Radioactive Activity
Compute the activity of a radioactive sample, A = λ·N, the product of the decay constant (λ) and the number of radioactive nuclei present (N). Activity, measured in becquerel (Bq = 1 disintegration/s) or curie, expresses how many nuclei decay per second. It is the fundamental quantity quantifying a radioactive source, and it decreases over time as nuclei decay. Enter the decay constant and the number of nuclei.
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Radioactive activity
Activity, A = λ·N, measures how "radioactive" a sample really is: how many nuclei decay per second. It is the product of the decay constant λ (the probability that any one nucleus decays in a second) and the number of radioactive nuclei present. The SI unit is the becquerel (1 Bq = 1 disintegration per second); the older unit is the curie (1 Ci = 3.7×10¹⁰ Bq, the activity of 1 g of radium-226). Because activity depends on how many nuclei are left, it falls over time as those nuclei decay — following the same exponential law, A(t) = A₀·e^(−λt). This is the quantity printed on the labels of sealed sources and radiopharmaceuticals, the one that tells a technician when a source is still useful and when it has decayed past the point of being worth handling. Enter the decay constant and the number of nuclei.
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Gamma Exposure Rate
Compute the exposure (or dose) rate of a point gamma source, X = Γ·A/d², from the exposure-rate constant (Γ, specific to the radionuclide), the source activity (A) and the distance (d). It combines the source strength with the inverse square law, allowing you to estimate the dose received at a given distance — fundamental in planning tasks with radioactive sources. Enter the gamma constant, the activity and the distance.
Radioactive Half-life
Compute remaining amount A = A₀·(1/2)^(t/T) for radioactive decay.
Decay Constant
Compute the radioactive decay constant, λ = ln(2)/T½, from the half-life (T½). The constant λ is the probability of a nucleus decaying per unit time — the larger it is, the more unstable the isotope and the shorter its half-life. It links the half-life (time for half the nuclei to decay) to the activity and to the exponential decay law. Enter the isotope's half-life.
Number of Half-Lives
Compute how many half-lives have elapsed, n = t/T½, and the fraction of radioactive material remaining, (1/2)ⁿ, from the elapsed time and the half-life. With each half-life the amount halves: after 1 half-life 50% remains, after 2 it is 25%, after 10 less than 0.1%. It is the intuitive way to assess how much of a source (or contamination) is left. Enter the elapsed time and the half-life.
Food Water Activity
Compute a food's water activity (aw) from the equilibrium relative humidity, aw = ERH/100. Water activity — the 'free water' available for reactions and microorganisms — is the most important conservation factor: below aw 0.6 no microorganism grows; bacteria stop at ~0.90, molds at ~0.70. Unlike total moisture, it explains why honey (moist but with low aw) does not spoil. Enter the equilibrium relative humidity (%).
Radiation Shielding Attenuation
Compute the radiation intensity after passing through shielding, I = I₀·e^(−μ·x), by the exponential attenuation law, from the initial intensity (I₀), the material's linear attenuation coefficient (μ) and the thickness (x). Unlike alpha and beta particles (which have a finite range), gamma rays and X-rays are only exponentially attenuated — never fully blocked. It is the basis of shielding calculation. Enter the initial intensity, the attenuation coefficient and the thickness.
The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.