A New Way of Looking at Nuclear Resonant Scattering

A new model for nuclear-resonant scattering of gamma radiation from resonant matter has been developed and is summarized here. This “coherent-path” model has lead to closed-form, finite-sum solutions for radiation scattered in the forward direction. The solution provides a unified microscopic picture of nuclear-resonant scattering processes. The resonant absorber or scatterer is modeled as a one-dimensional chain of “effective” nuclei or “effective” planes. The solution is interpreted as showing that the resonant radiation undergoes sequential scattering from one absorber “nucleus” or “plane” to another before reaching the detector. For recoil-free processes the various “paths” to the detector contribute coherently. The solution for this case gives calculated results that are indistinguishable from those using the classical optical model approach, although the forms of the solutions are completely different. The coherent-path model shows that the “speed-up” and “dynamical beating” effects are primarily a consequence of the fact that the single “effective” nuclear scattering processes are 180° out of phase with the incident radiation while the double nuclear scattering processes are in phase with the incident radiation. All multiple scattering paths are, and must be, included. The model can also treat the incoherent processes, i.e., processes involving gamma emission with recoil or conversion-electron emission. The source of the resonant gamma radiation can be from a radioactive source or from synchrotron radiation: both cases are treated. The model is used to explain and understand the results when each of the following experimental procedures is applied: time-differential Mössbauer spectroscopy, time-differential synchrotron radiation spectroscopy, enhanced-resolution resonant-detector Mössbauer spectroscopy, and the “gamma echo”.     

Nuclear resonant scattering using synchrotron radiation

A one-dimensional quantum model for nuclear resonant scattering using synchrotron radiation has been developed. This model gives a clear physical interpretation of the most prominent features of the coherent forward scattering process namely, the “speed-up” and “dynamical beat” effects. The form of the solution, for the time-dependent forward scattered intensity of the resonant radiation from the resonant medium after synchrotron radiation excitation, is a finite series. This unique solution can be interpreted in terms of a summation over all multiple forward scattering paths the radiation takes in reaching the detector. The resonant medium is represented by a linear chain of N effective resonant nuclei. The analysis starts from a coupled set of quantum mechanical equations for the relevant amplitudes in frequency space. Transformation to the time domain gives an analytical expression for the forward scattered intensity. The contribution of every order of the multiple scattering processes from the N effective nuclei appears naturally. The expression gives a clear physical understanding of all relevant aspects of resonant forward nuclear scattering. Furthermore, the present formalism allows the consideration of incoherent processes. This permits the study of processes in which there is gamma emission with recoil or emission of internal-conversion electrons. This revised version was published online in August 2006 with corrections to the Cover Date.     

Radiative process in strong magnetic fields

Abstract

The behavior of electromagnetic processes in strong magnetic fields is currently of great interest in high-energy astrophysics. Observations of neutron stars indicate that magnetic fields larger than 10¹² Gauss exist in nature. In fields this strong, where electrons behave much as if they were in bound atomic states, familiar processes undergo profound changes and exotic processes become important. Strong magnetic fields affect the physics in several fundamental ways: energies prependicular to the field are quantized, transverse momentum is not conserved and electron/positron spin is important. The relaxation of transverse mometum conservation allows first order processes and their inverses: one-photon pair production and annihilation, synchrotron/cyclotron radiation and absorption, which are kinematically forbidden under field-free conditions. The first two are essentially quantum-mechanical and hence significant only in fields whose strength approaches the critical field, B _cr = 4.414 × 10¹³ Gauss. One-photon pair production is likely to be the dominant source of e ⁺ -e ^? pairs in fields exceeding 10¹² Gauss. While synchrotron radiation and absorption are observable as classical electromagnetic processes in weak fields, they are considerably different in high fields, where the classical synchrotron radiation formulae can violate conservation of energy, and predict too large an emissivity and electron energy loss rate. The second-order processes: two-photon pair production and annihilation and Compton Scattering, are also modified in strong fields. The discreteness of e ⁺ - e^? pair states causes resonant behavior in the cross sections and decreases the second-order rates from their free-space values. These processes play an important role in modelling high energy emission from pulsars and gamma-ray bursts.     

