X-ray Production
- The x-ray imaging system is remarkable. It conveys to the x-ray tube target
an enormous number of electrons at a precisely controlled kinetic energy.
- At 100 mA, for example, 6 × 1017 electrons travel from the cathode to the anode of the x-ray tube every second. In an x-ray imaging system operating
at 70 kVp, each electron arrives at the target with a maximum kinetic energy
of 70 keV.
- Electrons, called projectile electrons, traveling from cathode to anode
constitute the x-ray tube current. When these projectile electrons hit the
heavy metal atoms of the x-ray tube target, they transfer their kinetic energy
to the target atom. These interactions occur within a very small depth of
penetration into the target. Where the projectile electrons slow down and
finally come nearly to rest.
- The projectile electron interacts with the orbital electrons or the nuclear field
of target atoms. These interactions result in the conversion of electron
kinetic energy into thermal energy (heat) and electromagnetic energy in the
form of infrared radiation (also heat) and x-rays.
- Approximately 99% of the kinetic energy of projectile electrons is converted to heat.
- The efficiency of x-ray production increases with increasing kVp. At 60 kVp,
only 0.5% of the electron kinetic energy is converted to x-rays. At 100 kVp,
approximately 1% is converted to x-rays, and at 20 MV, 70% is converted.
⭐. Characteristic Radiation
- If the projectile electron interacts with an inner-shell electron of the target
atom, characteristic x-rays can be produced.
- Characteristic x-rays are emitted when an outer-shell electron fills an innershell void.
- Although many characteristic x-rays can be produced, these can be produced only at specific energies, equal to the differences in electron-binding energies for the various electron transitions. Except for K x-rays, all of the characteristic x-rays have very low energy. The L x-rays, with approximately 12 keV of energy, penetrate only a few centimeters into soft tissue. Consequently, they are useless as diagnostic x-rays, as are all the other low-energy characteristic x-rays.
- Only the K-characteristic x-rays of tungsten are useful for imaging.
⭐. Bremsstrahlung Radiation
- Bremsstrahlung x-rays or “slowed-down radiation.” are produced when aprojectile electron is slowed by the nuclear field of a target atom nucleus.
- A projectile electron can lose any amount of its kinetic energy in an interaction with the nucleus of a target atom, and the bremsstrahlung x-ray associated with the loss can take on corresponding values. For example,when an x-ray imaging system is operated at 70 kVp, projectile electrons
have kinetic energies up to 70 keV.
- In the diagnostic range, most x-rays are bremsstrahlung x-rays.
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