Characteristic X-Rays

Characteristic x-rays are emitted from heavy elements when their electrons make transitions between the lower atomic energy levels. The characteristic x-ray emission which is shown as two sharp peaks in the illustration at left occur when vacancies are produced in the n=1 or K-shell of the atom and electrons drop down from above to fill the gap. The x-rays produced by transitions from the n=2 to n=1 levels are called K-alpha x-rays, and those for the n=3→1 transition are called K-beta x-rays. 特征X射线是在重元素的电子在较低的原子能级之间跃迁时发射出来的。图中左侧示意图中显示的两个尖锐峰对应于原子中n=1或K壳层中出现空位时，电子从更高能级跃迁填补空位所产生的特征X射线。由n=2到n=1能级跃迁产生的X射线称为K-alpha X射线，而由n=3到n=1跃迁产生的则称为K-beta X射线。 Transitions to the n=2 or L-shell are designated as L x-rays (n=3→2 is L-alpha, n=4→2 is L-beta, etc. ). The continuous distribution of x-rays which forms the base for the two sharp peaks at left is called "bremsstrahlung" radiation. 跃迁至n=2或L壳层的辐射被称为L射线（n=3→2为L-alpha，n=4→2为L-beta，依此类推）。形成左侧两个明显峰的连续射线分布称为‘制动辐射’。

X-ray production typically involves bombarding a metal target in an x-ray tube with high speed electrons which have been accelerated by tens to hundreds of kilovolts of potential. The bombarding electrons can eject electrons from the inner shells of the atoms of the metal target. Those vacancies will be quickly filled by electrons dropping down from higher levels, emitting x-rays with sharply defined frequencies associated with the difference between the atomic energy levels of the target atoms.

The frequencies of the characteristic x-rays can be predicted from the Bohr model. Moseley measured the frequencies of the characteristic x-rays from a large fraction of the elements of the periodic table and produced a plot of them which is now called a "Moseley plot".

Characteristic x-rays are used for the investigation of crystal structure by x-ray diffraction. Crystal lattice dimensions may be determined with the use of Bragg's law in a Bragg spectrometer.

Bremsstrahlung X-Rays

"Bremsstrahlung" means "braking radiation" and is retained from the original German to describe the radiation which is emitted when electrons are decelerated or "braked" when they are fired at a metal target. Accelerated charges give off electromagnetic radiation, and when the energy of the bombarding electrons is high enough, that radiation is in the x-ray region of the electromagnetic spectrum. It is characterized by a continuous distribution of radiation which becomes more intense and shifts toward higher frequencies when the energy of the bombarding electrons is increased. The curves above are from the 1918 data of Ulrey, who bombarded tungsten targets with electrons of four different energies.

The bombarding electrons can also eject electrons from the inner shells of the atoms of the metal target, and the quick filling of those vacancies by electrons dropping down from higher levels gives rise to sharply defined characteristic x-rays.