RU2399831C1 - Method of obtaining long phosphor persistence of optical emitters - Google Patents

Abstract

FIELD: physics. ^ SUBSTANCE: optical emitter phosphor based on a nanostructure ceramic based on Be2(Si0.8Ge0.2)O4 is exposed to radiation in the X-ray energy range or to electrons with energy of 10-300 keV and dose of 10-103 Gy, after which exposure is stopped. In order to regenerate luminescence intensity which drops with time to the required level, material of the phosphor is repeatedly exposed to ionising radiation and short-term optical stimulation is used to excite flashes of afterglow intensity. ^ EFFECT: broader functional capabilities of controlling photodetector devices, high level of radiation and environmental protection. ^ 5 dwg, 1 tbl

Description

The invention relates to a method for producing luminescent emitters of optical photons of the visible and infrared wavelengths, based on a long afterglow of phosphors, after the termination of their excitation by ionizing radiation. It can be used to configure photodetector paths in thermoluminescent dosimetry, nuclear spectroscopy, when performing photometric studies using spectrophotometers, fluorimeters, and other optical devices based on the registration of weak and super-weak light fluxes in a wide range of wavelengths.

At present, the spread of radio-luminescent emitters (RLI) of optical photons, which are fixed in space and time, a combination of radioactive substances and phosphor. The basis of the method for producing optical photons in X-ray radiation is the conversion of the energy of ionizing radiation in the substance of the phosphor, which is released during the decay of the radionuclide, into the radiation energy of the optical range. The design of the radar detector is isolated from the external environment by sealing and has an exit window for the output of optical photons. The intensity of the radioluminescent radiation is regulated by the activity, the type of radionuclide used, and its spectral composition - by the nature of the radioluminophore and the type of impurity activator ion.

The literature describes methods for producing radioluminescent emitters (G.A. Mikhalchenko. Radioluminescent emitters. M: Energoatomizdat, 1988) using solid, liquid, and gaseous media that convert real-time ionizing radiation energy into optical photon energy of various spectral ranges. At the same time, radar sensors have a number of significant drawbacks, the main of which are the following.

1. Biohazard in long-term applications.

2. The potential danger of radioactive contamination of the environment during depressurization during manufacturing, operation, storage or as a result of terrorist attacks.

3. The destabilization of the radioluminescent characteristics due to solid-phase chemical interactions between the radioactive substance and crystallophosphorus, stimulated by a high dose rate (0.1-1.0 Gy / s).

It should be noted that in practice, in the overwhelming majority of cases, the use of radar data for verification and calibration of optoelectronic receivers is episodic in nature, the time of direct work with them is incomparably shorter than the designated life of the source. Thus, most of their resources, radar data are in the storage position, requiring compliance with the norms and rules of storage of radioactive sources, which creates additional material costs when organizing work using radar data. Therefore, it is relevant to create sources of optical photons that can, in certain cases, replace X-rays, without the use of radioactive elements in them.

Another way to obtain a long afterglow, which is the basis for the creation of non-radioactive luminescent emitters, is based on the ability of some phosphors to emit optical photons after the termination of their preliminary excitation. The table below provides technical data on the chemical composition of some light-accumulating phosphors with long afterglow, their designation, spectral region and afterglow time (www.luminophor.ru/new/phLongAfter.htm).

Table Luminescence region Phosphor Type Chemical formula Afterglow period Green FV-540-1 ZnS: Cu 1-2 hours Green FV-530D SrAl ₂ O ₄ : Eu, Dy 8-10 hours * Turquoise AV-490D (Ca, Sr) Al ₂ O ₄ : Eu, Dy 8-10 hours Purple blue FV-440D CaAl ₂ O ₄ : Eu, Dy 8-10 hours * According to the terminology used in the technical literature, the afterglow of light-emitting phosphors of 8-10 hours is considered long or ultra-long

The latest effective light-emitting phosphor Penta L-1 (TU 2149-044-40245042-2003, www.kraspenta.ru) is a pale yellow-green powder with a particle size of 1-200 microns. Radiation excitation spectrum: visible or UV light, emission spectrum from 420 to 530 nm, afterglow period up to 8 hours.

