|MadSci Network: Physics|
Your question needs to be divided into two parts at the beginning. Monitoring levels of liquids in containers requires different techniques and radionuclide gauges than determining thickness of paper, plastic, or metal while being formed or manufactured. Liquid level gauges usually use a gamma-ray emitting radionuclide such as Cs-137 (half life approximately 30 years) or Co-60 (half life approximately 5 years). Co-60 emits a higher energy, more penetrating gamma ray than Cs-137. Generally, the higher the energy emitted, the shorter the half life of the emitting radionuclide. The main consideration in the choice of Cs-137 or Co-60 is the density of the material (water, other liquid, solids) that the beam of ganna rays penetrates before reaching the gamma-ray detector on the opposite side from the radiation source. Cs-137 is commonly used for liquid level determination is small containers such as packaged drinks, drug vials, liquids is aluminum cans (360 ml or 12 oz) and many other consumer packages. Co-60 is used in industrial applications such a determining coal level in a storage facility, density of materials flowing in steel pipe, and level determination in industrial sized distillation columns. Beta emitting radionuclides are commonly used in gauges to measure the thickness of films or webs that are continually produced (paper, plastic film, glass-fiber impregnated plastic film used to make circuit boards, or metal foils such as aluminum foil). The film runs at high speed between a radioactive source and a detector. The detector signal strength is used to control the plastic film thickness. In paper manufacturing, beta gauges are used to monitor the thickness of the paper at speeds of up to 400 m/s. Beta particles (fast moving electrons) are more easily absorbed than gamma rays, so that is why they are used for this purpose. The most common beta gauges for films having a low mass density (paper, plastic, fiberglass) use Kr-85, a radioactive noble gas with a half life about 11 years. For materials with a higher mass density, such as metals, a higher energy beta particle may be needed, so Sr-90 with a half life about 30 years is used in these gauges. For a film (foil, metal, plastic) travelling horizontally, called a "web", the source is housed in a shield with a small, shutter controlled, aperture that forms a beam directed through the web. The source holder is one one end of device that looks like a large tuning fork. The other leg of the device holds the detector exactly opposite the source aperture. A web gauge moves back and forth at right angles to the web, so it traces a repeating diagonal path across the web. Using known thicknesses of the material being gauged, the gauge is calibrated so that it can read out in a form equivalent to thickness of the web. For information on beta and gamma decay, interaction of radiation with matter, and a couple of pages on industrial process control, I recommend "Radiochemistry and Nuclear Methods of Analysis" by William Ehmann and Diane Vance, Wiley-Interscience, New York, New York, 1991 ISBN: 0-471-30628-2 One internet reference I found that may be useful is: http://www.accessexcellence.org/AE/AEC/CC/radio_table.html This is a table of uses of radionuclides, and includes level and thickness gauges.
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