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Journal of Physics Condensed Matter

Coherent re-emission of gamma -quanta in the forward direction after a stepwise change of the energy of nuclear excitation

Yuri V. Shvyd'ko, S. L. Popov, G. V. Smirnov

Published Mar 8, 1993• 29 citations
Publisher

Abstract

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.

DOI
10.1088/0953-8984/5/10/013

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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.
The decay rate of 57Fe{}^{57}\mathrm{Fe}57Fe nuclei in an 57FeBO3{}^{57}{\mathrm{FeBO}}_{3}57FeBO3​ crystal excited by 14.4 keV synchrotron radiation pulses was controlled by switching the direction of the crystal magnetization. Abrupt switching some nanoseconds after excitation suppresses the coherent nuclear decay. Switching back at later times restores it, starting with an intense radiation spike. The enhanced delayed reemission is due to the release of the energy stored during the period of suppression. Suppression and restoration originate from drastic changes of the nuclear states and of the interference within the nuclear transitions.
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.
The possibility to control the coherent decay of resonant excitations in nuclear forward scattering is investigated. By changing abruptly the direction of the nuclear hyperfine magnetic field, the coherent scattering of photons can be manipulated and even completely suppressed via quantum interference effects between the nuclear transition currents. The efficiency of the coherent decay suppression and the dependence of the scattered light polarization on the specific switching parameters is analyzed in detail. Using a sophisticated magnetic switching sequence involving four rotations of the hyperfine magnetic field, two correlated coherent decay pulses with different polarizations can be generated out of one excitation, providing single-photon entanglement in the keV regime. The verification of the generated entanglement by testing a single-particle version of Bell's inequality in an X-ray optics experimental setup is put forward.
Reversible coherent transients induced by a single photon in an ensemble of resonant particles are considered. It can be shown that a phase shift of a single photon wave packet produces its re-emission by these particles. As an example, the resonant interaction of a single gamma-photon with Mössbauer nuclei is theoretically analyzed. This example has interesting applications in such areas as information transmission and measuring of sub-angstrom vibrations of thin films for creating of sub-angstrom etalon displacements, which can be used for sensor calibration. We show that coherent transients that are governed by the time evolution of a population of nuclear excited states can be detected not only in the transmission of photons through resonant absorbers, but also directly in the time evolution of a population of nuclear excited states. These states experience radiative decay and also emit conversion electrons. The detection of conversion electrons provides information about the excited state population of nuclei located nearby the surface of the radiation incidence on the absorber. This is because of short mean free path (or escape depth) of these electrons in the bulk.
Classical phase-modulation theory is applied to describe the interference of the recoil-free gamma-radiaton fields emitted by a M\"ossbauer source and a resonant absorber in forward-scattering geometry. During the exponential decay of the excited state in the source the resonant absorber develops and emits a coherent field, which interferes destructively with the source field. A stepwise phase change of the source field relative to the absorber field is introduced by a fast mechanical displacement of the source. As a result, an intense pulse of gamma rays is observed in the time dependence of the resonantly filtered radiation. This coherent transient phenomenon is called a gamma echo. The time reference of the gamma-echo experiment is the moment of formation of the excited state in the source. This is in contrast to earlier transient M\"ossbauer experiments in which the measurements were synchronized to the phase of the modulation and averaging occurred over all possible times of formation of the excited state. By choosing a suitable shape for the phase-modulation function in the echo experiment, short pulses of gamma radiation with desired duration and amplitude can be generated within a time frame defined by the lifetime of the excited nuclear state. Here, a detailed description of the experimental and theoretical aspects of stepwise phase modulation in a coincidence scheme is given. Theoretical results are presented in a general form, taking into account the experimental nonidealities. The properties of the mechanical drive system are discussed in detail. Gamma-echo data are presented. Good agreement between the theoretical calculations and the experimental data has been established.
The so-called gamma-echo effect has been observed experimentally and analyzed using the semiclassical optical theory. Here the effect is reinterpreted using a new 1D quantum mechanical model. This leads to a different interpretation of the effect as a π phase-shift induced transparency. In the basic time-differential Mössbauer spectroscopic technique the forward-scattered recoil-free radiation is observed, in delayed coincidence, after passing through a nuclear-resonant absorber. The effect in question is produced most efficiently when the source of recoil-free radiation is moved abruptly causing a π phase shift of the source radiation during its radiative lifetime. Using the 1D model the effect is seen to arise from the constructive interference between the source radiation at a later time, and the radiation coming from the absorber excited at an earlier time. The exact form of the source modulation and the nuclear-resonant thickness of the resonant absorber determines the shape of the time-differential resonant gamma ray transmission spectrum. Numerical results are given using the familiar 57Fe recoil-free resonant transition. The π phase-shift-induced transparency allows the resonant gamma radiation, incident on the resonant absorber, to be transmitted through the absorber without appreciable attenuation.