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Radioactive Isotopes 7,942 views

Teacher/Instructor Jonathan Osbourne
Jonathan Osbourne

PhD., University of Maryland
Published author

Jonathan is a published author and recently completed a book on physics and applied mathematics.

Radioactive isotopes have unstable ratios of protons to neutrons in their atomic nuclei. An isotope is an element with a varying numbers of neutrons. Radioactive isotopes become more stable through nuclear reactions including alpha decay and beta decay. One example of a radioactive isotope is Carbon-14 which is used for carbon dating.

So let's talk about some radioactive isotopes. What is an isotope to begin with? An isotope is something that well, you call something an isotope, if it's got the same number of protons but a different number of neutrons. So we've got same number of protons, different number of neutrons. Same number of protons means it's the same element different number of neutrons means there's going to be a different mass number.

So here are some standard ones. Hydrogen 1 that's just a proton and that's standard hydrogen. Hydrogen 2 is called deuterium or heavy hydrogen and this occurs in the ocean very rarely but commonly enough that people say there's enough deuterium in the ocean to supply our energy needs from millions of years if we are able to do fusion properly which we are not yet but we'll see. Then there's hydrogen 3. So this is a proton and 2 neutrons, this guy is unstable unlike these two, they are both stable isotopes of hydrogen. This guy which is called tritium is very important in thermal nuclear devices and in fusion studies and he decays in about 12.32 years that's the half life into helium 3 an electron and an antineutreno.

Now most of the time, of course if we're going to do this reaction we'll want to put in the bottom numbers and so we've got 3=3+0 plus this guy's got to be 0. 1=2+-1 plus this guy's also got to be 0. So for that reason, I mean it didn't, it doesn't really it doesn't have any charge it doesn't have any mass. So it doesn't really interact in any of the nuclear reactions and so most of the time Chemists won't even write that guy. But he is there, alright?

So we've got electron and this is called an electron antineutreno alright, whatever. So these are some standard isotopes. We would say this is a radioactive isotope of hydrogen and these two are the stable isotopes of hydrogen. Some other important stable isotopes are lithium 6 and lithium 7. Both of these occur naturally in lithium. This one actually made a big big deal in the biggest thermal nuclear device, biggest hydrogen bomb that the United States ever set off. And this was in 1954, it's called the Castle Bravo explosion it was supposed to be about 5 mega tons but instead it ended up being 15 mega tons mainly because lithium 6 was used to make a solid lithium deutride fuel for the thermal nuclear reaction, they didn't think that the lithium 6 was going to participate in the reaction but it actually did.

Anyway, alright. So let's look at some standard radioactive isotopes. Alright. Uranium 238. That consist of the majority of the uranium that's found on the earth. It has a half life of 4 and a half billion years and decays via alpha emission. Uranium 235 is the big fissile. Fissile meaning I can use it in a nuclear reactor or in a nuclear bomb, isotope of uranium. It's the one that we want so if you say enriched uranium, it means that its uranium 235 content is much much much higher than that found in mined uranium. Now the reason, one of the reasons that uranium 235 is so rare in nature is that its half life is only 700 million years, whereas the half life of uranium 238 is 4 and a half billion years. So this is about the age of the earth. Many half lifes of uranium 235 have gone by in the lifetime of the earth so that means that a lot of the uranium 235 that was present, originally, when the earth formed is now gone. It's, it has decayed into thorium 231, alright. Thorium 232 is another very very very commonly found radioactive isotope on the earth. This consist of the majority of naturally occurring thorium on the earth because the half life of thorium 232 is about 14 billion years. He also decays via alpha emission.

Plutonium 239 is a very important isotope when looking at nuclear plants and when looking at nuclear bombs. The reason plutonium 239 is very very very important is that it's fissile and that means that you can take a neutron shoot it at this plutonium 239 and be almost certain that he'll fission and spit off some other neutrons and get going this chain reaction that we need to get going in a nuclear reactor. He alpha decays in 24,000 years. That means that he cannot be naturally occurring. Too many half lifes have gone by since the beginning of the earth for there to be any plutonium 239 left over even if there was any when the earth formed.

So, where do we get plutonium 239? Well we get it by making uranium 238 undergo a nuclear reaction. We shoot neutrons at it that are slow moving. It will accept some of them, become uranium 239 and couple of days later, that will have decayed into plutonium 239 and we'll have what we want. Alright?

What about radium 226? Radium 226 is a dangerous isotope. It appears in the uranium chain, so all these uranium that exists that has this half life of 4 and a half billion years, well it's going to decay at some point. And somewhere on that chain, you get radium 226. Radium 226 is the type of radium that was originally isolated by the curies in the late 1800s and this guy has a half life of about 1600 years. So you can purify it from uranium ores. Alright?

Then there's radon 222. This is the guy that you may have heard of if you've heard of radon poisoning in basements in the east and it's because of the uranium that's present in the bricks that will decay and eventually on the decay chain we get this radon. Radon is a gas and that means that when we get this radon it's just going to seep out of the bricks and you can breathe it in. You don't want to breathe it in though because if you do, the alpha decays in about 3.8 days and those alpha decays while your skin will stop them, your organs kind of won't. Alright? So if you breathe it in or if you eat an alpha emitter not so safe no more. Alright.

So let's look at the more dangerous guys. Carbon 14 is a beta emitter. His half life is about 5700 years. This guy actually occurs naturally in nature because of cosmic rays coming from the sun and their interactions with nitrogen and oxygen in the upper atmosphere. That will generate carbon 14 for us.

Potassium 40, also a beta emitter. Also is naturally occurring. His half life is about one and a quarter billion years. Strontium 90 and iodine 131 are both by-products of nuclear fission reactions. So if you've got a nuclear bomb that goes off you're going to have a lot of strontium and iodine 131 in the air right after that nuclear reaction. Now these guys are talked about a lot because they are beta emitters with appreciable half lifes and also because strontium and iodine will both be absorbed by the body and will be just held by the body. Strontium because it looks to the body like calcium and iodine because our bodies collect iodine in the backs of our brains.

Now, ordinary calcium, ordinary iodine would be fine. I mean, okay. Your body collects it for a reason. But strontium 90 is radioactive. His half life is about 29 or 28.9 years and that means that if our body, if you just take it in and the body says ooh, this is good. Let me use this to replace calcium in the bones. that's not good because when he decays just sitting there inside your bones that beta is just an electron moving very very very quickly and he's going to go through and wreak havoc in your cells. Same thing with iodine 131, 8 day half life. And so these things we want to avoid. And this is one of the big problems with nuclear fallout from nuclear bombs.

And then the last guy is polonium 210 which is an alpha emitter. Half life is 138 days and you may have heard of polonium 210 as the radioactive isotope that killed the Russian spy.

So those are radioactive isotopes.