X-ray Laser?

We know that there exist free electron X-ray lasers that have an electron moving past a rippled surface and emitting X-rays, as well as X-ray laser sources based on

plasmas such as the kinetic laser. However, an X-ray laser with a wavelength of about 1 A or 0.1 nanometer or less that can be operated on a tabletop would be convenient and would be able to resolve details down to nearly 1 wavelength. The uses in physics and medicine are expected to be many.

A very interesting tabletop device is the working monochromatic X-ray source shown in the illustration that emits very intense, narrow beams at the Cu 1.54 A characteristic emission line known as Kα₁. There is a special bimetal X-ray tube source of Cu-W that emits X-rays from both metals upon bombardment with high-energy electrons in the standard way. These X-rays exit the tube and then Bragg scatter in an external Cu crystal to produce a very narrow, intense beam of Cu Kα₁ X-rays. The first surprise is the enormous line intensity at a single wavelength, and the second surprise is that no Cu Kα₂ X-rays appear in the output from the external crystal. How does the external crystal affect the X-ray beam? Is this device an X-ray laser or a super-radiant X-ray source?

Answer

The mechanism for the intense X-ray source appears to be the following according to K. Das Gupta, the originator of this unique X-ray source. The W X-rays from the Cu-W X-ray tube knock out K shell electrons and others in the Cu atoms in the external Cu crystal to produce a temporary (about 10^–15 second) population inversion, and the Cu X-rays coming simultaneously from the same tube then stimulate transitions in these Cu atoms to produce the Cu Kα₁ line at the Bragg angle to the Cu(111) atomic planes. This mechanism is very selective, the line being so narrow and intense and the process being so efficient that one does not detect any of the competing Cu Kα₂ emission to the available 1s state. The single-frequency intense X-ray line has been used to analyze materials in minutes that formerly required hours to days to accumulate enough data.

Whether the population inversion for the 2p–1s transition in the external Cu atoms actually occurs is unknown. The emission X-ray line is uncharacteristically narrow and intense, and the absence of the other competing line indicates that whatever the selection process is doing must be very efficient. Other element sources such as nickel, based on the same mechanism, also have been made.