CA2283320C - Scintillating substance and scintillating wave-guide element - Google Patents

Abstract

The invention is related to nuclear physics, medicine and oil industry, name ly to the measurement of x-ray, gamma and alpha radiation; control for trans uranium nuclides in the habitat of a man; non destructive control for the structure of heavy bodies; three dimensional positron - electron computer tomography, etc. The essence of the invention is in additional ingredients in a chemical composition of a scintillating material based on crystals of oxyorthosilicates, including cerium Ce and crystallized in a structural type Lu2SiO5. The result of the invention is the increase of the light output of the luminescence, decrease of the time of luminescence of the ions Ce3+, increase of the reproducibility o f grown crystals properties, decrease of the cost of the source melting stock for growing scintillator crystals, containing a large amount of Lu2O3, the raise of the effectiveness of the introduction of scintillating crystal luminescent radiation into a glass waveguide fibre, prevention of cracking of crystals during the production of elements, creation of waveguide properties in scintillating elements, exclusion of expensive mechanical polishing of their lateral surface.

Description

SCINTILLATING MATERIAL (VARIANTS) AND
SCINTILLATING WAVE-GUIDE ELEMENT
The inventions relate to scintillating materials and they can be used in nuclear physics, medicine and oil industry for registration and measurement of an x - ray, gamma and alpha radiation; control for traps uranium radio nuclides in the habitat of a man (in particular, in the zones of Chernobyl catastrophe); sparing (non-destructive) control of the structure of solid bodies; three dimensional positron - electron computer tomography and x - ray computer 1 o fluorography without the use of photo films; as well as for the control of the level of liquid in oil reservoirs.
Known is the material of lutetium oxyorthosilicate with cerium CeZx Lu2 ~1-X>
Si05 where x is varying in the range from 2x10' to 3x10-Z (US Patent No. 4,958,080, 18. 09. 90, as well as Victorov L.V., Skorikov V.M., Zhukov V.M., Shulgin B.V. "Inorganic scintillating materials", Published by the Academy of Sciences of the USSR, series Inorganic materials, volume 27, N 10, pages 2005-2029, 1991). These scintillating crystals Ce2_X
Lu2 ~~_X~ Si05 have a number of advantages compared to other crystals: bigger density, high atomic number, relatively low refractive index, high light output, short time for scintillations decay. The drawback of the known scintillating material is a big scattering of the most important 2 0 scintillating parameters: the value of a light output, the position of a luminescence maximum and time of luminescence. This is clearly demonstrated by experimental results (J.D. Naud, T.A. Tombrello, C.I. Melcher, J.S. Schweizer "The role of cerium sites in the scintillation mechanism of LSO" IEEE Transactions on nuclear science, vol. 43, N 3, (1996), p. 1324 -1328.) 2 5 The scattering of parameters of scintillating elements from lutetium oxyorthosilicate with cerium is the result of a small coefficient of cerium ions distribution between a growing crystal and melt (Kce = 0.25), as a result of which a boule, grown by Czochralski method, has a concentration of cerium which is several times higher in the lower part than in the upper one. This causes the fact that a luminescence light output of samples is 2 - 5 times lower in 3 0 the lower part than in the top part, and a decay time. increases from 41 ns to 50 ns. Such scattering of parameters allows using only a small part of a crystal boule for the production of scintillating elements.
Known are glass optical wave-guides used for an optical transmission of information, see "Reference Book on Laser Technique", translation from German, V.N.
Belousov, Moscow, Energoizdat, 1991, p. 395//WISSENSSPREICHER LASERTECIIIVIK/Witolf Brunner, Klaus JungelVEB Fachbucherverlag Leipzig, 1987, in which for attributing wave guide properties to a fibre, it is manufactured with a refractive index gradient along its cross section at the expense of differences between chemical composition in its central and peripheral parts. However, the above fibres are not meant for the use as radiation sensors in scintillation detectors.
The closest analogue for the first and second variant of the claimed scintillating material, selected as a prototype, is a scintillating material of the company Hitachi Chemical Co. Ltd. (Tokyo, Japan), which crystals have a composition represented by the following chemical formula Gd2_ (x+y~LnxCeS,SIOs, where 0.035x<_1.9, 0.001<_y_<0.2. See application EP
0456 00281, 6.11.1996 "Single crystal scintillator and apparatus for prospecting underground strata using same". Inventor S. Akiyama, T. Utsu, H. Ishibashi, C.I. Melcher, J.S. Schweizer, Assignee: Hitachi Chemical Ltd.
The closest analogue for the third variant of claimed scintillation material, selected as a prototype, is a scintillating material of the company Hitachi Chemical Co.
Ltd. (Tokyo, 2 0 Japan), which crystals have a composition represented by the following chemical formula GdZ.~X+y~LnxCe~,Si05, where Ln -Sc, Tb, Lu, Dy, Ho, Er, Tm, Yb and 0.035x<_1.9, 0.OO15y50.2. See Patent US 5,26,154 "Single crystal scintillator", Inventor S.
Akiyama, H.
Ishibashi, T. Utsu, C.I. Melcher, J.S. Schweizer, Assignee: Hitachi Chemical Co. Ltd., 11.03.96 2 5 This patent (US Patent 5264154) practically completely repeats the invention claims of the above application EP 045600281, with one exception. In it, similar to the third variant of the material according to application CA 2283320, claimed is the material in which Lu can be or not be present in its composition, while in the material according to the application EP
045600281 the presence of Lu in the material is obligatory.

