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Radioactive Decay Calculator

Calculate current source activity from a reference date using half-life decay correction

Field guide: Source Activity and Dose Rate: What the Numbers Mean

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Free radioactive decay calculator for RSOs, radiation workers, radiographers, and health physics reviewers who need a local arithmetic prompt from an entered calibration activity, calibration date, isotope half-life row, and evaluation or lower target-activity prompt. The calculator applies the standard decay equation A = A0 x (1/2)^(t/T), unit conversion between Ci/mCi/GBq/MBq, elapsed-day prompts, decay factor, percent remaining, and projection rows. It is not source-certificate verification, an inventory action, a leak-test result, a dose-rate calculation, a radiation survey, transport-record support, waste-path determination, decay-storage acceptance, disposal or transfer authorization, medical-dose verification, RSO authorization, handling authorization, or legal compliance proof.

Pro Tip: Treat the decayed activity as a pre-review number. For short-lived rows such as Ir-192 and Se-75, the arithmetic can change quickly enough that certificate date, calibration time, source serial number, and current procedure matter. For long-lived rows such as Cs-137, Co-60, Am-241, and Ra-226, never let the decayed number alone drive a disposal or transfer decision - license, authorized-recipient, transport, financial-assurance, and regulator context controls the decision.

Review source-activity dose-rate prompts before survey confirmation

Activity to Dose Rate Calculator →

Review inverse-square dose-rate prompts after survey/source checks

Radiation Distance Calculator →

Review shielding HVL prompts after source and survey checks

Radiation Shielding Calculator →

Review stay-time arithmetic prompts after dose-rate confirmation

Radiation Stay Time Calculator →

How It Works

  1. Enter a Source Activity Prompt

    Enter the calibration activity and calibration date from the record you intend to review. Reconcile isotope, source serial number, activity unit, assay time, and certificate status outside the app.

  2. Select a Local Isotope Row

    Choose one of the supported local half-life rows. The rows point to NNDC/IAEA source data and must be checked against current isotope data, source certificate, manufacturer documentation, and procedure context.

  3. Review Arithmetic and Warnings

    Use evaluation-date or lower target-activity output as a local prompt. Resolve certificate, survey, inventory, transport, waste, medical, license, and RSO review gaps before using any value in records or field planning.

Assumptions

  • Radioactive decay follows first-order kinetics with a constant half-life.
  • No production of the isotope (no activation or ingrowth from a parent) is occurring during the decay period.
  • The source is a single isotope. Mixed-isotope sources require separate calculations for each component.
  • Half-life rows are local source pointers and must be verified against current source data and the source certificate.

Limitations

  • Does not model secular or transient equilibrium with daughter products.
  • Does not calculate dose rate, survey results, posting thresholds, stay time, or public-dose compliance.
  • Does not account for branching ratios in isotopes with multiple decay modes.
  • Does not handle time-of-day precision, medical assay times, inventory actions, source-transfer authorization, transport classification, waste-path review, or disposal authorization.

References

  1. NNDC/Brookhaven National Laboratory - NuDat 3 and evaluated nuclear data source pointer
  2. IAEA LiveChart of Nuclides - evaluated nuclear structure and decay source pointer
  3. 10 CFR 20.1501 - surveys and monitoring source pointer
  4. 10 CFR 20.2001 and 10 CFR 30.41 - disposal and transfer source pointers
  5. 49 CFR 173 Subpart I - Class 7 radioactive-material transport source pointer

Frequently Asked Questions

The half-life is the time required for half of the radioactive atoms in a sample to undergo decay. After one half-life, 50 percent of the original activity remains. After two half-lives, 25 percent remains. After ten half-lives, less than 0.1 percent remains. Half-lives are intrinsic physical properties of each isotope and cannot be changed by temperature, pressure, or chemical state. They range from fractions of a second to billions of years depending on the isotope.
Decay correction is reviewed whenever a procedure or record needs current source activity rather than calibration-date activity. The app can supply the arithmetic prompt, but final source records, leak-test context, inventory entries, shipment records, waste decisions, dose-rate calculations, and regulatory reports must be reconciled under current procedures, license conditions, and qualified review.
No. The target-date mode only answers when the entered activity would mathematically fall below a lower prompt. It does not approve replacement timing, source exchange, decay-storage handling, exemption-threshold status, waste path, release, transfer, transport, or disposal. Those decisions depend on source condition, license conditions, regulations, manufacturer/waste-broker options, surveys, and RSO or regulatory review.
This calculator only includes the listed isotope rows and whole-day date prompts. Short-lived medical materials often require assay time, radionuclide-specific procedure controls, radiopharmacy software, patient-dose checks, and medical-license requirements. Do not treat this app as a medical dose or administration verifier.
Activity is only one input to dose rate. Gamma constants, source encapsulation, shielding, device geometry, distance, scattering, collimation, instrument energy response, calibration, background subtraction, and survey method can all change the measured field. Use calibrated survey data and accepted references before using activity in dose-rate, boundary, posting, or stay-time prompts.
Disclaimer: This calculator applies the standard radioactive decay equation to values you enter. Local half-life rows point to NNDC/IAEA data but are not source-certificate verification. It does not replace source records, calibrated surveys, license procedures, radiation protection program review, transport/waste review, or RSO/qualified health physics review.

Guides for this job

Safety

Source Activity and Dose Rate: What the Numbers Mean

How to convert source activity (Curies, Becquerels) to dose rate using the specific gamma ray constant. Includes gamma constant reference table and worked field calculations.

Read Guide →
Safety

Radioactive Decay Activity Arithmetic and Review Boundaries

Decay equation prompts, half-life source rows, certificate and inventory boundaries, dose-rate survey limits, transport/waste caveats, and RSO-review gaps.

Read Guide →

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