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  • Hi. This is Mr. Andersen and today I'm going to talk about radioactive

  • decay or radiation for that matter. What is radiation? Radiation is essentially parts

  • of atoms that are given off or energy that's given off by radioactive atoms. And so we

  • measure that using a geiger counter. This is a geiger counter right here. And a geiger

  • counter, inside this one you have some inert gases that every time they get hit by a piece

  • of radiation they give off a little bit of an electrical charge which can be picked up

  • in here. So a regular geiger counter, when you turn it on, you're going to hear a little

  • bit of static. And the reason why is that there's always background radiation everywhere.

  • But is we put something radioactive in front of it. So let's say we put a big chunk of

  • uranium 238 here, it's going to give off a huge amount radiation and it will be able

  • to pick that up. We can use that to do a lot of important things. Like for example date

  • how old the earth is. And so there's a lot of stuff that comes out of that. So how does

  • radiation work? Well to understand radiation you have to understand the fundamental forces

  • that we have. And so if this up here is a nucleus. So the red ones are going to be the

  • protons. The blue ones are going to be the neutrons. What we find is if we look low on

  • the periodic table, so this is neon, neon is going to have 10 protons and 10 neutrons.

  • But as we move up the periodic table, so here we've got the same number of protons and the

  • same number of neutrons for calcium. When we move up to tin you'll find that the number

  • of protons is going to decrease compared to the number of neutrons that we have. And if

  • we get up to uranium we have 92 protons but 146 neutrons. Well, why is that? Well the

  • reason why is that the nucleus is held together using something called the strong nuclear

  • force. And so there are all these nucleons held together by this force which holds the

  • nucleus together. Now the nucleus would love to shoot apart. And the reason it would love

  • to shoot apart is you have all these positive charges up here. All these positive protons.

  • And when you have a small enough atom the strong nuclear force is able to hold it together.

  • But when you get something like uranium, it's got 92 protons in the center. And so you have

  • to have tons of neutrons to hold it together. And even with those tons of neutrons that

  • you have, sometimes it has a tendency to fall apart. And so this chart, it takes a second

  • to get your head around it, but essentially what we have on the bottom is the number of

  • protons. And then on the side we have the number of neutrons. And so you would think

  • if the number of protons and neutrons are always equal, then we have this perfect line

  • that goes up here. But as I showed you on the last slide, we tend to get more neutrons

  • the farther we go up. And so you can see that this graph tends to drift towards the neutrons.

  • So as you get way up here with like 82 protons, the perfect number of neutrons to have would

  • be 126. And so what you find is that if an atom exists on either side of this perfect

  • line right here, it's going to give off protons, neutrons. So sometimes they're going to change

  • just to get back to the equal point. Or that perfect ratio of neutrons to protons. And

  • so this chart is neat in that it shows you what kind of decay we have. So if you're on

  • this side of this line, you'll undergo what's called beta minus decay. If you're on this

  • side, you're going to undergo what's called beta plus decay. And then as we get up towards

  • here at the end you're going to have a lot of what's called alpha decay. And then eventually

  • we can have fission as we go far enough up. And so all of these types of radiation are

  • ways for an atom to get back to that perfect ratio of neutrons to protons. Okay. So I've

  • talked about why we have radiation. But what is this radiation? Radiation works like this.

  • You can imagine the first experiments where if we take a radioactive material and then

  • we allow it to shoot its way through, like this, and then we have a sensor over here.

  • If you have something radioactive here and then a tunnel right here, what you'll get

  • is a spot showing up or radiation showing up on this side. So what scientists want to

  • do was they wanted to figure out what is the nature of this radiation. So what they did

  • is they put a positive charge up here and a negative charge down here. If I remember

  • right they actually have negative one one side, but it doesn't matter. So what they

  • found is that you have one spot showing up down here. So there was some particle that

  • was going out that didn't have a charge. We had some that was actually being drawn in

  • this direction. And then we had some that was being drawing in the other direction.

  • And so we call these things alpha decay. Alpha decay has a positive charge. And so it would

  • be drawn towards the negative. So we're going to have alpha decay down here. Alpha decay

  • like that. We're going to have beta decay up here. Beta decay had a negative charge.

  • So it's drawn towards the positive. And then we have this gamma radiation here that didn't

  • have any kind of a charge. And so what are these types of radiation? First of all if

  • we look at alpha decay, alpha decay is simply two protons and two neutrons that are given

  • off. And so without electrons what is this? It's essentially helium. Helium has two protons.

  • It has a mass number of 4. And so that's alpha decay. Alpha decay can't even move through

  • a piece of paper. And so it's weak as far as that's going.

