Neutron Activation Calculator
Calculate induced radioactivity from neutron irradiation using activation analysis.
Why Activity Saturates
During irradiation, nuclei are being created by neutron capture and destroyed by decay simultaneously. Production is constant, but decay grows as activity accumulates — so activity climbs toward a ceiling where the two rates balance.
| Irradiation time | Fraction of saturation | Practical note |
|---|---|---|
| 1 half-life | 50% | Useful |
| 2 half-lives | 75% | Good return |
| 3 half-lives | 87.5% | Diminishing |
| 5 half-lives | 96.9% | Practical maximum |
| 10 half-lives | 99.9% | Reactor time wasted |
Irradiating beyond about three half-lives gives little additional activity for substantial reactor time. This is why irradiation schedules are set by the product's half-life rather than by convenience.
Why NAA Is So Sensitive
Neutron activation analysis detects elements at parts-per-billion levels because it measures gamma rays of characteristic energy from the activated nuclei. Each isotope emits at a specific energy, so identification and quantification happen together with very low background.
| Advantage | Why it matters |
|---|---|
| Non-destructive | Sample survives — used on artefacts and forensic evidence |
| Matrix independent | Neutrons penetrate; no dissolution or digestion needed |
| Multi-element | Many elements measured in one irradiation |
| Very low detection limits | Parts per billion for many elements |
The main constraints are access to a reactor and the half-life of the product. Very short-lived products must be counted immediately, while long-lived ones require cooling time before the sample can be handled safely.
Worked Examples
Common Mistakes
Activity is already at 87.5% of saturation and gains slow sharply. Longer irradiation mostly consumes reactor time without benefit.
Short-lived products decay measurably between the end of irradiation and measurement. Decay correction to a reference time is essential.
Only the specific target isotope activates. The natural abundance of that isotope must be included when converting sample mass to target nuclei.
Some have tiny cross-sections or produce products with inconvenient half-lives. NAA is excellent for many elements and poor for others.
Frequently Asked Questions
Formula Explorer connections
Interpretation: This relationship connects isotopic composition, decay, radiation, mass defect or nuclear energy to a measurable quantity. Assumption: Use the correct nuclide, decay constant, branching behavior and time units. Radiation estimates also depend on geometry, shielding and detector response.