Transcription
In the last talk, we went over a brief overview of the entire X-ray physics module. And don't worry if some of those concepts seem foreign to you and you're not exactly sure how the X-ray generation process works. We're going to take the next couple of talks to really dive deep into each of the various different components. And we're going to start by looking at the cathode because that is where electrons are produced and those electrons are used to create our X-rays.
After we've looked at the cathode and the anode and the X-ray tube, we will work backwards to the X-ray circuit because a lot of that will make more sense once we understand how the X-ray tube works. So, what exactly is the cathode? Well, the cathode is the negative electrode within our X-ray tube. The function of the cathode is to produce electrons via the process of thermionic emission. Those electrons are produced at the cathode, and only when we apply a voltage across this tube will those electrons be accelerated towards our anode. Those electrons striking our anode will then create X-rays.
So, if we have to tilt this cathode slightly and look at it front on, we can see the face of the cathode here. Now, the cathode, as I've said, is negatively charged, and within its face, there are two tungsten filament coils that have been machined into the face. If we were to cut this cathode in longitudinal section, we would get this cross-sectional view here. You can see our tungsten filament coils within the cathode itself, and this concavity here is what's known as a focusing cap, which we'll look at in a second.
Now, we produce electrons on the surface of our tungsten filament coil through a process called thermionic emission. Thermionic, temperature, emission, emission of electrons. Now, how exactly does thermionic emission work? Well, we run a current through this tungsten filament coil. Now, the tungsten filament itself has high electrical resistance, and that current running through it will generate heat. The tungsten filament coil will heat up more and more, and that heat energy will allow valence electrons in our tungsten atoms to become excited, to go to the surface of the tungsten filament coil, and that process is called thermionic emission.
So, why exactly do we use tungsten for our coil? Well, one, we can machine tungsten into a very thin coil and wrap that coil tightly within the cathode itself. That really thin tungsten will allow for a lot of surface area, so we can generate lots of electrons on the surface of the coil. Not only can we machine it into a very thin filament, but the tungsten itself has a very high melting point, and because this requires a lot of heat, we need tungsten to maintain its structural integrity at high temperatures. And lastly, tungsten has a high atomic number. It's got lots of valence electrons, and therefore, through the process of thermionic emission, we can have lots of electrons at its surface. It's a good thermionic emitter.
Now, these tungsten coils are machined into this focusing cap, which is normally made of a substance called molybdenum or nickel. Now, molybdenum has also got a high melting point, and we can make it negatively charged, but it's not a good thermionic emitter. We are not producing electrons on the surface of this molybdenum. So, when we produce electrons at the surface of our tungsten filament coil through the process of thermionic emission, we then apply a voltage across the X-ray tube. It's that voltage that will allow those electrons to accelerate towards our anode. Thermionic emission itself just makes those electrons available; it doesn't accelerate those electrons across the tube. It's the tube potential, that tube voltage, that accelerates those electrons.
Now, if we were to just accelerate those electrons towards the anode, they would spread out and hit the anode over a large surface area. And this is when we can use our focusing cap to focus down those electrons onto a smaller area on our anode. So, how exactly does the focusing cap work? Well, first, we can use what is known as an unbiased focusing cap. Our electrical supply to our filament and to our focusing cap is shared. As we increase the current through our filament, we increase the current through our focusing cap. Now, that current makes the focusing cap negatively charged, and there's an electromagnetic field that forms around the focusing cap. As we've seen in our electromagnetic force, that negative electromagnetic field will repel those negative electrons. So, electrons that spread out towards the focusing cap will be repelled away and be focused down onto a smaller area.
Now, when this is an unbiased focusing cap, we can't titrate the amount of focusing against the number of electrons we produce. They are linked together. If we want to focus down further without releasing more electrons, we need what's called a biased focusing cap, where we have a different supply, an independent supply to our focusing cap as opposed to our tungsten filament coil. So, we create electrons here, we then accelerate those electrons, and we can titrate the amount of negative charge on that focusing cap and really use it to focus it down over a very small actual focal spot on our anode. A grid-biased focusing cup is when we make the focusing cup so negatively charged that electrons actually can't be released from the surface of these tungsten filament coils because that negative charge prevents them from going out. They've repelled it into this concavity, into the focusing cap, and we don't get any electrons reaching our anode.
So, that, in a nutshell, is the cathode. The function of the cathode is to produce electrons on the tungsten filament coil. You'll see that we had two different sizes of our tungsten filament: a larger filament and a smaller filament. We can use the various different sizes depending on the size of the actual focal spot we want to create on our anode.
Now, in our next talk, we're going to look at the anode itself and how the anode deals with all the heat that is produced when electrons strike the anode because when electrons strike the anode, they can either produce heat or electromagnetic radiation in the form of X-rays. But for now, when we look at the cathode, the types of questions that come up over and over again: Why do we use tungsten for our tungsten filament? Why do we use molybdenum for our focusing cap? What is the process of thermionic emission? And how does the focusing cup work? All of these types of questions come up in the question bank that I've created and listed in the first line in the description. So, if you're interested in that, go check it out. Otherwise, I'll see you in our anode talk. Goodbye, everybody.