22 chromatography • spectroscopy
Fig. 3. Experimental ‘tilting’ Bridgman furnace used to grow the scintillator material, cerium tribromide.
“Gamma rays are electromagnetic radiation of high frequency that are produced by sub- atomic particle interactions such as electron- positron annihilation, radioactive decay, fusion and fission.”
an interface between liquid and solid, and it sets up a natural convection flow that is ideal for growing a big, single crystal. If a vibration disturbs the liquid directly above the forming crystal solid, it can change the convection patterns and multiple crystals will form from the previous single crystal. Which is contrary to our purpose of growing large, single crystals.”
“We looked into various active and air table vibration handling systems, and eventually selected Negative- Stiffness vibration isolation,” continues Harrison.
Negative-stiffness mechanism (NSM) isolators have the flexibility of custom tailoring resonant frequencies vertically and horizontally. Tey employ a completely mechanical concept in low-frequency vibration isolation. Vertical-motion isolation is provided by a stiff spring that supports a weight load, combined with a NSM. Te net vertical stiffness is made very low without affecting the static load- supporting capability of the spring.
Beam-columns connected in series with the vertical-motion isolator provide horizontal-motion isolation.
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Te horizontal stiffness of the beam- columns is reduced by the “beam- column” effect. A beam-column behaves as a spring combined with a NSM. Te result is a compact passive isolator capable of very low vertical and horizontal natural frequencies and very high internal structural frequencies.
Vibration transmissibility with Negative-Stiffness is substantially improved over air systems, which can make vibration isolation problems worse since they have a resonant frequency that can match that of floor vibrations. Transmissibility is a measure of the vibrations that transmit through the isolator relative to the input vibrations. Te NSM isolators, when adjusted to 0.5Hz, achieve 93 per cent isolation efficiency at 2Hz; 99 per cent at 5Hz; and 99.7 per cent at 10Hz.
NSM transmissibility is also improved over active systems. Because they run on electricity, active systems can be negatively influenced by problems of electronic dysfunction and power modulations, which can interrupt crystal growth continuity. Active systems also have a limited dynamic range – which is easy to exceed – causing the
isolator to go into positive feedback and generate noise underneath the equipment. Although active isolation systems have fundamentally no resonance, their transmissibility does not roll off as fast as Negative- Stiffness isolators.
“One of the concerns we had was surface waves coming across the ground, which induced vibration in the crystal growth system,” explains Harrison.
“We are located in a basement,” Harrison says. “Before we got the NSM system, I could actually see somebody walking down the stairs through the walls with a seismometer. With the Negative- Stiffness system in place, I can’t even tell when they are shelling at the nearby Fort Riley military base.”
Special nuclear materials Gamma ray detectors have been around for years, but they are either very low efficiency, poor performance or they require liquid nitrogen cooling, such as those employing germanium. Imagine the difficulty required to take liquid nitrogen into a remote desert searching for special nuclear material.
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