In prototype crystals it is possible to substitute a Gd3+ ion with a big radius for an ion with a small radius, for example, for Lu3+ ion. This allows to control some scintillation parameters, in particular, to shift a maximum peak of luminescence from 430 nm up to 416 nm - into the field of greater photo-electronic multipliers sensitivity. The change of prototype crystals composition also allows to smoothly change their density and to decrease the time of luminescence for Ce3+ ions up to 30 ns. Even with a non - significant content of Gd in a melt 20 mol %, it is possible to increase the homogeneity of the crystals grown because of the increase of cerium ions distribution coefficient.
The drawbacks of the prototype are the decrease of the light output of luminescence and of effective atomic number, compared to the known crystals of lutetium oxyorthosilicate.
The comparison of the light output of the prototype with the known crystals of Ce2_XLu2 ~;_ X?SiOs are made by the authors of the given invention and are summed in Table 1.
Table 1 Comparison of the light output and the effective atomic number of prototype crystals depending on the composition of a scintillating material Crystal Crystal composition Light output Effective atomic number Ce:LSO C.L. Melcher, Schlumberger -Doll Research 0.94 63.7 Ce: LSO Lul.9~aCeo.ooa6Si05 1.00 63.71 0.8LS0/0.2GS0L111.672Gd0.298~e0.0036S1O5 0.77 62.82 O.SLSO/O.SGSOLu1.,36Gdo.sa~Ceo.oo~zSi05 0.43 61.12 O.1LS0/0.9GS0Luo.,~3Gd~.83oCeo.o~2~Si05 0.29 57.66 Ce:GSO Commercial sample of Hitachi Chemical Co. 0.41 56.94 The drawbacks of the prototype also include that with the content of Gd of more than 50 at. % in the source melt, these materials are crystallized in a monoclinic syngony with a spatial group P21/c, Z=4. In crystals with such spatial group, deterioration of scintillation 2 o characteristics of Ce3+ ion is observed, compared to known crystals of Ce2_XLu2 ~~_x~Si05, which are crystallized in a structural type with a spatial group B2/b, Z=8.
So, for example, in crystals with the spatial group P2~/c observed are: the increase of a constant for the time of scintillations decay i up to 50-60 ns; the displacement of the peak of luminescence up to 430-440 nm, where the sensitivity of electronic photo multipliers is less: One more essential drawback of crystals with the spatial group P21/c is a strong cracking during crystal boule cutting and polishing, which drastically increases the cost of manufacturing elements of the size 2 mm x 2 mm x 15 mm for three dimensional positron - electron tomography with the resolution of 8 mm3.
The essential technical drawback of known scintillating crystals Ce2_xLu2 y-x)SiOs and prototype crystals is the growing of crystals from melting stock, containing an extremely expensive reagent Lu203 with a chemical purity of not less than 99.99%. A
common drawback of these materials is also the impossibility of creating scintillating wave-guide elements at the expense of refractive index gradient along the wave-guide cross section.
Known is a wave-guide element, selected as a prototype, which refractive index in the central part is greater than the refractive index in the peripheral part. By that, the element is two-layer one and each layer is made out of the scintillating material, with a radiation spectrum of the material of the peripheral part coinciding or overlapping the radiation spectrum of the central part (see USSR authorship certificate No. 1122113, G
O1 T 1/20, 1992).
2 0 This wave-guide element permits to provide a registration of nuclear and other radiation, and a delineation border of phases in the vicinity of two materials core-shell contact, where an additional scattering of light takes place, causes the reduction of light output, and the condition of coincidence or overlapping of absorption spectra in the central and peripheral part of the element brings about a strong scattering of the light output 2 5 depending on external conditions changes. Besides, the availability of a shell complicates the technology of wave-guide elements manufacturing, especially on the basis of crystals.
The technical task solved by the proposed group of the inventions is the increase of the light output of luminescence, decrease of the time of Ce3+ ions luminescence, increase of the reproducibility of properties of grown single crystals, reduction of the cost of source 3 o melting stock for growing crystals scintillators, containing Luz03 in great amount, the extension of the arsenal of technical facilities, implementing scintillating properties, the increase of effectiveness of the introduction of scintillating crystal luminescent radiation into a glass wave-guide fibre. In specific forms of implementation, the task of the invention is also the prevention of crystals cracking during cutting and manufacturing scintillation elements, creation of wave-guide properties in scintillation elements at the expense of refractive index gradient along its cross section, elimination of expensive mechanical polishing of a lateral surface of scintillating crystals at the stage of their growth.
The above tasks are solved in the following way:
In the known scintillating material on the basis of silicate crystal, including lutetium Lu and cerium Ce, in the first variant new is that the composition of the crystal is expressed by the chemical formula:
Lul:yMeyA1 _xCe,~Si05 Where:
A - Lu and at least one element of the group Gd, Sc, Y, La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb;
Me - at least one of the elements of the group Ti, Zr, Sn, Hf, As, V, Nb, Sb, Ta, Mo, W;
x - from 1 x 10~ fu. up to 0.2 fu.; and y - from 1 x 10-5 fu. up to 0.05 fu.
2 0 In the known scintillating material on the basis of silicate crystal, including lutetium Lu and cerium Ce, in the second variant new is that it contains oxygen vacancies ~ in the quantity of no more than 0.2 f.u. and has a composition, described by the chemical formula Lul _yMeyA~ _XCeXSi05_Z~Z
Where:
2 5 A - Lu and at least one element of the group Gd, Sc, Y, La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb;
Me - at least one of the elements of the group H, Li, Be, B, C, N, Na, Mg, Al, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb, Sr, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Cs, Ba, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, U, Th;
3 0 x - from 1 x 10'~ f.u. up to 0.2 fu.;

