RU2388017C1 - Film-type scintillator for detecting beta- and photon emissions - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: invention relates to radiometry of liquid, gaseous and solid media, as well as to dosimetry of ionising radiations. The film-type scintillator is made from a polycarbonate filled with a scintillating luminophor from a mixture of powdered crystalline ortho-silicate germanate of yttrium, gadolinium, lutetium and cerium, having general stoichiometric formula Y_2-x-y-z·Gd_x·Lu_y·Ce_z·O₃(SiO₂)_1-p(GeO₂)_p with stoichiometric index intervals: x=0.01…1.0, y=0.01…0.9, z=0.005…0.05, p=0…0.8 and powdered crystalline ortho-aluminate of yttrium, gadolinium, lutetium and cerium, having general stoichiometric formula Y_3-m-n·Gd_m·Ce_n·(AlGa)₅-O₁₂ with stoichiometric index intervals m=0.01…1, n=0.01…0.1. Maximum wavelength of the optical radiation of the mixture of luminophors ranges from 420 to 550 nm, and scintillation duration ranges from 30 to 40 ns.

EFFECT: shifting the spectrum of optical radiation from the green to the red region, which enables detection of radiation intensity using silicon-based photodiodes, as well as increase in relative light output of the scintillator when used in detectors with a large sensitive area for detecting low levels of radiation intensity.

3 cl, 3 tbl

Description

The invention relates to radiometry of liquid, gaseous, solid media, as well as to dosimetry of ionizing radiation, in particular to dispersed thin-layer film scintillation detectors of beta and photon radiation. Optimal is the use in scintillation detectors for measuring low levels of beta and photon radiation intensity in a wide energy range, as well as when using photodetectors based on silicon photodiodes.

Known scintillator based on sodium iodide activated by thallium, with a high relative light output relative to anthracene (Physical Encyclopedic Dictionary, M., SE, 1983, p. 733). However, high hygroscopicity excludes the possibility of its use as an active filler of a dispersed film scintillator.

A known plastic film scintillator based on a polymer from the group of poly-n-xylenes for detecting electrons and gamma fields (RU, 2150128, 05.27.2000). The known scintillator has an energy yield of ~ 7% and high speed (scintillation duration ~ 4 ns). A disadvantage of the known scintillator is the low value of the effective atomic number z _eff (z _eff ~ 7 carbon units), as well as the insufficient height of thermal stability of the energy yield (-25% at 25 ° C), which does not allow its wide use in the practice of radiometric and dosimetric measurements, especially in the region of low energies (E <20 keV) of photon radiation fields.

Closest to the proposed one is a known scintillator for detecting beta and photon radiation, made in the form of a film having a polycarbonate polymer base filled with scintillating phosphor from powdered crystalline yttrium, gadolinium, lutetium and cerium orthosilicate germanate (RU 2279692, G01T 1/20 2005). A disadvantage of the known scintillator is the relatively narrow spectral range of the optical radiation of the scintillator from λ = 420 nm to λ = 445 nm, which excludes the possibility of using the scintillator to detect the signal of semiconductor silicon photodiodes, the sensitivity of which is shifted to the green part of the visible spectrum (from λ = 520 to λ = 550 nm). Another disadvantage is the low sensitivity when used in detectors with a large surface, including for measuring low levels of radiation intensity.

The problem solved by the proposed invention is to expand the scope of this type of scintillator.

The technical result from the use of the proposed technical solution is the ability to shift the spectrum of the optical radiation in the green and red parts of the spectrum, which makes it possible to record the radiation intensity using silicon-based photodiodes, as well as to increase the relative light output of the scintillator radiation when used in detectors with a large sensitive surface for recording low levels of radiation intensity.

To achieve a technical result, a polycarbonate film scintillator is proposed, filled with a scintillating phosphor of powdered crystalline yttrium, gadolinium, lutetium and cerium ortho-silicate germanate, having the general stoichiometric formula

Y _2-xyz Gd _x Lu _y Ce _z O ₃ (SiO ₂ ) _1-p (GeO ₂ ) _p

at intervals of stoichiometric indices:

x = 0.01 ... 1.0,

y = 0.01 ... 0.9,

z = 0.005 ... 0.05,

p = 0 ... 0.8,

Additionally, a powdered crystalline ortho-aluminate of yttrium, gadolinium, cerium having the general stoichiometric formula is used as a scintillating phosphor

Y _3-mn Gd _m Ce _n (AlGa) ₅ O ₁₂

at intervals of stoichiometric indices

m = 0.01 ... 1,

n = 0.01 ... 0.1

in this case, the maximum wavelength of the optical radiation of the phosphor mixture is in the range from 420 to 550 nm, and the scintillation duration is from 30 to 40 ns.

