Beta Thickness Gauging
An ATI gauge uses the attenuation of beta particles to measure the thickness or weight of various materials, such as plastics, paper, and metal. What components make up a beta gauge? What are beta particles? Are they dangerous? In addition, ATI uses very small activity (size) sources. While conventional beta gauges use sources with hundreds or thousands of millicuries of activity, ATI sources typically range from 0.5 to 3 millicuries. This substantially reduces the amount of radiation present around the gauge. How are beta particles used to measure material? How do conventional beta gauges work? What's different about ATI's technique? Why do beta gauges have noise in their measurement? The standard deviation of the number which will decay is equal to the square root of the expected number. The square root of 500 is about 22. That means that, 67% of the time, we would expect 500 +/ 22 atoms to decay, or anywhere from 478 to 522 atoms. So, if we place a material in the beam that only stops say 10% of the beta particles, then it stops 50 of them, and we would expect to count 450 particles, with a standard deviation of 21. But it's clear that there really might be 429 to 471 particles, one standard deviation is 4.8%. The randomness of the number of beta particles detected is large compared to the change caused by the material. So we cannot make a good measurement. By increasing the number of atoms decaying, we can improve the measurement. If we instead have 500,000 atoms decaying, then a material that stops 10% of the beta particles would stop 50,000 of them, and there would be 450,000 to detect. The standard deviation is 671, or 0.15%, much less than the previous example. This lets us make a better measurement. So, by increasing the size of the source (or using a better detector that counts more of the incoming beta particles) we can improve the precision of the measurement. We can also do this by averaging the measurement over a longer time period. This gives us more particles to count, so we get a better average, with less variation. We can also reduce the air gap (distance between the source and detector). Finally, and perhaps most important, we can use the appropriate source for the measuring range. What thickness or basis weight range can be measured? The chance that a beta particle will make it through the material depends on how heavy the material is, and on the speed of the beta particle. A faster moving particle has a better chance of pasing through the material. Different sources produce beta particles with different speeds. So, by selecting the source that produces beta particles of the correct speed, we can match that speed to the weight of the material we are trying to measure. This is why several different sources are used in gauging. There are three commonly used beta sources: Krypton (Kr_{85}) Strontium (Sr_{90}) The above ranges are in g/m^{2}, grams per square meter. To convert from g/m^{2} to thickness in microns, multiply by the density of the material. To convert from microns to mils (thousanths of an inch) divide by 25.4. For most plastics or other materials with a density of one, one mil (thousandth of an inch) is 25.4 g/m^{2}.

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