3360 0006~"
y - from 1 x 10-5 fu. up to 0.05 fu.; and z - from 1 x 10-5 f.u. up to 0.2 f.u.
In the known scintillating material on the basis of silicate crystal, including Ce, new in the third variant is that it contains fluorine F and has a composition, described by the chemical formula:
AZ_X_yMeyCeXSi05_;F;
Where:
A is at least one element of the group Lu, Gd, Sc, 'S~, La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb;
1 o Me - at least one of the elements of the group H, Li, Be, B, C, N, Na, Mg, Al, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb, Sr, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Cs, Ba, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, U, Th;
x - from 1 x 10-4 f.u. up to 0.2 f.u.;
y - from 1 x 10-5 fu. up to 0.05 f.u.; and i - from 1 x 10-4 f.u. up to 0.2 fu.
Besides, the scintillating material in all three variants can comprise Ce+3 ions amounting from 0.00005 fu. up to 0.1 fu.
In the known wave-guide element from the scintillating material, in which a refractive index in the central zone is greater than the refractive index in the peripheral zone, new is that 2 0 the wave-guide element is made of a single crystal scintillation material with a refractive index gradient along the element cross section.
Besides, the lateral surface of the wave-guide element can be chemically polished.
A combination of the above features makes it possible to create a scintillating material with the increased luminescence light output, raise a reproducibility of grown single crystals, 2 5 reduce the cost of a technological process, create wave-guide properties in scintillation elements, etc.
The technical result is achieved due to the growing of crystals in a structural type Lu2Si05 with a spatial group B2lb (Z=8), as well as at the expense of an advantageous content of Ce3+ ions in a crystal. As our research has shown, oxyorthosilicates are crystallized with a spatial group B2/b only in the case if the content of lutetium in a crystal is not less than 50 at. % and/or the parameter of a scintillating material lattice does not exceed the following maximum values: a= 1.456 nm; b = 1.051 nm; c = 0.679 run; (3 = 122.4°.
In crystals with a spatial group B2/b (Z=8) an anomaly high scintillating light output for ions Ce3+ is observed, compared to all other known compositions of silicates, which as a rule have 2 - 5 times less light output during gamma excitation.
The share of x-ray radiation, transformed into the energy of primary electrons, and especially the effectiveness of interaction of gamma - quantum with the material of a scintillator, approximately depends in proportion to the cube of effective atomic number. For 1 o y - quanta with the energy of F,r < 1.022 MeV, interaction of Y quanta with the material of a scintillating crystal takes place due to the process of photo effect, non-coherent and coherent scattering. With the energies exceeding a doubled energy of electrons state of rest (Ey> 1.022 MeV), a process of formation of electron - positron pairs is also added. It is supposed that in the formation of a pair, each of interacted primary ~ quanta gives birth to at least three secondary scattered Y quanta. Two of which having an energy of 0.511 MeV each, and represent radiation, appearing in electron and positron annihilation. It is obvious from that that in a three dimensional positron - electron tomography it is preferable to use scintillating crystals with a greater effective atomic number. In the process of crystal growth heavy ions of Lu3+, which are replaced by lighter admixture ions Mel+, Me2+, Me3+, Me4+, Me5+, Meb+, can 2 0 cause the growth of a crystal with a smaller density of 7.2 - 7.4 g/cm3, and atomic number Z =
58-63. In gmwing large crystal boules by the method of Czochralski for compensating the charge and for the correction of effective atomic number, it is preferable to use heavy ions Hf +, Tas+ and W6+, which prevents the changing of physical parameters (density, refractive index) along the diameter of large crystals (40 - 80 mm) and additionally allows to receive 2 5 crystals with identical scintillation parameters, i.e. to increase the reproducibility of properties of scintillating elements, which are manufactured from grown single crystals.
The spatial group B2/b (Z=8) contains 64 ions in an elemental unit, in particular 8 ions of lutetium of the first type (Lut) and eight ions of lutetium of the second type (Luz). The energy of substitution Ce3+ ~ Lu1 is equal to +6.90 eV, and the energy of substitution of Ce3+
3 0 ~ Lu2 is equal to + 7.25 eV. In both the cases the energy of substitution is positive, as ion radius Ce3+ is greater than the ion radius Lu3+. Different displacement of oxygen ions after the substitution of Ce3+~ Lug, Luz in coordination polyhedron LuO~ and Lu06 determines principally different scintillation characteristics of the material. The light output, the position of the luminescence maximum and the constant of time for scintillations decay (time of luminescence) depend on the number of Ce3+, which substituted ions Lul and/or ions Lu2. So, in gamma excitation both centres of luminescence are always excited and luminescence simultaneously, and the constant of time for scintillations decay will depend both on the duration of luminescence of the first and second centres, and on the relationship of the concentration of ions of Ce3+ in coordination polyhedrons LuO~ and Lu06. The centre of luminescence Cel (polyhedron LuO~) has the time of luminescence of 30 - 38 ns and the position of the luminescence maximum 410-418 nm. The centre of luminescence Ce2 (polyhedron Lu06) has the time of luminescence of 50 - 60 ns and the position of maximum luminescence of 450-520 nm. The maximum technical result is observed in scintillating crystals containing ions Ce3+ only in coordination polyhedrons LuO~.
The simultaneous presence of Ce3+ ions in LuO~ and Lu06 decreases the light output 3 - 10 times, increasing the time of luminescence up to 40-50 ns and shifts the luminescence maximum into the area of less sensitivity of photo electron multipliers. The crystals containing ions of Ce3+. advantageously in coordination polyhedrons LuO~ are obtained from the melt additionally doped with ions of the following elements: Zr, Sn, Hf, As, V, Nb, Sb, 2 0 Ta, Mo, W. By that, ions Ti, Zr, Sn, Hf, Nb, Sb, Ta occupy a position with octahedral coordination (polyhedron Lu06) in the crystal lattice due to higher bond energies of these ions. In contrast, ions As, V, Mo, W, occupy tetrahedral positions, however with octahedral positions being strongly distorted.
An additional technical result is achieved by the possibility of using Lu203 with the 2 5 purity of 99.9% (or less) as a sources reagent instead of reagent Lu203 with a purity of 99.99% and purity of 99.999% used in the prototype for obtaining claimed variants of scintillation materials, which allows to decrease the cost of a melting stock for growing crystals 2.5 - 3 times. Some admixtures in the source reagent Lu203 with the purity of 99.9%
(or less) can reduce the Iight output of luminescence 2 -10 times. The reduction of the light 3 0 output occurs due to the formation of Ce4+ ions in heterovalent substitution, which takes place during the growth of crystal on the background of crystallization. Below listed are the simplest schemes of substitution:
(1) Lu3++ Si4+ ~ Ce3++ Si4+ - optimal substitution of lutetium ions by cerium ions.
(2) Lu3+ + Si4+ ~ Ce+4 + Me3+ - highly probable, harmful and undesirable heterovalent substitution with the compensation of charge for admixtures Me3+<_ Be, B, Al, Cr, Mn, Fe, Co, Ga, In.
(3) 2Lu3+ ~ Ce4+ + Me2+ - highly probable, harmful and undesirable 1 o heterovalent substitution with the compensation of charges for admixtures Me2+ = Mg, Ca, Mn, Co, Fe, Zn, Sr, Cd, Ba, Hg, Pb.
(4) 3Lu3+ ~ Ce+4 + Ce+4 + Me~+ - probable harmful and undesirable heterovalent substitution with the compensation of charge at big concentrations of cerium ions for admixtures Me+ _ Li, Na, K, Cu, Rb, Cs, Tl.
However, the additional introduction into the melt of at least one of chemical compounds (for example, oxide) of the elements of the group Zr, Sn, Hf, As, V, Nb, Sb, Ta, Mo, W in the amount 2 - 3 times greater than the summary concentration in atomic percent of 2 0 admixture ions (Me+ + Me3+ + Me3~ eliminated the formation of Ce+4 ions in the process of the crystal growth. This is related to the fact that at the background of crystallization there takes place a heterovalent substitution according to energetically more beneficial schemes with the compensation of charge.
(5) Lu3+ + Si4+ ~ Me2+ + Mes+
2 5 (6) Lu3++ Si4+ ~ Me+ + Me6+
(7) Lu3++ Si4+ ~ Me4+ + Me3+
Crystalline boules, containing heterovalent micro admixtures with a non-compensated charge, are responsible for cracking in the process of growth of a crystal and it's cutting. That 9.

