Radioactive decay

Now that you’re no longer convinced that you’ll melt when you come near anything nuclear, it’s time to talk about the various nuclear processes that take place.  Let’s start with:


Radioactive decay:

Some atoms are fine the way they are.  Carbon-12, for example, will hang around forever without doing anything weird.  It’ll just be good old carbon-12 for as long as you care to look at it.  Carbon-12 is said to be a stable nuclide, where the term “nuclide” refers to a particular atomic isotope.

Other atoms, however, act like your Uncle Jimmy.  Just like your Uncle Jimmy, carbon-14 isn’t quite right.  While carbon-14 will hang around for a good long while without doing anything weird, eventually it gets restless, ending up drunk and screaming on grandma’s grand piano.  Wait, that one was actually Uncle Jimmy.

Anyway, radioactive nuclides such as carbon-14 are referred to as radionuclides because they’re radioactive.  Any time you’ve got an atom with a nucleus that’s unstable for one reason or another, it will eventually fall apart via radioactive decay until it forms new atoms (called “daughter atoms”) that are stable.

Fig2-1therapy

Uncle Jimmy, on the other hand, usually just winds up back in therapy.


So, why are nuclei unstable in the first place?

Atomic nuclei become unstable mainly because the ratio of protons to neutrons isn’t quite right. Consider this:

The last time your Uncle Jimmy went to a bar, he and another guy were talking about what breed of dog is “best at messin’ up ‘them stealing folk'” when they’re trying to kidnap the pig.  Unfortunately, while Uncle Jimmy favored a pit bull, that other guy thought that a Doberman would be better for the job.  Eventually, Jimmy and the other guy got to the point where they wanted to punch each other, and Wayne The EMT had to break up the fight.

Fig2-2dogplate

They never agreed about which was a better watchdog, but both agreed that the pug is the tastiest dog around.

To understand what I mean, think of the bar as the nucleus, Jimmy and Wayne as protons, and the EMT as a neutron.  In this example, Jimmy and Wayne couldn’t stand each other (just as protons repel one another because they have the same charge) and the EMT served to keep them apart.  All is well.  However, let’s imagine that the argument contained five drunks instead of two.  In this case, the EMT wouldn’t have any chance at all of keeping things stable.  As a result, the bartender would have to kick people out until things calmed down.  This would restore the ratio of drunks to EMTs (protons to neutrons) such that everything was stable again.

Likewise, what we find is that if an atom has too many protons in the nucleus and not enough neutrons to keep them apart, it tends to undergo radioactive decay in such a way as to make the ratio more favorable.  This process will continue until the balance of protons and neutrons is enough to keep things stable.

Figure2-3stability

This chart depicts various isotopes by ratio of protons (x-axis, denoted by the variable Z) to neutrons (y-axis, denoted by N). Darker pixels indicate greater stability, and you can clearly see that there is a very narrow range along which stable isotopes can exist.

 

And how do the atoms decay?  I’m glad you asked!


 

Types of radioactive decay:

It turns out that there are a whole bunch of different ways that nuclei can fall apart.  I’ll talk in detail about a few, and just mention some of the weirder ones in passing.

Alpha decay (α)

Alpha decay occurs when a helium-4 nucleus is ejected from the nucleus of an unstable atom.  By getting rid of two protons and two neutrons, the remaining atom contains fewer protons, which leads to less repulsion between positive charges in the nucleus.  For this reason, alpha decay is most frequently seen in heavy nuclides.  Here’s an example:

Figure2-4alphadecay

In this equation, uranium-238 decays to make thorium-234 and helium-4.  Helium-4, as I mentioned before, is the same thing as an alpha particle.

One feature to keep in mind is that you can figure out what the products of nuclear decay will be by looking at the mass numbers and the nuclear charge number on the notation for each isotope.  For example, if I were to say that americium-241 undergoes alpha decay, you would know to write, if nothing else:

Fig2-5americium

 

After all, you can figure out the formula of americium-241 from the question and the formula of an alpha particle because it’s always the same as helium-4.

Now, let’s figure out what the [something] is.  If americium has a mass of 241 in this equation and one of the things it makes has a mass of 4, it kind of stands to reason that the mass of [something] must be 237.  Likewise, if americium has an atomic number of 95 and helium took two of those protons, there must be 93 left.  This leaves us with:

Fig2-americiumdecayfinished

And that’s how you do it!

Beta decay (β-):

Beta decay occurs when an electron is ejected from a nucleus.  This occurs when a neutron splits apart to form a proton and an electron, so the net result is to increase the atomic number by one without changing the mass of the nucleus.  Beta decay tends to occur in lighter nuclides than alpha decay does, usually when the number of neutrons is a little high.

One example of a beta decay process occurs when carbon-14 decays into nitrogen-14:

beta decay c-14

Note that the masses and charges add up as they did with alpha particles.  Additionally, beta particles are usually just shown as electrons, since that’s what they are.

Positron decay (β+):

Positron decay takes place when a positron (an antimatter version of an electron) is emitted from an unstable nucleus.  Because a positron is similar to an electron in that it has no mass, this doesn’t change the mass of the resulting nucleus.  However, a positron has positive rather than negative charge, so the atomic number of the resulting atom decreases by 1.  Positron decay occurs in lighter elements where there are too many protons:

positron-decay-potassium

Electron capture:

In electron capture, an electron from an inner orbital is captured by the nucleus, converting a proton to a neutron.¹  The net result is to decrease the atomic number, but leave the mass unchanged:

electron-capture

Gamma emission (ϒ):

Gamma emission takes place when a nucleus in an excited state gets rid of the excess energy in the form of a high energy photon – a gamma ray.  The symbol for the atom doesn’t change much, but the pre-decay form is sometimes shown with an asterisk above it to indicate that it’s in an excited state.  An example of gamma emission occurs with lutetium-177:

gamma decay

And how long does it take for these types of radioactive decay to occur?  You’ll just have to check out the next tutorial!


Footnotes:

  1. Electron capture is often further described by which energy level the electron came from.  As a result, terms such as K-capture and L-capture are common for this process.

 

Photo credits:

  • Jimmy and therapist:  Image courtesy of Ambro at FreeDigitalPhotos.net.  I tried to find an image of a therapist and patient that was more appropriate for the Jimmy character, but for some reason the stock photo people always imagine that female therapists are hot and male patients are handsome.  If this happens to be true, let me know via email because I’m kind of curious about that.
  • Woman putting puppy on plate:  Image courtesy of imagerymajestic at FreeDigitalPhotos.net. Incidentally, this picture is proof that any imagined event probably exists as a stock photo.  Sort of a Rule 34 for dumb pictures, if you will.

 

 

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