Abstract
The aging society comes with an increased number of people with serious health problems, including cancer and cardio vascular diseases. To combat these problems, early diagnosis is important and requires the development of new and improvement of existing medical imaging techniques. Positron Emission Tomography (PET) is the technique of choice
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when it comes to the precise location of tumors and it can also be used to monitor metabolic activity. An important challenge is to improve the spatial resolution. In order to do so, materials used in the heart of the PET scanners, the so-called scintillators, need to be improved. Especially their timing properties are a concern. The materials used as scintillators are inorganic crystals which are capable of absorbing high energy radiation and lanthanide ions are incorporated in the crystals to emit flashes of light after a high energy X-ray or γ-ray is absorbed by the crystal. The shorter the flash of light is, the higher the resolution of the PET-scanner can be. The typical decay time of the emission of nowadays used scintillators is around 40 ns. In order to improve their performance, the emission lifetime must be shortened, preferably below 20 ns. The presently used luminescent ion is the lanthanide ion Ce3+ and shortening the emission lifetime below 20 ns is not possible with this ion. Instead, a different lanthanide ion, Pr3+, may be used to lower the emission lifetime by a factor of 2-3. This is a direct consequence of the higher energy of the emitting state of Pr3+. Based on a famous law in physics (Fermi’s Golden Rule) it is well known that the higher the energy of the emission is (shorter emission wavelength), the faster the emission will be. A new class of the scintillator materials developed in the past decade are Ce3+ activated chloride host lattices. Their great advantage is a very high light output (with the brightest scintillator ever developed emitting 100 000 photons per MeV of excitation energy), very good energy resolution and a fast emission decay (~30 ns). As mentioned above, substituting Ce3+ ions by Pr3+ in such chloride host lattices could lead to further improved timing properties. In this project the luminescence properties of Ce3+ and Pr3+ activated inorganic host materials with an extremely fast response time for the use as scintillators in the Positron Emission Tomography were investigated. The results show that the very fast emission can be obtained in the chloride hosts with lifetimes down to 10 ns. However, the energy transfer from the host lattice to the Pr3+ ions after the absorption of highly energetic radiation is not efficient in these chloride crystals and this hampers application as a scintillator in PET-scanners. In addition systematic research has been done on the lifetime of the Pr3+ emission and it was found that the shortest lifetime that is possible for a luminescent ion is about 6 ns. This makes it possible to get a better estimate of the ultimate resolution that can be obtained with PET-scanners.
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