Antoine Henri Becquerel discovered radioactivity completely by accident when he exposed a chunk of uranium to a photographic plate. This opened up a whole new field of research to uncover the source of the mysterious energy.
The Discovery of Radioactivity
The Discovery of Radioactivity slideshow
Henri Becquerel discovers Radioactivity
Henri Becquerel Nobel Prize biography
Sur les radiations émises par phosphorescence (text in French)
Comptes Rendus translated by Carmen Giunta
Annotated Bibliography for Henri Becquerel
Transcription services provided by: GMR Transcription
Announcer: This episode of Astronomy Cast is brought to you by Swinburne Astronomy Online, the worlds’ longest running online Astronomy degree program. Visit astronomy.swin.edu.au for more information.
Fraser Cain: Astronomy Cast, Episode 373, the Becquerel Experiment. Welcome to Astronomy Cast, your weekly fact-based journey through the cosmos. We hope you understand not only what we know but how we know what we know.
My name is Fraser Cain. I’m the publisher of the Universe Today, and with me is Dr. Pamela Gay, a professor at Southern Illinois University Edwardsville and the director of CosmoQuest. Hey Pamela, how’re you doing?
Dr. Gay: I’m doing well. How are you doing Fraser?
Fraser Cain: Doing good although it is that season. It’s the season where we both complain about our allergies so I’m gonna complain about my allergies. I have allergies.
Dr. Gay: My eyes aren’t watering today, but I have all the windows shut and I’ve taken all the allergy meds, all of them.
Fraser Cain: Oh, good, good. Yeah, I know, it’s just in my – you probably hear it in my voice a bit. There’ll be a little bit of that. Cool, so let’s just do it because this is what we do. Let’s do one more reminder about the upcoming Hangout-A-Thon.
Dr. Gay: Yes, we have at the end of April, the CosmoQuest Hangout?A?Thon. It is April 25-26. You can learn all about it by going to cosmoquest.org/hangoutathon, all one word, and we’re still working and rounding up guests and door prizes, but it should prove to be an awesome 36 hours of me singing for our supper. It’s kind of our hope that this is the last time we have to do this. I’m in the process of writing a ginormous NASA grant that if we get it will mean we’re back out of the hole that sequestration put us in three years ago.
So, help us survive just until we get this grant hopefully, and if we don’t get this grant, well, there’s gonna be a whole lot of rethinking how we do things.
Fraser Cain: More, year after year, more Hangout?A?Thons then. Maybe we’ll just do them for entertainment as opposed to raise money.
Dr. Gay: Oh, that would be awesome. That would be truly awesome.
Fraser Cain: Yeah, yeah.
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Fraser Cain: So, Antoine Henri Becquerel discovered radioactivity completely by accident when he exposed a chunk of uranium to a photographic plate. This opened up a whole new field of research to uncover the source of this mysterious energy. So, this is a “that’s funny moment,” which as we’ve mentioned in the past are probably our favorite kinds of discoveries. So, when he performs the experiment, what was going on?
Dr. Gay: Well, it first of all wasn’t so much an experiment as an odd accident. This was back in the 1890s, and everything was new. They were still working on figuring out photographic plates, photographic film, radiation, electrons, all of these things, particles, and we still didn’t know all the particles that there were to find and we still didn’t know all of the things that particles could do and atoms and oh, there was just so much going on.
And Becquerel had recently seen a talk on X?rays. This was right after Röntgen had figured out that you could shine high-energy electromagnetic radiation, a fancy word for high-energy light, through someone’s hand onto special film and could actually see the bones inside the hand.
Fraser Cain: With no ill consequences.
Dr. Gay: Well, yeah, this led to the crazy decades of things like using X?rays to make sure your shoes fit correctly and things that I don’t think we’d quite be doing today.
But yeah, so Becquerel saw this presentation on X?rays and it got him to wondering about some effects he’d seen with uranium salt and he had this notion that maybe the effects he saw from uranium salt were due to some sort of a phosphorescence process.
Florescence, phosphorescence, these are processes where a material absorbs light, and in the case of florescence, emits it in real time at a different wavelength. So, you expose a rock, a shirt, something to ultraviolet light and then it gives off light. It reemits the energy at a lower wavelength that in some really awesome cases is visible to the eye. This is where if you stand under a black light in the right white cotton shirt, you’ll see your shirt fluorescing in some sort of purpley hew. Now, with phosphorescence, it’s a delayed response.
So, this is where you get calendars that glow in the dark. For instance, I had a Walt Disney one when I grew up and it used to creep me out as a small child. In this case, the glow?in?the?dark paint, the phosphorescent paint absorbs the daylight, the room light that hits it and then when all of the lights are off, the molecules reemit that light at a much slower pace, allowing you to see the glow?in?the?dark paint.