The ratio of crystalline yttrium, gadolinium, lutetium, cerium ortho-silicate germanate, and yttrium, gadolinium, cerium ortho-aluminate in the mixture is from 92-98% to 8-2% mass, respectively (or from 98: 2 to 92: 8).

The concentration of the mixture of scintillating phosphor in the film is from 10 to 75% mass.

An explanation of the physical features of the proposed scintillator. The composition of the proposed film scintillator includes a binary (double) composition of emitting materials. The first of these materials, the indicated orthosilicate, has a microcrystalline granular structure and luminesces with violet-blue light only when it is irradiated with electrons or high-energy gamma-ray photons. The second emitting component - yttrium orthoaluminate, gadolinium, cerium not only luminesces in high-energy photon fields, but additionally fluoresces when excited with short-wavelength quanta of visible light (violet or blue light). It was experimentally established that the introduction of a phosphor scintillator based on yttrium-cerium orthosilicate of rare-earth ions, such as gadolinium (Gd) and lutetium (Lu), into the main emitting substance dramatically increases the conversion conversion efficiency of the detector. Thus, the introduction of yttrium-cerium orthosilicate from x = 0.01 of the fraction of gadolinium ion to x = 0.5 makes it possible to increase the conversion efficiency by 25-30% when the detector is irradiated with photon or beta radiation with an energy of E≥45 keV.

One of the physical reasons for this phenomenon is the proximity of the K-orbit energy of the Gd ion with the energy of the exciting photon radiation. An increase in the value of the coefficient 0.5 <x≤10 does not significantly change the conversion efficiency of the detector, but it contributes to a longer wavelength shift of the spectral maximum of the scintillator radiation from λ = 420 nm to λ = 440 nm, which in turn reduces the fraction of the short-wavelength part of scintillations, which is usually absorbed by the polymer binder scintillator. If lutetium ion is introduced into the scintillating phosphor, replacing the original yttrium ion, then for excitation quanta with an energy of E = 45 keV, the conversion efficiency does not increase, however, a sharp jump in this parameter is observed for the photon energy with E> 60 keV.

When changing the value of the stoichiometric coefficient y from y = 0.01 to y = 0.6, the value of conversion efficiency increases by 36-42% for the exciting energy of quanta 60 keV <E≤100 keV. With an increase in the energy of exciting quanta over 100 keV, the conversion efficiency exceeds the values for the initial scintillating phosphor prototype by 32-35%. It was experimentally established that the increase in conversion efficiency achieved when a part of the ions is replaced by lutetium ions in the primary phosphor from yttrium-cerium orthosilicate is enhanced by the simultaneous presence of these elements in the phosphor. Thus, upon excitation by primary quanta with E = 120 keV, the conversion efficiency of the scintillation phosphor with the combined introduction of Gd and Lu, replacing up to 0.8 atomic fractions of Y, increases to a value 3-4 times higher than for the initial scintillating material . It is characteristic that the simultaneous presence of gadolinium and lutetium in the inorganic base of the orthosilicate is not accompanied by a change in the structure of the crystal lattice of the material.

At the same time, it was experimentally established that a change in the stoichiometric index z in the range from 0.005 to 0.05 allows one to increase the half-width of the spectral maximum of the scintillating phosphor from λ = 55 nm to λ = 62 nm. Since such an increase is accompanied by a partial long-wavelength shift favorable for conversion efficiency, it can be imagined that the optimal concentration of cerium ion in the matrix of the scintillating phosphor is z = 0.035-0.045 at. share. It was also found that an increase in the Ce concentration and the stoichiometric index z from z = 0.01 to z = 0.05 is accompanied by a significant decrease in the afterglow time constant by about 1.4-1.7 times. Such a change is associated with the concentration interaction of activating ions with each other, but unlike the initial scintillating phosphor, where the concentration interaction is accompanied by a loss in the conversion efficiency of the scintillator, this phenomenon does not occur in the proposed material. This is due to additional channels of the luminescence excitation of the Ce ⁺³ ion due to the presence of the gadolinium and lutetium ions in the phosphor. In detail, all samples of scintillation phosphors of various quantitative composition are given in Tables 1 and 2, where their parameters are compared, such as conversion efficiency ŋ and afterglow duration τ _e . A NaJTl scintillating crystal (Example 1-19) was used as a limiting standard, and a standard phosphor Y _1.96 Ce _0.04 SiO _{5 was used} as a prototype.

