Physics Question

This week we are going to investigate radioactive decays and radiometric dating. This will use two simulations, first “” by PhET, and second “” also by PhET. If prompted, both of these need to be run under the “Browser Compatible CheerPJ” option. Due to how rough these types of simulations run I am not embedding them in this page. Please only have one open at a time for the sake of your own computer.

You will also need a stopwatch for this lab. You can use your phone .

Part 1: Alpha Decay

1. Open the “Alpha Decay” simulation in a new tab or window.
   • In this section we are investigating the decay of Polonium-211 into Lead-207.
2. Briefly research both Po-211 and Pb-207. I recommend the and wikipedia pages.
   • Question 1: How does Po-211 tend to come into existence? How long is its half life? What mode of decay does it most experience? What does it decay into after that type of decay?
   • Question 2: Suppose you have 10 atoms of Po-211. How long would you expect it to take for half of them to decay to Pb-207?
3. Press the “Add 10” button under the Bucket o’ Polonium.
4. Wait until all of the Po-211 have decayed into Pb-207.
5. Count up how many atoms are to the left of the “Half Life” mark on the timeline.
   • Alternatively, it might be easier to count how many are on the right- then the number on the left is 10 – # on the right.
   • If it’s hard to see exactly which side of the line an atom is on, that’s okay! Just interpret the timeline the best you can.
   • Question 3: How many decayed before one half life? How many decayed after? Did you notice a pattern in the rate of decay of the atoms?
6. Simultaneously click “Reset All Nuclei” and start your stopwatch.
7. Stop your stopwatch when the final atom has decayed.
8. Record this time in a data table, repeating steps 6-8 for a total of five trials.
   • Here’s a sample data table:
     

     trial	time (s)
     1	14.50
     …	…
     5	18.61

   • Question 4: Was the amount of time for this final atom different each time? Was there any consistency to it at all? Did you observe any patterns? What can you say about the decay time for this final atom? (Note: the data table must be submitted within/with the lab)
     • As fun aside: some of the only “true random number generators” work by letting atom decays pick the numbers.
9. Switch over to the “Single Atom” tab.
10. Watch the nucleus and the alpha particles at the bottom of the simulation until one escapes.
11. You can click “Reset Nucleus” as many times as needed while observing this, as there will be a few questions on what’s going on.
    • Question 5: Describe the motion you observe inside the nucleus of the Po-211 atom, as well as the motion of the alpha particles at the bottom of the simulation.
    • Question 6: Before the decay occurs, are the alpha particles always confined inside of the blue potential energy well? What about after the decay?
      • You might have to watch for a little while to be sure either way!
    • Question 7: Paying close attention to the energy level of those alpha particles, what happens to all of their energy levels when one of them escapes?
      • Again, reset the nucleus as many times as needed to catch what happens.
12. This might require some recall of our discussion of the four fundamental forces, but the potential energy plot shows the sum of two potential energies, one being extremely attractive (negative) at the shortest distances and one being repulsive (positive) at short distances.
    • Question 8: Which two energies/forces (remember that potential energy is often simply the work done by a force when bringing an object into that position) are being combined to make the blue potential energy plot? Why is the positive force repulsive (why would the nucleus and alpha particle push each other away)?
13. Briefly research .
    • Question 9: Did you observe quantum tunneling in this lab? Why/why not?
14. Close the “Alpha Decay” simulation.

Part 2: Radioactive Dating Game

1. Open the “Radioactive Dating Game” simulation in a new tab or window.
2. Press the “Add 10” button and wait until the nuclei have all decayed.
   • Question 10: What differences, if any are there so far from the previous “Alpha Decay” simulation? What decay must C-14 undergo to become N-14? What is the half life of this decay?
3. Go to the “Decay Rates” tab.
4. Move the slider on the bucket all the way to the right.
   • Question 11: Describe the shape of the red line graph. Approximately what percent of the original C-14 was left after 1 half life? After 2 half lives? After 3 half lives?
5. Carbon-14 is super important for radiometric dating because it is constantly replenished as cosmic rays hit our atmosphere, turning N-14 into C-14 at a fairly constant rate. Since this is a continuously ongoing process that hasn’t really changed much since the Earth and its atmosphere formed, the fractional amount of C-14 expected in any organic object hasn’t really changed. This is because organic stuff (typically plants/ animals/ people) are always consuming and thus integrating C-14 into their bodies. They only stop adding more C-14 into their bodies when they die! Thus we can use the expected amount of C-14 at death compared to how much C-14 still exists in an object to determine the age of an organic object.
6. Switch to the measurement tab.
   • In this tab you have a special Geiger counter. It is a counter that does some math for us- knowing how much C-14 an object should have according to its mass (C-14 naturally occurs at a rate of 1/1000000000000 atoms of carbon. If the mass of the object is known, then the total amount of carbon is known, then the expected quantity of C-14 is known). The special Geiger counter tells you what percent of C-14 remains in the object.
7. Click “Plant Tree” and watch the grow, and die. 🙁
   • Question 12: How old was the tree when it died?
   • Question 13: How many years had passed when the probe read 50%? Explain why this number is not exactly the half life of C-14.
     • You might have to use the pause button a bit to get a good idea of the time passed at 50%
8. Select “Rock” from the right side of the screen and repeat the above process.
9. Switch the probe to “Uranium-238” and click “Erupt Volcano.”
10. Watch for when the probe starts measuring non-100% values.
    • Question 14: What change occured in the rock to start the U-238 decay process?
11. Do a bit of research on .
    • Question 15: Uranium-238, unlike C-14, is not produced anywhere outside of supernovae. Since we don’t have a steady supply of it, how is it possible to radio date rocks with it here on Earth?
12. Finally, switch to the “Dating Game” tab.
    • Question 16: Use the probe to estimate the ages of the following objects:
      • Living Trees
      • Wooden Cup
      • Rock 5
    • Your estimates are your answer for this question.
    • Note: You need to use carbon dating for things made out of once-living materials and uranium dating for rocks.

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