Tuesday, October 13, 2009

Positrons, Alpha Particles, and Gamma Rays

What happens when an atom doesn't have enoughneutrons to be stable?

That's the case with beryllium 7, 7Be4. Click on it in the applet and see what happens.

It decays to lithium 7--so a proton turns into a neutron. That makes sense...but how do you deal with the electric charge problem now? Going from Be to Li, you lose charge; emitting an electron would just make things worse.

Right...so instead you emit a positron--a particle that's just like an electron except that it has opposite electric charge. In nuclear reactions, positrons are written this way: 0e1.

So the reaction looks like this:

7Be4 => 7Li3 + 0e1

Good. The applet will show you many other decays that produce either electrons or positrons; it's easy to tell which, by the "direction" in which the decay moves. Sometimes it even takes more than one decay to arrive at a stable isotope; try 18Ne or 21O, for example.

So all radioactive isotopes decay by giving off either electrons or positrons?

No, there are other possibilities. Some heavy isotopes decay by spitting out alpha particles. These are actually helium 4 nuclei--clumps of two neutrons and two protons each. A typical alpha decay looks like this:

238U92 => 234Th90 + 4He2

There's also a third type of radioactive emission. After alpha or beta decay, a nucleus is often left in an excited state--that is, with some extra energy. It then "calms down" by releasing this energy in the form of a very high-frequency photon, or electromagnetic wave, known as a gamma ray.
Click on the advanced button for more information about why this happens.

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