So, Becquerel was thinking, huh, I wonder if one of these reactions, particularly phosphorescence might be occurring with uranium salt and he tried it. He exposed uranium salt to daylight, put it on film and he got a kinda ghostly image. So, he figured well maybe, maybe try this on an amazingly sunny day. He was working in Paris, he was working at the National Academy of Natural Sciences and the particular day he had his uranium salts and his films prepared, set up with a cross in between the salts and the film that he could use to try and image it as it blocked the high-energy particles.
As he set up all of this, the sun kinda failed to come out, and so here he was with his film in the drawer, his cross in the drawer, his uranium salts wrapped up in black paper on top of all of this in the drawer and no daylight.
Fraser Cain: Right, so we’ll come back to this experiment later, right? He’s like wait until the sun is out again and then we’ll try it again later.
Dr. Gay: See, you’d think that. That would have made sense. That’s probably what I would have done, but that’s not what Becquerel did and this is why we discovered radiation. Now, what actually happened is Becquerel had all of this and then decided to develop the film anyways.
Fraser Cain: To develop the film anyway, right? That’s it, that’s the moment. It’s crazy to me because who would do that?
Dr. Gay: And it’s effort, it’s stinky, smelly effort.
Fraser Cain: Yeah and there’s not gonna be anything on the film. It’s why bother?
Dr. Gay: This is one of the many moments in science where I look at what folks were doing in the late 1800s and the turn of the 1900s, and I’m just sort of like huh, why? Well, thank you.
Fraser Cain: I don’t understand why, but thank you for doing that.
Dr. Gay: And so for whatever reason, back in 1896, he opted to develop his film. And when he developed his film, he found a ghost of a smudge where the cross had been between his uranium salts and the film and a darker smudge where it had simply been the uranium salt and we actually still have that plate today. This is where we start to actually get photographic evidence of science, to be able to go and look at these things and that’s actually cool because you can look at the developed film and see his handwriting on it where he made notes. And this was that huh, huh, this is weird moment that sometimes occurs in science.
Fraser Cain: Yep, which heralds the beginning of the greatest discoveries. Okay, great, so he does this experiment in that he for some weird reason developed what should have been a completely blank piece of film, photographic plate so then what? Now he’s got this weird ghostly image and so he knows and so I guess what’s the conclusion that he reached?
Dr. Gay: Well, so, what he presented on March 1, 1896, was that there was these spontaneous emission of particles. His graduate student, who just happened to be Marie Curie, she named this radioactivity. There was spontaneous emission of particles that the film was able to capture, and then the true fun of sorting out what the insert expletives, probably in French, is going on moments began.
Fraser Cain: So, please continue with these moments.
Dr. Gay: Well, so the first thing was to try and figure out what is this, and they quickly realized that in this case, it wasn’t exactly X?rays like Röntgen had been working with, but this was something else. And as they started working with the uranium salts, they of course had to get more uranium so this is where Marie Curie and her husband, Pierre Curie, actually went and did some mining you might say. They went and collected uranium or processed it and had the weird moment of oh; it’s not just the uranium that’s causing this to happen. They actually found that in the garbage minerals they set aside, there was more radioactivity than in the uranium.
Fraser Cain: And did they realize that, I don’t know … that the uranium had caused other things to become radioactive? Had they realized that, you know, it’s almost like that fluorescence, that it’s the minerals store up the sunlight. In this case, they store up the radioactivity and release their own particles, right?
Dr. Gay: And they couldn’t figure out everything that was going on because one of the problems they kept running into is this was such a small fraction of the ore that it actually took years and years and years to pull apart all the minerals and figure out okay, so the uranium is causing this, there’s this stuff we didn’t even know existed that we’re gonna call polonium, it’s causing this other thing to happen. They started discovering radium, polonium, all of these radioactive elements within the ore and starting to put together a story. Now, of course, they had to start to figure out what are all the different characteristics of these materials.
Fraser Cain: Right, so I guess each of the different radioactive elements on the periodic table of elements, it’s gonna be radioactive in a different way and so I guess their process then was to try and figure out how can they perceive the different particles, the effect, what this radioactivity looks like?
Dr. Gay: And just as you’re struggling to find words, they were struggling to find questions. And in the next round of wow, that’s cool that they sorted that, the next thing that they started to do was to try and understand okay, what are the measurable characteristics of this radioactivity.
And so one of their starting points was to take a radioactive element, whether it be the polonium, the uranium salts, whatever and put it into a machine that allowed them to direct the particles through a collimator and then expose them to a magnetic field and then encouraged them via the whole once something’s in motion, it goes in a straight line unless acted upon a force, encouraged the particles via the magnetic force or lack thereof to hit a photographic plate. And what they found is some of the radioactive sources gave off particles that bent one way in relationship to the magnetic field.
