chromatography • spectroscopy 21
Research conducted on large crystal growth with high-Z semiconductor and scintillator materials has produced large crystal ingot yields.
since the early 1990s, yet a reliable system to produce large crystals at an economical cost has not been achieved until relatively recently.
Optical Sensing Technology for quality control and yield improvements
Fig. 2. Schematic of a Negative-Stiffness isolator.
Gamma rays are electromagnetic radiation of high frequency (very short wavelength), that are produced by sub-atomic particle interactions such as electron-positron annihilation, radioactive decay, fusion and fission. Gamma radiation is highly penetrating photons, extremely energetic. To directly detect them is very difficult, however – a material with a high Z number is needed, representing a high number of neutrons and protons in the nucleus. Tose nuclei tend to stop gamma rays much better than other elements such as hydrogen or helium, for example. A crystal with a high-Z number converts the gamma rays from electromagnetic waves to excited electrons. Te electrons move through the crystal or create light, one or the other, and produce something that is possible to be detected. If a crystal is very uniform, very homogeneous – it can be determined that a gamma ray interacted in the crystal by the effect that is observed in it.
Growing giant crystals At the SMART Laboratory, crystals of CdZnTe and the scintillator materials are grown via a vertical Bridgman furnace. In this process, molten material is directionally solidified from one end to the other to produce a large-volume ingot that is a single crystal. Methods to grow CdZnTe for gamma-ray spectrometers have been explored
Higher ingot yields enable smaller, faster and more accurate sensors, and allow gamma-ray detectors to be more economical and field-portable – benefits that can have a significant impact on national security objectives. Radiation detectors using CZT can operate in direct- conversion (or photoconductive) mode at room temperature.
Essentially, SMART Lab researchers encapsulate the material to be grown inside of a quartz ampoule under vacuum. Te quartz tube is put inside the furnace vertically bringing the material to a molten state between 500° – 1100°C. Ten they very slowly freeze the material from bottom to top. If the thermal gradients are correctly performed, a large crystal will develop. Once the crystal is grown, it is extracted from the tube, trimmed to size with a diamond wire saw and polished to produce a detector.
Vibration Isolation Critical to maximizing ingot yield is maintaining a stable crystal growth process through the elimination of external vibrations.
“Te general consensus within the crystal growth community is that uncontrolled vibrations can destabilize the growth interface,” says Professor Mark Harrison with SMART Lab. “As the material is freezing from bottom to top, there is
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