  • \b \b0 Next type is called beta decay. Beta decay

  • is an electron essentially. Electron has a minus one charge. And it has no mass. And

  • so that's beta decay. There's another type of beta decay, and it's weird to write an

  • electron here, but it has a positive charge. And it also has zero mass. And so we refer

  • to that as a positive electron or we call that a positron. But beta decay would be stopped

  • by a little sheet of aluminum foil. And then the last type of decay, the one that doesn't

  • have a positive or a negative charge is called gamma radiation. Gamma radiation occurs when

  • we have, remember, these strong nuclear forces that are holding this nucleus together. And

  • as they start to wiggle and these atoms or the nucleons wiggle underneath their force,

  • that energy is given off in the form of gamma radiation. It's not actually made of nucleons,

  • neutrons or protons. But is has a huge amount of energy. It's like x-rays. And it can only

  • be stopped if we move it through large amounts of soil or lead, as a way to stop it. And

  • so those are the types of radiation. At least the types of radiation that you should understand.

  • And so you also have to write nuclear formulas. And so when we did chemical reactions and

  • chemical formulas, remember we had to balance those equations. And you have to do the same

  • thing here. And so let's start with the one that I talked about at the beginning. So let's

  • start with uranium 238. Now uranium 238 is undergoing alpha decay. In other words it's

  • going to lose, let's go back for just a second. It's going to lose two protons and two neutrons.

  • And so what it's losing is actually a helium nucleus. And so that has a charge of two,

  • two protons and it has a mass number of four. Since there are two protons and two neutrons.

  • And so if you think about it, what does this become? Well I'm actually going to start with

  • my atomic number and my mass number up here. If you lose two protons, you're going to have

  • a mass or an atomic mass now of 90. And if you lose a mass of four, this is going to

  • become 234. Now what's interesting in a nuclear reaction is we've actually changed the number

  • of protons. Since you lost two protons it's not uranium anymore. And so if I look up here,

  • here's uranium on my periodic table. But if I lose two protons from that it's not uranium

  • anymore, it's going to be thorium. And so this is how you write a nuclear equation.

  • You have to make sure that these all balance. The mass numbers balance and the atomic numbers

  • balance as well. So what happens to uranium 238. It actually becomes thorium 234 over

  • time, as it loses these protons. Okay. Let's try another one of those. And so let's say

  • this time we're dealing with beta decay. So let's say we lose an electron. Electron remember

  • has a minus 1 charge. And it doesn't have a mass number. And so what's something that

  • undergoes beta minus decay, would be cesium 137. So cesium 137 has a mass number of 137

  • and an atomic number of 55. And so what happens when it undergoes beta minus decay? Well,

  • let's not write the symbol remember, because that symbol might change. If we've got 55

  • protons and we lose one electron, what we actually gain is a proton. So this becomes

  • 56. Where did it come from? One of those neutrons became a proton and gave off this electron.

  • So this becomes 56. What is the mass number? Well electrons don't have a mass, and so that's

  • going to be a mass of 137. Or it's a negligible mass. And so what does this become? Well we'd

  • have to find cesium on here. Cesium is 55. Okay. So here's cesium right here. Cesium

  • is 55 on the periodic table. But since we're gaining one proton that actually becomes barium.

  • And so this is going to become barium 137. Plus that electron which has a minus one charge

  • and no mass. And so this would be, we call this beta minus decay. Okay. Let's do another

  • type of decay. And this decay we'll do is beta plus decay. So what do you lose in beta

  • plus decay? You're losing an electron, but this is a positron. So it actually has a positive

  • one charge. It has a mass of nothing. What's something that undergoes beta plus decay would

  • be sodium. Sodium has an atomic number of 11 and a mass number of 22. And so if we lose

  • a positron, so if we lose one of these with a mass of this, what do we get? Well, we're

  • going from 11 mass number or an atomic number of 11. We're losing one of this. This now

  • becomes 10. This has not changed at all. So that remains at 22. And so let's look on our

  • periodic table. Well here's sodium right here. It's number 11. But if we lose one proton

  • we now become neon. And so when you're doing these equations and writing out the formulas,

  • make sure that you go all the way across the top. So we've lost one of these. All the way

  • across the top here. And make sure that we balance those out. In other words the the

  • atomic numbers and the mass numbers of the reactants and the products have to be exactly

  • the same. So that's radioactive decay. And I hope that's helpful.

Hi. This is Mr. Andersen and today I'm going to talk about radioactive

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放射線と放射性崩壊 (Radiation and Radioactive Decay)

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    kuoyumei に公開 2021 年 01 月 14 日
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