitted from the subject is absorbed by a photostimulable photoreceptor, and then this phosphor is excited by electromagnetic radiation (excitation light) such as visible light or infrared rays.
The radiation energy stored in the photoreceptor is emitted as fluorescence (stimulated luminescence), the fluorescence is read photoelectrically to obtain an electrical signal, and this electrical signal is visualized.

° According to the above radiation image conversion method, compared to the case of using conventional radiography, the radiation exposure is much lower.
AIT has the advantage of being able to obtain X-ray images with a rich amount of information. Therefore, this radiation image conversion method is 1
．． It is particularly useful in direct medical radiography such as X-ray shadows for medical diagnosis.

Stimulable phosphors used in radiation image conversion methods include:
The composition formula: % formula %
(At least one of Cd, X is at least one of C1, Br and I, A is Eu.

Tb, Ce, Tm, Ur, Pr, Ho, Nd.

At least one of Yb and Er, X and y are
It represents a number that satisfies the following conditions: 0≦X≦0.6 and O≦y50.2. ) JP-A No. 60-84382 Riko has a composition formula of 8% () (However, Ml is at least one alkaline earth metal selected from the group consisting of Ba, Sr, and Ca;
and X' is at least one kind of halogen selected from the group consisting of C1, Br, and (X is a numerical value in the range of 0 < and y is a numerical value in the range of Q<y≦0.2).

Among the photostimulable phosphors represented by the above compositional formula, 1. Barium fluoride bromide phosphor (BaFB-r:Eu2°) activated by divalent europium has been used for 390n
It is known that fluorescence (luminescence, exhaustion) having a peak wavelength around m has high brightness and is very practical. In addition, the photostimulation excitation spectrum of the above phosphor is 5
It is known that the emission intensity reaches its maximum between 00 and 700B (see FIG. 5).

As mentioned above, the radiation image conversion method using a stimulable luminescent material is a very advantageous image forming method, but this method also reduces the exposure dose to the human body, or makes it easier to use electrical processing later. Due to the need to facilitate this process, it is desired to further improve its sensitivity.

Regarding the stimulable luminescence mechanism of BaFX:Eu'' (X=Cj!, Br) phosphor, please refer to page 23 of the 201st Annual Conference on Fluorescent Materials Presentation Proceedings, published on June 29, 1980. ~ page 30, and concludes the following.

1) Under conditions where sufficient ultraviolet rays are irradiated to exhibit stimulated luminescence, the luminescence of Eu" decreases and the luminescence of Eu3+ increases. In other words, a portion of Eu3+ is ionized into Eu". . Further irradiation with visible light returns it to its original state.

2) The production spectrum, photoconduction spectrum, and absorption spectrum of the stimulated emission center match.

3) Under conditions exhibiting photostimulation, (1) the photostimulated excitation spectrum, (2) the photoconductive excitation spectrum, (3) the absorption spectrum, and the signal attenuation spectrum of (41E'SR) coincide with each other.

The stimulated luminescence mechanism is explained as follows. Excitation process jUV
Alternatively, due to X-ray excitation, a part of Eu"° is ionized to Eu34, and electrons are raised to the conduction band (E u"
+ h v-*E u co ゛ + e)e The electron is captured at the F center via the conduction band (F''+e-F), stimulated luminescence process: By red light irradiation, the electron at the F center is captured at the F center via the conduction band. (F-F”+6). The electrons pass through the conduction band and become Eu3°, i.e. ionized Eu”°
returns to , is captured, and changes it to the excited state of Eu2゛ (Eu”+ e−E u ”) a then Eu”
UV emission occurs due to (E u ”−4Euz′
″+hv).

(Problems to be Solved by the Invention) In the conventional radiation conversion method described above, the peak of the photostimulation excitation spectrum of the photostimulable luminescent material used in the method is 500.
``Since the wavelength is 700 nm, the following problems arose.

Stimulated emission wavelength (approximately 400no+) and excitation light wavelength (500no+)
~700 nm'), the scattered light of the excitation light tends to become a noise source in the read signal, and therefore a filter is required to cut off the scattered light from the excitation light, which inevitably reduces the read signal intensity.

Furthermore, since a pumping light source laser having an oscillation wavelength of 500 to 70 Qnm, such as a He-Ne laser, is large in size, the overall device becomes large and the cost increases.

