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Course: AP®︎/College Physics 2 > Unit 8
Lesson 3: Nuclear physicsBeta decay
During beta minus decay, a neutron in an unstable nucleus transforms into a proton, releasing a high energy electron and antineutrino in the process. During beta plus decay, a proton in an unstable nucleus transforms into a neutron, releasing a high energy positron and neutrino in the process. Beta particles carry away energy lost by the nucleus and are ionizing radiation. Created by Mahesh Shenoy.
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- This is a really interesting video, but I would never guess a neutron could turn into a proton! :D(5 votes)
- Why is nitrogen-13 unstable but carbon-13 isn't? The ratio of protons to neutrons in the nucleus of nitrogen-13 is 7 protons to 6 neutrons, and I thought that that's why an isotope can be unstable. However, carbon-13 has 6 protons and 7 neutrons, which still has a different number of protons to neutrons. Why is this stable?(2 votes)
- Thanks for the interesting and informative video!
However, I have a question about this video.
I have been researching neutrons and protons, and in my school textbooks, I learned that neutrons have a slightly greater mass than protons.
I also learned from the internet that antimatter is a substance whose mass is equal to a certain matter, but whose positive and negative properties, such as electric charge, are the exact opposite.
I thought that this difference in mass between protons and neutrons was because neutrons are made up of protons and electrons, and during beta decay, the electrons of the neutron are ejected together with the neutrinos to turn into protons.
However, the video explained that during beta-plus decay a positron is emitted and the proton turns into a neutron.
If positrons are the antimatter counterpart to electrons and have the same positive mass as electrons, then it sounds contradictory that positrons with positive mass are emitted and turned into neutrons with greater mass.
How can this phenomenon occur? Or is there an explanation for it if we proceed with the lesson?(0 votes)- A neutron is not composed of a proton and an electron. Similarly, a proton is not composed of a neutron and a positron.
Neutrons and protons are composed of particles called quarks, which come in different flavors. A neutron is made of one up quark and two down quarks. A proton is made of two up quarks and one down quark. During beta-minus decay, a down quark changes into an up quark, turning the neutron into a proton. (The opposite happens in beta-plus decay.)
This process is a result of the weak force. The weak force is mediated by particles called bosons. A down quark changes into an up quark through the emission of a weak boson. That boson then decays into an electron and an antineutrino, which are emitted from the nucleus. A similar process occurs during beta-plus decay.
As for the mass difference, this is related to energy. The higher energy state a system is in, the more massive it is. For example, a free neutron is unstable. It beta-minus decays into a proton (a H-1 nucleus). Energy is released during the decay. If you were to compare the mass of the parent neutron to the combined mass of the daughter proton, the beta particle, and the antineutrino, there would be a small difference in the mass. That difference in mass corresponds to the energy released during the decay by E=mc^2.
The free proton is stable, so it does not decay back into a neutron by beta-plus decay. However, some nuclei contain a certain number of protons and neutrons bound together that would be more stable if a proton became neutron. Those are the cases when beta-plus decay occurs. Again, there is a small decrease in mass from the parent nucleus to the decay products corresponding to the energy released.(5 votes)
- If you gain a proton and lose an electron, how does that not affect the charge?(1 vote)
- That's not what happens in beta decay.
In beta-minus decay, a neutron turns into a proton and an electron is produced. So, a +1 is added and a -1 is added, and there's 0 change in net charge.
In beta-plus decay, a proton turns into a neutron and a positron is produced. So, a +1 is lost and a +1 is added, and again 0 change in net charge.
What you describe sounds a bit like electron capture, which is a different process. It's basically the opposite of beta-minus decay. The nucleus captures an electron, which doesn't exist after. A proton turns into a neutron in the process. So, a +1 is lost and a -1 is lost, and there's 0 change in net charge.(2 votes)
- Do any elements exist that naturally tend to have an alpha decay or a beta decay? Is every decay possible for every radioactive element or is it somehow already figured out? For example if I have atask where I have to figure out what decay should happen and what the end products are how do I do that? I'm a bit confused.(0 votes)
- Radioisotopes tend to favor either alpha or beta decay. However, some isotopes can undergo either alpha or beta decay. Bismuth-212 is an example.
We know the decay type(s) for each radioisotope, as well as the energy released during its decay, based on measurements using radiation detectors. This information is available for all nuclei on the Chart of Nuclides. It's kind of like a Periodic Table, except it lists all known isotopes of all the elements and their decay information.
