17.2 Antimatter

Okay, we have a short lesson on antimatter, and it kind of sounds like a strange scientific sci-fi movie kind of word. But, it's actually a real thing. In 1928, Paul Dirac was using other people's works and Einstein's ideas, and he came up with an odd formula that sort of sounded weird. But his idea was that if an electron exists, then there should also be a positive particle of the exact same mass. Later on, in 1932, we saw a cloud chamber picture like we saw in the last lesson. It looked like two exact same things, but one went in the opposite direction, which would mean that it would have an opposite charge. Anderson verified that this is indeed true using the radius and charge-to-mass ratios of the electron. He saw that it would be the same thing, an exact same item, except a positive particle. And that was the first antimatter particle. Discovered, we tend to call it a positron because it's a positive electron. But we could also call it an anti-electron. So basically, up till now, we've only said there have ever been three particles: electron, proton, and neutron. But now, with the existence of antimatter, we actually have three more. We have the anti-electron or positron, we have a proton and antiproton, and we also have a neutron. So the antimatter counterpart would be an anti-neutron. So it kind of sounds weird that it's neutral, the opposite would be the opposite. But they both end up being the same mass, the same charge, but they're created through a different process. That's why we identify them as being something different. So in terms of labeling them, if it was an electron, it is usually written as "-" and if it's a positron, it is written as "e+". A proton is typically denoted as "p" and an antiproton as "pÃÑ". Plus an antiproton (bp-) and if it's a neutron because of zero, we usually just say Neutron. And then, for antineutron, we just put a little lion over top. A couple of key things that happen with antimatter and their particles is that if you collide an electron and a positron together, because of the exact same mass and exact same energy but opposite charges, what happens is that we get a process that we call total annihilation. So, what that means is both particles basically destroy each other and their entire mass gets converted into energy. And that energy then gets released as an EMR radiation. So, this first example says that particle and antiparticle annihilation happens in stars. One such reaction is when an electron and a positron collide, and they produce two identical gamma photons. So, the question is asking, what is the energy of one of the photons? These photons. So basically, we have the electron and positron colliding, so the charge is good, right? We have upheld the negative, some conservation of charge still occurs. But our energy, then we could calculate it by just using equals MC squared. So if we took the mass of an electron or positron (they're both the same) and multiplied it by the speed of light squared, we can actually calculate what the energy of that particle is once it converts into an EMR radiation. So if we take 9.112 x 10^-31 multiplied by 2 multiplied by 3 x 10^8 squared, we get an answer of 8.2 x 10^-14 Jules as the energy of the photon. And if you wanted electron volts We could do that by the charge of an electron, and that would give us 5.12 times 10^5 electron volts. Alternatively, we could say it's 512 kilo electron volts or even 0.5 mega electron volts. However, we want to do it, so the idea is pretty simple. These particles are opposites. If they hit together, they're going to totally annihilate each other and turn that mass into energy. Modern medicine has actually used some of this too to our advantage. A PET scanner, basically a positron emission tomography, injects an isotope like fluorine-18 into a person's body. As it moves through your body, it emits positrons through collisions. With electrons and beta-positive decay, we think, "Okay, well, beta-positive decay is bad because it's going to give off gamma radiation." But we can see that the energy levels are pretty low, so it's not a huge health risk. The idea is that as these rays leave the body, there are detectors placed around the person that can actually map out a 3D picture of what's going on inside. Something like a PET scanner can provide a pretty good diagram. With modern advancements, some hospitals are also replacing PET scanners with CT scanners, which are basically computer-generated scanners. So, with these combinations, we have a pretty good way of obtaining a digital image of internal organs or bone structures of people who have been injured or have medical issues. It's kind of a cool idea that you can actually take some of This physics can be applied to medicine for something good. And that's all for this lesson.