06 Dose
W/e and ion charge to dose
Energy spent per ion pair in dry air is 33.97 J/C. D_air = (Q/m)(W/e).
Listen
Listen · English
Simulation
W/e and ion charge to dose — 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.
Open this machineFormula
Variables
Results
D_air
Dose to air
0.0085Gy
D_air
Dose to air
0.849cGy
Explanation
What it means
W/e is the average energy spent per unit charge of ionisation in dry air, 33.97 J/C. Dose to air is (Q/m)(W/e). This is how a primary standard converts charge to gray before k_Q and stopping-power ratios take you to water. This is a working relation in Dosimetry.
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. Dosimetry is the chamber in water under the linac, or the well counter in the hot lab: TG-51, TRS-398, kerma, and recombination. These numbers are the calibration the rest of the department borrows.
Water phantom · Open this machineHow to use it
Enter collected charge and the air mass of the cavity (from volume × density at the measurement T,P). Then apply stopping-power ratio for medium dose. Change one input and watch the curve and the simulation follow.
Symbols
- QCollected charge12.5 nC
- mMass of air50 mg
Worked example
A typical case from the default values: Q = 12.5 nC (Collected charge); m = 50 mg (Mass of air). Substituting into the relation gives D_air = 0.0085 Gy; D_air = 0.849 cGy. These are teaching numbers — align them with your machine.
Typical values give
- D_air = 0.0085Gy
- D_air = 0.849cGy
Where it comes from
The displayed formula is the working relation. Energy spent per ion pair in dry air is 33.97 J/C. D_air = (Q/m)(W/e). Usual reference: Boutillon / Attix. Derive it in the specialty lesson, then return here to pin the numbers.
Reference: Boutillon / Attix
Assumptions & limits
Dry air, calibration quality. Humidity and W/e energy dependence are at the 0.1–0.5% level for most therapy beams.
Pitfalls
kQ is for that chamber and that beam quality — not a neighbour's value. Polarity and recombination are measured, not copied. A ⁶⁰Co N_D,w is not an MV calibration until kQ is applied. Dry air, calibration quality. Humidity and W/e energy dependence are at the 0.1–0.5% level for most therapy beams.
Keep this
Trace every gray back to a protocol, a chamber, and a quality index. Dry air, calibration quality. Humidity and W/e energy dependence are at the 0.1–0.5% level for most therapy beams.
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