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Radiation Distance Calculator

Apply the inverse square law to find dose rate at any distance from a point source

Free radiation distance screen for radiation safety officers, industrial radiographers, and health physicists who need a local inverse-square arithmetic prompt for external photon fields. Enter a known dose rate at a known distance and the screen returns the local dose-rate prompt at a new distance using I2 = I1 x (D1/D2)². You can also solve in reverse by entering a target dose-rate prompt. Results display in mR/hr, R/hr, mSv/hr, and μSv/hr with source warnings for point-source geometry, scatter, shielding, survey quality, 10 CFR 20 public-dose limits, radiation-area threshold context, and RSO review. The output is not field-ready posting approval, radiography boundary approval, dose assessment, work permit, ALARA review, or safe-entry authorization.

Pro Tip: The inverse square law assumes a point source in free air with no scatter. Backscatter from floors, walls, and equipment can change measured dose rates at close range, and shielding or collimation changes the field. Treat any calculated distance as a review prompt, then confirm the actual field with calibrated surveys, current procedures, license conditions, Agreement State requirements where applicable, and RSO review before posting or access decisions.

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Radiation Distance Calculator
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How It Works

  1. Enter Known Dose Rate and Distance

    Enter the measured or documented dose rate at a known distance from the source. This is typically taken from a survey meter reading or from the source manufacturer's certificate. Select the appropriate units for both dose rate and distance.

  2. Enter the Target Distance

    Enter the distance where you need a local dose-rate prompt. This could be a proposed work position, a boundary review point, or an unrestricted-area review point. The screen applies the inverse square law to compute the local prompt at this distance.

  3. Review Results or Solve for Distance

    The output shows the local dose-rate prompt at your target distance in multiple units. To work in reverse, enter a target dose-rate prompt such as 2 mrem/hr for an unrestricted-area hourly public-dose review point; the result still needs survey, procedure, and RSO review.

Assumptions

  • The source is treated as a point source (physical dimensions are small compared to the measurement distance).
  • No shielding or attenuation is present between the source and the calculation point.
  • Scatter contributions from nearby surfaces are not included in the calculation.
  • The medium between source and detector is air (no significant air attenuation at typical working distances).

Limitations

  • Does not apply to line sources, area sources, or extended source geometries.
  • Does not account for scatter, buildup, or reflection from nearby surfaces.
  • Does not include shielding attenuation. Use the Radiation Shielding HVL Screen only as a source-aware shielding prompt.
  • Does not model energy-dependent detector response or calibration factors.

References

  • 10 CFR 20 - Standards for Protection Against Radiation (U.S. Nuclear Regulatory Commission)
  • 10 CFR 34 - Licenses for Industrial Radiography and Radiation Safety Requirements for Industrial Radiographic Operations
  • ANSI N43.3 - Installations Using Non-Medical X-Ray and Sealed Gamma-Ray Sources, Energies Up to 10 MeV
  • Radiological Health Handbook (U.S. Department of Health, Education, and Welfare, 1970)

Frequently Asked Questions

The inverse square law states that radiation intensity from a point source is inversely proportional to the square of the distance from the source. If you double your distance from a source, the dose rate drops to one quarter. If you triple the distance, the dose rate drops to one ninth. This relationship holds for any source that is small compared to the measurement distance and where there is no shielding or significant scatter between the source and the measurement point.
The law breaks down in three common situations. First, when the source is not a point source, meaning you are close enough that the physical size of the source matters (a line source like an activated pipe follows a 1/r relationship, not 1/r squared). Second, when there is significant scatter from nearby surfaces that adds to the direct beam. Third, when there is intervening shielding that attenuates the beam. In practice, the inverse square law is a good approximation whenever you are at least three times the largest source dimension away from the source.
Under 10 CFR 20.1301, the dose to individual members of the public from licensed operations is limited to 100 mrem per year (1 mSv/yr), and the dose in any unrestricted area from external sources may not exceed 2 mrem in any one hour. That value is not the same as a radiation-area posting threshold. Use it as a public-dose review prompt and check your survey records, license conditions, RSO procedures, and applicable Agreement State requirements.
The most reliable method is a direct survey with a calibrated ion chamber or GM detector at a known distance. For sealed sources, the manufacturer's certificate lists the activity at a reference date, which you can convert to a dose rate using the specific gamma ray constant (see the Activity to Dose Rate calculator). If you use the certificate, remember to decay-correct the activity to today's date before calculating the dose rate, especially for shorter-lived isotopes like Ir-192 with a 73.8-day half-life.
For point-source external photon fields, distance reduces the direct-beam dose-rate prompt by the square of the increase. Doubling distance cuts the idealized prompt to 25 percent. Actual work still depends on source geometry, shielding, scatter, tools, procedures, training, access control, and survey confirmation, so the calculation should be treated as one input to the radiation protection program.
Disclaimer: This screen applies local inverse-square arithmetic for point-source external photon prompts in free air. Actual dose rates depend on source geometry, scatter, shielding, instrument response, and procedures. Field surveys with calibrated instruments and qualified RSO or health physics review are required before compliance, posting, access, or work decisions.

Learn More

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