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# Neutrons

## History of the neutrally charged particle and present day applications.

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Holding Things Together

### Holding Things Together

Credit: Fermi National Accelerator Laboratory
Source: http://commons.wikimedia.org/wiki/File:Standard_Model_From_Fermi_Lab.jpg

Scientists who probe the sub-atomic universe obviously have a sense of humor. Why else would they name quarks as “charm” or “strange?” And the colors: quarks are either “blue,” “red,” or “green.” In fact, the name “quark” comes from the nonsense poem, ''Three quarks for Muster Mark'' in James Joyce's Finnegans Wake. Who says that science can’t be fun?

#### Amazing But True

• Can you explain gravity to another person? Probably not. We can describe the effects of gravity, we can calculate how fast an object will fall, and we can explain the orbit of the moon using the idea of gravity. But exactly what is gravity? We say it is a force (whatever that is). It is not a tangible “something” that we can see and touch.
• When we look at the atom, we see some very strange things happening. We won’t worry about the electrons outside the nucleus – that can easily be explained. Two questions come up with regard to the nucleus – what are the neurons doing and why don’t the protons push each other apart? Protons are positively charged particles. We would expect these positive charges to repel one another. Common sense tells us that protons should push each other away, but they apparently don’t in the nucleus. How does that work?
• Credit: Wikimedia Commons
Source: http://en.wikipedia.org/wiki/File:Rutherford_1911_Solvay.jpg

Ernest Rutherford discovered the nucleus (1911) and the proton (1917) [Figure2]

• Then there is the neutron. A neutral particle (no electrical charge), so there is no reason for neutrons to cluster together. Current thinking is that the neutron somehow helps hold the protons together. If an atom has too many or too few neutrons in relation to the number of protons, that atom will be unstable and will decay to form another material. Even stranger, the quarks in the neutron are layered, with the outer edge and middle being negatively charged while the intermediate section is positive.
• So, here comes the strong force. This force is what is believed to hold particles together in the nucleus. It acts between proton and proton, neutron and neutron, and proton and neutron. The particles must be very close to one another for this force to come into play. Many descriptions of the strong force rely on complex mathematics and the interplay of different components of particle physics. Oh, we need to throw in Einstein’s E=mc2\begin{align*}E=mc^2\end{align*} since there are slight mass differences between the proton and the neutron that play a role in this puzzling phenomenon.

#### Show What You Know

1. List the four fundamental forces in nature.
2. What do we believe a proton is composed of?
3. What is the role of the neutron in the nucleus?
4. What is a neutron composed of?
5. How close do nuclear particles need to be in order for meson exchange to occur?

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