Table 1 №№ Stoichiometric indices in the composition 100% conversion efficiency Afterglow duration τ _e , ns Y Gd [x] Lu [y] Xie [z] SiO ₂ GeO ₂ [p] 1-0 1.95 0 0 0.05 one 0 one hundred 52 (prototype) 1-1 1.93 0.01 0.01 0.04 one 0 105 52 1-2 1.65 0.3 0.01 0.04 one 0 118 52 1-3 1.45 0.5 0.01 0.04 one 0 135 52 1-4 0.96 1,0 0 0.04 one 0 142 52 1-5 1.93 0.01 0.01 0.04 one 0 106 52 1-6 1.65 0.01 0.30 0.04 one 0 128 52 1-7 1.35 0.01 0.6 0.04 one 0 142 52 1-8 0.94 0.01 1,0 0.05 one 0 180 220 54 1-9 0.16 1,0 0.9 0.04 one 0 300 58 1-10 0.25 0.8 0.9 0.05 one 0 320 56 1-11 0 0.98 0.98 0.04 one 0 310 56 1-12 0.2 0.88 0.88 0.04 one 0 330 48 1-13 0.2 0.88 0.88 0.04 0.2 0.8 420 45 1-14 0.2 0.88 0.88 0.04 0.8 0.2 350 44 1-15 0.2 0.885 0.90 0.005 0.8 0.2 280 60 1-16 0.2 0.87 0.88 0.05 0.8 0.2 410 36 1-17 0.2 0.87 0.895 0,035 0.2 0.8 340 38 1-18 1.87 0.1 0 0,03 0.9 0.1 120 40 1-19 NaJTl 460 230

The second radiating material has the general stoichiometric formula Y _3-mn Gd _m · Ce _n · (AlGa) ₅ O ₁₂ . This material has a cubic crystal lattice with a garnet structure and belongs to the space group J3d. The rare-earth elements Y, and / or Gd, and / or Ce form a cathode sublattice in which the coordination number of ions is KI = 8. The coordination sphere Y and Gd includes alumina tetrahedra [AlO ₄ ], while the bond between the lanthanide ion and aluminum is realized through bridging oxygen.

The crystal lattice parameter of orthoaluminate is a = 12.01 Å and increases with increasing values of the stoichiometric indices “p” and “q”. So, when Gd with p = 0.5 atomic fractions is introduced into the aluminate, the crystal lattice parameter increases to a = 12.20 Å. An increase in the content [Gd] = 1 atomic fraction (respectively, with a decrease in the concentration [Y] = 1.95 atomic fraction), the crystal lattice parameter increases to a = 12.45 Å. Changing the concentration of Ce ⁺³ ion in the composition of the phosphor matrix from [Ce] = 0,01 to [Ce] = 0,05 changes the value of the lattice parameter in Δa≈0,1 Å, i.e. slightly.

The spectral-kinetic characteristics of the ortho-aluminate phosphor change as follows. The radiation of the activating Ce ³ ion for the composition Y _2.96 Gd _0.01 · Ce _0.003 · (Al, Ga) ₅ O ₁₂ is in the range λ _max = 539 ÷ 540 nm. An increase in the fraction of the Gd ⁺³ ion in the phosphor with an increase in the crystal lattice parameter is accompanied by a decrease in the intracrystalline field gradient, which causes a long-wavelength shift in the position of the spectral maximum. This very important experimental observation makes it possible to more accurately adjust the total radiation spectrum to the sensitivity of the light receiver.

