This converter estimates absorbed dose in a target medium from air kerma using an energy-dependent ratio and optional correction factors:
D ≈ Kair × R × B × (1 − g)
- Kair is air kerma (Gy).
- R is the mass energy-absorption coefficient ratio: (μen/ρ)material ÷ (μen/ρ)air.
- B is a buildup/scatter factor (dimensionless), default 1.0.
- g is the radiative loss fraction (dimensionless), default 0.0.
Preset ratios here are educational starting points. For professional use, insert a custom R derived from authoritative photon interaction data for your spectrum.
- Enter the measured air kerma value and choose its unit.
- Select the output unit for absorbed dose.
- Pick a target material and representative photon energy.
- For best accuracy, choose custom ratio and enter R.
- Adjust B and g only if your method requires them.
- Press Calculate to view results above the form.
- Use Download CSV or Download PDF for reports.
Example values below illustrate the workflow. Replace R, B, and g with values appropriate to your conditions.
| Air kerma | Material | Energy | R | B | g | Absorbed dose (Gy) |
|---|---|---|---|---|---|---|
| 0.010 Gy | Water | 100 keV | 1.00 | 1.00 | 0.00 | 0.0100 |
| 2.50 mGy | Soft Tissue | 60 keV | 1.02 | 1.05 | 0.00 | 0.00268 |
| 150 µGy | Bone (Cortical) | 60 keV | 1.30 | 1.10 | 0.00 | 0.000215 |
1) Air kerma versus absorbed dose
Air kerma (Kair) describes kinetic energy released per unit mass in air from indirectly ionizing radiation, commonly photons. Absorbed dose (D) is energy deposited per unit mass in the target medium. In photon fields, D in tissue often differs from Kair because interaction probabilities vary by composition and energy.
2) Units and practical scales
Both quantities use gray (Gy), where 1 Gy = 1 J/kg. For legacy reporting, 1 rad = 0.01 Gy, so 1 Gy = 100 rad. Diagnostic exposures are frequently in the µGy–mGy range, while therapy beams can reach Gy per fraction. This converter supports Gy, mGy, µGy, rad, and mrad for consistent workflows.
3) The key ratio R and what it represents
The conversion relies on an energy-dependent coefficient ratio R = (μen/ρ)material ÷ (μen/ρ)air. R links kerma in air to dose in a specific material by accounting for how efficiently photon energy is transferred and absorbed. For narrow spectra and known energy, using a custom R improves traceability and repeatability.
4) Material choice changes dose estimates
Water and soft tissue typically track close to air kerma at many energies (R near 1), which is why water is often used as a reference medium. Higher-Z materials can show larger deviations, especially at lower energies where photoelectric absorption dominates. Selecting the correct material is essential for bone, shielding metals, plastics, or lung-equivalent media.
5) Energy dependence and common reference bins
Photon interactions change with energy: tens of keV emphasize photoelectric effects, while MeV energies are more Compton dominated. The calculator offers representative bins (30, 60, 100 keV; 1 and 6 MeV) to match typical diagnostic and therapy contexts. If your source is polyenergetic, pick a representative effective energy or use a spectrum-weighted ratio from your protocol.
6) Buildup factor B and scatter geometry
B accounts for additional dose contribution from scattered photons in the medium. In free-in-air or tight-beam conditions, B can be close to 1.00. In thicker slabs, broad fields, or backscatter-rich geometries, B may exceed 1 and should be sourced from method-specific references. Keeping B explicit makes assumptions transparent in reports.
7) Radiative loss factor g in the dose model
The term (1 − g) allows a small correction for energy that escapes the region of interest as radiative losses from secondary charged particles. For many photon dosimetry applications, g is set to 0.00 as a reasonable default. If your standard defines g differently, enter the value used by your calculation or Monte Carlo model.
8) Reporting, QA checks, and exports
A practical quality check is dimensional consistency: convert Kair to Gy first, apply dimensionless factors (R, B, 1 − g), then convert to the output unit. Review that results scale linearly with Kair. Use the CSV export for logs and the PDF report for audits, lab notebooks, or coursework submissions.
1) When is absorbed dose equal to air kerma?
It can be close when the target medium behaves similarly to air and R is near 1, with minimal scatter corrections. Always verify the energy and material assumptions before treating them as equal.
2) What should I use for the ratio R?
Use an energy-dependent (μen/ρ) ratio from an authoritative dataset or your protocol. If you do not have one, start with a preset for learning, then replace it for formal work.
3) Why does bone often give higher dose estimates at low energy?
Bone has higher effective atomic number and density than soft tissue, increasing photoelectric absorption at tens of keV. That raises R relative to air, so the converted absorbed dose increases.
4) What is a reasonable default for B?
Use B = 1.00 if you are unsure and your geometry is close to free-in-air or narrow-beam assumptions. For broad fields or thick media, consult buildup references and update B accordingly.
5) Do I need to enter a nonzero g factor?
Often no. Many practical photon dose calculations set g = 0.00. Enter a nonzero value only if your dosimetry method explicitly includes radiative loss corrections.
6) Can I convert µGy to rad directly here?
Yes. The calculator converts internally through Gy. Remember: 1 rad = 0.01 Gy and 1 Gy = 100 rad. Choose µGy as input and rad as output to see the converted dose.
7) Is this suitable for regulatory or clinical decisions?
This tool supports education and documentation. For clinical or regulatory decisions, use validated workflows, traceable coefficients, and site-approved QA procedures. Always document energy, geometry, and coefficient sources used.