It's unlikely you'd be given a problem where you had to guess what type of decay a certain radioisotope will undergo. Instead, you might be given a parent nucleus and the type of decay, and be asked to predict the daughter nucleus. Or, you're given the parent nucleus and the daughter, and you need to figure out what type of decay occurred. But if you know what's involved in each type of decay, figuring that out would not require any guessing.(4 votes)
- why when its a 13 it converts to 6 neutrons like how do you get the 6 from the 13(1 vote)
Video transcript
- [Instructor] Did you
know that paper industries can use radioactivity to ensure consistent thickness
throughout the paper? That's right, but doesn't
it make you wonder, "How do you use radioactivity to do that?" Well, let's find out. If you have a very heavy nucleus, then there'll be too many protons in it, causing repulsion, making
the nucleus very unstable. When that happens, it just spits out a helium
nucleus and becomes lighter. Now, the dotted nucleus is more stable. This is what we call an alpha decay, and we've talked a lot
about this in great detail in our previous video on
alpha decay, but guess what? Even light nuclei can be unstable. For a completely different reason, it has now nothing to do
with the number of protons, but now it has something
to do with the ratio of protons and neutrons. Turns out that certain ratios
of protons and neutrons just doesn't work for the nucleus, making it very unstable. Now, what does it do now? How does it stabilize it? Well, now it undergoes a beta decay. In fact, we'll see that there
are two kinds of beta decay, but what exactly happens over here? What is a beta particle? Well, let's explore all of that by taking a couple of examples. For our first example,
let's consider carbon-14. It's a radioactive
isotope and it's unstable. And so, what it does is it
changes into nitrogen-14, spitting out a beta particle. So, again, the question is,
what's going on over here? What's this beta particle? Of course we can Google it, but where's the fun in that? Instead, let's put our thinking caps on and see if we can logically deduce this. The way I like to think about it is just keep track of
protons and neutrons. I mean, if you look at carbon,
it has six protons in it, and since the total
number of particles is 14, the remaining eight must be neutrons, so there are eight neutrons in it. Okay, what about nitrogen? Well, it has seven protons in it. And because mass numbers stayed the same, oh, it's still the 14, there must be seven more neutrons in it. Now, if you look at the protons carefully, you can see there is one
extra proton over here. But if you look at the neutrons, well, there is one
extra neutron over here. So, can we guess what might have happened? Well, we can guess that the neutron must have
converted into a proton. And guess what? That's exactly what happened over here. That's what a beta decay is, or at least one kind of beta decay is. A neutron gets converted into a proton. That's incredible, isn't it? But that's not it, folks, we can now also guess
what this particle is, or at least guess the
properties of this particle just by using charge conservation. I mean, whether you're dealing
with chemical processes or nuclear processes, the
charges must always be conserved. Now, for all the particles
that we've accounted for, of course, the charge stays
conserved, but look at this, a neutron is a neutral particle, but when it gets converted to a proton, you get a positive charge. Now, we need to account for it. The right-hand side
should also be neutral. That means along with the positive charge, there must be a negatively
charged particle that comes out. That's what a beta particle is. It must have an equal negative charge. And guess what that is? It turns out, you know, experimentally we found out that that is an electron. So, in this particular process, it's spitting out an electron. It's the good old electron
that we are all familiar with except for the fact that
it came from the nucleus. And so, whenever we have
electrons coming from the nucleus, we call it the beta minus particle, and this decay, we call
it the beta minus decay. Now, of course, we need to
write this in the same notation because this is still the nuclear process. So, how do we write that? Well, here we have a six, but here we have a seven. So, if I want this total
number to be six over here, I just have to subtract one, so I'll write this as minus one so that you have six here and
seven minus one, six here. And I know you must be
wondering, "Well, Mahesh, what does it mean to
have minus one over here? Because this is supposed to
be the atomic number, right? What does it mean to have a
minus one of an atomic number?" Well, don't worry too much about it. I like to think about it as
just it's a negative charge that was written over here. I mean, of course, it doesn't
make sense for an electron, you know, to have an atomic
number or a mass number, but it's just a way to make sure that our notations stay put. Okay, anyways, so that's for this one, what about the mass number? Well, the mass number did not change, so we have 14 here, 14 here. That's good, so we'll just write a zero. So, that's how we represent an electron in a nuclear process,
a beta minus particle. Okay, now you may be wondering, "What is this question mark over here?" We'll get to that, but before that we'll take another example. This time we have nitrogen
turning into carbon. Why don't you pause the video
and do the same analysis. Is even here is it the
same thing that's happening or something else is happening? Is this the same beta
particle or something else? Why don't you pause and give it a shot? All right, let's see. So, again here there are seven protons. And since the total mass number is 13, that means there are
six neutrons over here. And over here there are six protons. And again, the mass
numbers stayed the same so there must be seven neutrons over here. Again, if you try to account for them, you will see there is one
less proton over here, but there is one more neutron over here. So, what happened over here? Hey, it's the exact opposite, this time a proton got
converted into neutron. And again, if you try to