The inelastic channel in time-domain Mössbauerspectroscopy

Time-domain Mössbauer spectroscopy has been interpreted by Hamermesh using a classical optical model. One of the most interesting aspects of the experiments is the observation of a “speed-up” effect. This speed-up effect can be observed by measuring the gamma radiation coming from the source after transmission through a nuclear-resonant “filter”, i.e., the elastic channel, using the delayed-coincidence time-to-amplitude conversion method. This time-domain speed-up effect occurs in the coherent forward scattering of the nuclear-resonant gamma radiation. Time-domain nuclear-resonant forward scattering results observing the inelastic channel, i.e., the X-ray following internal conversion, using a radioactive source have not been obtained previously. Such results are presented for the radioactive-source case using both ⁵⁷Fe and ⁷³Ge. These two isotopes were chosen because of the differences in the values of the internal conversion coefficients α, i.e., α is 8 and 1310, respectively, for the two cases. In each case experimental data, using both the elastic channel and the inelastic channel, are given. Commercial sources and absorbers of iron-in-rhodium foils were used for the ⁵⁷Fe experiments. For the ⁷³Ge experiments, we prepared our own sources by electroplating ⁷³As onto Ge single crystals. The Ge absorber was the enriched ⁷³Ge single crystal originally prepared by Pfeiffer. Fits to the experimental elastic-channel data are given using the classical optical model. Our preliminary experimental results using the inelastic channel are inconclusive.     

The Coherent-Path Model for Nuclear Resonant Gamma-Ray Scattering: An Overview

Hyperfine Interactions - A “coherent-path” model for nuclear-resonant scattering of gamma radiation from resonant matter has been developed and is summarized here. The solution provides...     

Waves in resonant random media

Abstract

Electromagnetic properties of resonant inhomogeneous media are investigated. A new type of transverse electromagnetic wave arising due to the effective spatial dispersion is studied. It is shown that scattering on the fluctuations of the resonant frequency of the medium shifts the Cherenkov threshold and reduces the intensity of the Cherenkov radiation.     

Nuclear-resonance beamline at ESRF

Summary The Nuclear-Resonance Beamline at ESRF is dedicated to the excitation of nuclear levels by synchrotron radiation. The source of radiation and optical elements are optimized to provide an intense, highly monochromatic, collimated and stable X-ray beam of small cross-section at the M?ssbauer transition energies between 6 keV and 30 keV. The set-up of the beamline allows to perform studies in diffraction, small-angle scattering, forward scattering and incoherent scattering. Equipment is available to maintain the sample at variable temperature and magnetic field. Fast detectors and timing electronics serve to separate the delayed nuclear scattering from the ?prompt? electronic scattering and to measure the time spectra of nuclear radiation with sub-nanosecond resolution. The general layout and the parameters of the beamline are reported. Typical domains of applications are discussed and illustrated by first experimental results. Paper presented at ICAME-95, Rimini, 10–16 September 1995.     

Waves in resonant random media

Electromagnetic properties of resonant inhomogeneous media are investigated. A new type of transverse electromagnetic wave arising due to the effective spatial dispersion is studied. It is shown that scattering on the fluctuations of the resonant frequency of the medium shifts the Cherenkov threshold and reduces the intensity of the Cherenkov radiation.     

同步辐射时间分辨荧光光谱技术

时间分辨光谱技术是研究原子、分子和凝聚态物质的激发动力学，特别是研究发光动力学的有力工具，文章主要介绍以同步辐射为激发光源研究时间分辨光谱的两种方法－时间相关单光子计数法和相移调制法，并比较了两者的优缺点。还讨论了同步辐射光源与其他激发光源相比的特点。     

High pressure experiments with synchrotron radiation

Using a diamond anvil cell (DAC), high pressure ⁵⁷Fe Mössbauer spectroscopy has been performed with the nuclear forward scattering of synchrotron radiation. We used monochromatized synchrotron radiation from an in‐vacuum type undulator as a high‐density strong Mössbauer source with a quite small beam size. Pressure‐induced magnetic hyperfine interactions at ⁵⁷Fe in SrFeO_2.97 has been detected at 74 GPa by a quantum‐beat modulation of the decay rate after collective nuclear excitation with the synchrotron radiation pulse. Evidence for a transition from antiferromagnetism to ferromagnetism of Fe in SrFeO_2.97 at 74 GPa and 300 K has been obtained from the nuclear forward scattering under a transverse magnetic field.     

