RU2647222C1 - Method of controlling yield of scintillations and photoluminescence of powder scintillators and lyuminophores - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to the methods of controlling the characteristics of powder scintillators and luminophores produced by one of the known methods, for example, by the methods of co-deposition, solid-phase synthesis, etc., and used as an independent material. The method for controlling the yield of scintillations and photoluminescence of powder scintillators and luminophores contains the stages in which excitation of scintillations is produced by irradiating the surface of the sample with alpha radiation. The photons of scintillations arising under the action of alpha particles are detected by the optoelectronic system from the surface of powder scintillators and luminophores irradiated with alpha particles.

EFFECT: simplification of sample preparation, increase of productivity and increase of accuracy of measurements.

7 dwg, 1 tbl

Description

The invention relates to methods for controlling the characteristics of powdery scintillators and phosphors obtained by one of the known methods, for example, by methods of co-deposition, solid-phase synthesis, etc., and used as an independent material, as a pre-synthesized raw material for growing scintillation inorganic single crystals or as raw materials for ceramics, for use in ionizing radiation detectors in medicine, inspection technology, scientific research, etc., lighting imports and other light-emitting products.

The equipment for medical diagnostics, environmental monitoring, non-destructive testing, safety systems is based on a number of physical methods that use the registration of ionizing radiation. Scintillation detectors [Radiation detection and measurement. Glenn F. Knoll. Third edition. John Wiley & Sons Inc. New York, Chichester, Weinheim, Brisbane, Toronto, Singapore, 2000, 816 pp.]. The output of scintillations (light output) is one of the most important characteristics of a scintillation detector, a high value of this parameter is a necessary condition for the effectiveness of diagnostic equipment and a reduction in the dose received by a patient, as well as an increase in the performance of any measurements using ionizing radiation. Scintillation single crystals or scintillation ceramics based on them are often used as detector elements; the yield of detector scintillations depends on the output of scintillations of the used scintillation material.

Inorganic phosphors in the form of powders, coatings, screens, etc., designed to work when excited by ionizing and optical radiation, also have a wide range of practical applications - fluorescent and LED lighting devices, cathode phosphors, X-ray screens, etc. [Luminescent Materials. G. Blasse, B.C. Grabmaier. Springer-Verlag. Berlin, Heidelberg, 1994, 223 pp.]. For all the examples listed, the yield of photo- or radio-luminescence is a key operational characteristic of the material.

Therefore, an accurate, reliable and efficient way to control the yield of scintillations, radio and photoluminescence is an important link in the development and production of scintillators and phosphors.

A widely known method of controlling the light output of scintillation detectors by comparing the amplitude spectra measured by the test and reference detector. On the photodetector, for example, a vacuum photoelectron multiplier (PMT), the test and reference detector are alternately installed, and in the immediate vicinity of them is a source of ionizing radiation; source spectra are accumulated in a multichannel amplitude analyzer. If there are pronounced lines in the spectra, for example, the peak of total absorption of 662 keV from the Cs-137 gamma source, then the ratio of the peak position in the analyzer channels in the spectrum of the reference detector to that in the spectrum of the tested detector is equal to the ratio of their light outputs. One of the variants of this method is described in [A.Ya. Berlovsky and V.Ya. Zaslavskaya. A method for measuring the light output of scintillation detectors. USSR patent 392771. Publ. September 25, 74] and differs in that in order to exclude the influence of the inhomogeneity of the sensitivity of the photomultiplier photomultiplier, measurements are made using only part of its surface, limited by the diaphragm installed between the detector and the photomultiplier. This method is accepted by us as a prototype.

This method works well with scintillation materials and detectors with high optical transparency to their own radiation, in which the internal scattering and absorption of scintillation light are minimal and do not introduce an additional error into the measurement result. To implement the method in the compared spectra there should be pronounced lines or other features, the position of which in the channels (or in the energy scale) can be reliably determined. Otherwise, the measurement result of the light output will substantially depend on the geometric dimensions of the compared scintillation detectors, which determine the differences in the optical path traveled by the scintillation photons in the detector. The spectra taken for opaque materials in the geometry of "transparency" are a decreasing exponent, and it is impossible to distinguish the characteristic features of the curve, the position of which could be reliably set in the energy scale.

A known method of comparing the brightness of the phosphors when excited by optical or x-ray radiation using a selenium photocell or photometer [Kazankin ON, Markovsky L.Ya. Inorganic phosphors. - L .: Chemistry, 1975. - 173 p.]. The measurements are carried out in the current, rather than pulsed, mode, i.e. photocurrents are measured and compared, the value of which is determined, in addition to the efficiency of converting the energy (or number of photons) of the exciting radiation into the energy (or number of photons) of the luminescence (i.e., the energy or quantum yield of the luminescence of the phosphor), and also the length of absorption of the exciting radiation in the phosphor material , optical transparency and thickness of the studied material layer. This method is adopted for the second prototype.