From the above examples it follows that the known methods of exciting the luminescence of phosphors by pre-irradiating the material with photon radiation of the visible or ultraviolet range allow it to be used as a source of optical photons.

The disadvantage of the light emitting phosphors shown in the table, and the latest of them, Penta L-1, is that their emission spectra correspond to the visible region, which does not allow their use in the ultraviolet and near infrared ranges. The latter is particularly important in connection with the intensive use of a new class of photodetector based on pin diodes, the spectral sensitivity of which corresponds to the range of 600-1000 nm (GFKnoll. Radiation detection and Measurement. Third edition. John Wiley & Sons, Inc. 1999, p. 289 ) Another disadvantage of this group of light emitting phosphors is the need to use them in powder form and the inability to compact due to the strong absorption of exciting ultraviolet radiation in a dense material.

The closest in technical essence and the achieved result to the proposed should be considered a method of obtaining a long ultraviolet afterglow of phosphors based on BeO and Li ₂ O-MgO-SiO ₂ -Ce (RF Patent No. 2345274). In this method, adopted as a prototype, to obtain a long ultraviolet afterglow of phosphors of optical emitters based on BeO and Li ₂ O-MgO-SiO ₂ -Ce, they are irradiated with X-ray energy range or electrons with an energy of 10-300 KeV and a dose of 10-10 ⁴ Gy , after which the radiation is interrupted, and for the regeneration of the luminescence intensity that decreases with time to the required level, the irradiation of the phosphor substance is repeated many times. The luminescence spectrum of such phosphors is dominated by one band with a maximum of about 400 nm, which does not allow them to be used to control photodetectors based on photoelectronic multipliers and pin diodes with spectral sensitivity characteristics in the range of 600-1000 nm.

The basis of the invention is the task of expanding the boundaries of the spectrum of a long afterglow of phosphors from 300 to 800 nm, i.e. up to a wavelength range that matches the spectral detection ability of most photoelectronic, semiconductor, and thermal radiation detectors (MD Aksenenko, ML Baranochnikov. Optical radiation receivers. M: Radio and communications. 1987. P.296).

The solution of the technical problem is achieved by the fact that to excite long-term luminescence in the range of 300-800 nm, a phosphor based on nanostructured ceramics Be ₂ (Si _0.8 Ge _0.2 ) O _{4 is} selected; for its irradiation, ionizing radiation is used: x-ray or electron energies of 10-300 keV and a dose of 10-10 ³ Gy. The intensity of luminescence excited by ionizing radiation at an arbitrary time in the emission cycle is additionally increased by several tens of times by briefly illuminating the phosphor with optical radiation of the visible wavelength range. To regenerate the luminescence intensity that decreases with time and restore a new emission cycle, the irradiation of the phosphor substance of the emitter with ionizing radiation is repeated many times.

The lower limit of the energy of ionizing radiation is due to the need to excite a sufficient number of volume layers of the phosphor due to the penetrating ability of the exciting radiation.

The upper limit of the energy of ionizing radiation is due to the need for volumetric excitation of a phosphor 1-2 mm thick, without leading to chemical destruction of the phosphor material, the formation of new defects that can change the required properties of the material.

The lower limit of the radiation dose is associated with a minimum level of afterglow of the phosphor suitable for detection by optoelectronic receivers.

The upper limit of the radiation dose is associated with the saturation of the emissivity of the phosphors.

The essence of the invention is as follows.

In the proposed method, the phosphor material is irradiated at room temperature with x-ray or electron radiation in a selected range of energies and doses. Free charges, electrons, and holes formed under the action of radiation will be captured by traps due to intrinsic or impurity defects that are always present in the phosphor material. To obtain the afterglow of the desired wavelength, the phosphor is selected with a special chemical composition. Among the traps that captured free charges there will be those whose energy depth is relatively small, and they will be thermally unstable at room temperature. Such traps will lose trapped carriers spontaneously in time, the recombination of which with charges of the opposite sign is accompanied by luminescence. The afterglow duration is controlled by the radiation dose, and the spectral composition is controlled by the nature of the phosphorus material. The luminescence intensity and its regeneration can be set at any level by repeated exposure to ionizing radiation.

