by Jared Rovny

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    00:02 Our last topic in the Atomic Nucleus is, before we move to the electronic structure of atoms is "Radioactivity and Spectroscopy." First we'll discuss radioactivity.

    00:13 In radioactive decay usually occurs in context we're interested in, when an unstable nucleus admits some particle.

    00:21 We'll talk about three kinds of radiation.

    00:23 The alpha decay.

    00:24 The beta decay.

    00:25 And the gamma decay.

    00:26 And it sounds lot like these might be some new mysterious particles in, alpha, and the beta and the gamma.

    00:31 But it turns out as we see that these three particles are just things we've already familiar with in a new context.

    00:38 Historically, what happened is that somebody was looking at these kinds of decay and trying to measure how they were different.

    00:44 And when he put them in a magnetic field, he saw that one kind bent in magnetic field going one way.

    00:49 One kind bent in and went another way.

    00:51 And another one didn't bend at all.

    00:54 So he called these two that bent often in their own directions, the alpha and the beta.

    00:57 And then this one that didn't seem to change, or didn't seem to care about the magnetic field, he called gamma.

    01:03 But let's see what these three actually are if we pull back the curtain.

    01:07 First we have alpha decay.

    01:09 What's actually happening in alpha decay is that two protons and two neutrons are escaping from your unstable nucleus.

    01:16 When this happens the two protons just by definition are in fact a helium particle.

    01:21 And they are positively charge because they don't have any electrons going with them.

    01:26 And so have two protons and two neutrons.

    01:29 An alpha particle which is called a helium particle in reality, is moving away without any electrons and so has a positive charge.

    01:36 It's a relatively heavy particle.

    01:39 Because it's in fact just a helium nucleus.

    01:42 And again it's positively charge because it doesn't have any electrons with it.

    01:46 The fact that it's actually a helium particle and actually helium atomic nucleus, means that this kind of radiation will be short range.

    01:54 Because the thing that's being shot out from a particular unstable particle is actually such a heavy thing.

    02:00 So won't be able to travel very, very far distances before interacting too strongly with its environment.

    02:07 The other important thing about the helium particle from the alpha decay is that whatever atom it left has now lost two neutrons as well as two protons.

    02:17 The two neutrons could just change the isotopic identity of the atom that it left.

    02:24 But the fact that the originating atom lost two protons, means that whatever caused this alpha decay has now changed it's identity to be less by two protons for every alpha particle that has left the atomic nucleus.

    02:38 The second kind of decay is a beta decay.

    02:41 And there's actually a few different types of beta decay.

    02:44 One is the beta minus decay in which an electron ends up being emitted.

    02:49 And the other is called the Beta Plus Decay.

    02:51 In which case the positron which is in fact just the anti-particle of an electron and has a positive charge is emitted instead.

    02:59 So let's look at these two in slightly more detail.

    03:01 What's actually happening in a beta minus decay is that a neutron is decaying.

    03:07 And this neutron decays, turns into a proton, an electron.

    03:12 And then an extra thing which is in fact is an electron anti-neutrino.

    03:17 So the things that we should be particularly focused on and make sure that we understand is that a proton and an electron end up coming out from this beta decay.

    03:26 The electron is actually emitted and has very high kinetic energy as it leaves the atomic nucleus.

    03:32 And so it was this electron, this negative charge that was measured to have deflected because of the magnetic field in the original experiments.

    03:39 We should know again that a neutron turned into a proton.

    03:42 So whatever nucleus originated this reaction is going to have a change in identity by one proton.

    03:48 And also this last bit that left as well, this anti-neutrino, we call it an electron anti-neutrino, is emitted because it turns out of to be law of physics which you don't have to be to worried about or concerned about here.

    04:02 But that the lepton number has to be conserved.

    04:07 So there are a few different types of leptons that we talk about in each of certain particles.

    04:12 And in this case of these leptons is an electron.

    04:16 So to conserve this lepton number we also have an anti-electron neutrino.

    04:21 And this conserves are lepton number for us.

    04:24 But again the key components of the beta minus decay that you should certainly be familiar with, are first, that an electron, a negative charge is what's emitted.

    04:34 Again with very high energies.

    04:36 And also that neutron has turned into a proton.

    04:38 And this will change the identity again of whichever nucleus emitted this beta decay.

    04:44 The opposite of this the beta plus decay.

    04:46 We'll do the sort of the opposite.

    04:48 Instead of a neutron going into a proton and few other things, we have a proton turning into a neutron and few other things.

    04:55 Where these things are now a positron rather than an electron.

    04:59 And we now have an electron neutrino rather than an electron anti-neutrino.

    05:05 Because compared to the positron this will still conserve our lepton number.

    05:09 The important thing here of course that now we have a positive charge being emitted rather than a negative charge.

    05:14 And also that we have a proton turning into a neutron rather than the other way around.

    05:20 So whatever the originating nucleus is in this case, will lose one proton number from it's atomic number.

    05:27 Finally we have the gamma decay.

    05:30 And we in fact already introduced the gamma electromagnetic radiation.

    05:34 So we should already know by this point that the gamma decay is emitting a gamma ray photon.

    05:39 So really what's being emitted from a nucleus in this case is simply a photon of light, some amount of light.

    05:45 And this light has very, very high energy.

    05:47 We say that this kind of radiation is a highly penetrating radiation because these photons are able to travel very, very long distances.

    05:55 And go deep into materials when they hit them.

    05:58 This is exactly the kind of radiation that we should be worried about if we are out at the beach or something and experiencing to much of those cosmic rays over a long period of time.

    06:07 Or specially if we are going into space, where those cosmic rays are being filtered by something like our natural atmosphere.

    06:13 So again these gamma particles are in fact just gamma ray photons or photons at the gamma frequency of our electromagnetic spectrum.

    About the Lecture

    The lecture Radioactivity by Jared Rovny is from the course Atomic Nucleus.

    Included Quiz Questions

    1. Alpha, beta and gamma
    2. Alpha, beta and delta
    3. Gamma, delta and sigma
    4. Delta, gamma and beta
    5. Beta, gamma and sigma
    1. A nucleus emitting a helium particle
    2. A nucleus emitting a high-energy photon
    3. A nucleus emitting an electron
    4. A nucleus emitting a high-energy neutron
    5. A nucleus emitting a positron
    1. Beta minus
    2. Alpha plus
    3. Gamma
    4. Sigma
    5. Beta plus
    1. It drops by 1
    2. It increases by 1
    3. It remains the same
    4. It increases by 2
    5. It decreases by 2

    Author of lecture Radioactivity

     Jared Rovny

    Jared Rovny

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