Transcription
In our previous talk, we looked at the production of brim slang radiation. Now, let's shift our attention to characteristic radiation. The first thing you'll notice is that the spectrum, the characteristic radiation spectrum, looks vastly different from that brem salon radiation spectrum. And if you haven't watched that Brimstone radiation talk, please go and watch that one first, as this talk follows on from that one.
So, as we did before, we looked at where these x-rays are produced at the actual focal spot on the anode. The vast majority of this electron energy, as a reminder, is converted into heat at the anode, and less than one percent of that energy is converted into either Bram strong or characteristic radiation. Now, the vast majority of those x-rays will be brem's lung radiation, and a small percentage of that one percent will be characteristic radiation.
So, how exactly does characteristic radiation production occur? Well, when we looked at brem's lung radiation, we saw that that was an interaction between the striking electron, or the accelerated electron from the cathode, and the nucleus of our target material. Now, characteristic radiation differs from brem's lung radiation in the fact that this bombarding electron interacts with inner shell electrons of our anode target material. So, this accelerating electron will collide with an inertial electron of our target material, and if the kinetic energy of this electron is higher than the binding energy of this K shell, in this example, electron, then that K shell electron will be ejected from our target.
Now, that occurs only if the energy here is higher than the binding energy. And as I've said before, this energy of the colliding electron is determined by our tube potential, the KVP, the differential between the cathode and our anode that has accelerated this electron towards our anode. If that energy is higher than the binding energy, then we get the release of that K shell electron, known as a photoelectron, and this striking electron is deflected off at an angle.
Now, what's happened here is we have a vacancy within our K shell, and electrons from a higher energy shell, either an outer shell or an M shell electron, will drop down into this lower energy K shell and fill that vacancy. Now, because we are going from a higher energy shell to a lower energy shell, and energy in a closed system is conserved, we need to compensate for that loss of energy by releasing energy in the form of characteristic radiation.
Now, the energy of this characteristic radiation is determined by the difference in binding energies between our K shell and our L-shell electrons. And our binding energies, as we've mentioned before, are determined by the number of protons within the nucleus and the distance that that electron is away from the nucleus. So, this characteristic energy will be the same in a particular element, and that's why it's called characteristic radiation. The energy of those characteristic x-rays remains the same for a specific element; it is characteristic for that element. This energy is released in predictable, discrete, specific energy levels.
So, if we have a look at these electrons coming towards our target material here, now this color coding represents the different shells, our K, L, and M shells respectively. If these electrons have sufficient energy to displace a K shell electron, then we will get an L shell or an M shell electron dropping down into that K shell vacancy. If an O shell was to drop down into a K shell vacancy, the characteristic x-ray release will be the difference in binding energy between these two shells.
Now, we can see here that binding energy for a K shell electron is 69.5 Kev. Our Kev of these accelerating electrons towards anode needs to be more than 69.5 Kev in order to create characteristic radiation in the K Alpha spectrum here. Now, we know that this energy is determined by our KVP, and the number of electrons that are going towards our anode is determined both by our current and our KVP. So, the difference between our L and our K shell electrons varies between about 57 and 59 Kev. If an L shell electron drops into a K shell vacancy, that's what's known as K Alpha characteristic radiation. And if we are talking about tungsten here, which is what we are talking about when we're looking at these binding energies, then our K Alpha characteristic radiation in a Tungsten target material is between 57 and 59 Kev. We get these two peaks here called our K Alpha Peaks.
If an M shell was to drop into our K shell vacancy, the difference between our M shell and our K shell here is roughly 67 Kev, and that's what's known as our K beta characteristic radiation. So, you can see that these characteristic x-rays come at discrete energy levels that are predictable depending on our target material. These K Alpha and K beta characteristic radiations are independent of our energy of these striking electrons. We saw that in Branch lung radiation, as we increase the KVP and increase the energy of these electrons, our bremster lung radiation energy increases. So, long as these electrons have sufficient energy to displace a K-shell electron, then we will be getting these K Alpha and K beta characteristic radiations. If they were less than 69.5 Kev, we wouldn't get any K shell characteristic spectrum here.
Now, you may be wondering what happened if an L-shell electron was to be displaced and an M shell dropped in there. Well, then we would be getting characteristic x-rays that are below 10 Kev, and we know that the inherent filtration will filter out those electrons below 12 to 15 Kev on our spectrum here, just as we looked at in our bremshlang radiation. And if we were to add in added filtration, then none of those other characteristic x-rays would occur in our characteristic x-ray spectrum.
So, now let's have a look again at this graph front on, and we can see that our K Alpha and K beta Peaks, specific for Tungsten atom, occur here. Now, if we were to combine these with our bremster lung radiation curve, this is what's known as our x-ray spectrum, and you need to become very familiar with this spectrum and how it changes when we change various different parameters.
Now, there are two different things that we look at when looking at our x-ray spectrum. The first is known as x-ray beam quality, the average energy of this x-ray spectrum. And the second is known as x-ray beam quantity, the number of photons, the area under the curve of this x-ray spectrum. Now, the one thing to note when looking at the spectrum is it is a spectrum; it's a varied x-ray beam. There are multiple different energies within this x-ray beam, and x-ray spectrum is not monoenergetic. We can't create an x-ray beam that has one specific energy. When we look at the penetrability of an x-ray and how x-rays interact with matter, we make the false assumption that we are dealing with a specific monoenergetic x-ray beam, but it's always good to remember in the back of your mind, we are dealing with a heterogeneous x-ray spectrum that contains multiple different x-ray energies.
So, in the coming talks, I'm going to show you how changing the KVP, I'll generate a waveform, our current and exposure time, our filtration, and our target material will all change this x-ray spectrum to some extent. And we're going to look specifically at how it changes the x-ray beam quality and the x-ray beam quantity. And without a doubt, these changes, the changes in x-ray being quality and quantity, come up over and over again in exams. You'll very rarely find an x-ray physics exam without these questions coming up. And as always, I've included my question bank that I've collated multiple different past paper questions, and these types of questions occur commonly in that question bank. So, if you want to test yourself, go and check out that question bank below. Otherwise, let's move on to our next lecture where we talk about x-ray beam quantity versus x-ray beam quality. I'll see you all there.