SUMMARY OF THE INVENTION An object of the present invention is to provide a radiation image conversion method that reduces radiation exposure and provides clear radiation images.

A further object of the present invention is to provide a radiation image conversion method that can be implemented with a compact device and a phosphor for use in this method. [Means for solving the problem] According to Zero Speech, the above problem is caused by the activation of divalent europium, which is expressed by the following compositional formula (1), by the radiation that has passed through the camphor object or has been emitted from the object. It is an alkaline earth metal halide and has an absorption band region of 700 to 1500 nm.
By absorbing the phosphor 5 having a complex color center of a, and then irradiating the phosphor with electromagnetic waves in the wavelength range of 700 to 1500 nm, the radiation energy accumulated in the phosphor is converted into fluorescent light. A radiation image conversion method characterized by emitting fluorescent light and detecting the fluorescent light.

Compositional formula (1): MX2・aMX2' : bEuz''' (where M is Ca
* At least one kind of alkaline earth metal selected from the group consisting of Sr + and Ba; X and X' are at least one kind of halogen selected from the group consisting of C1, Br and I, and a is 0.5≦ a≦1.8 and b is 10-4≦bs10-! ), which is solved by .

According to the present invention, the above-mentioned problem is solved by a phosphor represented by the compositional formula (1) and characterized by having an absorption band region centered on a composite color in a wavelength range of 700 to 1500 nm.

Compositional formula (1); MX2・aMX, ": b Eu" (where M is at least one alkaline earth metal selected from the group consisting of Ca, Sr, and Ba; X and X' are C1, Br, and At least one halogen selected from the group consisting of I, and a is 0.5≦a≦1.
8 and b is 10-4≦b≦10-2), which is solved by.

[For production]

In the radiation conversion method of the present invention, when a phosphor is irradiated with radiation, in addition to F (color) centers that have captured electrons in the lattice vacancies of halogen anions, the phosphor and,
Due to the formation of a composite color center, which is an aggregate of F centers and holes, it has reading sensitivity in the infrared wavelength range, and therefore has an oscillation wavelength in the wavelength range of 700"1500n- from the phosphor. LD can be used. In this case, L
As D, (GaAIAs, InGaAsP), etc. are preferable.

When the phosphor used in the X491 image conversion plate of the present invention absorbs X-rays, in addition to the lower center where electrons are captured in the lattice vacancies of the halogen anions, the F center and the lower center and the vacancies are formed. A composite color center, which is an aggregate of

〔Example〕 .

Embodiments of the present invention will be described below based on the drawings.

Example 1 Barium bromide (Ba Brz) 297.148g, barium chloride (Ba Cj!t) 208.246g
, 0.783 g of europium bromide (Eu Br5) and 1000 g of distilled water were placed in a reaction vessel and dissolved.
Evaporate to dryness at a temperature of 100°C (wet reaction), then 1
The mixture was obtained by drying at a temperature of 00-120'C.

Next, the above mixture was put into an alumina port and fired at 400 to 900°C in air, and then further fired at 700 to 900°C in a hydrogen-nitrogen mixed gas (reducing) atmosphere. , crushed and divalent europium activated barium chloride bromide phosphor (Ba CI Br: 10 Eu”)
I got it.

According to Japanese Patent Application Laid-open No. 60-84382, the conventional manufacturing method is to dissolve the raw materials in distilled water, dry under reduced pressure at 60°C for 3 hours, and then vacuum dry at 150°C for 3 hours, and then put the mixture in an alumina crucible to 2.M 900 in a carbon dioxide atmosphere containing carbon dioxide
C. for 1.5 hours.

Example 2 The fluorescent powder obtained in Example 1, an acrylic resin as a binder, and toluene as a solvent were mixed and dispersed to give a weight ratio of fluorescent powder to binder of 8:1 and a viscosity of 3.
A 6 ps (25° C.) coating solution was prepared.

This coating solution was uniformly coated on a glass plate with a thickness of 0.501111 mm, and then dried at 60 tons to form a phosphor layer with a thickness of 300 g on the support. was created.

The measurement results of the stimulated emission spectrum of the obtained plate are shown in FIG. 1, and the measurement results of the stimulated excitation spectrum are shown in FIG. 2. As shown in FIG.
The relative stimulated emission intensity is large in the wavelength region of , and since it is far from the stimulated emission spectrum of 405 nm (Fig. 1), no noise is generated.

