00 Foundations

Exposure and air kerma

X = Q/m_air. 1 R = 2.58×10⁻⁴ C/kg → K_air = 0.00876 Gy/R (W/e = 33.97 J/C).

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Simulation

Exposure and air kerma — Change the numbers; the scene follows.

Where it works

Water phantom

Ion chamber

In the water tank under the linac, at the ion chamber — reference dosimetry happens here.

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Formula

X=\frac{Q}{m_{\mathrm{air}}},\quad K_{\mathrm{air}}=X\left(\frac{W}{e}\right),\quad 1\,\mathrm{R}=8.76\,\mathrm{mGy}

Variables

X · ExposureR

Results

X
Exposure
0.000258C/kg
K_air
Air kerma
0.00876Gy
K_air
Air kerma
8.7643mGy
K_air
Air kerma
876.426mrad

Explanation

X=\frac{Q}{m_{\mathrm{air}}},\quad K_{\mathrm{air}}=X\left(\frac{W}{e}\right),\quad 1\,\mathrm{R}=8.76\,\mathrm{mGy}

What it means

Exposure X is ionisation charge per mass of dry air — the oldest radiation quantity, still on older survey-meter scales (R, mR/h). Multiplying by W/e = 33.97 J/C converts charge to energy and gives air kerma. 1 roentgen = 2.58×10⁻⁴ C/kg = 8.76 mGy air kerma. Absorbed dose in tissue is then f-factor × exposure (see the f-factor calculator). This is a working relation in Radiation physics.

Where it is used

Clinically it sits on the Water phantom — Ion chamber. In the water tank under the linac, at the ion chamber — reference dosimetry happens here. Radiation physics lives at the x-ray target, the linac head, and inside the patient: how a photon is born, how it scatters, and how it dies. Use these relations before you trust a spectrum, a wall, or a kV-versus-MV contrast argument.

Water phantom · Open this machine

How to use it

Enter exposure in R (or charge and mass). Read C/kg, air kerma in Gy and mGy. A typical chest PA entrance exposure of 20 mR is 0.175 mGy air kerma. Change one input and watch the curve and the simulation follow.

Symbols

XExposure1 R

Worked example

A typical case from the default values: X = 1 R (Exposure). Substituting into the relation gives X = 0.000258 C/kg; K_air = 0.00876 Gy; K_air = 8.7643 mGy; K_air = 876.426 mrad. These are teaching numbers — align them with your machine.

Typical values give

X = 0.000258C/kg
K_air = 0.00876Gy
K_air = 8.7643mGy
K_air = 876.426mrad

Where it comes from

The displayed formula is the working relation. X = Q/m_air. 1 R = 2.58×10⁻⁴ C/kg → K_air = 0.00876 Gy/R (W/e = 33.97 J/C). Usual reference: ICRU 85 / Attix. Derive it in the specialty lesson, then return here to pin the numbers.

Reference: ICRU 85 / Attix

Assumptions & limits

Defined only for photons in air, under CPE. Not a tissue dose and not equivalent dose. W/e is for dry air at the calibration quality; humidity is a sub-percent correction (see humidity k_h).

Pitfalls

Do not mix free-electron Compton kinematics with photoelectric-dominated kV imaging. Check keV versus MeV, and never treat a spectrum as one photon. Defined only for photons in air, under CPE. Not a tissue dose and not equivalent dose. W/e is for dry air at the calibration quality; humidity is a sub-percent correction (see humidity k_h).

Keep this

Photons do not deposit dose; the electrons they set in motion do. Defined only for photons in air, under CPE. Not a tissue dose and not equivalent dose. W/e is for dry air at the calibration quality; humidity is a sub-percent correction (see humidity k_h).

In this specialty

Radiation physics