The present invention eliminates the influence on the measurement result of the scintillation yield and the photoluminescence yield of optical transparency and the thickness of the sample of the studied scintillation material, which makes it possible to study samples of a powdery scintillator or phosphor, which are obviously characterized by low optical transparency.

The technical problem that this invention solves is to create a method for controlling scintillation materials and phosphors, which can be used at various stages in the development and production processes. Compared with previously known methods, in particular, the methods described in the prototypes, the proposed method allows measurements of a greater number of different physical forms of materials, including a powder form, provides easier sample preparation and higher productivity with high measurement accuracy, and can be used on earlier stages of the process. This allows you to more effectively control the properties of the materials obtained in the processes of their development and production, which allows to obtain materials with higher and more predictable consumer properties compared to materials whose characteristics are controlled by the method described in the prototype. These advantages increase the quality of materials, expand the range of their application, increase labor productivity in their production.

To solve this problem, the control of scintillation yield (light output) and luminescence yield of powdery scintillators and phosphors is carried out by irradiating the surface of powdery scintillators and phosphors with alpha particles of standard radioisotope sources with alpha particle energies from about 4 MeV to 8 MeV, for example, ²⁴¹ Am source alpha particle energies of 5.44 and 5.49 MeV. Optical scintillation photons are recorded in geometry when the ionizing radiation source and photodetector (for example, a vacuum or solid-state photoelectronic multiplier) are installed on the same side of the irradiated surface (“reflection” geometry). Since the penetration depth of alpha particles into the studied sample materials does not exceed several tens of micrometers, the proposed method significantly reduces the influence of the thickness and optical transparency of the test sample on the measurement result compared to known methods when the radioisotope source and photodetector are located on different sides of the plane of the test sample (geometry “Transmittance”), or when the phosphor brightness is measured in the photocurrent measurement mode, when the measurement result is ok yvaetsya dependent absorption wavelength of the exciting radiation into the material of the phosphor, the optical transparency layer, and the thickness of the test material.

A powder sample can be prepared in various ways: using a bulk sample, preparing a composite with a polymer binder, pressing the powder, followed by sintering. Of these options, it is optimal to prepare a composite based on an inorganic powder of the measured sample and organic glue, for example, optical photopolymerizable glue. Also, the proposed method is suitable for measuring bulk materials such as single crystals, optical ceramics and glass.

The scintillation yield of the samples relative to each other is determined as the ratio of the positions of the peaks on the channel scale of the amplitude analyzer; the absolute yield of scintillations is determined relative to the calibration sample of the scintillator, as close as possible to the properties of the measured powder compositions, the light output of which was measured by standard methods, for example, a YAG: Ce single crystal (Y ₃ Al ₅ O ₁₂ : Ce).

The practical implementation of the invention is illustrated by the following examples.

Example 1

A sheet of polyethylene film with a thickness of 0.02-0.05 mm is laid on a glass substrate, an aluminum ring with an inner / outer diameter of 12/14 mm and a thickness of 1 mm is placed on top, a sample powder is poured into the ring so that the powder layer is flush with the upper plane rings. A drop of UV-cured optical glue (Norland 61 or the like) with a volume of 0.10-0.12 cm ^{3 is} placed in the center of the powder layer with a syringe; the workpiece is placed in total darkness for at least 2 hours. After the glue is completely absorbed, a second sheet of polyethylene film 0.02-0.05 mm thick is laid on top of the workpiece and covered with glass. The upper and lower glass are pressed together briefly to remove excess glue. The workpiece is placed under a UV radiation source with a wavelength of maximum emission of 360-370 nm, with a total power of at least 30 watts. Flare the workpiece for 4 minutes on each side (a total of 8 minutes); glass is removed from the preform. Next, the workpiece is kept for 48 hours at room temperature; then the polyethylene film is removed from the sample (polymerization procedures for Norland 61 adhesive are described; for other brands, polymerization conditions may differ). Excess glue is removed from the aluminum ring with a scalpel.

After these procedures, the sample is ready for measurements. The reflection geometry used for mechanically coupled samples is shown in FIG. 1. A source of ²⁴¹ Am alpha particles with an alpha particle energy of ~ 5.5 MeV is used (2). To reduce scintillation light losses, the sample and source are mounted on a thin wire frame; the entire structure is covered with Teflon reflector (3). A typical scintillation spectrometer is used consisting of a PMT (5) of the PHILIPS XP2020 brand with a spectral sensitivity range of 290-650 nm and a photocathode diameter (4) of 44 mm, a high-voltage source, spectrometric amplifier, and a multi-channel amplitude analyzer.

