[Measurement of depth dose curves of soft x-rays in a water phantom]

A device is described which allows to perform dosimetry with a usual flat chamber made of plexiglas (PTW soft ray chamber) in the water phantom. The depth dose curves measured for anode potentials of 10 to 100 kV were compared to the curves measured a) in a plexiglas phantom, b) in a M3 phantom using a soft ray chamber made of M3, c) in a mixed phantom consisting of a plexiglas bottom, a soft ray chamber made of plexiglas flush with its surface, and plates or foils made of M3, polystyrene, and frapan. The results show that there is no difference between the depth dose curves measured in water and those measured in the homogeneous M3 phantom if the curve is plotted over the mass related to the surface (g/cm2). There is also no difference to the dose curve measured in the mixed phantom according to c) if M3 plates determining the depth of measurement are layed on it. With the other substances, considerable deviations are found. An example is presented in order to warn against the uncontrolled use of "plastic foils" as phantom material for soft ray depth dose curves.