let's begin. before the turn of the century, radiations were discovered that weren't known about and they called these, guess what? x-rays. and these x-rays turned out to be simply a high frequency light. remember we talked about exciting atoms? we got an atom with the electrons going round and round and electron or light or something hits, makes electron go up, come back down, boom, off goes the energy in the form of light. the same thing happens when you excite the innermost electrons. if you excite those-- it takes a lot more energy-- and you're knocking electron way up and it comes way back down, boom, that light photon that emerges is beyond the range of seeing. it's even beyond the ultraviolet, and that's what x-rays turned out to be. simply high frequency light from the orbital electrons jumping orbits that correspond to great energies.

but after that, it was found that there were radiations coming from different minerals that did not have to do with the electron orbiting around nucleus. it had to do with the nucleus itself. and the radiations that were emitted, to make a long story short, were three different types. now you might think they'd call them maybe "a," b, c, right? but these are physicists, gang. we didn't call them "a," b, c. you know what we called them? alpha, beta, gamma. that's "a," b, c in greek, yeah? so alpha, beta, gamma ray. and it turns out that those radiations, you can find out these three different types of radiation by simply getting a lead block, boring a hole in it down to about the center and then putting in some radioactive substances. these are heavy minerals. and it turns out these radiations will be emitted in all directions but the ones coming out here will be comprised of beam. and then in that beam, if you put a magnet,

it turns out that in that magnetic field, the beams will separate. some will just keep right on going and some will bend this way and some will bend that way. and if you had a piece of photographic film up here, you could catch splotches here, here and here. three distinct splotches which would indicate there are three different kinds of rays coming up. we call one the alpha ray, we call the other the beta ray and we call the one that's un-deflected the gamma ray. so alpha rays are charged one way and beta rays are charged the opposite, opposite charged rays. so these rays will comprise of particles with electric charges. to make a long story short, the alpha particle turned out to be-- the alpha radiation turned about-- out to be the nucleus, the nuclei of helium atoms.

and it turns out that the beta particles were electrons. these plain, old electrons, like-- that would flows on an electrical circuit. and it turns out that the gamma-- alpha, beta, gamma-- is just your e and m radiation. it doesn't have any charge just like light ray doesn't have any charge. it's just pure energy, pure energy of electromagnetic fields, okay? and your beta particle is just electrons. and your alpha, helium nuclei. now those helium nuclei-- an alpha particle and a beta particle brought to rest will make up what's called a helium atom because this is the nucleus, this is the orbiting electron. and so the next time you see a kid walking by with a balloon filled with helium, tell the kid's mother, "hey, kid, you know what's in that balloon?" and just tell the mother, that's radioactive debris

from uranium and radium and dangerous things like that. true or false? true. that's true. that's true. but these particles now have lost their energy. if you walk by an old historic site and you see an old cannon and you see some of these cannonballs all sitting, all piled up. do you get scared when you walk by those cannonballs? you don't get scared when you walk by those cannonballs. they have no more energy. not the energy they had when they were projectiles in some war or something. so just as those cannonballs don't-- are not dangerous anymore, the alpha particles and the beta particles that make up helium gas in a balloon is not dangerous. they've lost their energy. this piece of chalk isn't dangerous. but if i put this piece of chalk in a shotgun and fired at you, then it is. and that's the difference between the particles that were rejected from nuclei. they come at very, very high speeds. high speeds and a lot of energy. they can tear in to you and maybe that's not so good. but once stopped, they're completely harmless. any questions at this point?

let's continue. begs the question, why is it that atoms are radioactive anyway? and it turns out all the atoms beyond atomic number 83, they're all radioactive, which is to say they're unstable. they decay. what's going on? remember we talked about the fundamental rule of electricity and that fundamental rule was that light charges repel and unlike charges attract? let's consider a proton here. and now another proton right next to it. according to what we learned about electricity and magnetism, what will happen here? begin with a r, end with epel. - try it. - repel. they're gonna repel. a lot or a little? answer begin with a l. -a lot. -a lot. there're gonna be a huge force of repulsion. but you know what? they don't do that, not quite.

