16.2 Nuclear Decay

All right, in this lesson, we're going to look at nuclear decays. We'll look at three different types: alpha, beta, and gamma. But before we get to that, let's take a look at a little bit of history and what each of them are. First of all, Henri Becquerel, a French guy, was sort of the first to discover that uranium did some weird things. Basically, what he found out was that when it was in a closed drawer and he happened to have some photographic plate in there (which is like film for old cameras), he found out that the uranium caused the plate to get exposed and it made sort of a weird picture. So he assumed that the uranium was giving off some sort of radiation, kind of like visible light. He was even able to show that a magnetic field could change the direction of that radiation. Shortly after, Marie and Pierre Curie were doing some experiments with radioactive elements like polonium and radium. They found out that they emitted radiation no matter what. They were the first to describe the idea that they figured that radiation was coming from the core of the atom nucleus. Marie Curie ended up being more famous later on for dying due to radioactive exposure and cancers and things like that. So now we know that basically what's happening in any kind of radioactive decay is coming from an unstable nucleus. The nucleus is not stable, it wants to sort of rearrange itself and change to try to get more stable. That's what happens when we have a decay process. If one element changes, let's suppose we have an element like carbon and it changes to something like oxygen or whatever, then we'd call that a transmutation. If it goes from one type of element to another. Rutherford was the guy that came up with the three different types, and he found that it sort of depended on how big they were, how powerful they were, what direction they went, and so on. So we'll go through each of the three types. The first is alpha radiation. He found out that it was pretty minimal, and we've already talked about alpha. We know that it is the same as a +2 elementary charge. He found that it was positive because in a magnetic field, it deflected to the left. Look at this diagram at the bottom. If you put your thumb upwards and your index finger into the page, you can see that your alpha particle should go to the left if you're using your right hand. So it has to be positive and it goes left. He found that it was pretty weak. It could go through about one piece of paper, and that's about it. Beta radiation, he found, was a little bit stronger, and it was negative, but it goes to the right. It can go through a thin sheet of aluminum. Then, gamma radiation. He found out that it was super strong and blasted ahead, and it could not be deflected. So let's start with alpha. Alpha particles - remember, we've been talking about these pretty much most of the year - and we know that an alpha particle is pretty much a helium nucleus. So we know that if it has two protons and two neutrons, it'd be a fully formed helium atom. So what happens is, whatever the chemical is, when it gives off an alpha particle, what it's doing is it's ejecting the two protons and neutrons that would have made up the original nucleus. This often makes it more stable, or it might even have to do this two or three times to get it more stable. But basically, it's getting rid of this alpha particle or helium nucleus. We don't call it helium because there are no electrons, and it's not just natural helium. It's sort of a byproduct of the nuclear decay. So we know it's positive in charge, and we know that if the chemical is... Gonna lose two protons and two neutrons, and likely the chemicals going to change. We can just basically do similar to what a chemistry equation would look like, and all we have to do is balance the numbers. So let's look at this example: we have 168-77 Karidian. 77 is the atomic number of iridium, and if we have 168, we know that if we take out an alpha particle, we basically just subtract two. So if we subtract two, we know it's a 75 element, which is rE. And if you look on here, your periodic table, 75 is rE, and that is ready and rhenium. I'm not trying to pronounce it, doesn't matter. And if we subtract 168 minus 4, that gives us 164. So they have to balance out. You basically just have to make sure your total protons, total nucleons, total up, and we call that the conservation of nucleons. So the total numbers always, always match. So you can see, you started with iridium, so that was our parent nucleus or our original amount. We take out the alpha particle, we're left over with a daughter nucleus, which was the 164-75 iridium. So during this process, if it took the total mass of the daughter, which is the mass plus the total mass of the alpha particle, we actually get less mass than what we started with. And the reason for that is to do that reaction, we had to convert some of the mass to energy. So we talked about this in the last topic, where we had a mass defect. Every time an atom undergoes a reaction, it's gonna lose a little bit of mass, because that mass was used up as energy to do the conversion. And we can actually still follow conservation momentum, all those kinds of things. All those rules still apply. So let's do another one. Let's do uranium-238. Sometimes we have 238 uranium. If we look on the periodic table, uranium is 92. So we know that it's gonna give off an alpha particle. So that would be our 4-2 helium. Then we know the rest of it is going to be the difference of 238 minus 4, which is 234. 92 minus 2 is 90, so we know that it changes to thorium. So we'd have 234-90 thorium. And that's it. So alpha decay, pretty simple. Just ones your numbers. And they're usually changing to an element that's two less on the periodic table. Now we'll go through beta decay. There are two different types: positive and negative. Negative is the most common, but positive does happen once in a while, so we have to look at that too. Basically, what happens is we're not removing neutrons and protons like we are with alpha decay, but we have what happens is in beta negative decay, one neutron actually splits up into a proton and an electron. So the total mass, if we look at the total masses, remember a neutron was slightly bigger than a proton, and some of that difference is due to the electron sort of being added on. So some scientists think a neutron is basically just a proton and electron merged together, which sour charged neutralizes. When our mass would end up being slightly bigger, so because it