Electron beams from an AECL Therac 20 accelerator  

The Therac 20 accelerator shown in fig.7 uses a scanned beam to achieve a flat beam profile. There are no scattering foils in this accelerator. The geometry consists of an exit vacuum window, a beam monitor ion chamber, a series of x and y jaws and electron beam applicators. The beam monitor chamber is made with aluminized Kapton and the applicator has thick scraper bars. For the Therac 20 the scan pattern (see reference [1]) was set for a 4040 field but the treatment field was 1010 .

The experimental data have been measured with a diode detector by Joanna Cygler of the Ottawa Regional Cancer Center.

Figures 30, 32, 33, 34 and 35 show the energy spectra and angular distributions of electrons and contaminant photons and comparison between calculated and measured central-axis depth-dose curves along with dose components contributed from each beam defining component for 6, 9, 13, 17 and 20 MeV beams respectively.

Due to the combination of no scattering foils and the design of the beam defining system, particularly the design of the applicator in the Therac 20, both the energy and angular spread of the electrons are small.

There are few contaminant photons in the beam. The large angular spread of contaminant photons is due to the fact that most contaminant photons are originated from the beam defining system.

Figure 31 presents a comparison of calculated and measured dose profiles at the depth of and at the phantom surface from various dose components for a 6 MeV beam from the AECL Therac 20. The agreement is excellent.