is why, for example, the addition into a scintillating material of a necessary quantity of ions, having the degree of oxidation of + 4, + 5, + 6 (for example, Zr, Sn, Hf, As, V, Nb, Sb, Ta, Mo, W, Th), permits to prevent the cracking of crystals in the process of growth, as well as during cutting single crystal boules and manufacturing elements. The above ions in an optimal concentration provide for the heterovalent substitution with the compensation of charge according to equation (5), (6), (7).
A separate variant of the proposed invention is growing the above crystal in inert, restoration or weakly oxidized environment. Under these conditions vacancies are formed in crystals in an oxygen sub-lattice and the composition of the crystal is described by the formula: Lul_yMeyAl_xCexSi05_Z~Z, where A-Lu and at least one element of the group Gd, Sc, Y, La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, x - the concentration of cerium ions, y - the concentration of ions of admixture elements, z - concentration of oxygen vacancies, which is determined based upon a consideration that at a small concentration of vacancies in an oxygen sub-lattice, the vacancies weakly affect the time of luminescence of Ce3+ ions and the light output of scintillating materials. However, the increase of concentration over an optimal limit causes a sharp decrease of the light output. The availability of oxygen vacancies completely suppresses the luminescence of admixtures rare earth ions Pr, Sm, Tb, Ho, Er, Tm and does not influence the luminescence properties of Ce3+, which permits to increase the stability of characteristics of the scintillating material and prevents cracking of crystals during 2 0 cutting, in the process of manufacturing scintillation elements. The presence in the source reagents or the addition of ions, having an oxidation degree of +4, +5, +6 (for example Zr, Sn, Hf, As, V, Nb, Sb, Ta, Mo, W, Th), into scintillation material in a necessary quantity, interferes with the formation of vacancies in the oxygen sub-lattice.
In the specific form of invention implementation the technical result, expressed in the 2 5 prevention of crystals cracking during cutting and manufacturing of scintillating elements is considerably improved by way of additional introduction into the material of at least one of the elements of the group H, F, Li, Be, B, Na, Mg, Al, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Rb, Sr, Zr, Nb, Mo, Ag, Cd, In, Sn, Sb, Cs, Ba, Hf, Ta, W, Hg, Tl, Pb, Bi.
An independent technical result - the creation of wave-guide properties in a 3 0 waveguide element along its cross section is achieved irrespective of spatial structure of oxyorthosilicate being crystallized, i.e. independently of the content of lutetium in a crystal because of the additional, compared to the prototype, content in a scintillating material of at least one elements of the group: H, F, Li, Be, B, C, N, Na, Mg, Al, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb, Sr, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Cs, Ba, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, U, Th. While the availability in the central zone of a scintillating element of ions F and/or H, Li, Be, B, C, N, Na, Mg, Al, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb, Sr, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Cs, Ba, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, Lu in a lesser concentration, and heavy ions of Hf, Ta, W, Re, Os, Ir, Au, Hg, Tl, Pb, Bi, U, Th in a greater concentration than in the peripheral zone of the volume - results in wave-guide properties of this element.
The introduction of F ions into the scintillating material composition permits to considerably decrease the constant of scintillations decay time (see Table 2), which increases the sensitivity of scintillation detectors.
Raising the efficiency of introducing radiation from scintillating crystal into the glass waveguide fibre is an independent technical task. This technical result is achieved by way of using wave-guide scintillating element from single crystal scintillation material, in which wave-guide properties are created in the scintillating element itself at the expense of the refractive index gradient along its cross section. The refractive index gradient appears in 2 0 crystal because of the difference of the chemical composition of its central zone from the chemical composition of its peripheral zone, but as the materials is a single crystal, there will not be a delineation border for phases between the central and peripheral zone of the wave-guide element and, consequently, there will not be additional light scattering, reducing the light output.
2 5 The refractive index of the central zone of the scintillating waveguide element should be grater than that of the peripheral zone In this case a scintillation element acquires an additional property: it focuses radiation along the axis of an element, as a result of which the radiation goes out of the scintillating element with a smaller divergence than from usual scintillating elements. This allows decreasing the divergence and, as a consequence, 3 0 decreasing the losses of radiation during its introduction into a glass fibre. The reduction of the refractive index of the peripheral zone of the scintillating element due to the change of the crystal composition can be achieved by any of the known methods or their combination:
- growing of a profiled crystal, which allows to immediately receive crystals, the composition of the peripheral zone of which is different from their central zone, - diffusion of light atoms from the melt, - diffusion from hard phase or gas phase into the surface layer of the scintillation element.
Additionally, for strengthening the waveguide effect, after growth and/or non-polished surfaces of scintillating elements can be polished chemically. Bt that all lateral surfaces can be polished simultaneously at scintillating elements in the amount 2 - 100 pieces (or more), for example, with the size 2 x 2 x 15 mm or 3 x 3 x 15 mm. For etching it is possible to use any polishing mixtures of acids, based on H3P04 with the addition of any acids, for example, HN03, HzS04, HCI, HF. For improvement of polishing properties any organic or inorganic salts containing ions H, Li, Be, B, C, N, F, Na, Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Br, Rb, Sr, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, I, Cs, Ba, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, U can be added to the mixture of acids. Comparison of scintillating elements with mechanically polished surfaces and chemically polished elements has shown that chemical polishing provides for the increase of the refractive capacity of the surface of any scintillating element, including a waveguide element.
2 o Both the growing of profiled scintillating crystals, and the additional chemical polishing of scintillating element surfaces, allows to achieve a positive technical result - the exclusion of expensive mechanical polishing of lateral surfaces of scintillating crystals, including that at the stage of their growth. It is necessary to point out that growing of profiled scintillating crystals allows to avoid an expensive polishing of lateral surfaces due to the 2 5 introduction into the material of the above admixtures. These admixtures, at certain concentrations, allow suppressing the evaporation of easily volatile components from the surface of the growing crystal. As a result the surface of blanks for scintillation elements is smooth, does not require further mechanical polishing. In separate cases an additional chemical polishing of the lateral surfaces of scintillating elements is required.

