7.3.2 X-ray Production
Reference: Weidner and Sells
Created October 19, 1995
184.108.40.206 Review of X-ray Definition
Recall from our initial physics review that x-rays are not defined in terms of their energy but rather in terms of their origin. To review, there are two sources of x-rays:
It turns out that for x-ray imaging both of these x-ray types comprise the typical x-ray spectrum illuminating a patient.
- Characteristic x-rays from inner shell vacancies
- Slowing or braking - Bremsstrahlung of electrons near the nucleus
220.127.116.11 X-ray Tube Basics
Although more properly discussed in the instrumentation course, we will introduce the components of the typical x-ray tube here. The following is a schematic based on Hendee (p. 68):
In this drawing the following points should be emphasized in x-ray production:
It is of interest to note that this reaction has been of experimental interest for the treatment of tumors by loading the tumor with boron and putting the patient in a neutron flux. The short range alphas have been shown to kill tumor cells.
- The tube must be evacuated in order to prevent the electrons from slowing down in the air
- The electrons are "boiled off" the filament by thermionic emission. A high AC current is used for this purpose.
- The electrons are accelerated down to the tungsten anode by very high voltages, up to 150,000 V. Special transformers called "x-ray generators" are used for the purpose of generating this voltage.
- The anode is made to rotate to dissipate the heat throughout the entire disk.
- when the electrons impinge on the tungsten, they undergo Bremsstrahlung, in which a broad spectrum, heavily weighted at the low energy and extending up to the kinetic energy of the electron (determined by the accelerating voltage, or kVp) is produced.
They also ionize the tungsten atoms, which then emit characteristic radiation in the form of K-alpha, K-beta, and L radiation.
- A filter is used to remove the excess low energy Bremsstrahlung photons, which would not emerge from the patient and only contribute to patient dose.
18.104.22.168 Typical Spectrum
The resulting x-ray spectrum is adapted from Hendee:
It shows the effects of the filter in removing the low energy photons preferentially, and the presence of the K-alpha and K-beta characteristic x-rays.
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Douglas J. Wagenaar, Ph.D., email@example.com