  Purpose: To analyze the air density dependence of the response of the new
SourceCheck 4pi ionization chamber, manufactured by PTW. Methods: The air
density dependence of three different SourceCheck 4pi chambers was studied by
measuring 125I sources. Measurements were taken by varying the pressure from
746.6 to 986.6hPa in a pressure chamber. Three different HDR 1000 Plus
ionization chambers were also analyzed under similar conditions. A linear and a
potential-like function of the air density were fitted to experimental data and
their achievement in describing them was analyzed. Results: SourceCheck 4pi
chamber response showed a residual dependence on the air density once the
standard pressure and temperature factor was applied. The chamber response was
overestimated when the air density was below that under normal atmospheric
conditions. A similar dependence was found for the HDR 1000 Plus chambers
analyzed. A linear function of the air density permitted a very good
description of this residual dependence, better than with a potential function.
No significant variability between the different specimens of the same chamber
model studied was found. Conclusion: The effect of overestimation observed in
the chamber responses once they are corrected for the standard pressure and
temperature may represent a non-negligible ~4% overestimation in high altitude
cities as ours (700 m AMSL). This overestimation behaves linearly with the air
density in all cases analyzed.