During the experiment, a short-wavelength shift of the spectral maximum of Ce ⁺³ radiation was also detected as a result of the partial replacement of the aluminum ion Al ⁺³ by gallium ions Ga ⁺³ . A similar shift is from 1 to 2 nm per unit of introduced gallium. At the same time, with the introduction of gallium, the atomic number of the matrix of the scintillating phosphor increases, therefore, the composition of the material Y _3-mn Gd _m · Ce _n · (Al _2.0 Ga _3.0 ) O ₁₂ was mainly used. It was experimentally established that the introduction of a second radiating material based on yttrium-gadolinium orthoaluminate into the film scintillator can significantly increase the light output of the device and increase light reflection. This effect is especially significant at high energies of the quanta of the exciting radiation.

Based on the studies of the second scintillating phosphor, Table 2 of spectral-kinetic parameters was compiled.

table 2 №№ Phosphor composition Spectral maximum λ _max , nm Afterglow duration, ns one Y _2.5 Gd _0.47 · Ce _0.03 · (Al ₄ Ga ₁ ) O ₁₂ 542 one hundred 2 Y _2.2 Gd _0.77 Ce _0.03 · (Al ₄ Ga ₁ ) O ₁₂ 546 98 3 Y _2.0 Gd _0.97 Ce _0.03 (Al ₄ Ga ₁ ) O ₁₂ 550 94 four Y _1.85 Gd _1.05 · Ce _0.1 · (Al ₂ Ga ₃ ) O ₁₂ 552 95 5 Y _2.0 Gd _0.95 Ce _0.05 (Al ₂ Ga ₃ ) O ₁₂ 543 90 6 Y _2.0 Gd _0.95 Ce _0.1 (Al ₂ Ga ₃ ) O ₁₂ 544 86

The spectral maximum of the radiation of the second phosphor has a range of variation from λ = 540 nm to λ = 550 nm, which ensures optimal operation of the combined scintillating phosphors with photomultipliers on multi-alkaline photocathodes.

When creating the invention, in the course of experiments, it was found that with the combination of these two emitting materials, a completely non-obvious synergistic effect occurs, which consists in increasing the light output of the proposed scintillator in comparison with the standard prototype scintillator. This increase in light output, as was shown when working on the invention, is determined by a number of parameters, specifically:

- the mass ratio between the two radiating components forming the filling of the proposed scintillator;

- the ratio of dispersion (grain size) of the radiating components;

- the ratio of the concentrations of yttrium and gadolinium ions in the basis of the fluorescent component - yttrium, gadolinium, cerium orthoaluminate.

The optimum dispersed composition of the luminescent material from yttrium-gadolinium-lutetium-cerium orthosilicogermanate was experimentally determined, comprising d _cp = 10–16 μm, d ₉₀ ≤24 μm. As will be shown below, in the synthesis of the fluorescent orthoaluminate of yttrium, gadolinium, and cerium, products with an average dispersion of d _cp = 10–12 μm and d ₉₀ = 20 μm or finer, for example, d _cp = 4 μm and d ₉₀ = 10 μm, can be obtained.

The optimal dispersion of the fluorescent component was determined by conducting a series of over a hundred optical spectral experiments. For these experiments, a standard luminescent material of the composition Y _1.65 Gd _0.2 Lu _0.1 Ce _0.05 (SiO ₂ ) _0.99 (GeO ₂ ) _0.1 , having an average grain diameter d _cp = 12 μm and d ₉₀ = 18 microns. This phosphor is designated by us in table 1 as h-1. A different amount of the second fluorescent component of composition Y _2.65 Gd _0.30 Ce _0.05 Al ₅ O ₁₂ , Ga = 0 with different dispersion was added to the specified phosphor. This material is indicated in table 1 as Ph-2. A scintillating film was made from a two-component mixture using polycarbonate, on which the optical spectral characteristics were measured:

- light output with respect to NaJ (T1),

- dominant wavelength of scintillating radiation λ _house. ;

- duration of scintillations, ns.

The experimental results are presented in table 3, which compares the data on the film scintillator, in which instead of a single-component phosphor, the radiation comes from a two-component composition.

