Step 7: Anode Current
While anode voltage controls the ‘hardness’ of the x-rays, the voltage applied to the tube’s heater gives independent control of the total x-ray flux. A hotter cathode boils off more electrons, reducing the impedance and allowing for a larger current to flow through the tub. One must be careful though, as too much power will damage the tube’s anode via excessive heating. Ideally, a medical x-ray generator would provide a short, high intensity burst of radiation to reduce the ‘shutter speed’ and overall dose absorbed by the patient.
Conveniently, these tubes usually come with some graphs to help one set parameters for the design of a machine; [image 1] displays the relation between heater voltage and anode current for the tube which I've chosen. From what the curve tells, one must apply 2.6V to the heater in order to allow a decent 3mA to flow through the tube’s anode. While 3mA might not sound like a lot of current, at 75,000V that is a respectable 225 watts of power! Assuming a typical 3% efficiency, this would equate to 6.7W of x-ray energy out.
When one thinks about how a 100W light-bulb emits on average 4 watts of visible light, it becomes quite clear that the tube will emit quite a hefty amount of radiation; certainly enough to expose a film.