these would and you won't find that in nature. you won't find in nature two protons stuck together like that that are stable. it turns out that the force of repulsion is like this. but there's another force inside just about stronger at this level and that force is called the strong interaction. and the strong interaction is holding these things together so the whole net force is about zero. but not quite, it's still unstable. so what you do is you put another particle here with no charge and that's a neutron. you want the protons and neutrons? both of which are made up of even fundamental-- more fundamental particles called quarks. but these neutrons don't repel. see, there's no charge. so neutron only holds. so it binds. it holds this to that. that neutron plays the role of like a nuclear cement. and if you have something like this, two protons with a couple of neutrons-- [makes sound] --the nuclear force is awesome compared to any repelling electric force. and that particle is stable.

nuclear force wins or the nuclear interaction wins. see, the nuclear interaction, as you read in the textbook, is a very, very short-range force over long distances, like a couple of diameters away. it's trivial but very, very close humungous force. but the electrical force, that peters off as the inverse square of distance. i mean, if you get twice as far away-- in fact, i wonder if you guys can answer this. will the electrical force-- if you got a couple of particles twice as far away, will the electrical force be weaker? i think people out in the street could answer that. but people out in the street couldn't say how much weaker. see if you're sitting next to someone who knows how much weaker will the-- electric force of repulsion be with particles twice as far apart. check. what's the answer, gang? four times. how many say fourth? show of hands. hey, hey, hey, we're learning the stuff.

that's a typical final exam question. now that one, yeah, huh, duck soup, yeah? i mean, you've learned that. inverse square law, twice as far away, 1/4 the effect, okay? so as these things get further and further away, the electrical force kind of peters down. but the strong interaction really, really peters down. and that means you can't really have a very big atomic nucleus. and here's why. let's suppose you have a big nucleus as this big. you got a proton over here and protons over here, they're so far apart. the nuclear interaction between these two is almost trivial. but between these two of the electric compulsion is not trivial. and so what happens? very, very large nucleus, just by virtue of the spreading apart, are unstable nuclei and that's beyond 83, atomic number 83. every atomic nucleus above atomic number 83 is unstable. and in that environment, you have then the ejection of alpha, beta and gamma particles. so it's almost simple geometry.

right now, this particle we'd call an alpha particle. this is a helium nucleus. and i'll write that helium nucleus like this. and i'll put a two down here and a four here. and this nomenclature, i hope you're familiar with. this is simply the atomic number. it tells you it's atomic number two in the periodic table of substances, yeah? it's got two positive charges. and the four refers to the atomic mass unit. and that's the number of nucleons altogether. i call this, by the way, nucleons. i can call you a people, a person, but a person can be a male or female. nucleon can be a proton or neutron. see what i'm saying? but there's four nucleons altogether, so it has a mass of four and atomic number two. so that would be the configuration. if i put another proton in there, then it would now be-- does anyone know? it would be the element lithium because now it would have three

and up here would be five. and any nucleus that has three protons, by definition and by chemical properties, will be that which we call lithium. and you guys know about that, the number of protons in the nucleus dictate what the nucleus is. okay. like what, 82 for lead. anything with 82 protons, honey, that's lead. anything with 92 protons, what's that? anything with 92 protons, what kind of age do we live in, gang? begin with a u. uranium, huh, it's uranium. now, we can have something like this. the most common form of uranium is uranium 238. we say this is the isotope of uranium, the isotope 238. you're on to what isotope means. you can have atoms with a variety of neutrons. this atom here has two neutrons. it would still be the same element if it had three neutrons, but now it'd be a different isotope. isotope is another word for kind of at the nuclear level, okay?

