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Laser Physics

Prospects for coherently driven nuclear radiation by Coulomb excitation

S. A. Karamian, J. J. Carroll

Published Feb 1, 2007• 10 citations
Publisher

Abstract

Possible experiments are discussed in which Coulomb excitation of nuclear isomers would be followed by sequential energy release. The possibility of coherent Coulomb excitation of nuclei ensconced in a crystal by channeled relativistic heavy projectiles is considered. The phase shift between neighbor-nuclei excitations may be identical to the photon phase shift for emission in the forward direction. Thus, the elementary string of atoms may radiate coherently with emission of characteristics nuclear γ rays, and the intensity of the radiation would be increased due to the summation of amplitudes. Mössbauer conditions should be important for this new type of collective radiation, which could be promising in the context of the γ-lasing problem.

DOI
10.1134/s1054660x07020053

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arXiv:1010.5134•Physical Review B•Apr 4, 2011•10 citations
Possible experiments are discussed in which Coulomb excitation of nuclear isomers would be followed by sequential energy release. The possibility of coherent Coulomb excitation of nuclei ensconced in a crystal by channeled relativistic heavy projectiles is considered. The phase shift between neighbor-nuclei excitations may be identical to the photon phase shift for emission in the forward direction. Thus, the elementary string of atoms may radiate coherently with emission of characteristics nuclear γ rays, and the intensity of the radiation would be increased due to the summation of amplitudes. Mössbauer conditions should be important for this new type of collective radiation, which could be promising in the context of the γ-lasing problem.
This report results from a contract tasking Moscow State University as follows: Exploration of methods to control the radiative gamma- processes in nuclei is currently one of the most promising investigation directions of the modern Quantum Nucleonics - a new realm of contemporary Physics which spreads the basic conceptual ideas and methods of optical Quantum Electronics of atoms and molecules to the gamma-ray range and atomic nuclei and even anti-particles. One of the most important problems in this field is the generation of coherent gamma-radiation by a Gamma-ray Lasing (GRL) process which makes use of excited nuclei as an amplifying medium. The most likely medium for gamma-ray lasers is an ensemble of metastable nuclear isomers, Direct excitation of these isomeric states by electrons or X-ray photons will also create a deeper understanding of nuclear structure and properties.
There is a possibility that an isomer can be found which could be used to obtain a sufficient population inversion density to bring about nuclear lasing. If it can be realized, a gamma ray laser ("graser") will be a new source of radiation in the sim\\simsim 1A region of unexcelled spectral brightness and coherence, and can be expected to have revolutionary consequences for physics, biology, and chemistry. The result of our research is that the effects of crystallinity are of crucial importance in considerations regarding gamma-ray lasers, bringing in new elements to laser theory which could very well prove decisive in determining whether grasing can or cannot be achieved.\nCertain multi-beam Borrmann eigenmodes of the radiation field in a perfect crystal have much stronger coupling to the emitting nuclei, and much less photoabsorption, than modes in the amorphous case. These modes may be selectively fed by nuclear transitions of the proper multipolarity. A crystal containing such emitters would require orders of magnitude less population inversion density in order to lase than would an amorphous sample of similar material.\nMulti-beam Borrmann modes also have several applications to single-photon optics, particularly the problem of anomalous emission of a gamma\\gammagamma-ray by an internal source of multipolarity M1 or higher. While this effect can occur for a 2-beam Borrmann mode, even stronger anomalous emission is possible into multi-beam Borrmann modes. In addition, certain multi-beam Borrmann modes can anomalously transmit resonant radiation through a Mossbauer crystal. (This cannot occur for the 2-beam modes, except for the case of an E1 transition, since the Borrmann mode couples strongly to the nuclei and thus will be strongly absorbed at resonance.)
The re-emission of gamma -quanta in the forward direction by an ensemble of excited nuclei after abruptly changing the energy of the nuclear excitation is studied. A sublevel of the 14.4 keV excited state with a definite spin projection of 57 Fe nuclei in a magnetic 57 FeBO 3 crystal is selectively populated. The abrupt change of the energy of the excited nuclear state is achieved by fast (<or=5 ns) reversal of the hyperfine (HF) magnetic field direction at the nuclei. The nuclear target was a black absorber prior to HF field reversal. A short (about 10 ns) intense flash followed by beats of gamma -radiation intensity is detected with some delay after reversal. The delay of the flash and the beat period depend on the HF nuclear transition excited. The duration of the flash depends on the effective thickness of the nuclear target. The theory developed interprets the time evolution of the re-emission as a result of the interference of two main spectral components of gamma -radiation. The first one has the original frequency, which represents the primary radiation. The second one has a shifted frequency, which represents radiation coherently re-emitted in the forward direction from the nuclear sublevel with energy changed by the HF field reversal. The observations reveal the feasibility of inelastic coherent gamma -resonance scattering and demonstrate the enhancement of the radiative channel in the coherent re-emission of gamma -radiation in the forward direction.
Abstract The narrow nuclear resonances of Mössbauer isotopes constitute almost ideal two‐level systems to study cooperative effects in the interaction of x‐rays with matter. Embedding an ensemble of resonant Mössbauer nuclei into a planar cavity facilitates the excitation of superradiant oscillatory eigenmodes via pulses of synchrotron radiation. These modes exhibit a large collective Lamb shift that can be controlled via the excitation conditions and the cavity geometry. Cooperative emission in connection with the radiative coupling of two resonant ensembles in a cavity leads to electromagnetically induced transparency (EIT). This contribution gives a review on cooperative effects in nuclear resonant scattering of synchrotron radiation. Special emphasis is given to the basic principles of recent experiments that led to the observation of the collective Lamb shift and EIT with nuclear resonances in the regime of hard x‐rays.
The interaction between super-intense coherent x-ray light and nuclei is studied theoretically. One of the main difficulties in driving nuclear transitions arises from the very narrow nuclear excited state widths, which limit the coupling between lasers and nuclei. In the context of direct laser–nucleus interaction, we consider the nuclear width broadening that occurs when in solid targets, when the excitation caused by a single photon is shared by a large number of nuclei, forming a collective excited state. Our results show that cooperative effects mostly contribute with a modest increase to the nuclear excited state population except in the case of 57 Fe, where the enhancement can reach almost two orders of magnitude. Additionally, an update is given of previous estimates of the nuclear excited state population and signal photons for x-ray lasers interacting with solid-state and ion beam nuclear targets taking into account the experimental advances of x-ray coherent light sources. The presented values are an improvement by orders of magnitude and are encouraging as to the future prospects of nuclear quantum optics.
Coherent control of nuclear pumping in a three-level system driven by x-ray light is investigated. In single nuclei the pumping performance is determined by the branching ratio of the excited state populated by the x-ray pulse. Our results are based on the observation that in ensembles of nuclei, cooperative excitation and decay leads to a greatly modified nuclear dynamics which we characterize by a time-dependent cooperative branching ratio. We discuss prospects of steering the x-ray pumping by coherently controlling the cooperative decay. First, we study an ideal case with purely superradiant decay and perfect control of the cooperative emission. A numerical analysis of x-ray pumping in nuclear forward scattering with coherent control of the cooperative decay via externally applied magnetic fields is presented. Next, we provide an extended survey of nuclei suitable for our scheme, and propose proof-of-principle implementations already possible with typical M\"ossbauer nuclei such as 57Fe^{57}\mathrm{Fe}57Fe. Finally, we discuss the application of such control techniques to the population or depletion of long-lived nuclear states.