Some of them bent the exact opposite way. This indicated that some of the particles had a positive electric charge; some of them had a negative electric charge. And then some of them, they just went straight on through. They were completely unaffected by the magnetic field, indicating that they were charged neutral.
Fraser Cain: They had a neutral charge. That sounds similar to some kinda particle that we’re aware of.
Dr. Gay: Well, in this case, it was actually just light, not neutrons. So, this is where we ended up with the whole original concept of the alpha, beta and gamma particles.
Fraser Cain: So, you’re looking for me to let’s get started then – alpha.
Dr. Gay: So, I have to start by admitting I utterly, totally and completely screwed up my discussion of alpha, beta and gamma particles a few episodes back. So, if I’m contradicting myself, it was because I had a moment a few episodes ago.
Fraser Cain: Yeah, historic Pamela was wrong.
Dr. Gay: Historic Pamela was wrong.
Fraser Cain: Yeah, the new Pamela is here to clear this up.
Dr. Gay: I am. So, the alpha particles that they found, these turned out to be nothing more than ejected helium that came out of these large atoms. As the large atoms broke apart into something new, something more stable hopefully, out popped a fast-moving, easy?to?stop helium. This got called alpha, and this is the particle that when I was young, I was annoyed to learn was nothing more than well, helium.
Fraser Cain: But just to be clear about this, this is this process of radioactivity where one kind of element decays into another kind of element and the way it’s doing this is by shedding –
Dr. Gay: Ejecting violently.
Fraser Cain: – sure, parts of itself, right, and in some cases, it’s doing it proton by proton or neutron by neutron and in other cases, it’s doing it in big clumps. In this case, the clumps are helium, which is what, two protons, two –
Dr. Gay: Neutrons.
Fraser Cain: – two neutrons.
Dr. Gay: And this is where we get a positively charged particle that is radioactive.
Fraser Cain: Right, okay. So, we’ve got some alpha particles coming off. What else have we got?
Dr. Gay: So, the other thing that we have is beta particles. And in this case, it’s nothing more than an electron. This is where we get the negatively charged high-speed particle coming out that can zot your DNA and cause cancer as unfortunately, Marie Curie discovered, but again, it’s just an electron that’s getting ejected at a high velocity from some sort of a radioactive decay process.
Fraser Cain: Okay. That’s two.
Dr. Gay: And finally what we have is gamma rays. This is where the decay process includes the emission of energy, high-energy photons that very little is going to stop. It takes significant lead or concrete to prevent gamma rays from zotting your DNA. With beta particles, all you need is heavy clothing; with alpha particles, paper clothing will do it; gamma rays, they’ll just kill you.
Fraser Cain: Right, well or turn you into the Hulk, right, obviously.
Dr. Gay. Something. Something.
Fraser Cain: Okay, and so the point being, I mean the three particles are connected together, like when you have the reaction, those three things will come out as part of this decay process, right?
Dr. Gay: And which ones you get depend on what the decay process is.
Fraser Cain: Right, exactly. So, if you could sort of stick your detector in front of a hail of particles coming off some element, you would know what that element was based on, what it was decaying into and the rates, things like that?
Dr. Gay: And the easiest way to tell them apart is you just look at is this positively charged, negatively charged, not charged and that tells you what you’re dealing with. And the amount of these that you receive depends on how much of that radioactive ore you have and what its half?life is, and this is all stuff they were trying to figure out.
The whole idea of half?life was something that came out of their research, and this all went fast and furious. And what really kind of amazed me was Becquerel and Curie; they won the Nobel Prize in 1903, just a few years after their initial announcement in 1896 of what they had discovered.
Fraser Cain: Okay, so hold on a second here, let me just see if I got this straight though. A researcher and his graduate student won the Nobel Prize together. Is that what you’re saying, his female grad student –?
Dr. Gay: I am. I know –
Fraser Cain: – won the Nobel Prize –?
Dr. Gay: – isn’t this it exciting?
Fraser Cain: – I know. That is the first and last time that has ever happened. No, I’m sure it’s happened other times, but –
Dr. Gay: It hasn’t happened often enough.
Fraser Cain: Right, but it definitely breaks the narrative that we’ve been going onto, which is that the researcher takes the credit and many times the grad student is the one who really comes up with the clever experiment and, but I mean Curie wouldn’t have done it without Becquerel. I mean without Becquerel kinda going you know what, I’m gonna do something really weird and develop a clean photographic plate, wouldn’t have occurred to her to start doing the research.
Dr. Gay: And what’s amazing about all of this is this was really early on in the existence of the Nobel Prize. You see this example of an advisor and the grad students winning the prize for amazing research, fast turnaround, everyone being acknowledged, and it’s just so sad that this precedent for awesomeness went away. Now admittedly, Marie Curie, while getting the Nobel Prize for her research also got cancer. So, apparently no story has a happy ending.