so this is the isotope 238. notice that there are many, many, many more neutrons than there are protons. if you subtract 92 from 238, i think you get, what is it, 146? 146 neutrons, but 92 protons. it turns out as your nucleus gets bigger and bigger and the same get further and further apart, you need more nuclear cement. you get the elements up to about 20. look in the periodic table and you'll see the atomic mass is always about twice the atomic number, see? when you get up at about 20, you get equal numbers roughly of neutrons and protons, but above 20, you need more nuclear cement and then you get-- you find your atoms have more neutrons. when we talk about-- oh, by the way, we have a little model here of the helium, the helium atom, inspired and brought here by paul casey. and what this is, we have a two protons, two neutrons for the nucleus and we have orbiting two electrons which make up a helium atom, a little model,

the planetary model of the helium atom. rip these two things away, and what do we call that helium nucleus? alpha. alpha what? - particle. - parti-- - cle. - cle. good, good cle. you get the cle. you get the cle? parti-cle, all right. good. hey, when we measure how much-- how radioactive something is-- and radioactive just means the things are unstable, they're busting apart, since emitting these particles. the measure of radio activity is properly called half-life. radioactive half-life, and that's an easy concept to get. how hot something is, hot-- you know why we say radioactive things are hot? why do we say radioactive things are hot? watch out for that uranium, that's hot. is it literally hot, temperature-wise? what do you suppose? answer end with a p. - yup. - yup. not p-e, just p. try it. - yup. - yup. yup, it is hot. why you suppose it's hot?

why would a piece of uranium be hotter than a piece of rock in the same environment? rock, meaning a rock with no uranium or radioactive minerals. why do you suppose? - releasing energy. - more unstable. because... check your neighbor in this one. this is something--you guys know enough physics to do this. what does it mean to say something's hot? hot, temperature-wise. well, they're emitting alpha and beta particles and those are going at high speed and hitting the other atoms-- yeah. --and it's making it move faster. yeah. just to say something's hot, you're gonna think it's moving faster, eh? little particles moving-- but guess what? they're moving faster when they're kicked around. what's kicking them around? how about some alpha particles there in you, honey, you'll be hot, too. they're gonna just kick around a little bit. that's all, it's that simple. so it turns out things would be-- why do you suppose we're getting all these volcanoes over there in the big island?

where's that heat coming from? you know, that's the earth, honey, the earth's natural heat. where did the earth get the natural heat? come on, where does it come from? where do volcanoes coming from? what's the source of energy making the volcano do its thing? - begin with r. - radioactivity. right, radioactivity, honey. radioactivity. and down there, there's a lot or a little? a lot. yeah. a lot of people think radioactive is something new, huh, new? radioactivity has been around before the world got here. this whole world is hot because of radioactive decay. next time you bathe in a nice hot spring, you say, "this is nature's hot water, honey." guess where that nature's hot water gets its heat? radioactive decay down underneath the ground, see? and so that--yeah, hot. that's where the heat comes from. natural? yeah, it's natural. it's been around a long time. yeah. so we talked about half-life in terms of how long does it take for half of the radiation rate to cut to half. or how long does it take for half of the atoms to decay, see? if you start off with something like some pure radioactive substance,

sometime later, that substance will all convert to something else. like all the uranium that exists in the world today will one day be lead. lead, whether the world survives or not, it will all be lead. now, how long does it take for half that uranium to decay? it turns out a long time. it turns out 4 1/2 billion years. yeah. and you know, when i used to prospect for uranium, i used to find uranium, you know? and that for every time i'd find uranium inside that rock, guess what also was there? - the lead. - lead. and guess how much lead compared to how much uranium? more. about same-same, which means that what, the age of that rock is about what? 4 1/2 billion year, that's the age of the earth. so 4 1/2 billion years is the radioactive half-life for the isotope uranium 238. that means in 4 1/2 billion years, all the 238 atoms around, half of them on the average will have decayed to something else. now, that's a long half-life. a shorter half-life is like a radium.

radium, i think, is 1,620 years, something like that. and radioactive carbon 14, that's 5,730 years. and little neutrons, it turns out a neutron by itself is radioactively decayed. and that will decay in about 12 minutes, half-life about 12 minutes. and then a little new-- at a 2 millionths of a second, half of them will decay, so there's a whole range of radioactive path lives. and here's the old story. let's suppose you have a gram of radioactive substance and it has a half-life of one day, something used maybe in medical research, yeah? one day, how much of that gram will be left at the end of one day and still be the kind of substance it is? begins with the h. - half. - very good. how much would be left at the end of two days? 1/4. quarter. right. don't be wimpish. come on, get it out.

