The half-life of technetium-99m is 6.0 hours. Every 6.0 hours, half of the atoms that are left decay, so after 2 half-lives (12.0 h) a quarter is left, and after 10 (60.0 h) about 0.10%.
Written as a rate, the decay constant is λ = ln 2 ÷ T½ = 3.209 × 10⁻⁵ per second, and the average atom survives for the mean life τ = 8.66 hours. One gram of the pure isotope contains 6.09 × 10²¹ atoms and has an activity of 195 PBq.
Technetium-99m decay table
Each row is a number of half-lives, the time that takes for Tc-99m, and what is left. The fraction is 2 raised to minus the number of half-lives, for any sample size.
| Half-lives | Time | Fraction left | Odds an atom survives |
|---|---|---|---|
| 1 | 6.00 h | 50% | 1 in 2 |
| 2 | 12.0 h | 25% | 1 in 4 |
| 3 | 18.0 h | 13% | 1 in 8 |
| 4 | 24.0 h | 6.3% | 1 in 16 |
| 5 | 30.0 h | 3.1% | 1 in 32 |
| 6 | 36.0 h | 1.6% | 1 in 64 |
| 8 | 48.0 h | 0.39% | 1 in 256 |
| 10 | 60.0 h | 0.10% | 1 in 1,024 |
N = N₀ × 2^(−t ÷ 6.0 hours). With only 400 atoms in the simulation the count scatters about the curve, because each atom decays at a random time.
How long until Tc-99m drops to a given level
Running the law backwards gives the time for the sample to fall to a chosen fraction, t = −T½ log₂(fraction). This is also how a measured fraction becomes an age.
| Fraction left | Time needed | Half-lives |
|---|---|---|
| 90% | 0.912 h | 0.152 half-lives |
| 50% | 6.00 h | 1.00 half-lives |
| 25% | 12.0 h | 2.00 half-lives |
| 10% | 19.9 h | 3.32 half-lives |
| 1.0% | 39.9 h | 6.64 half-lives |
| 0.10% | 59.8 h | 9.97 half-lives |
About technetium-99m
Technetium-99m is the most widely used medical radioisotope, in tens of millions of scans a year. The “m” stands for metastable: the nucleus sits in an excited state and drops to its ground state by emitting a 140 keV gamma ray, which a camera outside the body can see. The half-life of 6.0 hours is long enough to make and inject it and short enough that the patient’s dose falls fast.
It is made at the hospital from molybdenum-99, which has a half-life of 66 hours, in a device called a generator. After 24.0 h (4 half-lives) only 6.3% is left, so scans are usually timed within a day of the injection.
A worked example
A technetium-99m dose is drawn at 8:00 and injected at 14:00. What fraction is left?
6 hours is exactly 1 half-life, so 50% of the activity remains. By 20:00 it is 25%, and by 8:00 the next morning 6.3%.
Compare with other isotopes
The half-lives in the simulation span twenty orders of magnitude, from microseconds to billions of years. Each has its own page.
| Isotope | Half-life | Activity per gram |
|---|---|---|
| Carbon-14 | 5,730 years | 165 GBq |
| Uranium-238 | 4.47 billion years | 12.4 kBq |
| Iodine-131 | 8.02 days | 4.6 PBq |
| Polonium-214 | 164 microseconds | 11.9 YBq |
| Technetium-99m | 6.0 hours | 195 PBq |
This page is one case from the full guide, Half-life and radioactive decay simulator, which covers every case, the assumptions behind the model and what it leaves out.
Other cases
- Carbon-14 half-life 5,730 years
- Uranium-238 half-life 4.47 billion years
- Iodine-131 half-life 8.02 days
- Polonium-214 half-life 164 microseconds
Frequently asked questions
What is the half-life of technetium-99m?
6.0 hours. After that time half of any sample has decayed, and half remains.
How much Tc-99m is left after 3 half-lives?
One eighth, 13%, after 18.0 h.
Does the half-life depend on how much there is?
No. Each atom has the same chance of decaying in any moment, so the time for half of a sample to decay is the same for a gram or a ton. Heat, pressure and chemistry do not change it for ordinary decay.
When is Tc-99m gone?
It never reaches exactly zero, but after 10 half-lives (60.0 h) only 0.10% remains, which is the usual rule of thumb for “gone”.
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