Table 3 №№ The concentration of h-1, wt.% The concentration of Ph-2, wt.% The ratio of the average diameters h-1 and Ph-2, microns Relative light output with respect to NaI (Tl),% Scintillation duration, ns 1-1 90.0 10.0 d _h-1 12 0.7 37 d _Ph-2 12 1-2 92.0 8.0 d _h-1 12 0.72 40 d _Ph-2 12 1-3 94.0 6.0 d _h-1 12 0.63 42 d _Ph-2 12 1-4 96.0 4.0 d _h-1 12 0.58 44 d _Ph-2 12 1-5 98.0 2.0 d _h-1 12 0.55 44 d _Ph-2 12 1-6 99.0 1,0 d _h-1 12 0.52 46 d _Ph-2 12 1-7 99.0 1,0 d _h-1 12 0.65 44 d _Ph-2 2 1-8 98.0 2.0 d _h-1 12 0.78 42 d _Ph-2 2 1-9 96.0 4.0 d _h-1 12 0.91 40 d _Ph-2 2 1-10 94.0 6.0 d _h-1 12 1,04 40 d _Ph-2 2 1-11 94.0 6.0 d _h-1 12 0.93 38 d _Ph-2 1.0 1-12 NaJ (Tl) - - one 230 Single crystal 1-13 100 reference 0 d _h-1 12 0.69 56

As follows from the data in Table 3, the light output of the obtained model scintillators increases with an increase in the concentration of the fluorescent emitting material, however, this increase is not linear with respect to the increase in the concentration of the fluorescent emitter. With an increase in the concentration of the second phosphor Y _3-pq Gd _p Ce _q Al ₅ O ₁₂ from 1% mass gain is 36%, while the optimal concentration of fluorescent material is in the range from 8% to 1% mass. A possible mechanism for increasing the radiation of a two-component scintillator is the reabsorption of the primary emitted violet-blue glow emanating from the orthosilicogermanate phosphor, the grains of which are in contact with the grains of a fluorescent material with a garnet structure. If the average grain diameters of these materials are equal (d _cp ≈12 μm), up to 10 grains of the phosphor from orthosilicogermanate can be located around one grain of a garnet phosphor. The excitation of this phosphor in a powerful field of gamma rays or electrons is accompanied by intense scintillations of orthosilicogermanate. In this case, significant amounts of violet-blue quanta (hν≈2.9 ÷ 3.0 eV) are emitted, according to calculations up to N _q = 1 · 10 ⁴ quanta / 100 keV of photon field energy. These violet-blue quanta can be absorbed by grains of a fluorescent material having an intense yellow-orange color. Such an active absorption of primary violet-blue quanta causes, as we have shown when working on the invention, intense photoluminescence of orthoaluminate. The spectral maximum of this radiation is at a wavelength with λ = 550 nm. When the mass ratio in the scintillator of the orthosilicogermanate phosphor is up to 90%, the ratio of violet-blue quanta with yellow will be ~ 9: 1 (in this calculation, the quantum efficiency of the orthoaluminate is unity, which is close to the measured values).

Consequently, two-quantum radiation is generated at the exit of the scintillator, in which violet-blue quanta prevail. It has been experimentally proved that various photodetectors measure, when fixing this radiation, a different total wavelength of radiation that is different in magnitude. For example, the short-wavelength PMT (photoelectron multiplier) fixes the total radiation wavelength λ = (460 ± 5) nm, while when using a PMT with a multi-alkaline cathode, a significant long-wavelength shift of the dominant wavelength of the spectral maximum to 495 ÷ 500 nm is observed. If silicon photodiodes are used as the radiation detector of the proposed two-component scintillator, then the value of the dominant wavelength is shifted to λ _house = 515 ÷ 520 nm. At the same time, we found that the intensity of the noise processes accompanying the scintillation process decreases sharply (up to 4–5 times). The unusual and unpredictable property of the proposed two-component scintillator based on orthosilicogermanate and orthoaluminate also lies in the discovery of such a phenomenon of “noise suppression”.

The effect of a significant increase in efficiency arises, as follows from the data in Table 3, when using a fluorescent emitting material from yttrium orthoaluminate, cadmium gadolinium with a very small grain size, d _cp ≈ 2 μm in a two-component scintillator. With equal mass fractions of both components, for example, 98% and 2%, the light output increases for a scintillator with fine grain fluorescent garnet phosphor by 40%. When more finely dispersed grains of orthoaluminate are introduced into the scintillator, the light output of the radiation will exceed the value of this parameter for a single-crystal scintillator from NaJ (T1). In this case, the increase in light output in comparison with the standard is 50%. At present, we are not aware of scintillators for detecting beta and photon emissions of medium energies with a similar high yield. This is the advantage of the proposed two-component scintillator, characterized in that the optimal mass ratio of the orthosilicogermanate and orthoaluminate phosphors is respectively from 92-98% to 8-2% mass, while the average grain diameters of these materials are d _cp = 12 μm and d _cp = 2 microns.