you know--these-- you are university level, right? one-quarter, honey. now, what would the wimps say? nothing. zero. okay, they say zero, say, "when you lose a half one day, you lose the half the next day, it'll be gone." [makes sounds] what do we university types think about that? needs a little more careful scrutiny, right? because you could have started it up with a half to begin with, right? okay. it's gonna keep going half-- it's like the old bit about-- do you ever talk about this thing? we're gonna jump to the wall. and every time i take a jump, i get halfway to the wall. how many jumps before i hit the wall, what's the answer? - never heard. - you guys heard of that before. you'll never hit the wall if every time you jump you get halfway. let's suppose you wanna go from here to here. whoop, it's a half, then a half, then a half, then a half, then a half, then a half, then a half, then a half, then a half-- i never get to the wall. so in a similar sense, with radioactive decay, you never get to a point where there's no more atoms left. in a practical sense, sure you will. but when you're talking about radioactive decay, it's always nice to work with large populations. hence, half-life is more meaningful in that sense

than going for the life of the whole thing. you may have one that-- let's suppose you-- [makes sounds] --and there's one left and 20 years later-- [makes sounds] does it make sense to say the lifetime of that material is 20 years? no. take half of that-- [makes sounds] --that time, you got enough-- large numbers in there that your half-life, that your value can have more meaning. it can be more close to what is there. transmutation is the word given to atoms when they change from one form to another. the alchemist of all used to believe that you could transmute the atoms. you could start maybe with lead and go to gold or you could start with mercury and turn it silver and things like that. and it turns out they could never do that, and the reason they could never do that was because they were using chemical means. gang, when we talk about this nuclear stuff, we're talking way down beneath, way down beneath where the electrons are. way down in the nucleus itself. and in that nucleus, the reactions there

have nothing to do with what's going on outside like heat and temperature and pressure and things like that. okay. the nucleus is pretty well insulated from all that. and so the processes happen way down deep in the nucleus. in fact, the alchemists didn't know it, but radioactive decay is happening all around you. but they had no way of detecting it, and if they did have a way of detecting it, they wouldn't have understood it because they didn't have the nuclear model. though they have a model for hanging their ideas upon, that model was not developed then. so radioactive decay, the idea that atoms change from one atom to another is happening all the time. so all the uranium transmutes to other atoms in steplike fashion until they become lead. let's go through some nuclear reactions. this is kind of fun to see how these transmutations occur. a natural transmutation is uranium emitting an alpha particle.

we write it like this: we start with uranium, an element of uranium, and an alpha particle is emitted. and an alpha particle is simply a helium nucleus. now, this is a numbers game. what does the uranium turn into? well, maybe we don't know, but we know one thing, that the number of charges you began with will equal the number of charges you end up with. that's called the conservation of charge. you can never give a reaction with a net charge before and the net charge after is different. uh-uh. charge is conserved. so you start with 92 positive charges, you gonna end up afterwards with 92 positive charges, but two of them are here. guess where the others are? they're in this other element, and that must be 90. let me show you how that works. 90 plus 2 equals? - 92. - 92. yum, yum. okay? all right? now, there's another thing called the conservation of nucleons. here's something: when an atom decays,

the number of nucleons after is the same as the number of nucleons before. they might be rearranged but the same number. now, we have 238, but we got 4 there. so how many are gonna be left in this other particle? and that's gonna be 234. and then you can look up at the periodic table at the inside cover of your book-- and to find out what element has atomic number 90? - thorium. - thorium. - thorium. - and is that not thorium, gang? see, right in the periodic table, kwip, boom, thorium. and that's gonna be th. now, thorium itself is radioactive. thorium decays as well. if it decays into an alpha particle, you check with your neighbor what's gonna be the atomic number of what's left over?

