The discovery of X-rays was made in 1896 during experiments by the physicist W.C. Röntgen with a simple gas-filled tube. In 1915, when the Netherlands Cancer Institute started radiotherapy by X-rays, the manufacturers had developed quite advanced versions of this old type of X-ray tube. This example shows the last, most developed model of a sophisticated gas-filled tube which was manufactured up until 1920 by the company Müller in Hamburg . The NKI used many of such tubes until in 1919 all treatment units had been converted and only Coolidge tubes were used.
In contrast to the later developed Coolidge tubes, the gas filled tube had no filament to heat the cathode. In gas-filled tubes the cathode was heated by a bombardment of hydrogen gas ions released in the tube by arcing of the high voltage. The glass balloon is not under vacuum but contains a certain quantity of gas. Not too much as this would make the acceleration of the electrons difficult. In the picture, the cathode is on the left, the auxiliary anode is on the right and in the middle the massive anode, also called anti-cathode, where the X-rays are produced.
During use, the hydrogen gas in the X-ray tube is used up resulting in an increase of the resistance in the tube. The radiation production is thereby reduced and the high voltage rises causing an increase of the radiation energy. In order to prevent this, it is necessary to augment the gas supply in the tube. For this purpose, an extra glass tube containing a self-regulating regenerator is melted onto the tube. This contains palladium which gives off hydrogen gas when heated. If the voltage rises, sparking occurs resulting automatically in the release of extra gas in the tube.
Cooling with boiling water:
The anti-cathode (anode) becomes hot during treatment, the dissipated power being approximately 300 Watt. In this late type of X-ray gas tube the cooling of the anode is based on the boiling of the water. The spherical water reservoir is made of brass instead of glass to withstand the effect of the boiling. Boiling the water by the heated anode results in more effective heat extraction than by cooler water. Also the performance of the X-ray tube improves at the higher temperature.
X-ray tubes came in many shapes and sizes, made for hard and soft radiation and also for intra-cavitary irradiation. The tube most suited to the chosen treatment was mounted in the stand and connected with metal wires to the high voltage. The electric connections of the tube were supplied with eyelets on which the high voltage wires were hooked.