account for the charge because charges must be conserved, there's a positive charge
here on the left-hand side so on the right-hand side
there must be positive. This is neutral, that means this particle
must be positively charged, it must have the same
charge as the proton. So, what is it? Well, turns out this is
what we call a positron. But what exactly is a positron? Think of positron as kind of a twin, an evil twin of electron. It has almost all the properties similar to that of an
electron, like the same mass, it'll have similar spin and
all the quantum properties, but just one thing will be the opposite, its charge will be opposite. Okay, so this is probably
new particle for us, it's kind of like the electron
but with a positive charge, we call this positron. However, in general, if you have particles which have pretty much the same properties as some other particles except for a few that is the opposite, like maybe the charge or there are some other quantum properties that can be opposite as well, that case we call this an antimatter. So, this positron is an
antimatter of electron. Protons also have their anti-matter, it's called antiprotons. Neutrons will also have their anti-matter, it's called anti-neutrons,
and so on and so forth. And the beautiful thing about antimatter is that when antimatter
comes in contact with matter, they destroy each other,
they annihilate each other, giving out energy. But anyways, in this beta decay, we get a positron, an anti-electron, antimatter of an electron that comes out. And because it is positively charged, we call this beta plus decay. And just like before, just like before, we want to write it, you know, we want to write
it with the proper notations, this time we will write
it plus one over here. And again, don't worry too
much about what this is, we're just making sure
that this total number stays the same. And over here, because the
mass number never changes, we'll call it zero. So, this is how you write a
positron in a nuclear process, but that leaves with the last
piece of the puzzle over here, what exactly are these question marks? We have accounted for
all the particles, right? Well, let me ask you this. We know that in any radioactive process, things are supposed to become more stable. More stable means it should
have less energy, right? Well, where does the energy go? Well, the energy goes as the kinetic energy of these particles. But when we looked at it experimentally, we found that there was
some missing kinetic energy. And to account for that, we hypothesized that there
must be some other particle that is taking away that energy. It must be neutral because you've accounted
for all the charges, it must be very tiny, it must have very tiny mass, and it must not be interacting
with a lot of matter because we couldn't
detect it for a long time, but eventually we did. And you know what we call these particles? We call them neutrinos and anti-neutrinos. Even neutrinos have antimatter, okay? Now, the big question is
or which one comes where? Where do we get a neutrino and where do we get anti-neutrino? Well, it turns out that
wherever we get an electron, we get an anti-neutrino, and wherever we get an
anti-electron, that is a positron, that's where we get a neutrino. And so, the symbol for neutrino is nu. And the E over here just
represents that these neutrinos, anti-neutrinos came
along with the electrons. And of course, the bar over here represents it's an anti-neutrino. Now, we might be overwhelmed
thinking that, "Oh my God, there are so many
particles to keep track of. How do you remember this?" Well, most of it can be done logically. Well, first of all, if you zoom out, you can see a beta decay is basically neutrons
converting into proton or a proton converting into a neutron. And the reason they do that
is to improve the ratio. Remember, the whole reason was
the proton to neutron ratio didn't work for them,
making them unstable. So, by doing that, they
will change that ratio, that's the whole motivation over here. And then, you can use charge conservation to figure out where we'll
get a beta minus particle and where we'll get a beta plus particle. And finally, I remember that wherever we have electron matter, along with that I'll get the antimatter of neutrino, anti-neutrino. And wherever I have
antimatter the positron, which is the anti-electron, along with that I'll get
the normal matter neutrino, which is just the neutrino. And by the way, if you are
ever thinking about, "Hey, what allows this to happen? What kind of force allows this weird neutron to proton conversion and proton to neutron conversion?" Well, it's the weak nuclear force, the fourth fundamental force of nature. Now, that's answer the original question, how do industries use beta decays to ensure consistent thickness of papers? Well, turns out that beta decay, beta particles, sorry, both positive and negative beta particles, they have a much higher penetrating power compared to alpha particles. Remember alpha particles because they are have
a high ionization power because they have +2
charge and they're bulky, they can easily stop by even paper. Beta particles are much tinier because they have a single charge, they have smaller ionization power. And because they're much more tiny, they can easily pass through paper. In fact, you'll need something
like plastic or glass or maybe aluminum to stop it. So, now let's imagine what happens if you have a lot of beta particles coming and you keep paper in front of it. Well, some beta particles
will get absorbed, but a lot of particles will get through. Now, the amount of particles
that will get through will depend upon the
thickness of the paper, right? 'Cause if you have thicker paper, more beta particles get absorbed. So, by looking at how many particles are coming out from the back of the paper, you can figure out what the thickness at that particular point is. And this is how industry use beta decay to ensure that you have
a consistent thickness. You can see we can't use
alpha particles for that because it just gets stopped very easily. Beta particles have the
right penetration power to do that job. I find this fascinating because
I would have never imagined using beta decay for ensuring consistent
thickness in paper. I mean, that's just amazing if you ask me.