88. is it 88? how do you know it's 88? because 88 and 2, honey, is 90. yay, huh? same charge before and after, right? what's gonna be the mass of that particle? - 230. - 230. how do you know it's 230? because 230 and 4 is 234. okay? and that element is gonna be what? - radium. - radium. have you heard of raum, ga? l ght, rium, all right. ok. let's suppose it does not emit an alpha particle. it can emit a beta particle. if it emits a beta particle. beta particle, beta particle, beta--what's a beta particle? - electron. - electron. now we have to make the symbol for electron. for electron, get it? mm-hmm. at'she atomic nuer of an ectron? - negative one. - atom nber? negave one at is atomic numr, anyway? charge, right? what's the charge of aelectr?

- negative one. - negave one what's theass of an electron? zero. well, it's closer to zero than it is to one if one is e mass of a proton or neutron. and this has got, like, out 2,000th the mass. that's closer to zero than to one. so we round off atomic mass number, anyway. this is ze. now, what does the thorium become? ll, what do we got over here for an atomic number? 91 do you see it's 91? yeah. 91. and what's the mass gonna be? - 89. - now, what has 91? protactinium. - is it gonna be 91, gang? - yeah. let's check. - 91 minus 1 equals 90. - correct. it's 91. that's interesting. some people think that radioactive decay, you start with uranium and work down, down, down, down the periodic table approaching hydrogen. not true.

when beta particles are emitted, you climb up the table, gang. you see that? what happens in that nucleus? you got all these particles in that nucleus. i'm not gonna draw them all, but we've got 90 of them, right? and take away from 234, the rest are neutrons, so you got a lot of neutrons. what happens is this. e of these neutrons-- you can think of that neutron as being like th. no, it's n. you cak of it aseing a us and , whh is what? neutl. and what happens if a negive flies ou at's this beco - a plus. - a us. anthat's wt happen that's what happen one of these neutrons emits a beta particle, emits an electron, and becomes a proton. and this is the extra proton. see, you got the same number of nucleons. it's just that one of the neutral ones turned into a positive one. see that? so you're decaying up the table. and that element is what? 91? protactinium.

protactinium. pa. not common, huh? these are natural transmutations. there are artificial transmutations, transmutations that are induced. and one of the first that we have recorded is that which a fellow by the name of ernest therford made in, i think, 1919. and all rutherford did was he-- what did he do? he had some nitrogen gas in a container and he put inside a piece of uranium--or was it radium? i don't know--that was emitting alpha particles. and then he found the nitrogen gas later on contained trace amounts of oxygen. and what was happening was the-- the alpha particle coming out, hitting the nitrogen gas and boom, making the gas turn from one element to another. and that's where there was a nuclear interaction. and let me draw that or let me write that interaction on the board.

he had nitrogen, which is atomic number 7, got as many neutrons as nitrogen 14. you're breathi that all the ti, gang. it doesn't do anything for you. takes up space, give you the pressure in your lungs, yh? but when an alpha particle-- boom, bams into that, e followg was-- shown to be thca i think they took photographs with cloud chambers. and out of millions, they got something, millions of--i think a quarter of a million photographs, they found six reactions, which were this sort of thing here. it turns out, this knocked a proton loose from the nitrogen. and when it knocked a proton loose, what's left over, huh? what do we got here, gang? 8, 9, 1, 8. eight is what? biology types, chemist types, physics types, that's oxygen. what isotope of oxygen? most oxygen is 16, yeah? - this isotope is what? - 17.

if we put a 16 here, 16, 1 and 17-- 14, 18-- it's gotta be isotope 17. and that kinda made some history for thscientis succeed in deliberately changing one element from oneind tonother. artificial transmutation. at only occurs with nitren? that occurs with-- this is the historic one. now, you do it with all kinds of things. every time you slam particles down the end of an accelerator, you cause atomic-- nuclear transmutation. it's routine today. it's just that this historically was the first one that people were able to observe. that's artificial transmutation. slam nuclei into others and you rearrange them. that rearrangement process is called transmutation. paul. so what you're saying is today we can change lead to gold? yes, we can change lead to gold. would it stay that way or-- and the cost of doing that will not be economical. but would the stuff stay that way