Time-dependent polarization in Mössbauer experiments with synchrotron radiation (II)

The resonant forward scattering of X-rays from⁵⁷Fe nuclei is strongly polarization dependent. The broad band excitation provided by synchrotron radiation (SR) results in an interesting time-dependent polarization mixing. This mixing can be used to substantially reduce the nonresonant (nonrotated) scattering from electrons. The presented technique will allow the full utilization of next-generation synchrotron facilities as a source for Mössbauer experiments.     

Nuclear resonant scattering of synchrotron radiation

The principal ideas of the theory and the main results of the experimental studies of the coherent resonant scattering of-radiation by nuclear ensembles in matter are briefly over-viewed. An analysis of transmission of the Mössbauer-radiation and of synchrotron radiation through a nuclear resonant medium is suggested using an approach based on the optical theory. The feasibilities of the nuclear resonant scattering of synchrotron radiation as a new technique for studying the hyperfine interactions and some other phenomena of the physics of condensed matter are considered.     

The enrichment of physics and astrophysics: The legacy of Victor Hess

Summary The Third Victor F. Hess Memorial Lecture traces the evolution of the cosmic-ray discipline from Hess' discovery in his balloon flights of 1911. Serendipitously, this led to the genesis of two major fields of research, high-energy elementary-particle physics and cosmic-ray astrophysics. The subsequent development of the latter field is traced from the early decades when its promise was not widely appreciated. Epochal discoveries in the 1930's and 1940's stimulated widespread interest among physicists. Identification of the positron, the ?soft component?, muons, pions, and the elaboration of the electromagnetic theory of shower production were among the major advances. The nuclear character of the main primary component was demonstrated in the forties. Symbiosis with radioastronomy, notably the understanding of the synchrotron radiation emitted from the Crab Nebula, revealed the role of supernova explosions in providing the energy for the galactic cosmic radiation. Fermi's use of magnetohydrodynamics in his acceleration mechanisms, and their subsequest realization in shock-wave theory are sketched. Detailed investigations of composition—especially measurements of the Li−Be−B and of the³He components—yielded knowledge of the propagation and transformations of the relativistic nuclei in passing through the interstellar medium. This, in turn, made possible the evaluation of path length distributions, predictions of the arriving isotopic composition, the source composition, and cosmic-ray age. Finally, some tantalizing problems that remain are mentioned, notably those of the highest-energy and the lowest-energy cosmic rays. Invited talk given at the XXIV International Cosmic-Ray Conference, Rome, August 28–September 8, 1995.     

High-Pressure Studies of Magnetism and Lattice Dynamics by Nuclear Resonant Scattering of Synchrotron Radiation

A review is given on recent high-pressure experiments using nuclear resonant scattering of synchrotron radiation. We shortly introduce the methodological aspects connected with high-pressure experiments applying nuclear forward scattering and nuclear inelastic scattering of synchrotron radiation. Selected examples for the study of magnetism are given for the Laves phases LuFe₂ and ScFe₂, where we studied the variation of the magnetic ordering temperature and of the Fe band moment as a function of pressure. An actual example for the study of lattice dynamics is a recent investigation of the phonon density-of-states in metallic iron with special emphasis on hcp ε-Fe, where the pressure-induced texture is used, in conjunction with theoretical calculations, to extract density of phonon states as seen parallel and perpendicular to the hexagonal c-axis. This revised version was published online in August 2006 with corrections to the Cover Date.     