' , ' 3360 0006 Wave-guide scintillating elements with the refractive index gradient along its cross section allow for almost two times increase of the effectiveness of the introduction of radiation into a glass wave-guide fibre (with the length of 4 - 5 meters), which transmits radiation from a scintillation crystal to the photo electronic multiplier.
Wave-guide scintillating elements can be manufactured from any scintillating material, for example Ce: GdZSi05, Ce:Lu3A15012, Ce:YAl03, Bi4Ge301z and others.
The essence of proposed technical solutions is illustrated by the following graphical materials:
Fig 1. Scheme of luminescent radiation reflection and propagation in scintillating element (L»R) with a constant refractive index in known scintillation detectors.
Fig. 2. Scheme of luminescent radiation reflection and propagation in scintillating element (L»R) with a refractive index gradient along its cross section.
Fig. 3. Graph of the dependence of luminescence intensity on the wavelength after chemical polishing of a wave-guide element.
In the proposed first variant of the scintillating material based on known crystals of oxyorthosilicates, including cerium Ce, material characterised in that the composition of the crystal. is represented by the chemical formula:
Lul _yMeyA~ _XCeXSi05 Where:
2 0 A - Lu and at least one element selected from the group consisting Gd, Sc, Y, La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb;
Me - at least one element selected of the group Ti, Zr, Sn, Hf, As, V, Nb, Sb, Ta, Mo, W;
x is a value between 1 x 10'~ f.u. up to 0.2 f.u.; and y is a value between 1 x 10'5 f.u. up to 0.05 fu.
2 5 The lower limit of these elements is determined by the fact that at concentrations lower than the above limit of the technical result, the increase of the light output of luminescence, decrease of the time of luminescence for ions Ce3+, increase of the reproducibility of the properties of grown single crystals, decrease of the cost of source melting stock for growing crystals of scintillators, containing in great amount of Lu203 - are not observed. With the concentrations of the above elements lower that the above limit, the implementation of the technical task in individual forms of execution is also not achieved, namely it is not possible to prevent the cracking of crystals during cutting and manufacturing of scintillating elements, if as a source reagent used is Lu203 with the purity of 99,9% (or less).
The upper limit of these elements is determined by their maximum possible content in crystals, which are crystallized in a structural type Lu2Si05 with a spatial group B2/b (Z=8).
When their content is above the indicated limit, the destruction of the structural type Lu2Si05 takes place and the formation of inclusions of other phases, which determine the scattering of light and the decrease of transparency of a scintillating crystal.
The scintillating material comprises Ce3+ ions amounting from 0.00005 fu. up to 0.1.
f.u.
In the proposed second variant of the scintillating material based on known crystals of oxyorthosilicate, including cerium Ce, material characterised in that the composition of the crystal. is represented by the chemical formula:
Lug _yMeyA~ _XCeXSi05_Z~Z
where A - Lu and at least one element selected from the group consisting Gd, Sc, Y, La, Pr, Nd, Sm, 2 0 Eu, Tb, Dy, Ho, Er, Tm, Yb;
Me is at least one of the elements selected from the group consisting H, Li, Be, B, C, N, Na, Mg, Al, P, S, CI, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb, Sr, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Cs, Ba, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, U, Th;
2 5 x is a value between 1 x 10'~ f.u. up to 0.2 f.u.;
y is a value between 1 x 10'5 fu. up to 0.2 f.u.; and z is a value between 1 x 10'5 f.u. up to 0.2 fu.
In growing the above new scintillation materials in inert, restoration or weakly oxidized environment, oxygen vacancies are formed in crystals, which in small 3 0 concentrations weakly affect the achievement of the positive result of the invention. It is practically impossible to establish a lower limit of the content of oxygen vacancies in a scintillation material due to the lack of valid methodologies of determination of low concentrations of vacancies for oxygen, that is why the lower limit is equal to 1 x 10'5 f.u., which corresponds to the minimum concentration of heterovalent admixtures Me2+, the presence of which in a scintillator crystal brings about the appearance of vacancies in an oxygen sub-lattice.
The upper limit of the content of oxygen vacancies is determined by the fact that scintillation materials with oxygen vacancies in the material exceeding 0.2 fu. is not applicable for the use for its purpose - for registration of x-ray, gamma and alpha radiation.
In the proposed third variant of scintillating material on the basis of known oxyorthosilicate, including cerium Ce, material characterised in that the composition of the crystal. is represented by the chemical formula:
Az_X_yMeyCexSi05_;F;
Where:
A - at least one element of the group Lu, Gd, Sc, Y, La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb;
Me - at least one of the elements of the group H, Li, Be, B, C, N, Na, Mg, Al, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb, Sr, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Cs, Ba, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, U, Th;
2 0 x - from 1 x 10'~ fu. up to 0.2 fu.;
y - from 1 x 10'S f.u. up to 0.2 fu.; and i- from 1 x 10'~ fu. up to 0.2 f.u.
The lower limit of the fluorine ions is determined by the fact that at concentrations lower than the indicated level, a technical result, manifested in a considerable reduction of 2 5 scintillation decay time constant (Table 2) is not observed.
The upper limit of the fluorine ions is determined by their maximum possible content in crystals, which are crystallized in a structural type Lu2Si05 with a special group B2b (Z=8).
An individual case of the proposed inventions is a scintillating material wherein it contains ions Ce3+ in the range from 5 x 10'5 f. units up to 0. 1 f. units.

The lower limit for the ions of cerium is determined by the fact that with the content of Ce3+ in the quantity of less than 5 x 10-5 f, units, the effectiveness of a scintillation luminescence of Ce3+ becomes insignificant because of the small concentration.
It is necessary to point out that the limit of concentration interval for the content of cerium in a crystal is decreased two times. This is related to the fact that due to the use of the proposed scintillating matter a possibility of receiving scintillating materials -oxyorthosilicates with a maximum possible contents of ions of Ce+3 appears only in a coordination polyhedron LuO~.
The upper limit of the content of Ce3+ in a crystal is determined based on the fact that with the content of Ce3+ greater than 0.1 f. units, it is impossible to optically receive a high quality crystal. This is related to the high content of additional elements in a crystal, necessary for obtaining a maximum possible content of ions of cerium + 3 in coordination polyhedrons LuO~.
The proposed wave-guide element made of scintillating material having the refractive index at the central zone higher than the one at the peripheral zone, characterised in that the wave-guide element is made of a single crystal scintillating material with the gradient of the refractive index by the cross section of the element. Additional its lateral surface is chemically polished.
In known scintillation detectors, see Fig. 1, a scheme of reflection and propagation of luminescent radiation in scintillation element (L»R) with a constant refractive index in 2 o known scintillation detectors (R x R - cross - section of the element, L -its length, n -refractive index). Scintillation element 1 is shown, which has all six sides mechanically polished. To raise the efficiency of reflection it is possible to use metallic mirror coatings 2, for example from aluminum, or diffused reflecting coating 3, for example, from MgO, A1203, BN, Teflon or other white materials. Luminescent radiation 4, leaving from the end face of 2 5 the element is sent directly to a photo multiplier or is focused into a glass light guide for passing to the measuring device, located in several meters from the scintillation element.
In the wave guide element proposed in Fig. 2, where the scheme of reflection and propagation of luminescent radiation in scintillation element (L»R) with a refractive index along its cross section (R x R - cross section of the element, L - its length, n~- refractive 3 o index in the center of the element, n2 - the refractive index at the periphery of the element, a -angle of luminescent beam propagation) is given. Scintillation element 1 is shown, which has only one polished facet - through which radiation goes for registration.
Luminescent radiation 4, leaving from the end face of element, goes directly to the photo multiplier or focuses into a glass light guide for passing to a measurement device, located in several meters from the scintillation element.
As it is shown in Fig 3 as an example, a light output of scintillation element 1, made of crystal Lu~.99~Ceo.oozTao.oo~Si05,~4, after chemical polishing will be 5 times higher than the standard 2 element with the composition Bi4Ge30~2 with mechanically polished lateral surfaces.
1 o Measurements are made on samples of identical size and in the same conditions.
Examples of specific compositions of crystal and prototype, grown by Czochralski method, are shown in Table 2.
Table 2 Constant of scintillations decay time (T, ns) and light output (%) Constant Light of The composition of the meltingSize of the samplethe decay output, stock and purity of source reagents time i, ns Lu,.9gCeo.o2Si05 * 10 x 10 x 2 mm 42.3 100 ***

