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Australian Journal of Physics

Influence of Giant Nuclear-spin Polarisation on Resonant Gamma-ray Absorption and Emission

R. N. Shakhmuratov

Published May 29, 1998• 0 citations
Publisher

Abstract

We propose a new scheme of gamma-quanta amplification without inversion. Laser pumping of electron states creates giant nuclear-spin polarisation via the hyperfine interaction. This results in extreme cooling of the ground-state nuclear spin in a projection which does not absorb both laser pump and gamma-quanta according to selection rules for these transitions. Induced emission from the nuclear excited state is not influenced by the pump. Therefore gamma-quanta travelling inside the pump beam have an opportunity to induce stimulated emission without subsequent quenching by ground state nuclei.

DOI
10.1071/p97057

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We propose a new scheme of gamma-quanta amplification without inversion. Laser pumping of electron states creates giant nuclear-spin polarisation via the hyperfine interaction. This results in extreme cooling of the ground-state nuclear spin in a projection which does not absorb both laser pump and gamma-quanta according to selection rules for these transitions. Induced emission from the nuclear excited state is not influenced by the pump. Therefore gamma-quanta travelling inside the pump beam have an opportunity to induce stimulated emission without subsequent quenching by ground state nuclei.
We discuss a possibility for obtaining the self-amplified spontaneous emission of gamma radiation in an active medium with hidden population inversion of nuclear states. Hidden inversion appears in cooled nuclear ensembles due to assistance of the nuclear recoil in emission and absorption of gamma quanta. Establishing a hidden inversion allows an amplification of gamma radiation to occur in a particular frequency range without excess of the number of excited nuclei over the number of unexcited nuclei. We formulate a set of requirements to the pumping radiation and to the amplifying nuclear medium crucial to the development of gamma-ray lasing.
The features of the pump dynamics of isomeric nuclei excited by X-rays of a repetitively pulsed relativistic electron beam followed by the production of a medium with the negative absorption of gamma quanta are analysed. In the extended nuclear medium, the pump excites a travelling hidden-population-inversion wave with the anisotropic gamma amplification, which becomes positive in the case of the excess over the critical pump parameter equal to the product of the peak spectral power density of the X-ray source and the relative duration of an ultrashort relativistic electron bunch. In the alternative scheme with orthogonal directions of pumping X-rays and a flux of amplified gamma quanta, the absence of the amplification anisotropy opens up the possibility for constructing a standard two-mirror resonator with Bragg single-crystal reflectors. The critical peak value of the spectral pump power density is compared with the known characteristics of relativistic-electron X-ray sources by examples of some nuclides.
Abstract Nuclear quantum optics deals with the resonant interaction of gamma‐radiation and nuclei, generally incorporated in a solid‐state material. Single gamma‐rays interact with an ensemble of nuclei. The properties of the resonantly forward‐scattered gamma‐radiation depend on the details of the interaction. An external modification of the nuclear energies will influence the interactions, and hence the properties of the radiation. Several techniques are proposed to reduce the resonant absorption of the gamma‐rays, creating a nuclear medium that is partially transparent: nuclear‐level‐mixing induced transparency, modification of nuclear levels by means of an optical laser, phase‐shift‐induced transparency. Interference is crucial in all this. The partial transparency could be exploited to study special effects such as slow gamma‐radiation and gain without population inversion. It could also be of importance for the realization of a gamma‐ray laser.
Summary form only given. The gamma-ray laser dilemma states that incoherent pump required to produce a population inversion would unavoidable destroy the conditions of the Mossbauer and Borrmann effects which are crucial for realization of the net gain. For four decades this dilemma prevailed all over the researches on gamma-ray lasers, making hopeless any attempt to approach a problem of gamma-ray laser. Recently it was shown that laser driving of bounded electronic transitions in atoms provides efficient manipulation by the Mossbauer spectra of nuclear transitions. Under certain conditions it leads to suppression of resonant absorption by 3-5 orders of magnitude reducing it up to the level of off-resonant losses in the host matrix. In turn, suppression of the resonant absorption reduces by the same amount the requirement for incoherent pump and accordingly heating of the active sample. This allows to pump active nuclei in the host lattice without destroying the conditions of both Mossbauer and Borrmann effects. Moreover it allows to pump resonantly at the operating transition that increases additionally the efficiency of the incoherent pump by 1-2 orders of magnitude. All together gamma-ray laser schemes involving laser driving of electronic transitions reduce the laser threshold by 4-7 orders of magnitude and hence suggest a completely new path for solution of the gamma-ray laser problem.