Pub. 5 2015 Issue 3

19 MINING FOCUS When the machine is on, current goes through the cathode’s filament and heats it up. The heat causes electrons to go off the surface of the filament, and the positively charged anode then draws them across the vacuum tube. Since the voltage difference be- tween the cathode and the anode is large, the electrons speed through the vacuum tube with great force. • As some of the speeding electrons hit a tungsten atom, each collision causes an electron to jump tempo- rarily to a higher level and then drop back to its original level. When it falls back, the drop releases extra energy in what is called an x-ray photon. • Other electrons are attracted to the atom’s nucleus and whip around it much like a comet going around the sun. The change in direction as the electron goes around the atom’s nucleus slows the electron down. It then emits the extra energy it re - leased while it was slowing down as another x-ray photon. It’s interesting to note that mined materials make the x-ray machine pos - sible. For example: • Lead: Miners extract lead from ores. You can find lead in more than 60 different minerals, but miners focus on just three minerals for more than 95 percent of the lead they produce. The most common is named galena. Galena consists of lead and sulfur, along with traces of the metalloid arsenic and traces of other metals such as antimony, cadmium, cop- per, silver, and zinc. The other two minerals are called cerussite and anglesite. Since lead is mixed with other minerals such as marcasite, pyrite, and zinc blende, lead ore is often a byproduct of other metal min- ing such as zinc or silver. • Tungsten: The Swedes named tung- sten (tung sten in Swedish), and it means “heavy stone.” Tungsten can be found in tin ores, and the chemi- cal symbol for it is the letter “W,” representing the word “wolfram.” Medieval German smelters called it that because tin ores with tungsten had much lower yields than tin ores without it. The smelters used to say that tungsten ate tin “like a wolf.” Tungsten is used to make filaments for electric lamps, electrical contact points for automobiles, heating ele- ments for electrical furnaces, and other applications. • Aluminum: This is the second most abundant metallic element in the Earth’s crust. (First place goes to silicon.) Aluminum occurs naturally in an ore named bauxite, and it often comes from surface mines. • Oil: The oil is selected on the basis of dielectric strength. What does it mean to say that something is dielectric? It conducts electricity poorly but efficiently supports elec - trostatic fields. In other words, it only dissipates a small amount of energy as heat. This matters because if an electrostatic field becomes too intense, it can begin to conduct current. What are some common dielectrics? Think solids like porce- lain, mica, glass, and plastics. It can also be a liquid, like distilled water, or a gas like dry air. Vacuum is an ex - tremely good dielectric. It turns out that mineral oil is also a good dielec- tric. Different dielectrics have been used at different times. Dielectric fluids can be made from petroleum or can be made synthetically. Petro- leum, of course, is a fossil fuel. It can be black, clear, or green. It can be thin, like gasoline, or as thick as tar. • Glass (which is part of the vacuum tube): Glass is made from silicon dioxide, which is the primary ingredi - ent in the sand you find at a beach. To melt silicon dioxide, you have to heat it up to 3090 degrees Fahren- heit. Once it cools down, it becomes what is called an amorphous solid. The term means that in some ways it is like a solid and in other ways it is like a liquid. Glass is useful: it can be transparent; it’s cheap (keep in mind that there is more silica in the Earth’s crust than any other element); it is chemically inert, which means it doesn’t react with other elements; and it can be recycled. X-ray machines are one of the won- ders of our age and in many ways are at the center of modern medical practice. When you consider the materials used to produce x-ray machines, though, it’s sobering to realize that they would not exist without the mining industry. X A filter is located where the x-ray beam exits the box. In medical radiography , the filter is usually made of aluminum and is used to produce a cleaner x-ray image. This is done by absorbing x-ray photons that have lower energy and are more likely to scatter.