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typical rotating anode X-ray tube A considerable amount of heat is generated in the focal spot (the area where the beam of electrons coming from the cathode strike to) of a stationary anode. Rather, a rotating anode lets the electron beam sweep a larger area of the anode, thus redeeming the advantage of a higher intensity of emitted radiation ...
With rotating envelope tubes, the entire vacuum tube rotates with respect to the anode axis, versus rotating anode tubes, in which the target disk rotates inside a stationary vacuum tube. The target cools by conduction rather than radiation. Heat storage is less important, and waiting times are eliminated. [1]
The simplest and cheapest variety of sealed X-ray tube has a stationary anode (the Crookes tube) and runs with ~2 kW of electron beam power. The more expensive variety has a rotating-anode type source that runs with ~14 kW of e-beam power.
In EBCT, the X-ray tube itself is large and stationary, and partially surrounds the imaging circle. Rather than moving the tube itself, electron-beam focal point (and hence the X-ray source point) is rapidly swept along a tungsten anode in the tube, tracing a large circular arc on its inner surface.
In general, an X-ray's beam intensity is not uniform. When it focuses to a target, a conical shape appears (divergent beam). The intensity of the beam from the positive anode side is lower than the intensity from the negative cathode side because the photons created when the electrons strike the target have a longer way to travel through the rotating target on the anode side.
In X-ray tubes, the heel effect or, more precisely, the anode heel effect is a variation of the intensity of X-rays emitted by the anode depending on the direction of emission along the anode-cathode axis. X-rays emitted toward the anode are less intense than those emitted perpendicular to the cathode–anode axis or toward the cathode.
A beam tetrode, sometimes called a beam power tube, is a type of vacuum tube or thermionic valve that has two grids and forms the electron stream from the cathode into multiple partially collimated beams to produce a low potential space charge region between the anode and screen grid to return anode secondary emission electrons to the anode ...
Crookes X-ray tube from around 1910 Another Crookes x-ray tube. The device attached to the neck of the tube (right) is an "osmotic softener". When the voltage applied to a Crookes tube is high enough, around 5,000 volts or greater, [16] it can accelerate the electrons to a high enough velocity to create X-rays when they hit the anode or the glass wall of the tube.