Objective

In this activity, you will learn how to describe half-lives through descriptions, calculations, particulate diagrams, and graphs as well as analyze the benefits of long and short half-lives through the context of nuclear medicine.

Background

Half-life (t_1/2) is the time it takes for half of a radioactive sample to decay into a different isotope. Let's consider a radioactive isotope with a half-life of 8 days. This means every 8 days the amount of the radioactive isotope remaining after decay would be cut in half. Starting with a 100-gram sample of the isotope, 50 grams would remain after the first 8 days. After 16 days, 25 grams of the original isotope would be left. After 24 days, 12.5 grams of the original isotope would be left, etc. Every radioactive substance has a different half-life. Some can be longer and are measured in billions of years while other are shorter and are measured in milliseconds. There are benefits and drawbacks to both short and long half-lives.

Technetium-99m (Tc-99m) is a radioactive tracer used in medicine to produce images of a patient’s brain, thyroid, lungs, liver, gallbladder, kidneys, skeleton, blood, and tumors. It helps doctors diagnose Alzheimer's disease, cancer, and heart disease. Doctors attach Tc-99m to a drug administered to the patient, helping Tc-99m get to the desired location in the body. Tc-99m emits gamma radiation which exits the body and is captured by medical imaging equipment. It is used in 80% of nuclear medicine procedures. In the U.S. alone, Tc-99m is used in over 40,000 medical procedures every day. The very common cardiac “stress test” for heart attacks uses Tc-99m. Tc-99m is used in so many medical tests because of its short 6-hour half-life. In 24 hours, less than 10% of Tc-99m remains in the body, which is long enough to capture a medical image and short enough to keep the patient’s exposure to harmful radiation low. Though, its short half-life does have a downside as it makes it very difficult to transport to hospitals from nuclear reactors.

Molybdenum-99 (Mo-99) is a radioactive isotope that produces Tc-99m through beta decay. Mo-99 has a longer half-life of 66 hours, making it easier to transport. This longer half-life allows Mo-99 to be shipped from nuclear reactors to medical facilities on a weekly basis. At the hospital, Tc-99m is extracted or “milked” from the Mo-99 sample as it is produced. Although the half-life of Mo-99 is longer than Tc-99m, the half-life of Mo-99 is short enough that it still requires continuous production and “just-in-time” delivery.

The U.S. depends on a reliable supply chain of Mo-99 for its nuclear medicine procedures. Until very recently, the U.S. did not produce any Mo-99. The Mo-99 supply chain has faced challenges over the years. Global shortages have been a concern since the late 2000s due to aging reactors and efforts to minimize the use of highly enriched uranium. Most of the U.S. supply of Mo-99 was produced by Canada’s National Research Universal reactor. The reactor closed down in 2016 due to aging infrastructure. Since then, the majority of Mo-99 has been produced by research reactors in Australia, Belgium, the Netherlands, and South Africa, many of which were built in the 1960s and are constantly down for maintenance or failing safety inspections. Mo-99 is traditionally produced using highly enriched uranium. Using highly enriched uranium is a threat to society as it could be stolen and used to make nuclear weapons if in the wrong hands. The focus has shifted to finding new ways to make Mo-99 without highly enriched uranium. There are new reactors being built in Wisconsin, Michigan, and Missouri to supply Mo-99 without using highly enriched uranium, but at a higher cost.

Materials (per group)

• 2 pairs of scissors
• Yellow highlighter or marker
• Green highlighter or marker
• 2 stopwatches or timers (such as a cellphone or 6-second Interval Timer)
• Handout of Mo-99 sample (this is the sheet of paper with 4096 circles on it)

Instructions

Read all of the instructions below before starting the timer!

1. Work in groups of four students. Each group will need a Mo-99 sample handout (this is the sheet of paper with white circles on it). Trim the excess white space off from around the white circles.
2. Within your group of four, students should create two pairs of students. One pair will be in charge of the Mo-99 decay and the other pair will be in charge of the Tc-99m decay. Each pair of students will need scissors, a stopwatch, and a marker. The Mo-99 pair will need a yellow marker and the Tc-99m pair will need a green marker.
3. In each pair of students, assign one person to cut the sample in half (representing the radioactive decay) and another person to color the circles that were cut off (representing the change into a new isotope).
4. During this activity, time will be sped up. 1 hour will be represented by 1 second in the activity. Meaning, 66 hours = 66 seconds and 6 hours = 6 seconds.

