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Dose Rate / ALARA Foundation

Inverse square law calculator

Model-based estimate. Use the stated geometry, composition and measurement assumptions; see the tool scope for limitations.

Input

Point-source distance correction

Calculation mode
Scenario comparison
ScenarioDistanceUnitTimeUnit
AssumptionsPoint-source inverse-square geometry only.No shielding, attenuation, scattering, build-up, occupancy or source-size correction.Professional verification is required before radiation safety decisions.

Result

Dose-rate at target distance

25 µSv/h

Formula

D2 = D1 * (r1 / r2)^2
Known distance
1 m
Distance ratio
0.5
Dose-rate vs distance
40030020010000.51.081.652.222.8Distance, m
Scenario comparison
ScenarioAdjusted rateTotal doseBasis
Near task100 µSv/h25 µSv1 m, 15 min
Standard task25 µSv/h12.5 µSv2 m, 30 min
Remote check4 µSv/h0.666667 µSv5 m, 10 min
  • Point-source geometry only; source dimensions and detector geometry are not modeled.
  • No shielding, attenuation, scattering or build-up factor is included.
  • Dose-rate units are not converted across radiation protection quantities; known and target dose-rate units must be the same.

Dose Rate and ALARA Calculator

Inverse square law

The calculator computes the dose rate from a point gamma source and radiation protection parameters following the ALARA principle. By the inverse square law, dose rate falls off proportionally to the square of the distance from the source.

Five calculation modes

Modes: dose rate at distance, required distance, allowed working time, scenario dose and activity ↔ dose-rate conversion via the gamma constant. Built-in gamma constants are available only for validated values; others are entered manually with a stated source.

Model limitations

The model describes an unshielded point source in air, without shielding, scattering or build-up factor. Professional verification is required before any radiation safety decision.

Frequently asked questions

How do I calculate the dose rate from a source?

Pick the nuclide, enter the activity and the distance. Dose rate = Γ·A/r², where Γ is the H*(10) gamma constant at 1 m computed from ICRP-107 spectra including daughters in equilibrium (Cs-137/Ba-137m, Ra-226 series).

Which quantity is reported: effective dose, H*(10) or kerma?

By default the ambient dose equivalent H*(10), the quantity survey meters measure and limits are compared against. Air kerma is available separately; the H*(10)/Kₐ coefficients follow ICRP-74 (Co-60 1.16, Cs-137 1.21).

Why does the result differ from the source certificate?

Certificates often quote exposure rate (mR/h) or a different geometry and distance. Convert the units with the dose converter and make sure the distance is measured from the source centre to the point, not from the container surface.

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