Calculators / Decay
Tc-99m decay calculator
Technetium-99m has a half-life of 6.0072 hours (ENSDF), so a dose loses 10.9 % of its activity every hour. Everything below is computed by the same code as the app’s Decay tab.
Free · no account · works offline · iOS, Android and web
- HALF-LIFE
- 6.0072 h
- DECAY CONSTANT
- 0.1154 h−1
- SPECIFIC ACTIVITY
- 5.27 × 106 Ci/g
- DECAY
- IT 99.9963 %
The formula
A(t) = A₀ · 2^(−t / T½) = A₀ · e^(−λt) λ = ln 2 / T½ = 0.1154 h⁻¹
Tc-99m decays almost entirely by isomeric transition to Tc-99, whose 2.1×105-year half-life makes its own activity negligible, so the dose calibrator reading is the Tc-99m activity alone.
Decay table from 740 MBq (20 mCi)
| ELAPSED | REMAINING | ACTIVITY | IN mCi |
|---|---|---|---|
| 0.5 h | 94.4 % | 699 MBq | 18.9 mCi |
| 1 h | 89.1 % | 659 MBq | 17.8 mCi |
| 2 h | 79.4 % | 588 MBq | 15.9 mCi |
| 3 h | 70.7 % | 523 MBq | 14.1 mCi |
| 4 h | 63 % | 466 MBq | 12.6 mCi |
| 6 h | 50 % | 370 MBq | 10 mCi |
| 8 h | 39.7 % | 294 MBq | 7.95 mCi |
| 12 h | 25 % | 185 MBq | 5.01 mCi |
| 24 h | 6.27 % | 46.4 MBq | 1.25 mCi |
Worked examples
Forward. A 740 MBq (20 mCi) dose is calibrated at 07:00 and injected at 09:30, 2.5 h later:
A = 740 MBq × 2^(−2.5 / 6.0072) = 554.6 MBq (14.99 mCi)
Backward. To have 555 MBq (15 mCi) at a 13:00 injection, the activity to draw at 08:00 is the target decayed backward by 5 h:
A₀ = 555 MBq × 2^(+5 / 6.0072) = 988.2 MBq
The Decay tab does both from calendar dates and times, so an overnight gap or a clock change is handled for you.
Assumptions
- Physical decay only — no biological clearance.
- For Tc-99m still in a Mo-99 generator, ingrowth follows transient equilibrium; see the decay chain calculator.
Sources
- Half-life and branching: IAEA Live Chart of Nuclides, ENSDF evaluated levels.
- Decay law: Lamarsh & Baratta, Introduction to Nuclear Engineering, §2.
Numbers on this page computed by the physics code of Nuclear Toolkit 1.4.1, the same code as the app. How it is checked.