5. Mo-99 Instructions
1. The pair of students working on the Mo-99 sample will start with the Mo-99 sample handout (this is the sheet of paper with 4096 circles on it).
2. The Mo-99 sample is represented by white circles. There are dotted lines on the sample to help determine where “half” of the sample is located.
3. Start the stopwatch (do not stop the stopwatch until the very end of the activity).

4. The half-life of the Mo-99 beta decay is 66 hours. Every 66 seconds cut off half of the sample with scissors and quickly color it yellow to represent the creation of Tc-99m.
5. At 66 seconds, hand the yellow Tc-99m sample to the Tc-99 group.
6. Continue to watch the stopwatch. Make sure at 132 seconds the remaining white Mo-99 sample is cut in half again, colored yellow, and passed to the Tc-99m group.
7. Repeat this process for a total of 198 seconds or three half-lives of Mo-99.

2. Tc-99m Instructions
1. At 66 seconds into the activity, obtain a sample of yellow Tc-99m created through beta decay from the Mo-99 group. The Tc-99m sample is represented by yellow circles. There are dotted lines on the sample to help determine where “half” of the sample is located.
2. Start the stopwatch (do not stop the stopwatch until the very end of the activity). Alternatively, a 6-second interval timer can be used.

3. The half-life of the Tc-99m gamma emission is 6 hours. Every 6 seconds cut off half of the sample with scissors and quickly color it green to represent the creation of Tc-99. Color half of the sample without dotted lines on it.
4. At 6 seconds, put the green Tc-99 sample created off to the side of the table.
5. Continue to watch the stopwatch. Make sure at 12 seconds the remaining yellow Tc-99m sample is cut in half again, colored green, and placed to the side. Line the pieces of green Tc-99 up.
6. Repeat this process every 6 seconds for a total of 198 seconds. Note: If the Mo-99 group gives you more Tc-99m, cut the new sample down until the time is up.

Conclusion Questions

1. Complete the Venn diagram below to compare and contrast the nuclear decay of Mo-99 and Tc-99m that your group performed during the activity (cutting, coloring, etc.)

2. A medical lab has 120 grams of Mo-99 and Tc-99m. Graph the decay of each sample below over time. Connect data points with a smooth curved line.

3. How long does it take each isotope to go from 100 grams to 12.5 grams?
1. Mo-99
2. Tc-99m
4. Why is Mo-99 delivered to hospitals and not the Tc-99m needed for medical imaging?
5. Many nuclear scientists describe Mo-99 as the “cow” and Tc-99m as the “milk”. Explain why scientists use this cow/milk analogy when describing how hospitals use Mo-99. Hint: Think about how you cut the Mo-99 and Tc-99m samples in this activity to help you answer the question.
6. In 2019 only five nuclear reactors (Australia, Belgium, Canada, the Netherlands, and South Africa) supplied the world with Mo-99. In 2010, two nuclear reactors in Canada and the Netherlands were offline for repairs and unable to produce any Mo-99 for 6 months. A volcano in Iceland unexpectedly erupted, spreading ash across most of the northern hemisphere. The concentrated ash grounded most trans-Atlantic flights. Explain why this volcano eruption caused chaos in nuclear medicine departments across the United States.
7. Besides natural disasters, describe at least one other threat to the Mo-99 supply chain and offer at least one possible solution.
8. When Tc-99m is not available due to supply chain issues, Thallium-201 (Tl-201) can be used as an alternative. The half-life of Tl-201 is 73 hours. It takes approximately 10 half-lives to clear the radioactive material from the body after the medical test.
1. How many days does it take to clear Tc-99m from the body (10 half-lives)?
2. How many days does it take to clear Tl-201 from the body (10 half-lives)?
3. Why is Tc-99m the preferred isotope to use in nuclear medicine?
9. Describe the concept of radioactive half-lives in your own words. Describe why it is important to medicine.