## Nuclear Physics 101: What Holds the Nucleus Together?

Nuclear physics is a forbidding subject, even for trained physicists. To understand current news and discussion about nuclear science and technology, some background knowledge is required. The high school science picture of the atomic nucleus as a tiny ball of protons and neutrons needs more refinement.

Prepare to suspend all notions of common sense. Stuff at the atomic level gets pretty weird.

What Are Protons and Neutrons Made Of?

Every atom's nucleus is made up of a certain number of protons (atomic number, denoted Z) and a certain number of neutrons. To better explain nuclear processes, we need to know more about these two kinds of "nucleons."

Both protons and neutrons are made of a particular trio of particles called quarks. Quarks are difficult to study because they only come in pairs and trios, never by themselves. This is explained by something called color confinement. If you try to pull a pair of quarks apart, you don't get two individual particles. Instead the energy binding them together turns into mass (via Einstein's famous E=mc2 relation) and creates two new quarks, and hence, two new pairs.

Of the six types of quarks, only up and down quarks (yes, they have weird names) make up protons and neutrons. (The others are named topbottomcharm, and strange, and they degrade into up and down quarks.) Up quarks have a +2/3 charge, while down quarks have -1/3. Thus, two ups and a down make a proton (+1 charge), and two downs and an up make a neutron (0 charge).

If the nucleus is full of positively charged protons, why doesn't it fly apart?

This is a very good question. The electromagnetic force causes repulsion between like charges, so it makes sense that protons should all repel each other and blow the nucleus apart. However, there are two more exotic forces, called the strong and the weak, that are active within the nucleus.

While the electromagnetic force can attract and repel at large distances, the strong and weak forces within the nucleus are negligible at any distance larger than a few femtometers. (A femtometer is a millionth of a billionth of a meter, which is 100 billion times smaller than the width of a hair). At distances this minute, nuclear forces dominate.

The strong force is the real glue of the nucleus. It first holds the quarks together within protons and neutrons. Being 137 times more powerful than the electromagnetic force, the aptly named strong force also holds the protons and neutrons together within the nucleus, overwhelming their electromagnetic repulsion. The weak force is less powerful than the electromagnetic force, but still plays an important role in nuclear processes. We will see its odd workings later.

What's Next?

Now we know what holds the nucleus together. Next time we'll discuss a way that a nucleus can be torn apart by a burst of nuclear particles and forces: radioactivity. This powerful phenomenon can overcome the strong force's grip on the nucleus, unleashing the tremendous energy of the atom.

Image: NCSU Nuclear Physics

Tom Hartsfield
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