In the manufacture of a scintillator, a two-component mixture of phosphors is distributed in a polycarbonate solution. Scintillating polymer tape is formed by casting onto a movable metal substrate at a speed of its movement up to 5 m / h. The cast coating is dried with infrared lamps for 1 hour. Optimally, the cast film coatings have a width of 200 mm and a thickness of 40 to 220 microns. The proposed film scintillator has a yellowish color and intensively fluoresces under the action of scattered daylight.

In the process of experimental work on the invention, the possibility of increasing the conversion efficiency of the scintillator at high energies of electrons and gamma rays was studied. As the main mechanism for increasing efficiency, we used the technique of increasing the average effective number Z of emitting materials used in the scintillator. So, the average atomic number of the orthosilicogermanate Y _2-xyz Gd _x Lu _y Ce _z O ₃ (SiO ₂ ) _1-p [GeO ₂ ] _p is Z _cp ≈40 units at [Gd] = 0 and [Lu] = 0, but already increasing the content of [Gd] = 0.3 and [Lu] = l of the atomic fraction increases the average value of the atomic number to Z = 60 units. Increasing the content of [GeO ₂ ] = 0.2 increases the average atomic number to Z = 63 units. We have proposed controlling the average atomic number of the Y _3-pq Gd _p Ce _q (AlGa) ₅ O ₁₂ fluorescent phosphor by simultaneously increasing the content of two heavy elements Gd and Ga in the phosphor basis. An increase in the gadolinium content to [Gd] = 1.8–2.0 atomic fraction with an increase in the content of Ga = 3 atomic fraction increases the average atomic number to Z = 60 units. The introduction of two emitting materials with medium-high atomic numbers into the scintillation coating allows one to achieve an energy shift on the “efficiency-energy of quanta” curve towards higher energies. In this case, the energy curve becomes more gentle in the region of photon fields with E = 160–180 keV. This is also the advantage of the proposed scintillator.

The obtained thin-film scintillating coating based on a double mixture of phosphors is designed to create special detecting blocks using optically transparent light collectors, the geometric dimensions of which are selected from the conditions of their use. In the process of testing the new scintillator, thermal studies of its stability were carried out, which showed its high quality. In the mode of small, medium and large doses of radioactive radiation (electrons and gamma rays), high parameters of its radiation stability are established.

An installation series of new scintillators is currently ready for release.

Claims (3)

1. A film scintillator for detecting beta and photon radiation, made of polycarbonate filled with a scintillating phosphor of powdered crystalline orthosilicate orthosilicate yttrium, gadolinium, lutetium and cerium having the general stoichiometric formula
Y _2-x-y-z · Cd _x · Lu _y · Ce _z · O ₃ (SiO ₂ ) _1-p · (GeO ₂ ) _p
at intervals of stoichiometric indices:
x = 0.01 ... 1.0,
y = 0.01 ... 0.9,
z = 0.005 ... 0.05,
p = 0 ... 0.8,
characterized in that, in addition, a powdered crystalline orthoaluminate of yttrium, gadolinium, cerium with a general stoichiometric formula is used as a scintillating phosphor
Y _3-mn Gd _m Ce _n (AlGa) ₅ O ₁₂
at intervals of stoichiometric indices
m = 0.01 ... 1,
n = 0.01 ... 0.1,
the maximum wavelength of optical radiation of a phosphor mixture is from 420 to 550 nm, and the scintillation duration is from 30 to 40 ns. 2. A film scintillator for detecting beta and photon radiation according to claim 1, characterized in that the ratio of the crystalline orthosilicate co-yttrium, gadolinium, lutetium and cerium and orthoaluminate yttrium, gadolinium, cerium in the mixture is from 92-98 to 8-2 wt. % respectively. 3. A film scintillator for detecting beta and photon radiation according to claim 1, characterized in that the concentration of the mixture of scintillating phosphors in the film is from 10 to 75 wt.%.