Nuclear I PREFLIGHT
Please type your (first) name: Please type your LAST NAME and LAST FOUR SS# digits: IN:
The following three questions refer to the material you were to read in preparation for the lesson. Questions one and three require you to write a three or four sentence response. Number two is a multiple choice question. Click in the appropriate circle.

You may change your mind as often as you wish. When you are satisfied with your responses, click the SUBMIT button at the bottom of this page. Don't submit more than once. (If you absolutely HAVE to resubmit it, put a note on the end to that effect.)


1. Radioactive decay

The law of radioactive decay makes a prediction how the number of the not decayed nuclei of a given radioactive substance decreases in time. The yellow spheres of this simulation symbolize atomic nuclei of a radioactive substance. Pick an isotope from the menu and click the "start" button. In the top picture, you'll see the atoms change color as they decay; the lower picture is a graph showing the number of atoms of each type, (N/N0) at a given time t, predicted by the following law:

N   =   N0 ·   2-t/T 

N .... number of the not decayed nuclei
N0 ... number of the initially existing nuclei
t .... time
T .... half-life period

As soon as the applet is started, the atomic nuclei begin to "decay" (change of color from yellow).

It is possible to give the probability that a single atomic nucleus will "survive" during a given interval. This probability amounts to 50 % for one half-life period. In an interval twice as long (2 T) the nucleus survives only with a 25 % probability (half of 50 %), in an interval of three half-life periods (3 T) only with 12.5 % (half of 25 %) and so on.

You can't, however, predict the time at which a given atomic nucleus will decay. For example, even if the probability of a decay within the next second is 99 %, it is nevertheless possible (but improbable) that the nucleus decays after millions of years.


Why is it that something that has a constant probability of decaying generates an EXPONENTIAL curve for the number of atoms left? Why isn't the plot of the number of remaining radioactive atoms a straight line?



2. Probabilities and the Gambler's Paradox

Suppose your best friend bought you tickets for a weekend at Las Vegas, and you were losing money at the craps table. You had just rolled 4 snake eyes (two dice each showing one dot) in a row, and you thought to yourself, "the chance of rolling a fifth snake eyes is really low, because five times in a row is even more unlucky than 4 in a row." Now being a physics major you know that there are 6 sides to a die, so snake eyes is a 1/36 probability in a single throw of the dice. The question: is snake eyes on the next roll of the dice making that five in a row, greater than, less than, or equal to a 1/36 probability?

Less than.

Equal to.

Greater than.

Not enough information.


3. Nuclear Families

Using the applet above, indicate the decay mechanism (alpha, beta, gamma decay) for each step in the Uranium Actinium series. You can type it like: "Th 232 -a->Ra 228 -b->" , or any other convenient way.



honors extra

Because radioactive decay has a "built-in" clock, a half-life that doesn't depend on temperature, humidity, sunlight etc.--we can use that clock to figure out how old objects are. Since the atmosphere contains traces of radioactive Carbon-14 which is generated by cosmic rays hitting the upper atmosphere. Trees and plants process this C-14 into wood or food, and animals that eat the plants get C-14 in their bodies. Then when a tree or an animal dies, no more C-14 goes in, and the ratio of "normal" C-12 to C-14 starts to change by the built-in radioactivity clock. By measuring the ratios, one can determine how long ago the tree or animal died. The accuracy is usually from a few hundred years to about 50,000 years, or 8 half-lives. Originally people collected carbon and tried to count how much radiation was given off to determine the age. A much more accurate method has replaced it, which uses a mass spectrometer to directly count the ratio of C-14 to C-12, and precisely date the item to within 1%. That's the theory.

Despite its great precision, the method produced some really innaccurate numbers in its first decade or so, say, between 1950 and 1970. These early problems have been solved by using tree rings and coral rings to calibrate, and remove the offsets. Can you think of some reasons why the dating method failed to be as accurate as it was precise?




Below is a space for your thoughts, including general comments about today's assignment (what seemed impossible, what reading didn't make sense, what we should spend class time on, what was "cool", etc.):




You may change your mind as often as you wish. When you are satisfied with your responses, click the SUBMIT button.

I received no help from anyone on this assignment.