Gamma-ray laser

A gamma-ray laser, or graser, would produce coherent gamma rays, just as an ordinary laser produces coherent rays of visible light. It would be powered by nuclear transitions from a nuclear isomer. To construct a gamma ray laser, one must identify a suitable isomer, purify it, create a crystal from the purified material, and assemble a configuration that leads to the emission of a coherent gamma-ray beam. Because the wave length of gamma rays are shorter than that of x-rays, such a device, which has yet to be realized, would potentially be very useful in applications such as high-resolution imaging, surgery, and communications, as well as high-intensity applications.

Research to solve the difficulties inherent in the construction of a practical gamma-ray laser continues. In his 2003 Nobel lecture, Vitaly Ginzburg cited the gamma-ray laser as one of the thirty most important problems in physics.

The search for a gamma-ray laser is interdisciplinary, including quantum mechanics, nuclear and optical spectroscopy, chemistry, solid-state physics, metallurgy, as well as the generation, moderation, and interaction of neutrons, and involves specialized knowledge and research in all these fields. The subject involves both basic science and engineering technology.

The problem of getting a sufficient concentration of resonant excited (isomeric) nuclear states for collective stimulated emission to occur turns on the broadening of the gamma-ray spectral line. Of the two forms of broadening, homogeneous broadening is simply the result of the lifetime of the isomeric state: the shorter the lifetime, the more broadened the line.Inhomogeneous broadening is all the mechanisms by which the homogeneously broadened line is spread over the spectrum.

The most familiar inhomogeneous broadening is Doppler recoil broadening from thermal motion of molecules in the solid containing the excited isomer and recoil from gamma-ray emission, in which the emission spectrum is both shifted and broadened. Isomers in solids can emit a sharp component superimposed on the Doppler-broadened background; this is called the Mössbauer effect. This recoilless radiation exhibits a sharp line on top of the Doppler-broadened background that is only slightly shifted from the center of the background.

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