√ Decay and Transmutations | From Quanta to Quarks | Physics

hello everyone today we’ll be looking at decay and transmutation transmutation you might have heard it before it’s changing one element into another element and it’s what medieval alchemists try to do all those years ago on their quest for the Philosopher’s Stone so what is transmutation exactly well as I said transplantation is the process of taking one element and changing it into a different element so it can hit see here an example of taking carbon and changing it into nitrogen so this is very different to chemical reactions which you change the configuration of atoms any chemical so it can imagine for example taking hydrogen and adding it to oxygen to produce water this is a chemical reaction it is not a transmutation because on both sides of the equation we have hydrogen and on both sides we have oxygen when these equations stop balancing that’s when transmutation is occurring unless of course you’ve just got your equations wrong so transmutations change the atoms of the reaction means that we get something like carbon on one side and we end up with nitrogen on the other side so transmutations can only happen when the proton number of an atom changes remember the proton number is what gives an atom its element all carbon atoms have a proton number of six all nitrogen atoms have a program number of seven so if we want to change carbon into nitrogen we need to change the proton number now this can only happen during radioactive decay so this can happen naturally for some substances for other substances we need to fire neutrons at it until it decides it’s unstable enough to decay more about that later so radioactive decay is one of three different processes all of them release radiation but what sort of radiation depends on what process it is so radioactive substances are isotopes of elements that have unstable nuclei so some nuclei are stable some nuclei are unstable and even in a single element it can have a stable and unstable nuclei depending on the isotope so an unstable nucleus can undergo alpha decay or it can undergo be 50 gay or gamma decay and these are the three different types of radiation that I was talking about earlier so each one of these is linked to a different sort of particle is emitted from the nucleus we’ll start off with alpha decay now during alpha decay the nucleus emits an alpha particle and that’s what this object over here is so we can represent it with a Greek letter alpha now an alpha particle consists of two protons and two neutrons as we can see from the diagram but hang on that sounds a little bit familiar to protons isn’t that an atom well indeed yes is the same as a helium nucleus but it’s not the same as an atom because the helium nucleus in a helium atom tends to have electrons associated with it an alpha particle doesn’t have any electrons it’s just two protons and two neutrons so there are two kinds of beta decay our beta minus and b2 plus DK both of them look roughly like this though during beta minus decay the nucleus emits and electron which is odd because we don’t generally think of electrons coming from the nucleus we think of them being separate to the nucleus so something odd is going on here during beta plus decay which as I said is similar to beta minus the nucleus emits a particle of antimatter called an anti-electron or a positron now these are just the same as electrons except they’re positively charged instead of negatively charged and if one touches a an electron both of them will disappear obviously they’re called v2 minus and beta plus because the particles in beta minus decay are negatively charged and the particles in beta plus decay are positively charged finally we have gamma decay now during

gamma decay the nucleus emits a photon which we can see over here so unlike the others it’s not emitting a sort of solid particle the photon is called a gamma ray and has very very high energy now cast your mind back if you will to the electromagnetic spectrum when we went from radio waves microwaves infrared visible light ultraviolet light x-rays and right at the top we had gamma rays so gamma rays are the highest energy photons in all of the electromagnetic spectrum so you get a lot of energy coming up from these radioactive particles so how do you figure out what transmute into what well we use a conservation of mass and conservation of charge the atomic mass of the nucleus is conserved after decay after decays we have here an equation of decay polonium turning into lead and a helium nucleus which is an alpha particle so if we look at the mass of all the particles involved we have a ploy to map and weighing 210 and formic mass units turning into a lead atom that weighs 206 and an alpha particle which has two hydrogen sorry two protons and two neutrons giving an atomic mass of four so we can see that we have the same mass on both sides of the equation the other thing that’s conserved is charged if we look at the number of protons are on each side of the equation or the charge on each side of the equation we have 84 positive charges here a you two positive charges and two positive charges so that means that our charge is conserved we don’t gain or lose any charge during the transmutation this means that it’s possible to predict exactly which substance something will decay into just by knowing what sort of radiation its emitting so let’s go through them when a nucleus emit an alpha particle this is alpha radiation its mass decreases by four atomic mass units because an alpha particle is the same as a helium nucleus it has two protons and an atomic mass number of for its proton number decreases by two so you can see that in this case we’re starting with a proton number of 84 and ending up with 82 the other protons have gone into the alpha particle the daughter nucleus the product will have a different mass and a different proton number so we can see that both the mass and the proton number here have changed so that’s alpha decay what about beta decay so when a nucleus emits a beta particle its mass doesn’t really change electrons are so tiny compared two nucleons but they don’t really make much of an effect on how heavy the nucleus is so we can see that in these reactions the mass isn’t changing its staying at 44 this and at 22 for this however its charge will change now remember there are two different types of beta radiation there’s beta minus and beta plus so in each case we’ll get a change in the charge of the products or the change of the proton but which way it goes depends on the particle released so if we look at b2 minus then the electron emission has a charge of minus 1 so that means that the proton number of the daughter nucleus will increase if we add the charges together we have 19 on this side and 20 minus one on the other for the bottom DK beta plus we have 11 on this side and 10 plus 1 on the other so that means for beta plus decay the proton number decreases so the daughter nucleus will have the same mass but a different proton number so this is how we can transmute one element to another without it changing its mass finally we have gamma decay which is a little different to the first two there is no significant change in the mass or in the proton