- or would it be unstable? - oh, yeah, yeah, yeah, yeah. you can-- yeah, lead to gold, yeah. yeah. any another questions? you said they took some pictures. how did they do that? photographs. cloud chamber photograph. i won't take the time to talk about cloud chamber photographs, but did you ever look up in the sky sometimes and you can see where an airplane has been, you see the trail going through? what those are is ice crystals. and the airplane is interacting with the air up there-- [makes sounds] --a lot of h2o emitted with the exhaust and you'd actually see the track where the particle-- airplane has gone. even if it's so high that the airplane can't be seen, you can see the track. a similar thing happens when particles go through a region that's just gas, that's just about saturated. it's just gonna turn into little droplets and, boom, a little particle comes by, knocks these things, and they do turn into droplets and you can actually see the trail of these things. this can be seen with the naked eye, little cloud chamber tracks. if you go to a science museum sometime, you can actually see these things going on. you can look in this chamber, put a piece of radioactive material

and you see all-- [makes sounds] --you can see all the lines going out. you see, these are what? the little tracks of particles that are too small to be seen by the eye. the tracks can be seen. all the particles, can't, they're too small for us. but that's one way of doing it. carbon dating, transmutation happens in the atmosphere all the time. here's a common reaction that happens in our atmosphere. first of all, the earth is being bombarded all the time with high energy particles. it's cosmic radiation. radiations--particles are going through you all the time, splattering into the atmosphere of the earth, all of the earth. and a lot of these interactions eject neutrons. neutrons are flying all around. these neutrons hit the atoms of the air, we get kind of a interesting reaction. here's nitrogen again. it's easy for neutrons to slam into the nitrogen nucleus. very easy. it's easy for neutrons to slam into anything.

because the neutrons have no charge. see, it's hard to get two charged particles to slam together. very, very difficult. very, very rare. but it's very, very common for a neutron to splatter right into the nucleus of an atom because there's no repelling force. there's no electrical interaction. so a neutron slam right in. now, what's the neutron' atomic number? what's its charge? - zero. - zero. what's its mass? - one. - one. those two things slam. and when they slam, boom, again, a proton pops up and leaves wha well, whatev it es, it'sotta be--what's the-- what it's goa be over he, gang? - six. - six. and wh'she numr gonna be? - 13 - 14. yosee, it's 14. y, this is not twice this. th is twe and then some. this atom here has extra neutrons. it turns out that atom is unstable. that atom is carbon 14. most of the carbon in your envinment is carbon 12,

but somef the on youenvinment is carbon 14. and that carbon isioactive. and that carbon emits a beta particle. could you tell me what ileft? some people can. some people can't. most of you, of course-- begins with a c. can't. the shorter version of c. check your neighbor. what are the numbers gonna be, gang? what's the number gonna be over here? - seven. - seven? - is it seven? - yeah. - if it's seven, does it check? - yeah. seven minus one is six. what's seven? - nitrogen. - nitrogen. - what do we got for up here? - 14. what goes around comes around. nature is cyclic, gang. you started ofwith nitrogen, quick, boo you end up with nitrogen. but not right away. not right away. the half-life of that reaction is 5,730 years.

t, the radioactive half-life is 5,730 years for carbon 14 to decay back into nitrogen. that means this. everyone take a breath. well, you don't have to. it's all right. come on, here we go. ah, you know what you breathe in? - air. - some co2. now, you think all that co2 is c12, right? beginning with a w. some carbon 12. so as soon as you take a breath-- [makes sound] --you blow right back out again, right? but it all didn't come back out. what comes out is not the same thing that went in. why clorets? [laughter] some go in and stays. and when it stays, it stays part of you, but it's radioactive, ready to fire at any moment. okay? and for every carbon 14 you bring in, you take on the average 5,730 years for him to fire off again, okay?