ends up being changed into a proton, we end up getting one more proton in the reaction. And then we get this extra electron that gets ejected, and it gets ejected as a regular electron, but because it wasn't original, we call it a beta particle. And we write that as -1/0 beta. So the -1 takes care of the charge and the fact That we're gaining a proton, and then the 0 just means there are no protons or neutrons in that ejected electron. So, the only... let's go back here. So, we look at this example: we just have calcium-46 (46Ca) and we have negative 1 0 beta (-1β). So, all we have to do is add one, so the 20 (+21) plus the negative 1 would give us the 20. So, our new chemical ends up being 46Sc21. Okay, so there's one other slight thing that we've got to be careful of. When we have a decay-type equation, this only had 46Ca and it changed to 46Sc. Which is 46Sc21 (Scandium). So, then the way to be careful is if we worry about conservation of momentum, what ends up happening is that the ejected electron ends up moving in one direction. So, let's sort of picture our calcium being an original stationary element. And when it undergoes this chemical reaction, it changes to scandium and we get one electron being emitted. So, using conservation of momentum, that means there had to be something going the other way to balance things up. So, what they found out was there is a small particle that accounts for the mass difference, and that particle ends up being neutral. So, they call it a neutrino. And in this case, we call it an anti-neutrino (we'll get to that a little later on). But we write it as a V (velocity) of some neutral object. We put a little slash over it to indicate that it's an anti-neutrino. So, the neutron then goes from a neutron to a proton and ejected electron, plus this anti-neutrino. The other type is a positive decay, and in beta-positive decay, the same, basically the exact same process occurs, except it's the opposite sign. So, we start with a proton and it splits up into a positron (which is like an electron, but positive) and a neutrino as well. So, in terms of a chemical reaction, you basically do it in exactly the same way, except your beta-positive particle would be just write it as a 0 and +1. So, when you do the math on it, you would subtract one number from your periodic table. So, if we go through this example, we've got potassium-40 (40K19). And if we go through beta-positive decay and get our beta with a 1-0, so we know then the original or the daughter element has to be 40Ar18. So, the 18 plus 1 would add up to the 19. And then we write our neutrino, which is just a B. So, the difference is in beta-positive decay, we get a neutrino being emitted, and in negative decay, we get an anti-neutrino. And the "anti" just means it's sort of like the opposite type of particle. We're going to get into that in the next chapter, so you don't have to worry about it too much just yet. And our last type is gamma decay. Gamma decay is the exact same thing as we talked about earlier, but it's just electromagnetic radiation. Gamma is our highest frequency, highest energy of the electromagnetic radiation. So, what happens is when we do alpha and beta decay questions or scenarios, kind of think of the nucleons. The protons and neutrons being all jumbled up, so they get sort of scrambled up and some of them getting mixed and changed and so on. So, then during the process of getting those to be reformed into a nucleus again, a burst of energy is given off. So, every time they get sort of rearranged, some of the mass gets lost and gets changed into energy. So, the gamma radiation is that energy and that's given off as a gamma wave or an EMR wave. And remember, it's high-energy, so that means it'll pass through lead and other sources like that. And the math equation behind it is really easy, you just have to basically show the exact same thing and show the gamma wave. So, write a little symbol for T 18 argon for this example, stays this 40:18 argon, but it just rearranges itself. We get off this little gamma radiation symbol, so it's really easy formula to work with. It's by far the simplest of the three, and quite often we don't worry about it too much because nothing really changes anyway. So, the last thing we'll look at is sort of what happens if you have more than one decay series. Quite often, you'll get a question where it just gets you to do several decays one after the other. So, in this example, you can see with 226 thorium, it undergoes alpha decay and it changes to 228 or 222 radium. And then it says that radium undergoes that they had a negative decay, so it changes to actinium. And that one undergoes another alpha, and then that results undergoes to Ralph again. So, if you get one of these questions, they usually just give you an original element, and it tells you to do an alpha, a beta, and alpha beta whatever in each step to change it to. And obviously, with any kind of radiation or emission, there's going to be some sort of risks. So, here are some of the units you'll see: rads, grays, and sieverts. And they're just telling you how much energy could be absorbed by tissue. So, rad is one kilogram of tissue would only be absorbing 0.01 joule, so a pretty minimal amount. Gray is about a hundred times that much, and a sievert is way, way bigger. It's in the thousands or millions. So, in a normal situation, a person can get about 0.5 millisieverts a year to be safe. Anything bigger than about six or so would kill you. And in terms of what you'd feel, if you do undergo any kind of radiation exposure, the first type is radiation sickness. So, that's usually where if you're getting cancer treatments and things like that, you're usually quite sick after because it kills some of your cells. Plus, it sort of ionizes some of them and causes you to feel like sort of sick, tired, flu-like symptoms. And if it's severe enough, obviously, it would kill you later on. So, the nuclear bombs in Japan and World War II, the people who survived the initial blast usually died a few days later with radiation sickness. And the second type is if you get enough radiation internally into your cells, it'll actually mess up your DNA a little bit. And the cells are growing a little bit differently. So, basically, what happens is you start modifying or changing the genetic structure, which usually leads to the formation of some sort of cancer cells and so on. And that's about it. So, we talked about the three types, we looked at how to do each of the three math formulas for them, and that's all.