Lu203, CeOz, Si02, purity 99.995%

Lu~.98Ceo.ao3SiOs * 10 x 10 x 2 mm 44.1 98 ***

Lu203, CeOz, Si02, purity 99.995%

Lup.99Gd0.99Ce0.00,25105 **

****
Lu203, Ce02, Si02, Gd203 5 x 5 x 5 mm 33.9 43 purity 99.995%

Lul.9aCeo.oo3Si05 Luz03, purity 99.8% 10 x 10 x 2 mm 43.8 31 ***

CeOz, SiOz, purity 99.995%

Lu,.9~sCeo.ozTao.oosSiOs.oo2 Lu203 purity 99.8% 10 x 10 x 2 mm 38.3 100 ***

Ce02, Si02Ta205, purity 99.995%

Lu~.9~~Ceo.o2wo.oo3SiOs.oo2 Lu203, with the purity 99.8% 10 x 10 2 mm *** 39.2 100 Ce02, Si02, W03, purity 99.995%
Lu, .9~aCeo.o2Cao.oo~ Tao.osSi04Fo.o6 Luz03 with the purity 99.8% 10 x 10 x 2 mm *** 32.1 102 Ce02, Si02, Ta20s, purity 99.995%
CaO, CeF3, purity 99%
Lu~.9~sCeo.ooo2sTao.oosSiOs.ooz Lu203, Ce02, SiO2Ta20s with 10 x 10 x 2 mm *** 38.0 6 the purity of 99.995%
Notes: * the known scintillating crystal is indicated ** prototype crystal is indicated *** two surfaces 10 x 10 mm are mechanically polished **** all surfaces S x 5 mm are mechanically polished Example 1. Growing of crystals with a structural type Lu2SiOs and a spatial group B2/b (Z=8), additionally containing at least one element of the group Ti, Zr, Sn, Hf, As, V, Nb, Sb, Ta, Mo, W. Growing of these crystals was conducted according to the general scheme - by way of extruding from melt by any method, in particular by Czochralski method.
A scintillating crystal, grown of a melting stock Lu~.9~~Ceo,oxWo.oo3SiOs,~2 on the basis of Lu203 (purity 99.8%), additionally containing the ions of tungsten in the range of 1.2 x 10'9 atom/cm3, has a position of a maximum of luminescence about 418 nrn and the time of luminescence (decay of scintillations) T = 39 ns, compared with i = 42 ns for crystal, grown from the melt with the composition of Lu~.98Ceo.oZSiOs (Table 2).
These data confirm the possibility of growing crystals, containing ions of Ce3+
advantageously in coordination polyhedrons LuO~, if the melt is additionally doped with ions of the following elements: Ti, Zr, Sn, Hf, As, V, Nb, Sb, Ta, Mo, W, which occupy in a crystal an octahedral polyhedron Lu06 or tetrahedral positions. All these admixtures ions have an increased concentration in the diffused layer at the crystallisation front, as their 2 o coefficients of distribution are small (K< 0.2). An increased concentration of admixtures with the charge 4+, 5+, 6+ in a diffused layer interferes with the incorporation into the crystal of cerium atoms in the form of Ce4+, and does not affect the competing process of the substitution of Ce3+ ~ Lut, when it becomes the main one.
Example 2. Obtaining a scintillation material on the basis of oxyorthosilicate crystal, including cerium Ce, the composition of which is expressed by the chemical formula AZ_ XCeXSiOs, wherein A is at least one element of the group Lu, Gd, Sc, Y, La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, Yb, as well it contains fluorine F and/or at lest one of the elements of the group H, Li, Be, B, C, N, Na, Mg, Al, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb, Sr, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Cs, Ba, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, U, Th.
1 o The data of Table 2 demonstrate the possibility of using reagent Lu203 with the purity of 99.8% instead of a more expensive Lu203 with the purity of 99.995%. The introduction of additional compensating ions while using reagent Luz03 with the purity of 99.8% eliminates the possibility of deterioration of the most important parameter - the constant of time of scintillations decay T, for example, for crystals grown of the melting stock of the composition L111.974Ce0.02Ca0.001Ta0.OSS1O4.94F0.06 and Lu1.97sCe0.02Ta0.OSs1Os.002~
For growing the crystal of lutetium - cerium - tantalum orthosilicate by the method of Czochralski the melting stock of the composition of Lu~,9~sCeo.ozTao.oosSiOs,oo2 was used, which contained micro admixtures of Na, Mg, Al, Si, Ca, Ti, Cr, Mn, Co, Ni, Cu, Zn, Mo, Ba, Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, W, Pb, Th -which were 2 0 present in the source reagent Lu203 (99.8%) in the range from 1 x 10"
atom/cm3 up to 1 x 10'9 atom/cm3. By that, the following method of receiving samples was used:
source reagents lutetium oxide and silicon oxide were thoroughly mixed, pressed in tablets and synthesised in a platinum crucible during 10 hours at 1200° C. Then by means of induction heating the tablets were melted in an iridium crucible in a sealed chamber in the atmosphere of nitrogen 2 5 ( 100 volumetric % of N2). Before growing, cerium and tantalum oxide were added into the melt. A crystal was grown out of iridium crucible with the diameter of 80 mm with the volume of the melt of 330 cm3. At a speed of crystal pulling of 3 mm/hour and the frequency of crystal rotation of 20 rounds per minute. After detachment of the grown crystal from the melt, the crystal was gradually cooled down to a room temperature during 40 hours.

