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.
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 crystalize an appreciable number of excited nuclei by conventional radiochemistry is at least a few seconds. To have the inversion persist, the lifetime of the excited state 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 occurs in a parent-doped converter where it generates Mössbauer radiation, which 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, 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.
- Balko, B.; Cohen, L.; Sparrow, D. A.; eds. (1989). Gamma-Ray Lasers. Pergamon. ISBN 978-0-08-037015-6 http://www.sciencedirect.com/science/book/9780080370156 Provides a definitive overview of the current status of gamma-ray lasers.
- Killus, J. (2006). "The gamma laser". Unintentional Irony. A review for laymen.
- Baldwin, G. C. (1979). "Bibliography of GRASER research" (PDF). Los Alamos Scientific Laboratory Report LA-7783-MS.
- Stevens, C. B. (1986). "A graser breakthrough at Los Alamos". EIR Science & Technology. 13 (43): 22–23.
- Ginzburg, V. L. (2003). "On superconductivity and superfluidity". The Nobel Prize in Physics 2003: 96–127.
- Baldwin, G. C.; Solem, J. C.; Gol'danskii, V. I. (1981). "Approaches to the development of gamma-ray lasers". Reviews of Modern Physics. 53: 687–744. doi:10.1103/revmodphys.53.687.
- Baldwin, G. C.; Solem, J. C. (1979). "On the direct pumping of gamma-ray lasers by neutron capture". Nuclear Science & Engineering. 72 (3): 290–292.
- Vali, V.; Vali, W. (1963). "Induced gamma y-ray emission". Proceedings of the IEEE. 51 (1): 182–184. doi:10.1109/proc.1963.1677.
- Letokhov, V. S. (1973). "On the problem of the nuclear-transition gamma-laser". Journal of Experimental and Theoretical Physics. 37 (5): 787–793.
- Kamenov, P.; Bonchev, T. (1975). "On the possibility of realizing a gamma laser with long-living isomer nuclei". Bolgarskaia Akademiia Nauk, Doklady. 28 (9): 1175–1177. Bibcode:1975BlDok..28.1175K.
- Il'inskii, Yu. A.; Khokhlov, R. V. (1976). "Possibility of creating a gamma-laser". Radiophysics and Quantum Electronics. 19 (6): 561–567. doi:10.1007/bf01043541.
- Baldwin, G. C. (1977). "On the feasibility of grasers". Proceedings of the Fourth Workshop on Laser Interaction and Related Plasma Phenomena, Troy, NY, November 8–12, 1976. Schwarz, H. J.; Hora, H.; eds. 4A: 249–257. doi:10.1007/978-1-4684-8103-7_13.
- Andreev, A. V.; Il'inskii, Yu. A.; Khokhlov, R. V. (1977). "Role of collective and induced processes in the generation of Mössbauer gamma radiation". Journal of Experimental and Theoretical Physics. 46 (4): 682–684.
- Hien, P. Z. (1970). "Spontaneous emission of gamma quanta by a system containing identical nuclei". Journal of Experimental and Theoretical Physics. 31 (1): 83–86.
- Gol'danskii, V. I.; Kagan, Yu. M. (1973). "Feasibility of the nuclear-transition gamma laser (Graser)". Soviet Physics Uspekhi. 16 (4): 563–565. doi:10.1070/pu1974v016n04abeh005305.
- Namiot, V. A. (1973). "Stimulated line narrowing and the Mössbauer effect for long-lived isomers". JETP Letters. 18 (6): 369–373.
- Andreev, A. V.; Il'inskii, Yu. A.; Khokhlov, R. V. (1974). "Narrowing of gamma resonance lines in crystals by continuous radio-frequency fields". Journal of Experimental and Theoretical Physics. 40 (5): 819–820.
- Baldwin, G. C. (1979). "Time-domain spectroscopy of recoilless gamma rays". Nuclear Instruments and Methods. 159 (2-3): 309–330. doi:10.1016/0029-554x(79)90656-6.
- Terhune, I. H.; Baldwin, G. C. (1965). "Nuclear superradiance in solids". Physical Review Letters. 14: 589–591. doi:10.1103/physrevlett.14.589.