As a result of applying the described method, it is possible to obtain a compact, autonomous, non-radioactive, regenerated luminescent emitter of optical photons in the wavelength range of 300-800 nm with a peak in the band of 400 and 700 nm with an afterglow of more than several tens of hours. The emitter can be in the form of a plate with a surface area of about 1 cm ² and a thickness of 0.5-1.0 mm, which makes it convenient to place it in small and inaccessible places of optical systems.

The implementation of the proposed method for obtaining an ultra-long afterglow of phosphors based on Be ₂ (Si _0.8 Ge _0.2 ) O _{4 is} illustrated by the drawings below.

Figure 1. The integrated yield of X-ray luminescence and afterglow of samples of nanostructured ceramics based on Be ₂ (Si _0.8 Ge _0.2 ) O ₄ , Be ₂ (Si _0.6 Ge _0.4 ) O ₄ , Be ₂ (Si _0.4 Ge _{0, 6} ) O ₄ , Be ₂ (Si _0.2 Ge _0.8 ) O ₄ . X-ray irradiation (40 kV, 10 mA, W, D = 10 ² Gy).

Figure 2. Dependence of the afterglow intensity of nanostructured ceramics based on Be ₂ (Si _0.8 Ge _0.2 ) O ₄ on time 5 hours after the end of preliminary irradiation with x-ray radiation (40 kV, 10 mA, W, D = 10 Gy).

Figure 3. The optical spectrum of the afterglow of nanostructured ceramics based on Be ₂ (Si _0.8 Ge _0.2 ) O ₄ excited by an electron beam with a dose of 100 Gy.

Figure 4. The phosphorescence spectrum of lithium glass doped with cerium Li ₂ O-MgO-SiO ₂ -Ce, excited by the method adopted for the prototype.

Figure 5. Afterglow of a phosphor based on nanostructured ceramics Be ₂ (Si _0.8 Ge _0.2 ) O ₄ . 1 - after excitation by x-ray radiation; 2 and 3-induced afterglow with flashes of light with a power of 2 mW with a wavelength of 480 nm for 60 and 10 seconds, respectively.

1. Samples and equipment

To verify the operability of the proposed method, samples of synthesized nanostructured ceramics with a variation in the silicon and germanium contents of Be ₂ (Si _0.8 Ge _0.2 ) O ₄ , Be ₂ (Si _0.6 Ge _0.4 ) O ₄ , Be ₂ (Si _0.4 Ge _0.6 ) O ₄ , Be ₂ (Si _0.2 Ge _0.8 ) O ₄ . The samples were plane-parallel plates with a thickness of about 1 mm and a surface area of about 1 mm ² . Irradiation was carried out using an X-ray apparatus of the URS-55 type or a pulsed electron accelerator of the Mir-2D type, equipped with a tube with the release of electrons. The integral luminescence of the samples at the time of irradiation (radioluminescence) and after irradiation was recorded with an FEU-142 type photomultiplier, and the spectral composition of the luminescence was measured using a DMR-4 monochromator equipped with an FEU-106 photomultiplier. A comparative analysis of the integrated yield of x-ray luminescence and afterglow (Figure 1) showed that the most effective is a phosphor based on nanostructured ceramics Be ₂ (Si _0.8 Ge _0.2 ) O ₄ . This compound was chosen to solve the problem posed in the present invention.

2. The methodology of the proposed method

2.1. Figure 2 shows the dependence of the intensity of the afterglow of Be ₂ (Si _0.8 Ge _0.2 ) O ₄ from time to time 5 hours after the end of the preliminary x-ray irradiation (40 kV, 10 mA, W, D = 10 Gy). Similar measurements were carried out under irradiation with doses of 1 · 10 ² and 1 · 10 ³ Gy. The nature of the curves in this case did not differ from that shown in Fig. 1, they shifted parallel to the curve of Fig. 2 upward along the afterglow intensity axis in accordance with the radiation doses. With an increase in the radiation dose, the intensity and duration of the afterglow increase. The region with a particularly slow decay of phosphorescence at t> 5 hours after irradiation is extrapolated by the equations:

where I (t) is the luminescence intensity, imp./s; t is the time after the end of irradiation, hours; 0,029 - attenuation constant, 1 / hour.