number of the nucleus instead the nucleus goes from an excited state where the nucleus is very excited and energetic to a lower energy state bear in mind that this doesn’t affect the electrons only the nucleus so this process is usually part of a chain of reactions so in alpha decay will lead into something like excited nickel and then that excited nickel will undergo gamma decay to become unexcited nickel so it’s a very important decay but it doesn’t change the mass or the charge of the of the original nucleus and once again just to make sure it’s in your head this has nothing to do with the electrons we can forget about the electrons completely we’re dealing with a nucleus this is nuclear physics so in a Nicholas decays the progress is usually unstable so we start with something unstable we decay we end with something unstable and that means that the new nucleus will eventually decay too so we can see here we have something sign with polonium creating lead the letters on stable so it turns into bismuth the business that is unstable so it turns into thallium the thallium is on state also it turns into lead and this isotope of red is stable so at this point the atom will stop decaying the resulting series of nuclear reactions is called a decay chain because we can see that all of these are all these reactions that change together it’s worth pointing out I think that we have led appearing twice but the first time it appears when we have led 211 it’s an unstable isotope of lead and so it will decay into bismuth over here but when we reach ledge 207 this is in fact a stable isotope of lead and will not decay further so that’s the end of the theory we’ve gone through all three different types of radiation let’s quickly go on to some questions question 7 what type of nuclear decay is this well we have uranium here thorium here and I helium nucleus here so what does that mean well it’s not be TDK because that’s an electron or positron emitted we can’t see any of those it’s not gamma decay because there are no gamma photons and it is nuclear decay because we are having one atom in changing it into a different sort of Adam so it’s got to be alpha decay we can see that a is the correct answer and if we remember what alpha decay means it means that we spit out a helium nucleus an alpha particle from the original nucleus and so that’s this part of the equation here question 8 something similar what sort of decay is this if its decay at all so in this case we’re not spitting out an alpha particle so it’s not a we have carbon 14 turning into nitrogen-14 and an electron so we have an electron coming out of the nucleus this is not gamma decay because gamma decay does not change the proton number it is nuclear decay because the proton number does change it is in fact beta decay so b is the correct answer beauty decay is the emission of an electron or positron from the nucleus in this case it’s a negatively charged particle so it’s an electron question nine complete each decay equation so we have americium 241 turning into something plus the helium nucleus which is an alpha particle so during alpha decay our atomic mass decreases by 4 because it goes into the alpha particle and our program number decreases by 2 because goes into the alpha particle so that means that our new mass will be 237 it’s a 240 1-4 and our new program number will be 95 minus two which is 93 so to figure out what element that is we can look it up on the periodic table as it turns out the element with 93 protons per atom is

neptunium a very close relative of uranium so we can see that on both sides of the equation the mass is conserved and the charge is conserved now a bit more quickly we’ll go through our three more be 233 palladium our undergoes beta minus decay so its mass doesn’t change its got to stay as 233 and its proton number will increase by one so the charge is conserved so what has an atomic number of 92 its uranium so our answer is 233 our atomic mass units and 92 proton 92 protons for uranium this is uranium 233 see ledge 209 undergoes beta minus decay so its mass doesn’t change but its proton number increases by one and it becomes bismuth 209 finally our we have francium undergoing alpha decay or francium sorry so it’s mass decreases by four and it’s charged decreases by two and we’ll end up with a 13 217 we can see that in each of these equations the charge is conserved and the mass is conserved all right question 10 classify each transmutation is alpha beta or gamma decay so we’re trying to figure out what’s the missing particle start off with we have a 2 neptunium 237 into palladium 2 33 so if we look at the program numbers of these two we’re actually going from 93 to 91 so that’s not plating is protecting him and our mass is decreasing from 237 to 233 so if I mass decreases by 4 and our charge decreases by 2 we must be releasing an alpha particle so this is alpha decay be 4m 229 into radium 225 so once again will notice that the mass is going down by four and if we look at the proton number of these elements on a periodic table will notice that the proton number is going from 92-88 so that means if we lose for mass and we lose two protons its alpha decay see radium 225 into actinium 225 so in this case our mass isn’t changing so we know it can’t be alpha decay but our proton number is it’s moving from 88 to 89 if we’re going to conserve charge it means that we need 88 goes to 80 9-1 and so will emit a beta minus particle this means that if we look at the number of total charge on both sides will have 88 on this side the 88 protons and 89 minus one makes 88 on the right side so a charge is conserved finally d astatine 217 into bismuth 213 in this case our atomic mass is changing down by 4 and our proton number is changing it goes down by two so once again it’s alpha decay alright so that’s fairly simple one last question question 11 write a series of nuclear equations showing the transmutation of lead 197 into gold 197 so in in this question we’re turning lead into gold which is exactly what those alchemists tried to do so many centuries ago so if our mass starts off at 197 and ends at 197 there can’t be any alpha decay we just lose too much mass so we need a way of changing our proton number so that it’s decreasing without emitting alpha particles so how do we do that that’s right beta plus decay so the atomic mass doesn’t change it so there can be no alpha decay but we can use beta plus decay so we have a lead 197 into thallium 197 and a positron remem remember that a positron is the same as an electron except it’s positively charged all right so now we have 197 thallium what do we do with that well same thing thallium 197

into mercury 197 plus another positron and finally a mercury 197 turns into goals 197 and loses another positron so in each equation we can serve the charge and the mass of both sides of the equation now it is in fact possible to make lead 197 by taking a different isotope of lead and hitting it really hard with things like neutrons the thing is it’s so expensive to do and can only be done in such small quantities that it is not economically viable at all to turn lead into gold I’m afraid well let’s the end of the questions so in this section we’ve gone through nuclear decay alpha beta and gamma decay we’ve talked about how transmutation can occur and what we mean by chains of nuclear reactions