In addition, the results of measuring the stimulated luminescence intensity when the IP obtained in Example 2 was irradiated with X-rays with a tube voltage of 75 KVp and excited with an LD with an oscillation wavelength of 830 n++, and the conventional B a F
Regarding the X-ray image conversion plate prepared with Br: 1O-3Eu"°, the measurement results of stimulated emission intensity when excited with a He-Ne laser were compared and are shown in Table 1.

Table 1 As is clear from Table 1, the relative stimulated luminescence intensity of the phosphor according to the present invention is 30 times that of the conventional one.

Next, the X-ray diffraction pattern of the phosphor of the present invention is shown in FIG. The crystal structure is tetragonal. Further, regarding a conventional phosphor having the same composition as the phosphor of the present invention, the photostimulation excitation spectrum is shown in FIG. 5, and the X-ray diffraction pattern is shown in FIG. For example, the photostimulation excitation spectrum of the conventional material has a peak at 500 to 700 nm, but the present invention has a peak at 850 nm, and the peak intensity of the X&I diffraction pattern of the conventional material is expressed as (2
00,004), (032).

(121)... However, the present invention provides (121)...
) , (200°004) (022). This suggests subtle differences in crystal structure due to differences in manufacturing methods, and it is thought that color centers formed in the crystal also differ due to differences in crystal structure. It is clear that both data are different from the phosphor of the present invention. In addition, FIG. 4 shows BaC1°・a according to the present invention.
This is a correlation diagram between the a value and the stimulated luminescence intensity at Ba Brz: 10-'Eu'', and 0.5≦a≦1.8 is good.

Regarding the differences between the X-ray diffraction pattern and photostimulation excitation spectrum of the phosphor of the present invention and the X-ray diffraction pattern and photostimulation excitation spectrum of a conventional phosphor having the same composition as the phosphor of the present invention, the present inventor We believe that this is due to the following mechanism.

+al Regardless of the type of color center, the shape of the crystal lattice does not change. There is no change in the diffraction angle (2θ).

However, the intensity in the plane index is determined by the presence or absence of lattice points.
−, depending on the location of the defect and the type of color center.
It is believed that the intensity distribution of the X-ray diffraction pattern changes.

(bl The principles of recording and reading X-ray images are described in Section 201 above.
- BaFX: Eu” (X = Cl, Br,
Regarding I), it is considered as follows.

(1) Recording = When irradiated with X-rays, the emission center (Eu”
) electrons fly out. These electrons are captured by lattice defects via the conduction band (solid line process in Figure 7).

(2) Reading: After recording, excitation light (He-Ne
When irradiated with a laser (or LD), the electrons in the lattice defect return to the emission center (Eu'') via the conduction band. At this time, stimulated luminescence is generated (the process indicated by the broken line in Figure 7). The emitted light is received by a photodetector and converted into an electrical signal.

In the diagram regarding the difference in photostimulation excitation spectra between the phosphor of the present invention and a conventional phosphor, El is the energy level at the lower center of the conventional one, and E2 is the energy level at the center of the composite color of the present invention. .

The energy distribution of the state of this composite color center and F center is calculated from the stimulated excitation spectrum. The complex color center is shown as an example in Figure 8, and many absorption bands are caused by alkaline and
Observed in the optical spectra of halides, they are thought to be due to transitions at defects consisting of F-centered clusters or aggregates. These defects occur when a crystal containing an F center is irradiated with F-band light.
The t-band is created first by this process, followed by the F-band. In both cases, the band is such that it is observed at a lower photon energy than the F band.

In the case of the present invention, by irradiating X-rays, the lower center and
We believe that complex color centers are formed simultaneously and therefore lead to different stimulated excitation spectra.

In the present invention, whether such an F center and a composite color center are formed at the same time, or when a composite color center is formed,
Wet mixing reaction, first firing step in air at 400-900°C and 700°C in hydrogen-nitrogen mixed gas atmosphere
We believe that this is due to the second firing step of firing at ~900°C.

When the phosphor used in the X-ray image conversion plate of the present invention absorbs X-rays, in addition to the F centers that capture electrons in the lattice vacancies of the halogen anions, the
A composite color center, which is an aggregation of centers and holes, is formed in the phosphor, and has a photostimulation excitation spectrum peak in the infrared wavelength region.