Experimental research of the relationship of the constant of the time of decay of scintillations (i, ns) and the light output in the area of 400 - 430 nm, depending on the chemical composition of crystals, was carried out using the radiation of radio nuclide 6°Co, similar to the methodology of E.G. Devitsin, V.A. Kozlov, S.Yu. Potashov, P.A.
Studenikin, A.I. Zagumennyi, Yu.D. Zavartsev "Luminescent properties of Lu3A1501z crystal, doped with Ce", Proceedings of the International Conference "Inorganic scintillators and their applications"(SCINT' 95), Delft, the Netherlands, Aug. 20 - 1 Sept. 1995. The results of measurements are shown in Table 2.
Example 3. Scintillating material based on the crystal of orthosilicate additional containing oxygen vacancies. For creating oxygen vacancies in crystalline samples, obtained by the method of Czochralski, their heating in vacuum during 2 hours at the temperature in the interval of 1200° C - 1620° was used. The formation of oxygen vacancies insignificantly affects the scintillation parameters of crystals, grown from reagents with the purity of 99.995%. On the contrary, oxygen vacancies bring about the decrease by 20% -70% of the light output of crystals, additionally doped, for example, by ions of Mo, W, Ta, due to the formation of dying centres.
The presence of oxygen vacancies completely suppresses the luminescence of admixture rare earth ions Pr, Sm, Tb, Ho, Er, Tm, and does not affect the luminescence properties of ions of Ce3+. In crystals of oxyorthosilicate additionally containing oxygen 2 0 vacancies completely suppressed and absent is the luminescence of ions of Tm3+ at 452 nm, ions Pr3+ at 470 - 4$0 nm and 520-530 nm, ions Tb3+ at 544 nm, ions Ho3+ at 550 nm, ions Er3+ at 560 nm, ions Sm3+ at 593 nm. The time of luminescence (decay of scintillations) of ions Pr, Sm, Tb, Ho, Er, Tm, is for several orders of magnitude longer than for ion of Ce3+, that is why the suppression of luminescence of admixture rare earth ions in the visible and 2 5 infra red area of the spectrum is necessary for the preservation of quick operation of elements based on Ce3+ ion, which is experimentally observed in silicates crystals, additionally containing oxygen vacancies.
Example 4. Scintillating material on the basis of oxyorthosilicate crystal, which contains Ce3+ ions in the quantity of 5 x 10'5 f., units up to 0.1 f. units. For growing by Czochralski 3 0 method of lutetium - cerium - tantalum orthosilicate crystal, containing Ce3+ ions in the range of 5 x 10's f. units, the melting stock was used with the chemical composition of Lu,.9~sCeo.ooo2sTao.oossiOs.ooz on the basis of source reagents (Luz03, CeOz, Si02, Ta20s) with the purity of 99.995%. The crystal was grown out of the iridium crucible with the diameter of 60 mm at a speed of pulling of 3 mm/hour and frequency of rotation of 20 rounds per minute.
At a contents of Ce3+ in a crystal in the amount of less than S x 10's f.
units, the effectiveness of the scintillation luminescence of Ce3+ becomes insignificant due to a small concentration, as a result of which the light output (Table 2) does not exceed 6% for samples, made of the top and bottom part of the crystalline boule with the weight of 1040 g.
An important technical advantage of scintillation crystals of oxyorthosilicates, 1 o containing small quantities of Ce3+ ions (5 x 10'~ - 5 x 10's f. units), is the possibility to use 100% of the melt in the process of crystal growth, which considerably increases the time of operation of iridium crucibles, and, consequently, decreases the cost of scintillating elements.
Example 5. Chemical polishing of the lateral surface of a scintillating element obtained by Stepanov's method or any other similar method allows to grow scintillation crystals with a necessary cross section (2 x 2 mm or 3 x 3 mm), which allows to eliminate the operation of cutting a large boule. Chemical polishing permits to polish all lateral surfaces simultaneously at scintillating elements in the quantity of 2 - 100 pieces (or more), for example, with the size of 2 x 2x 15 mm or 3 x 3 x 15 mm. By that, the lateral surface can have any form: cylindrical, conical, rectangular, polygonal or random. Cheap chemical polishing allows excluding an 2 o expensive mechanical polishing of the lateral surface of scintillating elements in the process of their manufacturing.
The crystal Lu~.99~Ceo.oozTaa,ooISiOs.~~ was grown by the method of Czochralski. 40 scintillating elements were cut out of a crystalline boule (10 elements of the size 2 x 2 x 15 mm, 10 elements of the size 2 x 2 x 12 mm, 10 elements of the size 3 x 3 x 15 mm, 10 elements of the size 3 x 3 x 20 mm). All 40 elements were simultaneously subjected to chemical polishing at temperature of 260° C in the mixture of the following composition:
H3P04 (30%) + H2S04 (61%) + NaF (4%) + NaCI (5%). The concentration is indicated in weight percent.

Optimal time of chemical etching is 30 minutes. As a result of chemical polishing an optically smooth lateral surface was obtained at which there are no pyramids of growth and etching pits.
The light output of a scintillating element Lu1.99~Ceo.oo2Tao.ooiSiOs.oooa after chemical polishing is more than 5 times higher than with the standard one used in electron - positron tomography Bi4Ge30,z with mechanically polished lateral surfaces (Fig. 3).
Example 6. The creation of waveguide properties in scintillating elements at the expense of the refractive index gradient along its cross section In the process of growth of a profiled crystal from melt, the form of a melt column determines its cross section. Different physical effects are used for the shaping of the melt.
The creation of the melt column of a certain form with a help of a shaper is known from the source Antonov P.L, Zatulovskiy L.M., Kostygov A.S. and others "Obtaining profiled single crystals and articles by Stepanov's method", L., "Nauka", 1981, page 280, as Stepanov's method for growing profiled crystals.
The application of Stepanov's method opens the possibility of growing scintillating crystals of the size of 3 x 3 x 200 mm with the formation of a wave-guide nucleus in the crystal in the process of growth. The wave-guide nucleus appears if there are admixtures in the melt, which depending on the distribution coefficient are concentrated in the central part (K> 1 ) or in the peripheral part (K< 1 ) of the growing crystal. Fig. 2 shows non-uniform 2 0 distribution of admixture along the crystal cmss-section (nl refractive index in the centre of a crystal and n2 - refractive index at the periphery of the crystal). Non-uniform distribution of admixture ions along the cross section (3 x 3 mm) of the crystal brings about the refractive index gradient along its cross section, and if n>>n2, a wave-guide effect takes place.
The wave-guide effect causes focusing of a light flow along the axis of an element and 2 5 increases the amount of light, leaving the end plane of the scintillating element, which in the long run determines the effectiveness of an actual gamma ray detector. The increase of the light flow from the end plane of the scintillating element occurs due to the decrease of the summary losses of scintillating radiation during reflection from a lateral surface.

