Activity to Dose Rate Approximation Calculator

Calculator

This calculator estimates external dose rate from a gamma-emitting point source using an inverse-square approximation:

• R is dose rate (µSv/h).
• Γ is gamma constant (µSv/h per GBq at 1 m).
• A is activity (GBq).
• d is distance from the source (m).
• F is attenuation factor (dimensionless).

When shielding is selected and layer data is available, the attenuation factor is estimated using half-value layer (HVL) and tenth-value layer (TVL) models.

1. Select a nuclide, or choose Custom and enter Γ.
2. Enter activity and choose the correct unit.
3. Enter distance between the source and the point of interest.
4. Optionally select shielding material and thickness, or use a manual attenuation factor.
5. Enter exposure time to estimate the total accumulated dose.
6. Press Calculate to show results above the form.
7. Use Download CSV or Download PDF to export results.

1) What this tool estimates

This calculator approximates external gamma dose rate from a radioactive source. It combines a nuclide-specific gamma constant, the entered activity, and distance. Results are shown in µSv/h and mSv/h, plus total dose for your exposure time.

2) Why gamma constants matter

A gamma constant (Γ) expresses how strongly a nuclide irradiates at 1 meter. Here it is treated as µSv/h per GBq at 1 m, which supports fast field estimates. Example values used include Cs-137 ≈ 78, Ir-192 ≈ 117, and Co-60 ≈ 305.

3) Distance drives the biggest change

Dose rate decreases with the inverse-square law, so doubling distance cuts rate to one quarter. This is why small changes near the source matter greatly. Use consistent geometry and measure distance to the point of interest. For quick checks, compare 0.5 m vs 1 m vs 2 m to see scaling. Keep shielding and source orientation the same when comparing scenarios.

4) Activity units and conversion accuracy

You can enter activity in Bq, kBq, MBq, GBq, or curie-based units. Internally, activity is converted to GBq, using 1 Ci = 3.7×10¹⁰ Bq = 37 GBq. Enter the best available calibration value for your source. For unit context, 1 mSv = 1000 µSv, and 1 µSv/h sustained for one hour gives 1 µSv. If you enter time in minutes, it is converted to hours for dose integration.

5) Shielding is modeled as attenuation

When shielding is enabled, the calculator estimates an attenuation factor using half-value layer (HVL) and tenth-value layer (TVL) behavior for lead or concrete. This supports quick comparisons, but real shielding depends on energy spectrum, buildup, scatter, and gaps. Thin shields can reduce primaries yet increase scatter contribution. Treat the shielding output as an order-of-magnitude guide for setup decisions.

6) Manual attenuation for custom scenarios

If you already know a transmission factor, enter it directly as a number from 0 to 1. The manual factor overrides the layer model and is useful for composite shields, broad-beam corrections, or measured survey reductions.

7) Interpreting dose rate and total dose

Dose rate describes instantaneous exposure intensity, while total dose multiplies that rate by time. For example, 50 µSv/h over 30 minutes yields about 25 µSv. Always compare results to your site’s control levels and procedures.

8) Limitations and professional practice

This is a point-source approximation intended for planning and training. Complex geometries, self-shielding, skyshine, and scatter can shift real dose rates. For compliance or work authorization, confirm with calibrated instruments and qualified radiation protection guidance. Apply ALARA principles: increase distance, reduce time, add shielding. Document assumptions alongside your results when exporting.

1) What does “dose rate” mean here?

Dose rate is the estimated external gamma dose per hour at your distance, reported in µSv/h and mSv/h. It is a planning approximation, not a survey-meter reading.

2) Why does distance change the answer so much?

The calculator uses an inverse-square relationship. If you double the distance from a point source, the estimated dose rate becomes one quarter, assuming the same source and shielding.

3) When should I use the Custom gamma constant?

Use Custom when you have a trusted reference Γ value for your specific nuclide, source form, or energy spectrum. Enter Γ as µSv/h per GBq at 1 m.

4) What is the difference between HVL and TVL?

HVL is the thickness that halves intensity, while TVL reduces intensity to one tenth. The calculator uses simplified layer behavior to estimate attenuation from lead or concrete.

5) Should I enter shielding thickness in cm or mm?

Either is fine. Choose the correct unit in the thickness selector. The calculator converts internally to millimeters to apply the HVL/TVL layer estimates.

6) What is “manual attenuation factor” used for?

It lets you apply a known transmission factor from measurements or detailed analysis. Enter a value between 0 and 1. It overrides the built-in layer model.

7) Is this suitable for safety decisions?

Use it for planning and education only. For radiation protection decisions, follow your local rules, verify with calibrated instruments, and consult qualified professionals.