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 γ or \gamma), is a penetrating form of electromagnetic radiation arising from the radioactive decay of atomic nuclei. It consists of the shortest wavelength electromagnetic waves, typically ...

, 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" is an acronym for "light amplification by stimulated emission of radiation". The fir ...

produces

coherent Coherence, coherency, or coherent may refer to the following: Physics * Coherence (physics), an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference * Coherence (units of measurement), a deri ...

rays of visible light. Potential applications for gamma-ray lasers include medical imaging, 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 a fundamental theory in physics that provides a description of the physical properties of nature at the scale of atoms and subatomic particles. It is the foundation of all quantum physics including quantum chemistry, ...

nuclear Nuclear may refer to: Physics Relating to the nucleus of the atom: * Nuclear engineering *Nuclear physics *Nuclear power *Nuclear reactor *Nuclear weapon *Nuclear medicine *Radiation therapy *Nuclear warfare Mathematics *Nuclear space *Nuclear ...

and

optical spectroscopy Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wav ...

chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...

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

, 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 sc ...

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

neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beh ...

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

engineering Engineering is the use of scientific method, scientific principles to design and build machines, structures, and other items, including bridges, tunnels, roads, vehicles, and buildings. The discipline of engineering encompasses a broad rang ...

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 th ...

to occur turns on the broadening of the gamma-ray

spectral line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...

. 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 of frequencies of electromagnetic radiation emitted due to an electron making a atomic electron transition, transition from a high energy state to a lower energy st ...

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 The Mössbauer effect, or recoilless nuclear resonance fluorescence, is a physical phenomenon discovered by Rudolf Mössbauer in 1958. It involves the resonant and recoil-free emission and absorption of gamma radiation by atomic nuclei bound in a ...

. 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 Gain or GAIN may refer to: Science and technology * Gain (electronics), an electronics and signal processing term * Antenna gain * Gain (laser), the amplification involved in laser emission * Gain (projection screens) * Information gain in de ...

. 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 The Borrmann effect (or Borrmann–Campbell effect after Gerhard Borrmann and Herbert N. Campbell) is the anomalous increase in the intensity of X-rays transmitted through a crystal when it is being set up for Bragg reflection. 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 Radiochemistry is the chemistry of radioactive materials, where radioactive isotopes of elements are used to study the properties and chemical reactions of non-radioactive isotopes (often within radiochemistry the absence of radioactivity leads to ...

is at least a few seconds. To ensure the inversion persists, the lifetime of the

excited state In quantum mechanics, an excited state of a system (such as an atom, molecule or nucleus) is any quantum state of the system that has a higher energy than the ground state (that is, more energy than the absolute minimum). Excitation refers to a ...

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, ...

occurs in a parent-doped converter, where it generates Mössbauer radiation that is then absorbed by

ground-state The ground state of a quantum mechanics, quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than t ...

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. 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