Crystalline rare-earth activated oxyorthosilicate phosphor

Details Crystalline rare-earth activated oxyorthosilicate phosphor Crystalline rare-earth activated oxyorthosilicate phosphor

2002-05-31

2004-02-10

Koslow, C. Melissa (Department: 1755)

Compositions

Inorganic luminescent compositions

Compositions containing halogen; e.g., halides and oxyhalides

C250S483100, C250S36100C

Reexamination Certificate

active

06689293

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to phosphors and more particularly to a crystalline, transparent, oxyorthosilicate phosphor activated with a rare-earth metal dopant.
BACKGROUND OF THE INVENTION
Phosphors are currently used in many important devices that include fluorescent lamps, RGB (red, green, blue) screens, lasers, and crystal scintillation detectors for radiography. Perhaps the most important property of any phosphor is its brightness, i.e. its quantum efficiency. The quantum efficiency of a phosphor is the ratio of the number of photons that a phosphor absorbs to the number of photons that a phosphor emits. Other important properties of a phosphor include its spectral region of maximum emission, its optical absorption (in particular, its self-absorption), its decay time, and its density. Superior phosphors have a high quantum efficiency, good linearity, fast decay time, and minimal self-absorption.
An exceptionally good phosphor is cerium-activated lutetium oxyorthosilicate. This material has been conveniently abbreviated in the literature as either LSO:Ce or Ce:LSO, and will be referred to herein as LSO:Ce. LSO:Ce is a crystalline solid composed of a host lattice of lutetium oxyorthosilicate (Lu
2
SiO
5
abbreviated LSO) that is activated with a small amount (e.g. 0.25 atomic percent of Ce relative to Lu) of the rare-earth metal dopant cerium (Ce). Cerium is an excellent activator because both its ground state and excited states lie within the band gap of about 6 eV of the host LSO lattice. LSO:Ce is a very bright crystalline phosphor, i.e. it has a very high quantum efficiency. LSO:Ce also has a high density (7.4 gm/cm
3
), a fast decay time (about 40 nanoseconds), a band emission maximum near 420 nanometers, and minimal self-absorption. Oxyorthosilicate phosphors, including LSO:Ce, have been documented in the following reports and patents.
“Czochralski Growth of Rare-Earth Orthosilicates (Ln
2
SiO
5
)” by C. D. Brandle et al (J. Crys. Growth, vol. 79, p. 308-315,1986), describes yttrium oxyorthosilicate (YSO) activated with Ce, Pr, Nd, Sm, Gd, Tb, Er, Tm, or Yb. While gadolinium oxyorthosilicate doped with Tb (GSO:Tb) appears in Table 3 of Brandle, its use is not described.
Single-Crystal Rare-Earth-Doped Yttrium Orthosilicate Phosphors” by J. Shmulovich et al. (J. Electrochem. Soc.:Solid-State Science and Technology, vol. 135, no. 12, p. 3141-3151, 1988), describes single crystals of rare-earth activated YSO prepared according to aforementioned C. D. Brandle et al. Among the phosphors described is a green phosphor containing YSO activated with a mixture of Tb and Gd, and a red phosphor containing YSO activated with a mixture of Tb and Eu.
“Czochralski Growth of Rare Earth Oxyorthosilicate Single Crystals” by C. L. Melcher et al. (J. Crys. Growth, vol.128, p. 1001-1005, 1993) describes the preparation of single crystals of GSO:Ce, LSO:Ce, and YSO:Ce by the Czochralski method.
“Czochralski Growth and Characterization of (Lu
1−x
Gd
x
)
2
SiO
5
” by G. B. Loutts et al. (J. Crys. Growth, vol.174, p. 331-336, 1997), describes the preparation and properties of single crystals of cerium-activated oxyorthosilicates having a crystal lattice of lutetium and gadolinium.
U.S. Pat. No. 4,647,781 to K. Takagi et al. entitled “Gamma Ray Detector,” which issued on Mar. 3, 1987, describes a cerium-activated oxyorthosilicate scintillator having the general formula Gd
2(1−x−y)
Ln
2x
Ce
2y
SiO
5
wherein Ln is yttrium and/or lanthanum, wherein 0≦x≦0.5, and wherein 1×10
