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Medium energy X-rays (MEX)

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NOTE: All MEX kilovoltage services are currently unavailable due to tube replacement.

Medium energy X-rays (MEX)

Ionisation chambers are calibrated directly against the primary standard of air kerma for medium-energy X-rays, the Medium Energy Free Air Chamber (MEFAC). During a measurement, the MEFAC is placed in the beam and the air kerma rate determined. The MEFAC is then replaced with the chamber to be calibrated, and the same X‑ray beams are used to expose the chamber. A monitor chamber allows for variations in the X‑ray tube output (Figure 1).

Chambers are calibrated directly against the primary standard (the Medium Energy Free Air Chamber - MEFAC) by shifting each chamber into the beam consecutivelyFigure 1: Chambers are calibrated directly against the primary standard (the Medium Energy Free Air Chamber - MEFAC) by shifting each chamber into the beam consecutively.


Currently, medium energy X-rays are provided by a Seifert Isovolt generator. The X-ray tube has a stationary tungsten target. The beams used for radiotherapy calibrations are listed in Table 1. The calibration procedure is largely automated and so all the beams (ST1 to ST17) are included for a single fee.

Many therapy ionisation chambers such as the Farmer chamber have build-up caps designed for megavoltage radiation. This cap is removed for kilovoltage beams. The highest energy beams ST16 and ST17 are sometimes measured with the build-up cap on for the purposes of calculating a calibration coefficient for 192Ir.

Table 1: Medium energy therapy radiation qualities available from the Seifert X-ray generator for a source-detector distance of 1331 mm, nominal beam diameter 11 cm.

Beam ID kVp Additional filtration Air kerma
rate (mGy/s)
First HVL
(mm AI)
First HVL
(mm Cu)
Effective energy (approx.)
(keV)
AI
(mm)
Cu
(mm)
Sn
(mm)
ST1 30 0.32 0.3 0.35 19
ST2 50 0.32 0.9 0.56 26
ST3 75 0.32 0.9 0.77 34
ST4 100 0.32 0.9 1.01 42
ST5 75 1.05 0.9 1.50 37
ST6 100 1.05 0.9 2.06 45
ST7 135 1.05 0.9 2.93 54
ST8 100 3.45 0.9 3.94 0.14 50
ST9 150 3.45 0.9 5.81 0.27 62
ST10 135 1.05 0.25 0.8 8.42 0.47 66
ST11 135 2.02 0.27 0.8 8.86 0.51 68
ST12 150 2.02 0.27 0.9 0.60 71
ST13 180 2.02 0.54 0.9 0.99 84
ST14 250 2.02 0.27 0.9 1.17 89
ST15 250 2.02 0.54 0.9 1.53 97
ST16 250 0.99 1.86 0.9 2.52 123
ST17 300 1.95 3.94 0.25 0.8 3.98 165


Note that polarity and recombination corrections are not provided for kilovoltage calibrations. The uncertainties resulting from these omissions are small for the air kerma rates used in the calibration and the chamber types calibrated.

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