An analysis of equations (1-3) and experience show that the measured phosphorescence intensity, one to two orders of magnitude higher than the intrinsic background of the used photodetector, equal to 1-5 pulses / s, can be detected 200-300 hours after irradiation. The specified time interval is sufficient for the practical application of the proposed invention. In addition, in contrast to the method adopted for the prototype, to excite a long afterglow by the proposed method, due to the extremely developed effective surface of the nanostructured ceramic Be ₂ (Si _0.8 Ge _0.2 ) O ₄ , the dose range is reduced by an order of magnitude.

2.2. Figure 3 shows the phosphorescence spectrum of nanostructured ceramics, and Figure 4 shows the afterglow spectrum of the phosphor obtained by the method adopted for the prototype. A comparison of these two spectra shows that the afterglow spectrum of the phosphor excited by the proposed method contains all the components of the spectrum excited by the method adopted as a prototype, and their main difference is that the afterglow spectrum excited by the proposed method is wider ( 250-800 nm), has a well-defined band in the near infrared region (about 700 nm), an approximately equal ratio of band intensities of about 400 and 700 nm. The set of marked differences in the afterglow spectra of phosphors excited by the proposed method and adopted as a prototype significantly expands the class of optical photodetectors controlled by regenerated luminescent emitters.

2.3. In the process of implementing the proposed method, the photosensitivity effect of the phosphor used was revealed, namely, that illumination of the phosphor excited by the ionizing radiation of visible light leads to an increase in the afterglow intensity by one or two orders of magnitude relative to the level of glow excited by the ionizing radiation only. The effect is observed only after irradiation of the phosphor with ionizing radiation. Without such preliminary excitation by illumination of the phosphor with the visible and ultraviolet range of wavelengths, the photosensitivity of the phosphor was not observed.

Studies have shown that the compound used in the present invention is an effective storage thermoluminophore. On the curve of its thermal emission after excitation by ionizing radiation and a decrease in the intensity of the afterglow to the background level, there are two peaks at about 300 and 500 ° C. The peak intensity at 300 ° C decreased if the phosphor was illuminated with visible light before measuring the thermoluminescence. The magnitude of this decrease depended on the wavelength, intensity and time of illumination of the phosphor. An analysis of these facts allows us to conclude that the physical cause of photosensitivity is the optically stimulated transfer of charge carriers from deep traps to smaller ones, the devastation of which occurs even at room temperature, their recombination enhances the intensity of the afterglow.

Thus, it is possible at an arbitrary time in the afterglow cycle and beyond, induced by ionizing radiation, using optical excitation, to cause a sharp increase in the intensity of the afterglow. This feature allows in addition to spectral sensitivity to control the dynamic range of photodetectors.

2.4. To establish the optimal regimes of optically stimulated flares on the afterglow curve, an optical monochromator and an incandescent lamp were used as a light source. The measurements showed that the most efficient excitation is realized by light with a wavelength in the range of 450-500 nm. The energy of quanta of this wavelength corresponds to the optical depth of the traps responsible for the thermoluminescent peak of about 300 ° C. The optimal optical stimulation time was 10-60 seconds. The optimal optical radiation power at the phosphor location was 1–5 mW. The results of these measurements are presented in FIG. 5. In this drawing, curve 1 shows the afterglow of a phosphor based on nanostructured ceramics Be ₂ (Si _0.8 Ge _0.2 ) O ₄ , curves 2 and 3 - induced by flashes of light with a wavelength of 480 nm for 60 and 10 seconds, respectively.

The figures show the possibility of obtaining ultra-long glow of phosphors of small-sized, regenerated emitters of optical photons based on nanostructured ceramics Be ₂ (Si _0.8 Ge _0.2 ) O ₄ , not associated with the use of radioactive isotopes.

Claims (1)

A method of obtaining a long afterglow of optical emitter phosphors, including irradiating the phosphor with X-ray energy or 10-300 keV electrons and a dose of 10-10 ³ Gy, after which the irradiation is interrupted and, for regeneration, the luminescence intensity decreases with time to the required intensity level phosphor ionizing radiation is repeated many times, characterized in that the substance of the phosphor of the optical emitter choose nanostructured ceramics n based Be ₂ (Si _0,8 Ge _0,2) O _4, and for the excitation of afterglow intensity flashes optical stimulation is used with a wavelength of 450-500 nm, with a duration of 10-60 and 1-5 mW output power.

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