A gamma-ray laser, or graser, is a hypothetical device that would produce coherent

gamma rays A gamma ray, also known as gamma radiation (symbol ), is a penetrating form of electromagnetic radiation arising from high energy interactions like the radioactive decay of atomic nuclei or astronomical events like solar flares. It consists o ...

, just as an ordinary

laser A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word ''laser'' originated as an acronym for light amplification by stimulated emission of radi ...

produces coherent rays of visible light. Potential applications for gamma-ray lasers include

medical imaging Medical imaging is the technique and process of imaging the interior of a body for clinical analysis and medical intervention, as well as visual representation of the function of some organs or tissues (physiology). Medical imaging seeks to revea ...

, spacecraft propulsion, and cancer treatment. In his 2003 Nobel lecture, Vitaly Ginzburg cited the gamma-ray laser as one of the 30 most important problems in physics. The effort to construct a practical gamma-ray laser is interdisciplinary, encompassing

quantum mechanics Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Addison-Wesley, 1989, It is ...

, nuclear and optical spectroscopy,

chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a physical science within the natural sciences that studies the chemical elements that make up matter and chemical compound, compounds made of atoms, molecules a ...

solid-state physics Solid-state physics is the study of rigid matter, or solids, through methods such as solid-state chemistry, quantum mechanics, crystallography, electromagnetism, and metallurgy. It is the largest branch of condensed matter physics. Solid-state phy ...

, and

metallurgy Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys. Metallurgy encompasses both the ...

—as well as the generation, moderation, and interaction of

neutron The neutron is a subatomic particle, symbol or , that has no electric charge, and a mass slightly greater than that of a proton. The Discovery of the neutron, neutron was discovered by James Chadwick in 1932, leading to the discovery of nucle ...

s—and involves specialized knowledge and research in all these fields. The subject involves both basic science and

engineering Engineering is the practice of using natural science, mathematics, and the engineering design process to Problem solving#Engineering, solve problems within technology, increase efficiency and productivity, and improve Systems engineering, s ...

technology.

Research

The problem of obtaining a sufficient concentration of resonant excited (isomeric) nuclear states for collective

stimulated emission Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level. The liberated energy transfers to ...

to occur turns on the broadening of the gamma-ray

spectral line A spectral line is a weaker or stronger region in an otherwise uniform and continuous spectrum. It may result from emission (electromagnetic radiation), emission or absorption (electromagnetic radiation), absorption of light in a narrow frequency ...

. Of the two forms of broadening, ''homogeneous'' broadening is the result of the lifetime of the isomeric state: the shorter the lifetime, the more broadened the line. ''Inhomogeneous'' broadening comprises all 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 The emission spectrum of a chemical element or chemical compound is the Spectrum (physical sciences), spectrum of frequencies of electromagnetic radiation emitted due to electrons making a atomic electron transition, transition from a high energ ...

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. With the inhomogeneous background removed, and a sharp line, it would seem that we have the conditions for gain. But other difficulties that would degrade gain are unexcited states that would resonantly absorb the radiation, opaque impurities, and loss in propagation through the crystal in which the active nuclei are embedded. Much of the latter can be overcome by clever matrix crystal alignment to exploit the transparency provided by the Borrmann effect. Another difficulty, the ''graser dilemma'', is that properties that should enable gain and those that would permit sufficient nuclear inversion density seem incompatible. The time required to activate, separate, concentrate, and crystallize an appreciable number of excited nuclei by conventional radiochemistry is at least a few seconds. To ensure the inversion persists, the lifetime of the

excited state In quantum mechanics Quantum mechanics is the fundamental physical Scientific theory, theory that describes the behavior of matter and of light; its unusual characteristics typically occur at and below the scale of atoms. Reprinted, Add ...

must be considerably longer. Furthermore, the heating that would result from neutron-pumping the inversion ''in situ'' seems incompatible with maintaining the Mössbauer effect, although there are still avenues to explore. Heating may be reduced by two-stage neutron-gamma pumping, in which

neutron capture Neutron capture is a nuclear reaction in which an atomic nucleus and one or more neutrons collide and merge to form a heavier nucleus. Since neutrons have no electric charge, they can enter a nucleus more easily than positively charged protons, wh ...

occurs in a parent-doped converter, where it generates Mössbauer radiation that is then absorbed by ground-state nuclei in the graser. Two-stage pumping of multiple levels offers multiple advantages. Another approach is to use nuclear transitions driven by collective electron oscillations. The scheme would employ a triad of isomeric states: a long-lived storage state, in addition to an upper and lower lasing state. The storage state would be energetically close to the short-lived upper lasing state but separated by a forbidden transition involving one quantum unit of spin

angular momentum Angular momentum (sometimes called moment of momentum or rotational momentum) is the rotational analog of Momentum, linear momentum. It is an important physical quantity because it is a Conservation law, conserved quantity – the total ang ...

. The graser would be enabled by a very intense optical laser to slosh the electron cloud back and forth and saturate the forbidden transition in the near field of the cloud. The population of the storage state would then be quickly equalized with the upper lasing state whose transition to the lower lasing state would be both spontaneous and stimulated by resonant gamma radiation. A "complete" chart of nuclides likely contains a very large number of isomeric states, and the existence of such a triad seems likely, but it has yet to be found. Nonlinearities can result in both spatial and temporal harmonics in the near field at the nucleus, opening the range of possibilities for rapid transfer from the storage state to the upper lasing state using other kinds of triads involving transition energies at multiples of the optical laser quantum energy and at higher multipolarities.

Research

See also

References

Further reading