In FIG. 2 shows the amplitude spectra recorded from an opaque powder sample (sample 6) in the standard geometry "in the light" and in the "reflection" geometry, implemented in the proposed method. It can be seen that the spectrum recorded in the “clear” geometry is monotonically decreasing and does not contain features that could be used to determine the yield of scintillations from the sample. The spectrum recorded in the “reflection” geometry contains a local maximum, the position of which on the energy scale (channel number) can be reliably determined.

The light yield of the samples is measured relative to a YAG: Ce single crystal with dimensions of ∅12 × 1 mm. Also, the position of the peak from alpha particles in the spectra of a YAG: Ce single crystal is used to control the stability of the measuring stand. To process the spectra — determining the positions of the peaks and errors in their determination of samples and a YAG: Ce single crystal — the ROOT v.5.26 software package (https://root.cern.ch/) is used. Examples of processing the spectra accumulated during accumulation 600 s are shown in FIG. 3-5. Table 1 shows the values of the positions of the peaks in the energy scale and the calculated values of the scintillation outputs.

Example 2

Sample preparation for measurements can be carried out in another way, providing a mechanical bond of the powder in the sample, for example, by means of preliminary compaction and subsequent sintering. It is also possible to use a loose unbound sample poured into a Petri dish with a diameter of 10-20 mm with a layer of 0.5-1 mm. In this case, a photodetector (PMT) and a ²⁴¹ Am excitation source are located above the sample cup. The use of a loose unbound sample is less preferable, as it leads to a gradual contamination of the measuring installation.

Example 3

In FIG. Figure 6 shows the positions of the peaks of alpha particles (horizontal RL scale) of four powdered YAG: Ce samples, which were heat-treated at different temperatures, as a result of which they have different photo and radioluminescence intensities (Table 1, samples 4-7). Heat treatment of sample 4 was carried out at a minimum temperature (1000 ° C), and of sample 7 at a maximum temperature (1500 ° C). The peak positions were measured as described above; a YAG: Ce single crystal (sample 8) was also used for calibration. It can be seen that as the heat treatment temperature rises, the light yield of the samples increases.

The photoluminescence yield of the same YAG: Ce samples was measured by us by a direct method under optical excitation by a light-emitting diode with a wavelength of 440 nm. In measurements with optical excitation, a “reflection” geometry was also chosen in which the luminescence light from the surface of the sample is collected by the optical system and sent to the SDH wide-band spectrometer manufactured by Solar Laser Systems through the optical fiber. Using a white diffuse reflector, the emission spectrum of the LED was measured separately. The photoluminescence spectrum of sample 7 with a peak of exciting radiation present on it is shown in FIG. 7.

The photoluminescence yield was determined by the peak amplitude at the maximum of photoluminescence; previously, from the spectra (similar to those shown in Fig. 7), the baseline and the maximum normalized emission spectrum of the LED were subtracted. In FIG. Figure 6 shows the obtained positions of the photoluminescence peaks upon excitation by a 440 nm (PL) light emitting diode of four powdered YAG: Ce samples on the vertical axis. A clear correlation is seen between the positions of the peaks of ²⁴¹ Am alpha particles and photoluminescence, which indicates the applicability of the proposed method for controlling the photoluminescence yield of powdered phosphors.

Thus, the proposed method is suitable for measuring scintillation light yield and photoluminescence yield of powder samples, and can also be used to compare these values with values for single-crystal materials.

Table 1 and FIG. 1-7.

Table 1 - peak positions and light output values for measured samples.

FIG. 1 - Scheme for measuring the yield of scintillations upon irradiation with an alpha source, where 1 is a sample; 2 - alpha source; 3 - reflector; 4 - photocathode; 5 - PMT.

FIG. 2 - The amplitude spectrum of the sample of an opaque powder sample, taken in the geometry of "reflection" and "clearance".

FIG. 3 - Amplitude spectrum of a YAG: Ce single crystal (sample 1). The peak of alpha particles in the channel is 235.9 ± 0.08.

FIG. 4 - Amplitude spectrum of a powdered GGAG: Ce sample (sample 2). The peak of alpha particles in the channel is 563.1 ± 14.26.

FIG. 5 - Amplitude spectrum of a powdered GGAG: Ce sample (sample 3). The peak of alpha particles in the channel 421.1 ± 4.08.

FIG. 6 - Peak positions of ²⁴¹ Am alpha particles (horizontal RL scale) and photoluminescence upon excitation of 440 nm (PL) of four powdered YAG: Ce samples (samples 4-7).

FIG. 7 - Photoluminescence spectrum of a sample with a peak of exciting radiation present on it, measured by a SDH spectrometer.

Claims (1)

A method for controlling the output of scintillations and photoluminescence of powdered scintillators and phosphors, characterized in that the scintillations are excited by irradiating the surface of the measured sample with alpha radiation, while the scintillation photons arising under the influence of alpha particles are detected by an optoelectronic system from the surface of the powdery scintillators -particles.

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