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Activity to Dose Rate Calculator

Convert source activity in curies or becquerels to dose rate using specific gamma ray constants

Free activity to dose rate calculator for RSOs, industrial radiographers, and health physicists who need to determine the exposure rate from a sealed source at a given distance. Enter the source activity in curies (Ci), millicuries (mCi), or becquerels (Bq), select the isotope to load its specific gamma ray constant (also called the exposure rate constant), and enter the distance. The calculator returns the dose rate in R/hr, mR/hr, mSv/hr, and μSv/hr. The specific gamma ray constant (typically given in R/hr per Ci at 1 foot, or R·cm²/mCi·hr at 1 cm) is a published value that relates source activity to exposure rate for a given isotope. Values used in this calculator are from the Radiological Health Handbook and peer-reviewed references. This calculation is essential when you have a source certificate showing activity but need to know the expected survey meter reading at a working distance, or when you need to establish restricted area boundaries before exposing a source.

Pro Tip: The gamma constant published in older references (R/hr per Ci at 1 foot) is sometimes called the "R-factor" or "gamma factor." Be careful with unit conversions: 1 R/hr/Ci at 1 ft = 1.3 R·cm²/mCi·hr at 1 cm. If your reference uses CGS units and your calculator expects R/hr/Ci at 1 foot, dividing by 1.3 converts correctly. Also note that gamma constants for multi-line emitters like Ir-192 vary slightly between references because different authors include or exclude low-energy lines below 100 keV that may not penetrate the source capsule. For Ir-192, values between 4.69 and 5.9 R/hr/Ci at 1 foot appear in the literature. Know which value your regulatory guidance expects.

Apply the inverse square law to find dose rate at other distances

Radiation Distance Calculator →

Calculate shielding needed to reduce this dose rate

Radiation Shielding Calculator →

Decay-correct the source activity to today's date

Radioactive Decay Calculator →

Calculate allowable work time at this dose rate

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Activity-to-Dose Calculator
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How It Works

  1. Enter Source Activity

    Enter the activity from the source certificate or decay-corrected activity. Select the unit: Ci, mCi, μCi, GBq, MBq, or TBq. If the certificate date is not today, use the Radioactive Decay Calculator first to get the current activity.

  2. Select the Isotope

    Choose the isotope from the dropdown. The calculator loads the specific gamma ray constant for that isotope automatically. Common sealed source isotopes include Co-60, Cs-137, Ir-192, Se-75, Yb-169, and Ra-226. If your isotope is not listed, you can enter a custom gamma constant.

  3. Enter Distance and Review

    Enter the distance from the source in feet, meters, or centimeters. The calculator returns the dose rate at that distance in multiple units. Compare the result against regulatory limits, posting thresholds, or your facility's ALARA action levels.

Assumptions

  • Source is treated as a bare, unshielded point source. Capsule attenuation is not included.
  • Gamma constants assume all gamma and X-ray emissions above the low-energy cutoff contribute to dose.
  • Distance is measured from the center of the source to the point of interest.
  • No scatter, buildup, or intervening shielding is modeled.

Limitations

  • Does not account for source capsule or housing attenuation (gives unshielded dose rate only).
  • Does not model contributions from bremsstrahlung or characteristic X-rays from surrounding materials.
  • Does not include beta dose rate for isotopes with significant beta emissions.
  • Custom isotopes require the user to supply a valid gamma constant from a published reference.

References

  • Radiological Health Handbook (U.S. Department of Health, Education, and Welfare, 1970)
  • 10 CFR 34 - Licenses for Industrial Radiography and Radiation Safety Requirements
  • NNDC/Brookhaven National Laboratory - Nuclear Decay Data (NuDat database)
  • IAEA Safety Reports Series No. 16 - Calibration of Radiation Protection Monitoring Instruments

Frequently Asked Questions

The specific gamma ray constant (also called the exposure rate constant) is a number that converts source activity to exposure rate at a standard distance. It is defined as the exposure rate in R/hr at a distance of 1 foot (or 1 cm in CGS units) from a 1-curie point source of that isotope. It accounts for the energy and yield of all gamma ray lines emitted by the isotope. For example, Co-60 has a gamma constant of about 1.32 R/hr per Ci at 1 foot, meaning a 1-curie Co-60 source produces 1.32 R/hr at 1 foot.
Several factors cause variation. First, the inclusion or exclusion of low-energy photons (below 50 to 100 keV) that may be absorbed by the source capsule affects the total. Second, older references may use different nuclear decay data than current evaluations. Third, some references give the constant in air kerma (Gy) rather than exposure (R), requiring a conversion factor. For regulatory calculations, use the gamma constant specified in your license conditions or the reference your regulatory body recognizes. For Ir-192, the NRC commonly uses 5.9 R/hr/Ci at 1 foot in 10 CFR 34 guidance.
1 curie (Ci) equals 3.7 x 10^10 becquerels (Bq), or 37 gigabecquerels (GBq). 1 millicurie (mCi) equals 37 megabecquerels (MBq). 1 microcurie (μCi) equals 37 kilobecquerels (kBq). The curie is the traditional unit used in U.S. regulations and industry. The becquerel is the SI unit used internationally and in most scientific publications. Both describe the same physical quantity: the number of nuclear disintegrations per second.
No. The gamma constant assumes an unshielded point source. In reality, the source capsule, the pigtail assembly, and the camera housing all provide some attenuation. For a source inside a radiography camera, the camera's depleted uranium shielding reduces the dose rate by a factor of 100 to 1,000 compared to the bare source. This calculator gives the unshielded dose rate, which is what you need for boundary calculations when the source is in the exposed position.
A common industrial radiography source is 100 curies of Ir-192. Using a gamma constant of 5.9 R/hr/Ci at 1 foot, this source produces about 590 R/hr at 1 foot. At 10 feet, the inverse square law gives 5.9 R/hr. At 100 feet, the dose rate drops to 0.059 R/hr or 59 mR/hr. This is why radiography operations require restricted area boundaries at distances of 50 to 150 feet or more, depending on the source strength and local shielding conditions.
Disclaimer: Gamma ray constants used in this calculator are from published references and are suitable for preliminary estimates and boundary calculations. Actual dose rates depend on source encapsulation, self-shielding, scatter conditions, and detector response. Regulatory compliance requires field surveys with calibrated instruments.

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