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  • When a nucleus is very big it becomes unstable. It's a bit like when you build a sandcastle

  • and when you get a really big sandcastle the last bit of sand just keeps falling off. That

  • last bit of sand is a bit like an alpha particle. You put it in and eventually, bang, it comes

  • shooting out.

  • So what is an alpha particle? How do we know they're there? What do they leave behind

  • in the nucleus? These are all questions that we're going to try and answer.

  • Let's look at how to build a really big nucleus first. You may not know this but most of the

  • most common atoms have a mass number which is a multiple of four and a proton number

  • which is a multiple of two. Let's look at helium first. Helium has a mass number of

  • four, a proton number of two. Now let's go onto Carbon. Carbon has a mass number of twelve

  • and it's got a proton number of six. So it could be made up of three of these blocks

  • of two neutrons and two protons. Then finally let's look at Oxygen. Oxygen has got a mass

  • number of sixteen and a proton number of eight. And it can be made up of four of these building

  • blocks. This is because the easiest way to build a bigger and bigger nucleus is to add

  • two protons and two neutrons at the same time. Just like this block here.

  • So if you can imagine, if you have a really big nucleus, that's so big that it starts

  • to become wobbly or unstable, then wouldn't it be easier to just get rid of one of these

  • blocks. So here's your unstable nucleus and bang, out comes two protons and two neutrons

  • together in an alpha particle. When an atom emits this particle, this is what we call

  • an alpha particle. So there you have it, an alpha particle is two protons and two neutrons.

  • It's the heaviest type of radiation and the two protons give it a charge of plus two.

  • One, two.

  • What element does this sound like? Also, because it's very heavy, it travels slower than other

  • particles. Well in particle terms anyway. They come out of the nucleus at 30,000 km/s.

  • You might find that a bit fast but wait until you see the other particles.

  • You can use these properties to detect alpha radiation. Because it is big and has a large

  • charge, you can make it move through an electric field. So here's your alpha particle coming

  • out of the nucleus and it's going through an electric field. As it goes through, it

  • bends. If it comes across another atom it's pretty likely to take an electron with it.

  • Here's the alpha particle, here's an atom. The alpha particle comes across it, as it

  • gets near, it makes it start to wobble a bit and as it comes away it pulls off an electron

  • with it. This makes it very, very ionising. However, it also loses energy more quickly

  • than the other particles so it slows down and eventually stops. This means it's not

  • very penetrating. You can even stop it with tissue paper. Once you stop it, it sits and

  • waits for a couple of electrons. So remember we said it's got two protons and two neutrons.

  • When it gathers those electrons it becomes a helium nucleus, which is pretty harmless.

  • So there you have it, an alpha particle is a helium nucleus waiting for two electrons.

  • What about the nucleus it leaves behind? This nucleus is called the daughter nucleus. This

  • has lost two protons and two neutrons. The newly formed daughter nucleus loses four in

  • its mass number and two in its proton number.

When a nucleus is very big it becomes unstable. It's a bit like when you build a sandcastle


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B2 中上級

アルファ線とは?| 放射能について|ヒューズスクール (What is Alpha Radiation? | Radioactivity | The Fuse School)

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    kuoyumei に公開 2021 年 01 月 14 日