Fraser Cain: Yeah, yeah, but I guess it didn’t even occur to them to –
Dr. Gay: They didn’t know.
Fraser Cain: – yeah, like before we do any experiments with this brand new, amazing substance, let’s pile a whole bunch of it next to some lab animals and see if there’s any problem, but it’s also kind of amazing to think that they got the Nobel Prize in the early 20th Century for the discovery of it and yet 40 years later, it was used as the most powerful bomb ever created in war.
Dr. Gay: And it’s true the work that they did, identifying radioactivity, the properties of energy being released as these atoms break into smaller pieces versus fission processes of smaller atoms combining and giving off energy as they form larger atoms, as these decay processes were discovered, they were putting together a whole new picture of how matter works. I don’t think until their work came along that we really had this understanding that atoms weren’t forever.
They were the ones who started putting together that picture of things can transform one atom into another through these radioactive processes that can serve charge by giving off positrons and electrons that can serve so many other properties that lead to child particles that we can start counting to figure out – well, carbon dating is possible due to looking at the child atoms that come from that decaying parent carbon.
Fraser Cain: And you also get situations like the discovery of neutrinos, which were fairly difficult to discover, but the math said they had to be there.
Dr. Gay: We needed that one more conservation particle. And all of this came out of essentially a photographic plate in a drawer with uranium salts and a cross.
Fraser Cain: Yeah. So, I’d love to go a little deeper just into some of the other experiments that they went into. So, once Becquerel had done that original discovery, once the Curies started to radiate themselves too soon, what kinds of experiments did that lead on to? What other parts of this did they uncover and what did it lead to?
Dr. Gay: I think that starts to get to be an entire new show.
Fraser Cain: Well, we’ve done one on radioactivity and –
Dr. Gay: Right, and it went on to trigger Rutherford to start to do work where he was bombarding gold foil with alpha particles and this allowed us to map out the shape of an atom. We couldn’t have done that without alpha particles. We’re gonna be talking about more of these experiments as we go. It’s kind of amazing to think that this also was what led us to start finding new atoms where now atoms get found by colliding things together in high-energy accelerators.
Prior to this, atoms got found by just kind of noticing the world around you was made of gold and silver and stuff, but Marie Curie went on to get a second Nobel Prize in 1911 because as they went through that ore looking for the uranium, they kept finding this other radioactive stuff and discovered this was new atoms that hadn’t been previously classified. So, this was simply foundational work that led people to literally go digging through rocks looking for things that fell apart in the night and gave off high-energy particles.
Fraser Cain: Right, which of course as we now know are the remnants of supernova explosions, right?
Dr. Gay: Yeah, that’s entirely true. Out of the Big Bang, we got hydrogen, helium, bits of lithium and beryllium, but everything else in the universe came from stars in one way or another, and all of these heavy atoms, all of these radioactive atoms, they came straight out of supernovae. And we actually use several of these radioactive atoms to start getting at the ages of stars in some cases where we can start looking at the ratios of different specific isotopes of things like strontium to pull apart how old some stars are.
Fraser Cain: Well, I think that – well, it was true, but I think it’s a bit of a misnomer because when you think about a main sequence star like our own sun, which is forming hydrogenated helium, when it runs out of that, it’s gonna start forming helium into heavier elements, but when it dies, it’s gonna hang on to it all.
So, none of that oxygen or carbon or any of that stuff that’s in the sun is ever getting back out again because it’s just gonna hold on with gravity.
Dr. Gay: It’s trickier than that actually. So, there are two myths. One is all atoms come from supernova, other than the ones that came out of the Big Bang, and the other myth is that all the carbon stuff in a star gets rereleased. It’s actually somewhere in between because in the outer atmosphere of stars, which are quite happily going to get emitted out in planetary nebula form, there are different reactions that go on.
There’re occasionally things like convective overshoot that can change the composition of the outer layers of the stars, all sorts of different things occur that do influence the composition of the atmospheres of stars, which do get given off to get recycled into planets and other stars.
Fraser Cain: So, some of it is gonna make its way back out into the universe?
Dr. Gay: Yeah.
Fraser Cain: Okay, well, I think after that complete diversion, I think we can wrap this episode of Astronomy Cast up. Thanks Pamela.
Dr. Gay: Thank you.
Fraser Cain: Thanks for listening to Astronomy Cast, a nonprofit resource provided by Astrosphere New Media Association, Fraser Cain and Dr. Pamela Gay. You can fine show notes and transcripts for every episode at astronomycast.com. You can email us at email@example.com, tweet us @astronomycast, like us on Facebook or circle us on Google+. We record our show live on Google+ every Monday at 12:00 p.m. Pacific, 3:00 p.m. Eastern or 20:00 GMT. If you miss the live event, you can always catch up over at cosmoquest.org.
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Duration: 29 minutes