- Baldwin, G. C. (1973). "Is there a high frequency limit to laser action?". Proceedings of the Third Workshop on Laser Interaction and Related Plasma Phenomena, Troy, NY, August 13–17, 1973. Schwarz, H. J.; H. Hora,H.; eds. 3B: 875–888. doi:10.1007/978-1-4684-8416-8_23.
- Andreev, A V.; Il'inskii, Yu. A. (1975). "Amplification in a gamma laser when the Bragg condition is satisfied". Journal of Experimental and Theoretical Physics. 41 (3): 403–405.
- Il'inskii, Yu. A.; Khokhlov, R. V. (1974). "On the possibility of observation of stimulated gamma radiation". Soviet Physics Uspekhi. 16 (4): 565–567. doi:10.1070/pu1974v016n04abeh005306.
- Baldwin, G. C.; Solem, J. C. (1997). "Recoilless gamma-ray lasers". Reviews of Modern Physics. 69 (4): 1085–1117. doi:10.1103/revmodphys.69.1085.
- Borrmann, G. (1941). "Über Extinktionsdiagramme der Röntgenstrahlen von Quarz". Zeitschrift für Physik. 42: 157–162.
- Kagan, Yu. M. (1974). "Use of the anomalous passage effect to obtain stimulated emission of gamma quanta in a crystal". JETP Letters. 20 (1): 11–12.
- Andreev, A. V.; Il'inskii, Yu. A. (1976). "Possible use of the Borrmann effect in the gamma laser". Journal of Experimental and Theoretical Physics. 43 (5): 893–896.
- Baldwin, G. C.; Solem, J. C. (1979). "Maximum density and capture rates of neutrons moderated from a pulsed source". Nuclear Science & Engineering. 72 (3): 281–289.
- Baldwin, G. C.; Solem, J. C. (1995). "Kinetics of neutron-burst pumped gamma-ray lasers". Laser Physics. 5 (2): 326–335.
- Gol'danskii, V. I.; Kagan, Yu.; Namiot, V. A. (1973). "Two-stage pumping of Mössbauer gamma-ray lasers". JETP Letters. 18 (1): 34–35.
- Gol'danskii, V. I.; Kagan, Yu. (1973). "The possibility of creating a nuclear gamma laser". Journal of Experimental and Theoretical Physics. 37 (1): 49.
- Baldwin, G. C.; Solem, J. C. (1980). "Two-stage pumping of three-level Mössbauer gamma-ray lasers". Journal of Applied Physics. 51: 2372–2380. doi:10.1063/1.328007.
- Baldwin, G. C. (1984). "Multistep pumping schemes for short-wave lasers". Proceedings of 6th International Workshop on Laser Interaction and Related Plasma Phenomena, Monterey, CA from October 25–29, 1982. Hora, H.; Miley, G. H.; eds. 6: 107–125. doi:10.1007/978-1-4615-7332-6_8.
- Solem, J. C.; Biedenharn, L. C. (1987). "Primer on coupling collective electronic oscillations to nuclei" (PDF). Los Alamos National Laboratory Report LA-10878.
- Biedeharn, L. C.; Baldwin, G. C.; Boer, K. (1986). "Nuclear excitation by laser driven coherent outer shell electron oscillations". Proceedings of the First International Laser Science Conference, Dallas, TX, November 18–22, 1985. Stwalley, W. C.; Lapp, M.; eds. 146: 52–53.
- Solem, J. C.; Biedenharn, L. C.; Rinker, G. A. (1987). "Calculation of harmonic radiation from atoms subjected to strong laser fields and the possibility of nuclear excitation". Journal of the Optical Society of America A. 4: P53. Bibcode:1987JOSAA...4...53S.
- Solem, J. C.; Biedenharn, L. C. (1988). "Laser coupling to nuclei via collective electronic oscillations: A simple heuristic model study". Journal of Quantitative Spectroscopy and Radiative Transfer. 40 (6): 707–712. doi:10.1016/0022-4073(88)90066-0.
- Solem, J. C. (1988). "Theorem relating spatial and temporal harmonics for nuclear interlevel transfer driven by collective electronic oscillation". Journal of Quantitative Spectroscopy and Radiative Transfer. 40 (6): 713–715. doi:10.1016/0022-4073(88)90067-2.