Scintillating substance and scintillating wave-guide element

Details Scintillating substance and scintillating wave-guide element Scintillating substance and scintillating wave-guide element

1999-09-10

2001-08-21

Sugarman, Scott J. (Department: 2873)

Optical waveguides

Having particular optical characteristic modifying chemical...

C117S012000, C117S013000, C250S370110, C250S483100, C252S301170, C385S012000, C385S129000

Reexamination Certificate

active

06278832

ABSTRACT:

FIELD OF INVENTION
The invention is related to nuclear physics, medicine and oil industry, namely to scintillating materials, and is meant for: registration and measurement of an x-ray, gamma and alpha radiation; control for trans 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 hard bodies; three dimensional positron-electron computer tomography and x-ray computer fluorography without the use of photo films; as well as for the control of the level of liquid in oil reservoirs.
DESCRIPTION OF RELATED ART
Known is the material of lutetium oxyorthosilicate with cerium LU
2(1−x)
Ce
2x
SiO
5
where x is varying in the range from 2×10
−4
to 3×10
−2
(U.S. Pat. No. 4,958,080: date of Patent Sept. 18, 1990, “Lutetium orthosilicate single crystal Scintillator detector”, Inventor C. I. Melcher, W. Redding Assignee: Schlumberger Technology Corp., 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 Lu
2(1−x)
Ce
2x
SiO
5
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 scintillating parameters:
the value of a light output, the position of a luminescence maximum and time of luminescence. This is explicitly 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.)
The scattering of scintillating elements patameters of lutetium oxyorthosilicate with cerium is the result of a small coefficient of cerium ions distribution between a growing crystal and melt (Kc
e
=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 brings about the fact that the light output of samples luminescence is 2-5 times lower in the lower part than in the top part, and the decay time is increased from 41 ns to 50 ns. Such scattering of parameters allows to use only a small part of a crystal boule for the production of scintillating elements.
As a prototype for the proposed invention it is possible to select scintillating crystals of the company Hitachi Chemical Co. Ltd. (Tokyo, Japan), having the composition, represented by the following chemical formula Gd
2−(x+e)
Ln
x
Ce
y
SiO
5
, where Ln=Sc, Tb, Dy, Ho, Er, Tm, Yb and 0.03≦x≦1.9, 0.00≦y≦0.2 (European patent ER 0456 002B1: Date of publication Jun. 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., as well as U.S. Pat. No. 5,264,154: date of Patent Mar. 11, 1996, “Single crystal scintillator”, Inventor S. Akiyama, H. Ishibashi, T. Utsu, C. I. Melcher, J. S. Schweizer, Assignee: Hitachi Chemical Co. Ltd).
In prototype crystals it is possible to substitute a Gd
3+
ion with a big radius for an ion with a small radius, for example, for Lu
3+
ion. This allows to control some scintillation parameters, in particular, to shift a maximum peak of luminescence from 430 nm up to 416 nm—in the field of a greater sensitivity of photoelectronic multipliers. The change of prototype crystals composition also allows to smoothly change their density and to decrease the time of luminescence for Ce
3+
ions up to 30 ns. Even with a non-significant content of Gd in 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 known crystals of lutetium oxyorthosilicate. Comparison of the light output of the prototype with the known crystals of Ce
2−x
Lu
2(1−x)
SiO
5
are made by the authors of the given invention and are summed up in table 1 (G. B. Loutts, A. I. Zagumennyi, S. V. Lavrishchev, Yu. D. Zavartsev, and P. A. Studenikin “Czochralski growth and characteristics of (Lu
1x
Gd
x
)
2
SiO
5
single crystals for scintillators”. J. Crystal Growth, Vol. 174 (1997), p. 331-336).
To the drawbacks of the prototype can also be referred that with the content of Gd of more than 50 at. % in the melt, these materials are crystalized in a monoclinic syngony with the spatial group P2
1
/c, Z=4.
In crystals with such a spatial group, deterioration of scintillation characteristics of ion Ce
3+
is observed, compared to known crystals of Ce
2−x
Lu
2(1−x)
SiO
5
, which are crystallized in a structural type with a spatial group B2/b, Z=8. So, for example, in crystals with a spatial group P2
1
/c observed are: the increase of a constant for the time of scintillations decay &tgr; up to 50-60 ns; the displacement of the peak of luminescence up to 430-440 nm, where the sensitivity of electronic photomultipliers is less. One more essential drawback of crystals with a spatial group P2
1
/c is a strong cracking during crystal boule cutting and their polishing, which sharply increase the cost of manufacturing elements of the size 2 mm×2 mm×15 mm for three dimensional positron-electron tomography with the resolution of 8 mm
3
.
The essential technical drawback of known scintillating crystals Ce
2−x
Lu
2(1−x)
SiO
5
and crystals of the prototype is the growing of crystals from melting stock, containing an extremely expensive reagent Lu
2
O
3
with the chemical purity of not less than 99.99%. The common drawback of these materials is also the impossibility of creating scintillating waveguide elements at the expense of refractive index gradient along the waveguide cross section.
SUMMARY OF INVENTION
The technical task of the invention is the increase of the light output of luminescence, decrease of the time of luminescence of ions Ce
3+
, increase of the reproducibility of properties of grown single crystals, decrease of the cost of source melting stock for growing crystals scintillators, contained in great amount of Lu
2
O
3
, the extension of the arsenal of technical facilities, implementing scintillating properties, the increase of effectiveness of the introduction of scintillating crystal luminescent radiation into glass waveguide 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 waveguide properties in scintillation elements at the expense of refractice index gradient along its cross section, exclusion of expensive mechanical polishing of the lateral surface of scintillating crystals at the stage of their growth.
The technical result is achieved due to the growing of crystals in a structural type Lu
2
SiO
5
with a spatial group B2/b (Z=8), as well as at the expense of an advantageous content of Ce
3+
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 nm; &bgr;=122.4°.
In crystals with a spatial group B2/b (Z=8) an anomaly high scintillating light output for ions Ce
3+
is observed, compared to all other known compositions of silicates, which as a rule have 2-5 times less light output during gamma excitation.
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