SCINTILLATORS - GOING DIGITAL
Evaporation solves the challenges for “thick” layer deposition to enable the latest large area digital scintillator technology for medical and industrial use. Evatec Product Manager Kurt Flisch tells us how.
FPD - A growing market in healthcare
Digital X-ray detectors used in Flat Panel Detectors (FPDs) convert X-rays into electronic data that a computer can process and convert to an image quickly. These FPDs first convert X-rays into visible light through a scintillating medium and this visible light is then converted into electrical charge by a photodiode or TFT. The most common digital scintillators are based on materials like Caesium Iodide (CsI) and Thallium Iodide (TlI).
FPD based systems are increasingly replacing the traditional analog and computed radiography (CR) systems used by the healthcare industry until now which have disadvantages of relatively high radiation exposure, poor image quality, long diagnosis time and the need for chemical processing. Some analysts expect that the FPD market in this sector will reach USD 1,700 Million dollars by 2021.
Evaporation solves the manufacturing challenges for CsI/TlI
Deposition of the layers required for the x-ray detector sets some interesting challenges to be solved
-
The typical layers required are thalium doped CsI with typical doping rates of TlI of between 0.2% and 3.0%.
-
A whole variety of panel sizes and shapes must be handled: square, rectangular up to a typical maximum of 17” x 17” (chest x-rays).
-
The layer thicknesses required are much higher than in typical thin film deposition processes at 500 – 600 μm, and over 700 μm for moving image detectors.
-
The typical substrate temperature must be kept below 150C during the whole deposition time of 5 or more hours.
CsI and TlI exhibit unique non linear temperature gradation making control of source temperature during both ramp-up (shutter closed) and evaporation essential to maintain source stability during an extended process time. Accurate rate control of both materials during the entire evaporation process is also essential as scintillator performance is highly sensitive to small changes in TlI doping level.