Broad beam attenuation measurements in lead in kilovoltage X-ray beams

Broad beam attenuation is dependent on several parameters, such as the beam energy, thickness of attenuator, distance between the detector and attenuating material, atomic number of the attenuator, and the size and the volume of the ionization chamber. Lead is a common shielding material and hence the lead attenuation was studied in the context of the distance between detector and attenuating material and the chamber volume. Lead sheets of size 30 x 30 cm2 and high purity and precise thickness of 0.1 mm to 4.5 mm were used for the broad beam attenuation measurements of X-ray beams from a Philips Super 80CP unit with the range of 50 kVp-125 kVp. Six different chambers of 6 cm3, 15 cm3, 60 cm3, 150 cm3, 180 cm3 and 600 cm3 volume from two manufacturers were used. The source to the attenuator distance was kept constant at 1 m and the distance between ionization chamber and attenuating material (Pb) was varied in the range of 0-20 cm. The results show that the broad beam attenuation depends on the distance between chamber and attenuating material for each kV and ionization chamber volume. Shorter distances between chamber and attenuating material provide higher transmission. The attenuation differences were more pronounced at lower transmission values (< 10(-3)). The choice of the detector was shown not to be a critical factor except for the large volume of the 600 cm3 ionization chamber. In conclusions, broad beam attenuation measurements used for shielding calculation is relatively independent on the measuring devices except for a very large ion chamber. However, the distance between the attenuator and the detector is critical for the accuracy of attenuation measurements especially for smaller transmission values, which are most critical for public exposure.