measurement of absorbed dose

Measurement of Absorbed Dose (5)

參考資料：1. The Physics of Radiation Therapy. Faiz M. Khan2. Introduction to Radiological Physics and Radiation Dosimetry. Frank H. Attix

The Bragg-Gray cavity theory

• Chamber Volume• The quantity Jg can be determined for a

chamber of known volume or known mass of air in the cavity if the chamber is connected to a charge-measuring device.

• However, the chamber volume is usually not known to an acceptable accuracy.

• Indirect method of measuring • Jair is to make use of the exposure calibration of

the chamber for Co-60 γ ray beam


• Chamber Volume• Suppose the chamber with this build-up

cap is exposed in free air to a Co-60 beam and that a transient electronic equilibrium exists at the center of the chamber.

• assume initially that the chamber wall and the build-up cap are composed of the same material (wall).


• Chamber Volume• if the chamber (plus the build-up cap) is

replaced by a homogeneous mass of wall material with outer dimensions equal to that of the cap,


• Chamber Volume• where is the ratio of electron fluence

at the reference point P (center of the cavity) with chamber cavity filled with wall material to that with the cavity filled with air.

• This correction is applied to the Bragg-Gray relation to account for change in electron fluence


• Chamber Volume• ψ in the above equation can be replaced by

Ψ , provided a transient electron equilibrium exists throughout the region of the wall


• Chamber Volume• If Dair is the absorbed dose to air that would

exist at the reference point with the chamber removed and under conditions of transient electronic equilibrium in air,


• Chamber Volume


• Chamber Volume• Also Dair (under conditions of transient

electronic equilibrium in air) can be calculated from exposure measurement in a Co-60 beam with a chamber plus build-up cap,


• Chamber Volume• where k is the charge per unit mass

produced in air per unit exposure ( 2.58 x 10-4 C kg-1R-1 )

• M is the chamber reading normalized to standard atmospheric, 22 , 760 mmHg℃

• Aion is the correction for ionization recombination under calibration conditions,

• Pion is the ionization recombination correction for the present measurement.


• Chamber Volume


• Chamber Volume• consider a more realistic situation in which

the chamber wall and build-up cap are of different materials.


• Chamber Volume


• Effective Point of Measurement• Plane Parallel Chambers

• Since the front plane (toward the source) of the air cavity is flat and is exposed to a uniform fluence of electrons, the point of

measurement is at the front surface of the cavity.


• Effective Point of Measurement• Cylindrical Chambers

• Electrons (from an electron beam or generated by photons) transversing a cylindrical chamber of internal radius r will enter the sensitive volume of the chamber (air cavity) at different distances from the center of the chamber.



• Dutreix and Dutreix showed that theoretically the point of measurement for a cylindrical chamber in a unidirectional beam is displaced by 0.85r from the center and toward the source.

• the effective point of measurement is influenced by the number of electrons entering through a surface area ds at A of the chamber and the track length of these electrons in the cavity.



• Dutreix and Dutreix :

measurement of absorbed dose

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