College Physics : Radioactive Decay

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Example Questions

Example Question #1 : Radioactive Decay

Radiocarbon dating is a method that allows scientists to estimate the age of organisms after they have died. A fairly constant amount of radioactive remains in the animal while alive, but once it dies the degrades over time into . By measuring the relative amount of these two in a dead organism compared to a living one,  along with knowing that the half-life of is years, it can be determined how long ago the organism died.

If a scientist finds that a fossil contains amount of the normally found in the atmosphere at that time, approximately how old is this fossil?

Possible Answers:

Correct answer:


In this question, we're given a brief description of radiocarbon dating. We're given the amount of that has been found in a fossil sample and we're asked to find the approximate age of the fossil.

First, let's briefly go over radiocarbon dating. This method essentially assumes that the amount of radioactive carbon within an organism remains fairly stable at any given time while the organism is alive. Moreover, this amount of radioactive carbon is related to the amount of radioactive carbon in the atmosphere. Once the organism dies, however, it ceases to gain any radioactive carbon; rather, the that was present now begins to decay into . Thus, by measuring the amount of in the organism and comparing it to the amount in the atmosphere, the age at which the organism died can be approximated.

Since we're dealing with the decay of , this is a radioactive decay problem. Recall that all radioactive decay reactions follow first-order rate kinetics. What this means is that the rate of decay is only dependent on the amount of radioactive material at any given instant. Hence, we can use the first-order rate equation.

We can further rearrange this expression to isolate the variable for time.

In arriving at this expression, we see that we need to know the rate constant, , in order to solve for . To do this, we can use the equation that relates the half-life to the rate constant for a first-order process.

Rearranging, we can find the rate-constant.

Now that we have the rate constant, we can plug this value into the previous expression to solve for .


Example Question #2 : Radioactive Decay

Suppose that  of a radioactive drug is injected into a patient. After  minutes pass by, another  is injected. After another  minutes pass by, how much of the drug is expected to be in the patient's body?

Note: The half-life of the drug is  minutes.

Possible Answers:

Correct answer:


For this problem, the first step is to find the rate constant for the decay reaction. Since we're given the half-life, we can calculate this value using the following equation.

Now, we can use the first order rate equation to find out how much drug will be left after the first time interval of  minutes.

So, after the first  minutes, there will be  of the drug in the patient's body. But from the question stem we're told that an additional  of the drug is injected. Hence, there is now  of the drug present.

With this new amount in mind, we'll need to calculate how much of the drug will be present after more time has elapsed. We can do this by using the same equation as before.

This is the final amount of drug that is expected to be present in the patient's body at that instant of time.

Example Question #3 : Radioactive Decay

Iodine-131 has a half life of 8.02 days and undergoes beta decay.

If a  sample of iodine-131 is left out for 1 week (7 days), how much of the sample will still be iodine-131? What will Iodine - 131 decay into?

Possible Answers:

 of iodine-131 will be left and will decay into antimony-127

 of iodine-131 will be left and it will decay into xenon-131

 of iodine-131 and decays into tellurium-131

 of iodine-131 will be left and it will decay into xenon-131

 of iodine-131 will be left and it will decay into antimony-127

Correct answer:

 of iodine-131 will be left and it will decay into xenon-131


Finding the amount is a simple half life problem. Start with setting up what you know for any half-life problem.

,  is the rate of decay, 200 is the initial amount, and 100 is the half left after 8.02 days.

From there simplify. 

. To solve this take the natural log of both sides so that 

Now plug this into the equation and find what happens after 7 days.

Beta decay is when a neutron releases an electron and becomes a proton. Therefore, the atomic number goes up by one but the atomic mass remains the same. This results in xenon-131.

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