Attenuation of 4-20 MV x rays by a new compensator material of cement.

The properties of a new cement based material for production of compensators are presented. Broad beam attenuation of 4-20 MV x rays by slabs of the material have been measured at various field sizes and depths in a large water phantom. For comparison the attenuation of aluminum was determined at some of the photon energies and it was found that the attenuation properties of the cement based material are very close to those of aluminum. At 6 and 18 MV, a comparison of different phantoms for attenuation measurements was carried out. For this investigation the ionization chamber was placed in a 50x50x50 cm3 water phantom, a 20x20x20 cm3 water equivalent plastic phantom, and a cylindrical mini-phantom. Agreement was obtained between the measurements in the large water phantom and in the water equivalent plastic phantom. The measurements carried out with the mini-phantom in a 6 MV x-ray beam gave a higher transmission versus absorber thickness than the transmission found with the water phantom resulting in a lower value of the effective attenuation coefficient. At 18 MV x rays the difference between the measurements in the water phantom and in the mini-phantom was less. This shows that a water equivalent plastic phantom with an area comparable with the largest field size applied can be used for measurements of effective attenuation coefficients, whereas a mini-phantom cannot be used directly, especially at low photon energies.