Scintillator can be regarded as phosphor materials for
radiation rays, which converts high energy particles such as a-ray, g-ray
and X-rays into visible or UV light. Application of scintillator has
a wide variety. Usually it combined with photodetectors and used for
example medical devices such as X-ray CT, PET/SPECT, high energy
physics, and a familiar example is the baggage inspection in airports.
Recently, there are new demands for scintillators for example emission in infrared region or superfast decay time for new detectors like avalanche photodiode and utilization of time of flight(TOF) information. At present development of new scintillators is mainly based on Ce3+ 5d-4f luminescence, but to satisfy such new demands, new concept is needed. We are trying to develop scintillator with new mechanisms of luminescence, for example Pr3+ 5d-4f luminescence, charge transfer luminescence of Yb3+, room temperature exciton luminescence in wide bandgap semiconductor and so on using micro pulling down method, by which screening of the single crystalline materials possible.
Scintillator based on 5d-4f luminescence of Pr3+
One of the most important application for scintillator crystals is positron emission tomography(PET) . An advantage of PET compared to CT or MRI is the ability to make functional image of human body, which can easily detect cancer or other disease, but low spatial resolution and low S/N ratio of the image restrict the ability. Decay times of Ce based oxide scintillators are around 40 -100ns, but scintillators with shorter decay time needed to shorten time gate and utilize time of flight information. To achieve shorter decay time, we paid attention to the relation between transition probability and emission wavelength. High efficiency luminescence around 300nm is suitable to compatible with fast decay time and detection with conventional photomultiplier. 5d−4f luminescence of Pr3+ was selected to achieve such demands. Pr3+ ion shows 5d-4f luminescence like Ce3+ in some host and energy gap between 5d-4f excited state and 4f ground state is bigger than that of Ce3+ so that shorter decay time is expected. On the other hand, compared to 5d-4f luminescence of Ce3+, there are many restriction of host crystal for Pr3+. Bigger band gap and higher crystal field are needed and many 4f levels interfere the fast 5d-4f luminescence. Our approach for the search of Pr based scintillator was the screening of the host materials by the micro pulling down method. In addition to conventional demand for host material, serveral principle such as band gap, crystal field an so on were included for investigation of host materials. In particular, Lu3Al5O12(LuAG) host shows best properties for Pr3+ 5d-4f luminescence. Decay time under UV excitation was less than 20ns with more than 3 times higher light yield compared to conventional BGO. We have developed mass production technology of this crystal with Furukawa co. ltd. Up to 2 inch size crystals with homogeneous properties were achieved. Pr:LuAG scintillator was adopted for JST Regional Research and Development Utilization Program, "High resolution PEM apparatus for next generation detection for breast cancer" Basic scientific study of this material is also in progress. We are also developing other Pr based scintillators for other applications.
2-inch size Pr:LuAG single crystals and polished samples
Energy spectra of Pr:LuAG under Cs137 excitation
Investigation of scintillators with new mechanisms
Charge transfer(CT) luminescence of Yb3+ is Laporte and parity allowed
transition like 5d-4f luminescence of Ce3+, but much little studied.
We pay attention to this emission as a new mechanism for scintillator
and investigate luminescence properties in several host crystals. By
using m-PD method, luminescence properties of CT were studied in garnets such as LuAG,
YGG, YGG, LuGG and perovskites such as YAP and LuYAP. Luminescence based on CT transition was
very fast, For example, Yb:YAP has a decay time of 0.6 ns and Yb:LuAG 0.5 ns at room temperature,
which is very much faster than former materials and suitable for TOF.
Recently new photodetectors such as avalanche photodiode were developed and they show even higher
properties compared to conventional photomultiplier. But sensitivity of these detectors are in long
wavelength region so that new scintillators for them are needed. For this purpose, we are developing
new scintillators such as the materials based on Bi3+ luminescence.
Moreover, new scintillators with new luminescence mechanisms such as exciton luminescence of
semiconductors, perturbed luminescence of Ce3+. scintillators for neutron detection are under development by us.
The figure below is a comparison of current scintillators and materials under development by our group.
Based on our research, we will develop new scintillators to satisfy not only conventional demands like
high efficiency, high density but new demands like superfast decay time, emission in new wavelength region.
Comparison of current scintillators and materials under development by our group