Specific Activity Calculator

The Specific Activity Calculator finds how radioactive a substance is per unit mass — a critical value in nuclear medicine, radiochemistry, and radiation safety work. Enter a radionuclide's half-life (in seconds, minutes, hours, days, or years) and molar mass, then select your preferred output unit system (SI Bq-based or non-SI Ci-based) and scale to get the specific activity. Secondary outputs include half-life in seconds, decay constant, and atoms per gram — plus an optional mass-to-activity conversion if you enter a sample mass.

g/mol

Optional: for mass-activity conversions

Results

Specific Activity

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Half-life (seconds)

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Decay Constant

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Atoms per Gram

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Frequently Asked Questions

What is specific activity in radioactivity?

Specific activity is the radioactivity per unit mass of a radioactive material, typically expressed in Bq/g or Ci/g. It indicates how much radioactive decay occurs per gram of the substance.

How is specific activity calculated from half-life?

Specific activity is calculated using the formula: A = (λ × N₀)/M, where λ is the decay constant (ln(2)/t₁/₂), N₀ is Avogadro's number, and M is the molar mass. The decay constant relates directly to the half-life.

What's the difference between Bq and Ci units?

Becquerel (Bq) is the SI unit for radioactivity, representing one decay per second. Curie (Ci) is the older unit, where 1 Ci = 3.7 × 10¹⁰ Bq. Bq-based units are preferred in scientific applications.

Why do I need to enter molar mass?

Molar mass is essential for converting from atomic-level decay rates to mass-based specific activity. It represents the mass of one mole of the radioactive isotope in grams.

How do half-life units affect the calculation?

The calculator converts all half-life inputs to seconds internally before calculating the decay constant. Longer half-lives result in lower specific activity values since the material decays more slowly.

What is the decay constant?

The decay constant (λ) represents the probability of decay per unit time. It's calculated as ln(2)/half-life and is fundamental to determining specific activity and radioactive decay rates.

Can I convert between different activity units?

Yes, the calculator provides output in both SI (Bq-based) and non-SI (Ci-based) units with various scale multipliers like kBq, MBq, GBq, µCi, mCi to match your preferred units.

What does 'atoms per gram' represent?

This shows the number of radioactive atoms present in one gram of the material. It's calculated using Avogadro's number and helps understand the atomic composition of the radioactive sample.