Modern Physics
Tc-99m medical tracer
Short half-life tracer — activity drops quickly on the decay graph.
Tc-99m medical tracer — interactive Modern Physics simulation. Short half-life tracer — activity drops quickly on the decay graph. Free browser-based virtual physics lab with live SI measurements and a guided tutorial.
Medical tracer
Short half-life tracer — activity drops quickly on the decay graph.
Technetium-99m (T½ ≈ 6 h) is a gamma-emitting tracer chosen so activity A = λN drops quickly after imaging, limiting patient dose. Press Play: the grid changes color rapidly as N/N₀ = (½)^(t/T½). Short T½ means large λ = ln2/T½ and high initial activity for a given N₀.
- Short T½
- Activity A = λN
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
Technetium-99m (used in medical imaging) has a half-life of only about 6 hours — far shorter than carbon-14's 5,730 years. Does its radioactive ACTIVITY (decays per second) also decrease over time, or does activity stay constant while only the total count of atoms drops?
Predictions to weigh
- Activity decreases too, following the same exponential decay as the atom count, since activity is proportional to how many radioactive atoms remain.
- Activity stays constant even as the atom count drops.
- Activity actually increases as fewer atoms remain, since each remaining atom is more likely to decay.
Variable roles
What you set:
- Elapsed time t (h)
What you measure:
- Activity A (MBq)
How the investigation runs
- Open the tc99m-medical preset and press Reset. The half-life is fixed at 6 hours; the sample starts with 10¹⁰ atoms.
- Enable the activity readout.
- Set the elapsed time for each trial and record the sample's activity.
Governing equation
Radioactive Activity — A = λN
A sample's activity — its rate of decay events per second — is A = λN, where λ = ln(2)/T½ is the decay constant and N is the number of undecayed nuclei remaining. Activity falls exponentially in lockstep with N.
What the printable worksheet asks students to work out
- For one trial, compute N(t) = N₀·(1/2)^(t/T½) using T½ = 6 hours, N₀ = 10¹⁰, then A = λN using λ = ln(2)/T½. Compare to the table.
- Explain why activity A = λN follows the exact same exponential decay curve as N(t) itself — activity is just the atom count scaled by the fixed decay constant λ.
Where this shows up beyond the lab
- Tc-99m's short half-life is exactly why it's used in medical imaging: it delivers a strong, useful radioactive signal quickly, then decays away to a safe level within about a day, minimizing a patient's long-term radiation exposure. Explain why carbon-14 (with its 5,730-year half-life) would be a terrible choice for this application.
- Because Tc-99m decays so quickly, hospitals can't stockpile it — it's typically 'milked' on-site from a longer-lived parent isotope (molybdenum-99) shortly before use. Explain, using the decay law, why shipping pre-made Tc-99m from a distant factory would waste most of the dose in transit.
- AP Physics 2 — Unit 15: Modern Physics
- IB Physics — E.3 Radioactive decay
- General High School Physics — Modern physics intro
- NGSS High School Physics — Wave-particle duality of light
- Welcome to Tc99m Medical
- Select the nuclide
- Press Play
- Medical half-life
- Open the Properties panel
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.