Reading Quiz 16 Key

Due before class Wednesday, Aug 1st

Physics 106, Summer 2012

Reading: Chapter 29 All

Did you complete the reading assignment?


Did you complete the entire quiz or spend at least 15 minutes working on this quiz and the attached links?


1) A half-life is the approximate time in which half of a population of  radioactive atoms will decay.

2) A nucleus is unstable when there are too many protons in a nucleus, when the neutron to proton ratio is too high, or when the energy of the nucleus is too high.

3) Alpha decay is when a helium nucleus (2 protons and 2 neutrons) is ejected from a nucleus.

4) Beta decay is when an a neutron changes into an electron and proton, a positron is emitted, or an electron is captured.

5) Gamma decay is when a high energy photon is emitted.

Exercise 1

We refer to the speed in which radioactive elements decay by their half-life. To see how the half life of an element works, go to this applet:

This one is really easy to use. Adjust the length of the half-life using the scroll bar and then press the 'Start' button to watch the affect.

Are all the particles decayed after two half lives?


Do exactly one half of the particles decay during the half-life time? Why or why not?

Exercise 2

To get a better understanding of what is happening during alpha decay, go to the following applet:

Click the 'Add 10' button right under the Bucket o' Polonium and watch what happens. What is being shot off the Polonium atoms (there is a legend in the upper right corner of the applet)?

2 protons and 2 neutrons

After the simulation finishes, press the 'Reset All Nuclei' button which makes them all radioactive again. Did the exact same thing happen (check a few times if you are not sure)? Why or why not?

Click on the 'Single Atom' tab at the top of the applet. Pay special attention to the bottom part. This is what is called a potential well. Inside this potential well, it shows the potential energy that the alpha particles within the nucleus are at. If any of the particles energy rises above that of the top of the well, it will "fall" out of the nucleus. Press the 'Reset Nucleus' button and watch what happens in the well and to the nucleus.

What causes the varying energy in the alpha particles for the unstable nucleus?

Electrons attracting the protons out of the nucleus.
Protons repelling each other out of the nucleus.
Too much mass in the nucleus causing it to fall apart.

What happens to the other alpha particles in the well when one particle is emitted from the nucleus?

They move around more
They move around less
They move up in energy
They move down in energy

Why is the new nucleus more stable than the old one after alpha decay?

Exercise 3

Different kinds of decay review:

What is the cause of alpha decay?

Too many protons in a nucleus
The neutron to proton ratio is too high or too low
Nucleus energy is too high

What is the cause of beta decay?

Too many protons in a nucleus
The neutron to proton ratio is too high or too low
Nucleus energy is too high

What is the cause of gamma decay?

Too many protons in a nucleus
The neutron to proton ratio is too high or too low
Nucleus energy is too high

How are regular beta decay, positron emission, and electron capture different?

If you are having difficulty figuring out what type of decay a nucleus will go through, here is a chart that will help you recognize the trends:

To search for a specific group of atoms, scroll through the select nuclei scroll box and hit start. You can also see what the decay chains and reaction path look like for the selected nucleus by clicking on those buttons.


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Was there anything that you didn't understand in the reading assignment?  What was confusing to you?

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