Radioactivity: Expect the unexpected - Steve Weatherall

926,587 views ・ 2012-12-10

TED-Ed


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Transcriber: Andrea McDonough Reviewer: Bedirhan Cinar
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It is only in the last 100 or so years that human kind has understood
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that the nucleus of the chemical elements is not always fixed.
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It can change spontaneously from one element to another.
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The name for this process is radioactivity.
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You probably already know something about the nucleus:
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it's much tinier than the atom,
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it's made of particles called protons and neutrons,
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there are electrons orbiting around it.
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And though the atoms can share or swap electrons when they bond together,
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the nuclei themselves never change.
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Right?
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Well, no.
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Certain nuclei are not stable in that way.
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This means they may change suddenly,
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spontaneously.
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The radioactive nucleus flings out a small particle
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and transforms into another element, just like that.
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For example,
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the carbon nucleus can eject a fast-moving electron
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and turn into a nitrogen nucleus.
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There are two different particles that can be emitted from radioactive nuclei,
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but never together.
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The very fast electron is known as a beta particle.
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If you know a little bit about electrons, you may be thinking,
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"What was the electron doing in the nucleus in the first place?"
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The answer is there is a neutron in nucleus
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spontaneously changed into a proton,
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which stayed behind,
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and the electron flew out as a beta particle.
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This is not what chemistry has taught us to expect.
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The nucleus is supposed to be stable.
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Neutrons don't change into protons.
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Except, sometimes they do!
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The other particle it emits spontaneously
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from an unstable nucleus is alpha.
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An alpha particle is 8,000 times more massive than beta,
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and it's a bit slower.
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Alpha is made from two protons and two neutrons.
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If we trap all those alpha particles together,
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we get helium gas.
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Alpha is a helium nucleus.
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Like the beta particle, you would not have expected
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a heavier nucleus to throw out helium.
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But again, it happens,
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and the nucleus becomes a new element.
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So, is radioactivity useful or just dangerous?
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Wherever you are sitting,
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it is quite likely that there is a device nearby
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which contains a source of alpha particles:
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a smoke detector.
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The source is radioactive Americium.
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You are totally safe from these alpha particles,
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which cannot travel more than a few centimeters in air.
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Beta particles penetrate much farther
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through materials than alpha can.
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Radioactive atoms are used in medicine as traces,
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to show where chemicals travel in the patient.
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Beta particles are emitted and have enough energy
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to emerge from the body and be detected.
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There is a third type of nuclear radiation:
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gamma, which is not a particle at all.
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It is an electromagnetic wave,
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like microwaves, or light,
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but it is actually 1,000 times more energentic than visible light.
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Gamma rays may pass right through your body.
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Gamma is used to zap the bacteria in fruit to increase its shelf life,
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or in radiotherapy to kill cancer cells.
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Radioactive substances get hot,
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and this heat can be used to generate power.
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This heat has been brought to you since space probes,
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and, in the past, in pacemakers for hearts.
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The more abruptly nuclear radiation is slowed down,
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the more damage it does to the atoms it hits.
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This is called ionization.
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Alpha causes the most ionization as it crashes into other atoms
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and gamma the least.
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In humans, the most serious effect of radiation
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is the damage that it can cause to our DNA.
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Although alpha cannot penetrate your skin,
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if you inhale or injest a radioactive nucleus,
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the health consequences can be severe.
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Radioactivity is both useful and deadly,
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but it is all around us as a background to the natural world.
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