the half-life is that long. but, gang, guess who's radioactive, me or you? - or all of us? - all of us. all of us? all of us, gang. we're radioactive. okay? 'cause we're composed of carbon. okay? and a lot of that carbon is carn 14. and it's radioactive. now, what happens when the bad day comes and all of a sudden you check out and you're no longer breathing in anymore? do you become less radioactive or more radioactive? more. that's because you're under the ground. oh, you don't be seeing that. right. you don't be seeing that. so long as you keep breathing in, you're breathing radioactive materials in, right? they become part of you. but when they become part of you, don't they decay? once a carbon 14 decays, is it radioactive anymore? - no. - no. now, it's nitrogen. huh, nice and stable nitrogen. so what happens when you die, you become less and less radioactive. you are the most radioactive right now. now, if you stop breathing, you become less and less radioactive

and stop breathing. every time you eat tomatoes, it's carbon, isn't right? carbon is in everything you eat. is there any food you eat that doesn't have carbon in it? okay? and some of those carbons is carbon 14. so when you die-- or when anything that breathes in, that takes in rbon 14, dies, it becomes less and less radioactive; hence, radioactive dating, carbon dating. you've heard of that before. you wanna know how old an old tree is or something like that. you need to take a little-- that being the half-life, let me give you an example. you're with your friends, you're up country. you go into an old cave. and you say, "hey, you know what i think there was people in this cave long ago." and you dig down, you find evidence of an old camp fire. and down in that campfire, you find an old ax, maybe it's a caveman's ax or something, yeah? and that's the ax--the ax handle and everything. what you do is you take that ax handle, you come here to the university, go to the chemistry department. at first, just take a little tiny slit. they might wanna hog the whole thing for themselves, right? it's your artifact, right? take just the littlest, tiniest bit of flake of wood off there, okay?

take that wood flake off. go to the chemistry department. but before you go in the door, take your jackknife and take a little flake of wood off a tree that's living. now, you got two splinters of wood, yeah? one's the old. one's the light, yeah? bring that to the chemistry department. chemistry tech. yeah, the chemistry tech opens the door. can you do something for me? can you get the carbon out of these things? yeah. when can you do it? come back tomorrow. you come back tomorrow. oh, and would you measure them up by the way so i get the same mass in each. you come back the next day. you get two envelops. one says old. one says new. you're telling the guy what's going on, yeah? and you get the old and the new. now, they're both carbon. when you open, they look soot. chemist-type, can i borrow a geiger counter. yeah, right over there. you et the geiger counter. you know what a geiger counter is? a radioactive-- radiation detector. and you put the carbon under the geiger counter, okay? you put the new one in there. the new one. from the little twig. huh? you put that under there. click, click, click, click, click, click, click, click,

click, click, click, click, click, click, click. maybe 20 counts per minute. make believe. 20 counts per minute. now, you take that away. now you take the old one. put it underneath there. click, click, click, click, click, click, click, click. 10 counts per minute. ooh. how old is the ax handle? check your neighbor. how many of you say, "oh, i think it's something like more than 5,000 years old." show of hands. yay. 'cause it's a half-life. what if you put it under there and it went click, click, click, click. five counts per minute. and the new stuff is 20. ooh. then how old is the ax handle, gang? 11,000. 11,000 or something. 11,460. twe that? yay. maybe twice as old as the half-life.

you got it? all right. how do you determine those numbers? how do you determine those numbers? yeah. some guy was really, really patient and put some carbon in there and kept waiting, waiting, waiting. and when it got down to half way, he just sort of looked at the counter and see how many years went by. not really. i'm joking. i'm joking. but how--it turns out the radioactive decay rate and the half-life are related. if something has a very, very high rate of radiation, then its half-life is gonna be greater. and a simple computation does that. very, very-- short, short half-life, high rate. a long half-life, low rate. let's suppose i have two substances, a gram of each. and one gram goes-- [makes sounds] --like that in geiger counter. the other one goes-- [makes sounds] ...guess which one lasts for a longer time? check your neighbo [makes sounds] check. which one gets the shortest half-life?

let's hear it. [makes sounds] you guys can't do that? the shorter half-life, okay? --is running out. it's running dow you guys are on wednesday. wednesday, we talk about fission and fusion. [music] captioning performed by aegis rapidtext

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