基于FPGA的数字反符合γ谱仪系统设计

本文设计了基于FPGA的数字反符合γ谱仪系统来降低天然本底和康普顿散射对低活度放射性测量的影响。该系统选用Φ145 mm×95 mm×80 mm的NaI （Tl）环形探测器与Φ75 mm×75 mm的NaI （Tl）主探测器构成反符合探测器,采用FPGA和高速ADC同步采样主探测器脉冲信号和反符合环形探测器输出信号。在FPGA中实现了核脉冲信号采集、缓存、反符合甄别、梯形成形等相关算法。在天然本底测量实验中,数字反符合γ谱仪系统的计数率为191.80 cps,本底抑制系数为2.69;对137Cs放射源的测量实验表明,在反符合探测器端面中心处,反符合测量峰总比为0.41,能量分辨率为6.99%;在反符合探测器侧面中间部位,反符合测量峰总比为0.30,能量分辨率为7.48%。实验结果表明,基于FPGA的数字反符合γ谱仪系统明显降低了天然环境本底和康普顿散射对测量的影响,适用于低活度放射性测量、现场就地放射性测量。     

High-pressure Mössbauer spectroscopy with nuclear forward scattering of synchrotron radiation

Abstract

Using a diamond anvil cell, high-pressure ⁵⁷Fe Mössbauer spectroscopy has been performed with the nuclear forward scattering of synchrotron radiation. A pressure-induced magnetic hyperfine interaction at ⁵⁷Fe in SrFeO_{2, 97} has been detected at 44 GPa and 300 K for a first time by a quantum-beat modulation of the decay rate after collective nuclear excitation by the synchrotron pulse. The basic concept and method used to detect nuclear forward scattering with synchrotron radiation are discussed.     

Parametric excitation of an extremely low-frequency radio pulse by an extragalactic gamma-ray burst on December 27, 2004

We simulate an electromagnetic pulse excited by a sharp descent of the charged upper wall of the Earth–ionosphere cavity. The ionosphere descent by 20 km was detected over the dayside Earth’s hemisphere during a giant extragalactic gamma-ray burst in the constellation of Sagittarius (SGR 1806–20). We show that a sharp change in the ionosphere altitude can cause a discrete pulse of extremely low-frequency radiation with the spectrum having some specific features and the amplitude significantly exceeding the natural regular noise background. The time of arrival of the radio pulse to an observer coincides with the gamma-burst time.     

Scattering of electromagnetic wave from perfect electromagnetic conductor cylinders placed in un-magnetized isotropic plasma medium

The two dimensional analytical formulation of scattering of electromagnetic wave from a perfect electromagnetic conductor (PEMC) cylinder placed in un-magnetized isotropic plasma medium is presented. The extended classical scattering theory is used. The incident wave is taken as linearly polarized wave. The analytical theory is formulated for parallel polarization (TM) and also for perpendicular polarization (TE). The numerical computation results show that backscattering and forward scattering are sensitive to electron density and effective collision frequency of plasma medium. We placed different types of cylinders (PEC, PMC and PEMC) in un-magnetized plasma medium and concluded that stealth capability of plasma increases with the placement of PEMC cylinder in plasma medium.     

Heterodyne detection of synchrotron radiation

A time integral method for the study of resonant nuclear scattering of synchrotron radiation in the forward direction is presented. The method relies on the interference of radiation scattered by nuclei in two samples, one moving with respect to the other. The method, termed heterodyne detection of synchrotron radiation, gives the same information on hyperfine parameters as the well known differential method. The general formalism is developed for the case where the reference is a single line sample and the investigated sample has magnetic or quadrupole splitting. The first experiments are discussed. A comparison of time differential synchrotron radiation spectroscopy, heterodyne detection and Mössbauer spectroscopy is given.     

Streuung elektromagnetischer Oberflächen- und Volumenwellen an rauhen Oberflächen. Teil I. Allgemeine Grundlagen

Scattering of Electromagnetic Surface and Bulk Waves from Rough Surfaces. Part I. General Theory A classical theory of scattering of electromagnetic surface and bulk waves on rough surfaces (dielectrics or conductors) is given. The primary wave creates a surface polarization on the rough boundary presenting the source for the radiation of scattering. These roughness-caused polarization must be considered in the wave equations and in the boundary conditions of the fields, too. General expressions for the spectral power density of surface and bulk scattering waves will be given. The excitation of surface polarization can result from primary waves homogenously or inhomogenously.