,' ~ ~ 3360 0006 The second important advantage of scintillating elements (size 3 x 3 x 15 mm after cutting of a crystal rod into several elements) with a wave-guide effect compared to the elements 3 x 3 x 15 mm, manufactured from a large crystalline boule, is 1.5 -1.6 times greater effectiveness of the input of light beams into a glass light guide, which is responsible for the transfer of scintillating radiation from a scintillating element to the photo-electronic multiplier in a new type of medical 3-dimensional tomographs, in which two different physical methods of obtaining brain image of a man are used simultaneously:
electron -positron tomography and magnetic resonant tomography.
The growing of a profiled crystal by Stepanov's method was conducted using an iridium crucible with an iridium former, having a cross section of the outer edge of 3 x 3 mm, which was determining the cross section of the growing crystal. Transportation of melt out of crucible took place along a central capillary with the diameter of 0.9 mm due to capillary effect. For example, for obtaining a lutetium - gadolinium - cerium orthosilicate crystal with a focusing waveguide effect a melting stock with the composition Lu~,6~2Gdp.298Ce°.oo36S1O5 was used, using the following methodology. Source reagents: lutetium oxide, gadolinium oxide and silicon oxide were thoroughly mixed, pressed in tablets and synthesised in a platinum crucible during 10 hours at 1200° C. Then, the tablets were melted in an iridium crucible in a sealed chamber in the atmosphere of nitrogen (100 volumetric %
N2) by means of induction heating. Cerium oxide was added to the melt before growing. The former 2 o allowed growing from one to four profiled crystals simultaneously. Etching was performed to the crystal Lu2Si05, cut in a crystallographic direction (001), i.e. along the axis of optical indicatrix, having the greatest refractive index ng. Profiled crystals were pulled out of melt at a speed of 4 - 15 mm/hour without rotation. Growing a profiled crystal at a speed of higher than 20 mm/hour brings about the growth of crystal of a permanent composition along the rod 2 5 cross section. Upon the crystals reaching the length of 50-90 mm they were torn from the shaper by a sharp increase of the speed of pulling. The grown profiled crystals were cooled to a room temperature during 12 hours.
Profiled crystalline rods were cut into several scintillating elements of the size of 3 x 3 x 15. One sample with mechanically polished 6 surfaces was used for the determination of 3 0 composition with a help of electronic microanalysis (Cameca Camebax SX-50, operating at ' ~ 3360 0006 20kV, 50 ~A and diameter of the beam of 10 microns). For a profiled crystal, grown at a speed of pulling of 4 mm/hour, a crystalline rod in the centre had a composition Lu~.~BGdo.ZO2Ceo.oo~sSiOs and lateral surfaces had a composition in the range Lu~,s~_~.6oGdo.3o-o.ooassi05.
Gradient of the refractive index along a crystal cross section was determined from the interference picture: n~ - nZ ~ 0. 006, where n~ is a refractive index at the centre of a crystal and n2 is a refractive index at the periphery of a crystal. The presence of a refractive index gradient causes focusing along the axis of a waveguide scintillation element of all beams of scintillating radiation thanks to a complete internal reflection, if an angle between an optical axis and the direction of scintillation radiation is less than the angle a"~x,, calculated according to the formula , sina",aX. _ ~ n2 k - n2 m where nm the refractive index of the coating (periphery) of a light guide and nk is a refractive index of the core of the optical wave-guide. See "Reference book on laser technique".
Translation from German B.N. Belousov, Moscow, Energoizdat", 1991, page 395//
WISSENSSPREICHER LASERTECH1~TIK/Witolf Brunner, Klaus Junge. / VEB
Fachbucherverlag Leipzig, 1987.
For a scintillating element with the value of a refractive index gradient along the crystal cross section equal to nl - nz = 0.006 a complete internal reflection of all scintillating 2 o beams will take place if the angle of their spread is less than angle amaX. = 8.4 degrees. It is necessary to point out that a complete internal reflection of scintillation beams, having the direction of a < a~x,, takes place irrespective of the fact if the lateral surface of a scintillating element is polished or not. For scintillating elements widely used in computer tomography with a cross section of 2 x 2 mm or 3 x 3 mm and length of 15 - 20 mm with the angle of 2 5 complete internal reflection a",~, = 8.4 degrees there will take place 2 -3 complete internal reflections of scintillating beams before their leaving the element (Fig.2).

Claims (6)

What is claimed is:

1. Scintillating material based on a silicate crystal comprising lutetium (Lu) and cerium (Ce) characterised in that the composition of the crystal is represented by the chemical formula Lu1-y Me y A1-x Ce x SiO5 where A is Lu and at least one element selected from the group consisting Gd, Sc, Y, La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, and Yb;
Me is at least one element selected of the group Ti, Zr, Sn, Hf, As, V, Nb, Sb, Ta, Mo, and W;
x is a value between 1 x 10-4 f.u. up to 0.2 f.u.; and y is a value between 1 x 10-5 f.u. up to 0.05 f.u.

2. Scintillating material based on a silicate crystal comprising lutetium (Lu) and cerium (Ce) characterised in that it contains oxygen vacancies (.quadrature.) at the quantity not exceeding 0.2 f.u. and its chemical composition is represented by the formula Lu1-y Me y A1-x Ce x SiO5-z(.quadrature.)z where A is Lu and at least one element selected from the group consisting Gd, Sc, Y, La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, and Yb;
Me is at least one element selected from the group consisting H, Li, Be, B, C, N, Na, Mg, Al, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb, Sr, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Cs, Ba, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, U, and Th;
x is a value between 1 x 10-4 f.u. up to 0.2 f.u.;
y is a value between 1 x 10-5 f.u. up to 0.05 f.u.; and z is a value between 1 x 10-5 f.u. up to 0.2 f.u.

3. Scintillating material based on a silicate crystal comprising cerium (Ce), characterised in that it contains fluorine (F) and its composition is represented by the chemical formula A2-x-y Me y Ce x SiO5-i F i where A is at least one element selected from the group consisting of Lu, Gd, Sc, Y, La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, and Yb;
Me is at least one element selected from the group consisting of H, Li, Be, B, C, N, Na, Mg, Al, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb, Sr, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Cs, Ba, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, U, and Th;
x is a value between 1 x 10-4 f.u. up to 0.2 f.u.;
y is a value between 1 x 10-5 f.u. up to 0.05 f.u.; and i is a value between 1 x 10-4 f.u. up to 0.2 f.u.

4. Scintillating material according to Claims 1, or 2, or 3, characterised in that the content of the Ce3+ ions is within the range of 0.0005 f.u. to 0.1 f.u.

5. A wave-guide element comprising a scintillating material and having a refractive index at a central part greater than a refractive index at a peripheral part, wherein the wave-guide element is made of a single crystal scintillating material with a composition according to any one of claims 1, 2 and 3, and has a refractive index gradient along an element section.

6. The wave-guide element of Claims 5 characterised in that its lateral surface is chemically polished.