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Radioactive Decay Calculator

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

Free radioactive decay calculator for RSOs, radiographers, and nuclear medicine technologists who need to determine the current activity of a sealed source or radioactive material. Enter the original activity from the source certificate, the assay date, and today's date (or any target date). Select the isotope to load its half-life, or enter a custom half-life. The calculator applies the standard decay equation A = A0 x (1/2)^(t/T) to return the remaining activity, the decay factor, the number of half-lives elapsed, and the percentage of original activity remaining. It also shows a decay timeline table with activity at regular intervals. This is a routine calculation that every holder of a sealed source license must perform. Source certificates list the activity at a specific assay date, but regulatory records, shipping papers, waste disposal manifests, and survey calculations all require the activity corrected to the date of use. Getting this wrong means your boundary distances, shipping category, and waste classification may all be incorrect.

Pro Tip: For Ir-192, activity drops about 1 percent per day. A 100 Ci source decays to roughly 50 Ci in 74 days and to about 12.5 Ci in 222 days. Many radiography companies swap sources at 25 to 30 percent of original activity because exposure times become impractically long. When you receive a new source, calculate the swap-out date in advance and put it on the calendar. This prevents the situation where a crew shows up to a job with a source too weak to complete the exposures in the allotted time.

Convert decayed activity to a dose rate at distance

Activity to Dose Rate Calculator →

Calculate boundary distance from the decayed source

Radiation Distance Calculator →

Determine shielding for the current source strength

Radiation Shielding Calculator →

Calculate allowable stay time with updated activity

Radiation Stay Time Calculator →

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Radioactive Decay Calculator
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How It Works

  1. Enter Original Activity and Assay Date

    Enter the activity listed on the source certificate (in Ci, mCi, GBq, or MBq) and the assay date. The assay date is the date the manufacturer measured the activity, which is printed on every sealed source certificate.

  2. Select the Isotope or Enter Custom Half-Life

    Choose the isotope from the dropdown to auto-load its half-life, or enter a custom half-life with a unit (seconds, minutes, hours, days, or years). Common sealed source isotopes: Ir-192 (73.83 days), Co-60 (5.271 years), Cs-137 (30.17 years), Se-75 (119.78 days).

  3. Set Target Date and Review

    The calculator defaults to today's date but you can enter any past or future date. The output shows remaining activity, decay factor, number of half-lives elapsed, and a decay timeline. Use the decayed activity for dose rate calculations, shipping papers, and regulatory records.

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 values are from the NNDC evaluated nuclear data library.

Limitations

  • Does not model secular or transient equilibrium with daughter products.
  • Does not calculate dose rate directly. Use the Activity to Dose Rate Calculator for that conversion.
  • Does not account for branching ratios in isotopes with multiple decay modes.
  • Does not handle time-of-day precision. Elapsed time is calculated in whole days unless the user provides fractional day inputs.

References

  • NNDC/Brookhaven National Laboratory - Evaluated Nuclear Structure Data File (ENSDF)
  • 10 CFR 20, Appendix C - Quantities of Licensed Material Requiring Labeling
  • 49 CFR 173 - Shippers: General Requirements for Shipments and Packagings (DOT)
  • Cember, H. and Johnson, T. - Introduction to Health Physics, 4th Edition (McGraw-Hill)

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.
You should decay-correct whenever you perform a calculation that depends on current activity: pre-job boundary surveys, shipping papers (49 CFR 173), waste manifests, quarterly inventory reports, and leak test documentation. For Ir-192, which loses about 1 percent per day, weekly corrections are standard practice. For Cs-137 with a 30-year half-life, annual corrections are sufficient because the activity changes less than 2.3 percent per year.
When a sealed source decays to below the exempt concentration or quantity listed in 10 CFR 20, Appendix C, it may be disposable as non-radioactive waste, depending on your license conditions and state regulations. However, sealed sources typically contain long-lived isotopes (Co-60 at 5.27 years, Cs-137 at 30 years) that take many decades to reach exempt levels. Most sealed sources are returned to the manufacturer or sent to a licensed waste broker long before they decay to exempt quantities. Short-lived isotopes like Ir-192 (74 days) or Se-75 (120 days) may decay to exempt levels within a few years of final use.
Yes. The decay equation is universal. For Tc-99m (6.01 hours), F-18 (109.77 minutes), I-131 (8.02 days), and other medical isotopes, enter the half-life and the assay time. For short-lived isotopes calibrated to a specific time of day, enter the assay date and time if the calculator supports it, or calculate the elapsed hours manually and divide by the half-life in hours. For a Tc-99m generator elution at 7:00 AM, the activity at 1:00 PM (6 hours later) is exactly half the elution activity.
Three common reasons. First, the gamma constant you used may not match the constant your reference expects (different low-energy cutoffs or unit systems). Second, scatter from nearby surfaces adds to the direct dose rate, especially indoors or near concrete. Third, if the source is inside a device, the device geometry may focus or collimate the beam in one direction. Always verify calculated values with field measurements. The calculation gives you the expected value; the survey gives you the actual value. If they differ by more than 20 to 30 percent, investigate the discrepancy before proceeding.
Disclaimer: This calculator applies the standard radioactive decay equation. Half-life values are from NNDC/Brookhaven National Laboratory. Decay-corrected activities should be verified against field survey measurements for regulatory compliance. This tool does not replace required instrument surveys.

Learn More

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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.

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Radioactive Decay and Source Lifecycle Management

Decay equation fundamentals, half-life reference data from NNDC, source replacement scheduling, decay-in-storage programs, and regulatory tracking requirements.

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