The x-ray beams of the 1 MV machine are measured in a plywood phantom with a density of 1, built up of plates of 30 cm x 30 cm x 1 cm. For superficial measurement the ionisation chamber is positioned halfway into the wood.
It proved to be impossible to generate a stable 1 MeV beam. In clinical practice the machine is operated at a tube voltage of 825 kV. The tube current is 1 mA. The focus-phantom distance is 125 cm. The radiation is filtered in a sandwich of 2 mm Cu + ½ mm Sn + ½ mm Al.The resulting dose rate is about 20 r/min.
Percentage depth dose of the 825 kV x-ray beam from the Millionaire
Isodose curves for the 825 kV X-ray beam from the Millionaire
With 825 kV X-rays the radiation dose at greater depth is not much higher than with the 200 kV radiation which was already available. There is no skin sparing with 825 kV. Due to the low beam intensity of 20 r/min, the radiation treatments take a long time, and because of the static projection through the floor of the cellar, the positioning of the patient is problematic. All things considered, the Millionaire as a treatment machine is not a success and it is taken out of service in 1952. Note: It is uncertain whether the available ionisation chambers were suitable for superficial measurements.
With the monochromatic radiation from Cobalt 60 (1.17 and 1.33 MeV photons) some skin sparing will be realized. However, effective improvement in skin sparing, depth dose and reduced bone dose can only be obtained with photons generated with a voltage of 8 MV and higher. The technical developments after 1945 are directed to adress the need for high voltage X-ray treatment units.