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Scientist on the Prowl. on April 4, Chapter: 1: "Experiments in.
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The work of J. Thomson , Marie Curie , Ernest Rutherford , James Chadwick , and their contemporaries established that atoms were in fact composed of smaller particles: Furthermore, radioactive decay showed that by rearranging these particles atoms of one element could turn into atoms of a different element. This was in fact what radioactive decay was. The early researchers named three different types of radioactive decay, based on their properties. They chose the first three letters of the Greek alphabet to refer to them: Alpha decay is when an atom emits an energetic alpha particle, an alpha particle being composed of two protons and two neutrons bound together.

This is actually the same thing as the nucleus of a helium atom. But what's left behind is an atom with two fewer protons and two fewer neutrons than it started with. And an element is defined by the number of protons in the nucleus of its atoms, also called the atomic number. So, for example, let's start with an atom of uranium, which has 92 protons. In particular, let's use the most common isotope of uranium, with neutrons, known as uranium where is the total number of protons plus neutrons, which is called the mass number. This atom is unstable, and will eventually decay by radioactivity, although it will probably take a very long time to do so.

When it decays, it emits an alpha particle. This changes the atom's atomic number by -2, and the atom now has only 90 protons and neutrons, since it's lost two of those as well. This makes it an atom of the element known as thorium , specifically the isotope thorium Thorium is also unstable, and a lot more so. It decays within a matter of days.

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But it does so by a different method: The thorium atom emits a beta particle, which is actually just an electron. But this electron is not one of the electrons that surround the nucleus. It is actually ejected from within the nucleus. But there are no electrons in the nucleus, so where did it come from?

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The answer is that the electrically neutral neutrons themselves are inherently unstable particles, and can decay into a positively charged proton plus a negatively charged electron, plus a third particle which I haven't talked about before, called a neutrino. Neutrinos are electrically neutral, extremely low in mass, and actually not that important for what I'm talking about today, but I wanted to mention them for completeness. The important thing is that when a neutron inside a nucleus decays, causing beta decay, the produced electron and the neutrino gets ejected with high energy, while the new proton sticks in the nucleus.

The element with atomic number 91 is protactinium. So thorium beta decays and turns into protactinium The mass number remains the same. Protactinium is also unstable and can decay by beta decay.


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This increases the atomic number from 91 to 92, which is uranium again! But fear not, we have not entered a vicious cycle, because the total number of protons and neutrons is now We have an atom of uranium , not the uranium we started with. Overall, we've lost four neutrons along the way, though they were ejected as two protons and two neutrons the alpha decay , plus two electrons the two beta decays.

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Uranium decays by alpha decay, forming thorium, which decays by alpha to form radium Another alpha decay takes us to radon, then another to polonium, and yet another which produces lead Even this is not the end of the road, as lead decays by beta decay into bismuth, and from there it pinballs around a bunch of other radioactive isotopes before we finally reduce the mass number enough and end up at lead This, at long last, is the end of the line, as lead is completely stable and will not undergo radioactive decay.

The chain of isotopes from uranium to lead is known as a decay chain. You may have noticed that the mass number only ever decreases by 4 alpha decay or stays the same beta decay. So if we instead start with the different radioactive isotope uranium, we will never land on any of the same isotopes. We go down a completely different chain, which ends in lead And there are two other radioactive decay chains, corresponding to the other two remainders left over when you divide the mass number by 4.

Every heavy radioactive element isotope belongs to one of these four chains.

A complication of both alpha and beta decay is that often the newly produced nucleus is left in an unstable energy state itself. It's a bit like taking a block out of a game of Jenga. With all the pre-existing protons and neutrons in an atom, it's nice and solid. Toss out an alpha or beta particle, and what's left can be a bit rickety. The leftover protons and neutrons might need to rearrange themselves a bit to get comfortable and stable again.

Just as in Jenga, the collapse is triggered because there's an excess of potential energy that can't be supported by the structure. A Jenga tower gets rid of the excess energy by collapsing loudly. A nucleus gets rid of the excess energy as it relaxes by emitting it in the form of electromagnetic radiation - the nucleus produces a photon of light. The energy involved is quite high though, and the resulting photon has a lot more energy than visible light.

It belongs to the high-energy part of the electromagnetic spectrum now known as gamma rays , because it corresponds to the third type of radiation named by early radioactivity researchers.

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Typically these gamma rays are emitted within a second or so of the alpha or beta decay for any given atom. The atom might then wait for some time before undergoing another alpha or beta decay and continuing down its decay chain. Pitchblende, a uranium ore.

The amounts of time that all of these radioactive decays take is random for any given atom. But there are isotopes that tend to decay faster, such as thorium which typically decays within a few days, and isotopes that decay more slowly, such as uranium which typically lasts billions of years before decaying.

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These decay times can be characterised by what's known as the half-life. Each isotope has a particular half-life, which is an amount of time. For example, the half-life of thorium is The half-life of uranium is 4. What does a half-life mean in practice? It's usually stated that the half-life is the amount of time it takes for exactly half of the atoms in a sample of a radioactive isotope to decay. Say we are given a single atom of thorium as a New Year's gift at midnight when the fireworks go off by some sort of mad scientist, presumably.

If you're lucky, you can keep cherishing your thorium atom a bit longer. The overall chance that you still have your gift is now down to You can see that for every period of It is possible for it to last a year or more, but the odds gradually become extremely small. Now let's extend this to a large sample of a radioactive isotope. Say your mad scientist friend gives you a block of thorium Besides thinking that he might be trying to kill you with radiation poisoning, you'd be delighted.

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Since there are zillions of atoms in there, very close to half of them will in fact have decayed. Think of it this way: If it comes up heads, that atom will decay at some time within the first Toss zillions of coins, and by far the most likely result is that very close to half of them will come up heads. Yes, it's possible that they'll all come up heads, but the odds of that are stupendously small.

The result is that very close to half of your original kilo of thorium has now decayed. You now have half a kilo of thorium, plus or minus a very tiny bit due to the random fluctuation of the exact numbers of heads and tails. Leave a Reply Cancel reply Enter your comment here Fill in your details below or click an icon to log in: Email required Address never made public. Doubtful News Skeptical analysis of news stories with updates Guerrilla Skepticism on Wikipedia A user group devoted to learning to improve Wikipedia and ensuring skepticism-related content is well-cited.

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