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Types of Decay


Now let's walk through a few of the decay modes. We'll start with alpha decay. Here an atom splits into a smaller atom and an alpha particle which is just a helium nucleus. So if we look at this we have a large nucleus that breaks resulting in our alpha particle or our helium nucleus.

If we were to write this we could say that 10, 5 boron goes through alpha decay and produces a 6, 3 lithium. Now remember, nucleons are conserved. So 10 will not change for this and we have on here 4 plus 6 equals 10. For the alpha decay our number of protons stays the same because we're just splitting off the helium nucleus, so 2 plus 3 equals 5.

So in an alpha decay we take a nucleus and split it into 2 smaller ones, one of which is just essentially the helium nucleus. Now our next decay mechanism is beta decay. Here a neutron turns into a proton by ejecting a beta particle which is essentially an electron. So if we consider that a neutron has both positive and negative charges.

With the neutron ejects the negative portion as a beta particle it then becomes a proton. So for you to see this visually, we have our nucleus, it'll eject the beta particle, which then causes the neutron to turn into a proton. An example of this 10, 5 boron going through a beta decay becomes 10, 6 carbon.

So the total number of nucleons is the same here, however our protons have increased by one because a neutron has been converted into a proton. So we go from 5 to 6. Next decay mechanism, positron decay. In positron decay a proton turns into a neutron by ejecting a positron, which is essentially a positive electron.

So here we have our positive charge that's ejected as a positron, and we go back to being a neutron. Now, if we look at this visually, we'd have our nucleus that would eject the positron and one of our protons is going to become a neutron. So for the case of 10, 5 boron, a positron decay results in 10, 4 beryllium. So again, the total number of nucleons is conserved, however, a proton turns into a neutron, which reduces this number from 5 to 4.

Electron capture is when a nucleus 'catches' an electron and combines it with a proton to make a neutron. So if we have our nucleus and an electron out there, it catches the electron and then adds it to a proton to convert it into a neutron. So for this case, proton with an incoming electron absorbs that and we get a neutron.

For 10, 5 boron, an electron captured turns it into 10, 4 beryllium. Again nucleons are conserved. Adding this electron makes a proton turn into a neutron or 5 goes to 4. The last decay mode we'll look at is gamma decay. This is the result of a rearrangement of nucleons in the nucleus. Doing so releases extra energy in the form of a gamma ray.

Looking at a visual model, when a nucleus releases a gamma ray it does so because it has rearranged to some lower energy state. So for 10,5 boron undergoing a gamma decay, we essentially get 10,5 boron but it's in some new energy state as indicated by the stars. So gamma decay does not change the element but it does change the position of the nucleons in the nucleus.

They move round a little bit and that moving results in lower energy states, and the release of the excess energy as a gamma ray. Now let us do a few decay reactions. We'll fill in blanks for the following. In the first one we have berkelium-249 undergoing a beta decay. Now here in a beta decay, a neutron is converted into a proton.

When elements are written in this form, just showing the protons plus neutrons is often not helpful in decay reactions. Instead we wanna write them with their mass number, 249, with the atomic number underneath and berkelium happens to be 97. So now we know the protons plus neutrons plus the number of protons. So in this beta decay we're gonna convert one of these neutrons into a proton raising the atomic number without changing the total number of nucleons and we'll have 249, 98 californium.

This next one is a gamma decay resulting in plutonium-236. Now remember, gamma decays don't affect the nucleons, so on this side we're still gonna have plutonium-236, it's just that the nucleus has a slightly different orientation after the gamma decay. So, for this instance we don't actually need to look up the atomic number for the plutonium because it's not gonna be affected.

If we did however we would see that this would be 236, 94 plutonium, because plutonium has 94 protons, it's element 94. Our next reaction, technetium-88 undergoing a positron decay. Now in a positron decay, we're going to turn a proton into a neutron. So again we hit the periodic table and we see that technetium, element 43, So we change one of these protons into a neutron and we get 88, 42 and this element is molybdenum.

The last one we had polonium-208 undergoes some change to become lead-204. Now notice here 208 goes to 204 meaning that we've lost 4 nucleons. This only happens in alpha decays because alpha is a helium 4, 2 nucleus. We could have written this out and said the polonium was 208, 84 Po and lead is 204, 82 Pb and then these would balance, 208 equals 4 plus 204, 84 equals 2 plus 82.

However, all the other decay modes we've talked about don't affect the mass number. Only an alpha-decay, where we actually split the nucleus and send off the nucleons in a different direction, will affect the total number of protons plus neutrons. Nucleons are still conserved, they just are now split into an alpha particle and the new nucleus.

Now, if we were to do an electron capture, It has the same result as a positron decay. So technetium-88 plus an electron would also result in molybdenum-88. Because in an electron capture a proton is converted into a neutron. So the result of electron capture or positron decay is the same.

They're just different modes to get to the same product. That concludes this portion of the physics two lecture.

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