The factors governing the efficiency of stimulated luminescence are the electron capture rate at the F center during the excitation process (recording) after X-ray irradiation,
The luminous efficiency of the luminescent center in the stimulated luminescence process can be considered.

It is believed that the electron capture rate of F centers depends on the number of F centers and the number of defects with low energy levels caused by impurities. In other words, if the number of F centers is small, electrons excited by X-rays will quickly recombine with the original hole, and if captured by a low-energy level defect caused by an impurity, they will be quickly conducted using thermal energy. It is raised to the band, recombines with holes, and does not contribute to stimulated luminescence.

The luminous efficiency of the luminescent center is considered to depend on the number of impurities. In other words, if electrons moving through the conduction band from the F center are captured by luminescent impurities, they may emit light at a wavelength outside the photomultiplier tube's sensitivity range, and they may also be affected by paramagnetic impurities. As a result, the spin state of the electron changes and it becomes deactivated in a non-radiative process, so it does not contribute to stimulated luminescence.

From the above, it is considered that the stimulated luminescence rate can be further improved by increasing the number of F centers and reducing impurities.

〔Effect of the invention〕

As explained above, according to the present invention, the infrared wavelength region (
700 to 1500n*), it is possible to use a phosphor with extremely strong reading sensitivity compared to visible light wavelengths and a high-output LD, and there is no need to attach an excitation light source cut filter in front of the detector. Therefore, it is possible to convert an X-ray image into an X-ray image with higher efficiency, that is, with a lower dose of radiation than conventional X-ray image conversion methods. Furthermore, when an LD is used, it is possible to downsize the radiation image conversion device.

[Brief explanation of drawings]

FIG. 1 shows a phosphor according to the present invention, Ra CI Br:
FIG. 2 is a photostimulation spectrum diagram of the phosphor according to the present invention, and FIG. 3 is a diagram showing the stimulated emission spectrum of the phosphor according to the present invention. FIG. 4 is a diffraction pattern diagram of BaCj!1-aB according to the present invention.
Fig. 5 is a correlation diagram between the a value and the stimulated luminescence intensity for a Br, :10-'Eu''.
2.Ba BrziO-3Eu" is a photostimulation excitation spectrum diagram, FIG. 6 is a diagram of a conventional X-ray diffraction pattern of the phosphor, and FIG. 7 is a diagram of the principle of recording and reading in radiation X-ray image conversion, and Figure 8 is a conceptual diagram of the center of complex colors. Stimulated emission spectrum '$1 Figure wavelength (nm) Luminance excitation spectrum Figure 2 Angle (0) '$3 Figure a value Figure 4 De °C α °C 7 ( 1) Da℃ γ℃ 1
α℃ Fig. 5 Conventional example Fig. 6 Fig. 70 N(u) hν(eV) F center and composite color center No. 8

Claims (1)

[Scope of Claims] 1. Radiation transmitted through or emitted from the subject is a divalent europium-activated alkaline earth metal halide represented by the following compositional formula (1), and is 700 to 1
The radiation energy stored in the phosphor is absorbed by a phosphor having an absorption band centered on a composite color at 500 nm, and then irradiated with electromagnetic waves in the wavelength range of 700 to 1500 nm. 1. A radiation image conversion method characterized by emitting a fluorescent light and detecting the fluorescent light. Composition formula (1): MX_2・aMX_2':bEu^2^+ (where M is at least one alkaline earth metal selected from the group consisting of Ca, Sr, and Ba; X and X' are Cl, Br and I, and a is 0.5≦a≦1.
8 and b is 10^-^4≦b≦10^-^2)
. 2. The method according to claim 1, wherein the electromagnetic wave is laser diode (LD) light. 3. Represented by compositional formula (1), and 700 to 1500n
A phosphor having an absorption band region centered on a composite color in m. Composition formula (1): MX_2・aMX_2':bEu^2^+ (where M is at least one alkaline earth metal selected from the group consisting of Ca, Sr, and Ba; X and X' are Cl, Br and I, and a is 0.5≦a≦1.
8 and b is 10^-^4≦b≦10^-^2)
. 4. The phosphor according to claim 3, wherein the phosphor is manufactured by carrying out a wet mixing reaction at a temperature of 60 to 100°C. 5. The fluorescent material according to claim 3 or 4, wherein the fluorescent material is heat-treated in air and further heat-treated in a reducing atmosphere after undergoing the wet mixing reaction. body.