−3
≦y≦0.1.
U.S. Pat. No. 4,958,080 to C. L. Melcher entitled “Lutetium Orthosilicate Single Crystal Scintillator Detector,” which issued on Sep. 18, 1990, describes LSO:Ce.
U.S. Pat. No. 5,025,151 to C. L. Melcher entitled “Lutetium Orthosilicate Single Crystal Scintillator Detector”, which issued on Jun. 18, 1991, describes an apparatus that uses the LSO:Ce scintillator of the '080 patent to investigate subsurface earth formations.
U.S. Pat. No. 5,264,154 to S. Akiyama et al. entitled “Single Crystal Scintillator,” which issued on Nov. 23, 1993, describes a single crystal cerium-activated oxyorthosilicate scintillator having the general formula Gd
2−(x+y)
Ln
x
Ce
y
SiO
5
wherein Ln is Sc, Tb, Lu, Dy, Ho, Er, Tm, or Yb, wherein 0.03≦x≦1.9, and wherein 0.001≦y≦0.2.
U.S. Pat. No. 6,323,489 to K. McClellan entitled “Single Crystal Scintillator,” which issued on Nov. 27, 2001, describes a single crystal, cerium activated oxyorthosilicate scintillator having the having the general formula Lu(
2−x−z
)Y
x
Ce
z
SiO
5
, wherein 0.05≦x≦1.95 and 0.001≦z≦0.02.
While the properties of LSO:Ce are exceptional, high quality single crystals of LSO:Ce are difficult and expensive to prepare. The high cost, which is at least partly due to the high cost of starting materials (high purity Lu
2
O
3
powder) and equipment (iridium crucibles for containing the Lu
2
O
3
powder, which melts at about 2150° C.) also limits efforts to develop other types of crystals with an LSO host lattice. Yet, there remains a need for crystal phosphors and scintillators with exceptional properties.
Therefore, an object of this invention is to provide a crystalline, rare-earth-activated oxyorthosilicate phosphor.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
SUMMARY OF THE INVENTION
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention as embodied and broadly described herein, the invention includes a crystalline, transparent phosphor consisting essentially of lutetium yttrium oxyorthosilicate activated with a rare-earth metal dopant M and having the general formula Lu(
2−x−z
)Y
x
M
z
SiO
5
, wherein 0.00≦x≦1.95, wherein 0.001 ≦z≦0.02, and wherein M is selected from Sm, Tb, Tm, Eu, Yb, and Pr.
The invention also includes a radiation detector having a transparent crystalline phosphor that consists essentially of lutetium yttrium oxyorthosilicate activated with a rare-earth metal dopant M and having the general formula Lu(
2−x−z
)Y
x
M
z
SiO
5
, wherein 0.00≦x≦1.95, wherein 0.001 ≦z≦0.02, and wherein M is selected from Sm, Tb, Tm, Eu, Yb, and Pr. The radiation detector includes a photodetector that is optically coupled to the phosphor to detect light from the phosphor.
The invention also includes a transparent crystalline phosphor consisting essentially of lutetium gadolinium oxyorthosilicate activated with a rare-earth metal dopant M and having the general formula Lu(
2−x−z
)Gd
x
M
z
SiO
5
, wherein 0.00≦x≦1.95, wherein 0.001 ≦z≦0.02, and wherein M is selected from Sm, Tb, Tm, Eu, Yb, and Pr.
The invention also includes a radiation detector having a transparent crystalline phosphor that consists essentially of lutetium gadolinium oxyorthosilicate activated with a rare-earth metal dopant M and having the general formula Lu(
2−x−z
)Gd
x
M
z
SiO
5
, wherein 0.05≦x≦1.95, wherein 0.001≦z≦0.02, and wherein M is selected from Sm, Tb, Tm, Eu, Yb, and Pr. The radiation detector includes a photodetector that is optically coupled to the phosphor to detect light from the phosphor.
The invention also includes transparent crystalline phosphor that consists essentially of gadolinium yttrium oxyorthosilicate activated with a rare-earth metal dopant M and having the general formula Gd(
2−x−z
)Y
x
M
z
SiO
5
, wherein 0.00≦x≦1.95, wherein 0.001≦z≦0.02, and wherein M is selected from Sm, Tb, Tm, Eu, Yb, and Pr.
The invention also includes a radiation detector having a transparent crystalline phosphor cons

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