字幕表 動画を再生する 英語字幕をプリント 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.