Recently, in the field of medical care, diagnosis using a positron emission tomography (which will be abbreviated herein as “PET”) scanner has been carried out more and more often. Thus, to realize a PET scanner with even higher performance, searches for better scintillator materials have been conducted.
The scintillator materials for use to make a PET scanner need to detect a γ ray. To meet those needs, single crystal scintillator materials, including BGO (bismuth germanium oxide), LSO (lutetium silicon oxide), and GSO (gadolinium silicon oxide), have been used so far to make a PET scanner. The properties of a scintillator material are evaluated by its fluorescence output, fluorescence decay time, and energy resolution, for example. All of the single crystal materials mentioned above have properties that are good enough to use them to make a PET scanner. And as for a method of growing such a single crystal, a melt growth process such a Czochralski process or a Bridgman process has been used extensively on an industrial basis.
To make the PET more popular, however, the throughput of the diagnosis should be increased. But the throughput cannot be increased unless a single crystal scintillator material, of which the intensity of emission is greater than, but the fluorescence decay time is shorter than, conventional scintillator materials, is developed.
Patent Document No. 1 discloses GSO activated with a dopant Ce (cerium). On the other hand, Patent Documents Nos. 2 and 3 disclose cerium doped lutetium borate materials. Cerium doped lutetium borate has a high intensity of emission and a short fluorescence decay time, and therefore, is regarded as a promising scintillator material. Patent Document No. 3 also suggests that the cerium doped lutetium borate material be applied to the field of PET. However, the cerium doped lutetium borate material disclosed in that document is just powder. Thus, nobody has ever reported a technique for forming a single crystal of cerium doped lutetium borate that is big enough to use it in the PET.
As for lutetium borate, its phase transition point (of about 1,350° C.) that involves a significant volumetric change is located in a lower temperature range than its melting point (of 1,650° C.). That is why according to a conventional single crystal growing process, in which the starting material should be heated to a temperature that is high enough to melt or dissolve the material, when the melt being cooled passes the phase transition point, its volume will expand so much that the crystal will collapse, which is a serious problem. For that reason, it has been impossible to produce a single crystal of cerium doped lutetium borate according to the conventional method. And nobody has ever reported that they succeeded in making a single crystal of cerium doped lutetium borate, of which the properties are good enough to use it to make a PET scanner.
On top of that, what is realized by Patent Documents Nos. 2 and 3 is either powder or a tablet obtained by compacting a dry mixture and baking the compact.
Other materials such as LaBr3 have also been proposed. However, all of those alternative materials have some problem in terms of their chemical stability or density. Therefore, it is sad to say that a perfect single crystal scintillator material that will satisfy all of those requirements has never been reported so far.                Patent Document No. 1: Japanese Patent Application Laid-Open Publication No. 2003-300795        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2005-298678        Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 2006-52372        