摘要:

We review the phenomenon of coherent bremsstrahlung (CB) of electrons in a crystal. This well-studied process in a new light after the realization by Andersen [J. U. Andersen,Nucl. Inst. and Methods170, 1 (1980)] that CB and the phenomenon of channeling radiation (CR) are two aspects of the same physical process. These two types of radiation may be described in a unified framework by the use of Bloch functions for the electron in the crystal. The kinematics of CB is discussed from this viewpoint and a model calculation for CB in a strained-layer superlattice is presented.

ISSN:1042-0150    

DOI:10.1080/10420159108211483

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

When multi-GeV electrons and positrons penetrate single crystals near axial/planar directions dramatic effects appear. The large gamma factors (105−106) enhance the electric fields along axes/planes to overcritical values within the electron restframe. Overcritical in this context means that perturbation theory (used hitherto) is no longer applicable, but a full description of QED in strong fields has to be used. Bremsstrahlung becomes a highly non-perturbative process resulting in a much enhanced cross section. Originally a peak in the photon spectra from 150 GeV electrons penetrating Ge-crystals was detected. In the literature different suggestions for processes leading to this peak is found, i.e. “new” physics, radiation cooling, multiphotons etc. Along axial/planar directions these effects were investigated in detail by our CERN-collaboration (NA-43) and surprising effects were found. 150 GeV axially channeled electrons radiate 80% of the total energyEoin 1 mm Ge – 20 times more than in amorphous material. The peak in the photon spectra around (0.8–0.9) ×Eois a multiphoton effect and found both in Si and Ge, but not for thin Si-crystal and not for incident angles larger than ∼ 0.5 × the channeling angle ψ1. The influence of strong fields on shower formation was also measured along axial directions. Radiation lengths are (10–50) times shorter than Bethe-Heitler values. The angular dependence is in agreement with the critical angle θ0=Uo/mc2∼ 0.5 mrad calculated by Baier et al. for strong field effects.

ISSN:1042-0150    

DOI:10.1080/10420159108211484

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

A crystal has a regular structure, therefore every motion in such a structure seems to be regular. However, it is not actually so and even in perfect crystals the particle motion may be either regular or chaotic. Everything depends on the number of integrals of motion determining a particle trajectory.

ISSN:1042-0150    

DOI:10.1080/10420159108211485

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

The process of relativistic particle radiation in an external field has been studied in the semi-classical approximation rather extensively. The main problem arising in the studies is to express the formula of the quantum theory of radiation in terms of classical quantities, for example of the classical trajectories. However, it still remains unclear how the particle trajectory is assigned, that is which particular initial or boundary conditions determine the trajectory in semi-classical approximation quantum theory of radiation.

ISSN:1042-0150    

DOI:10.1080/10420159108211486

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

A Monte–Carlo program to investigate electromagnetic shower formation in single crystals for particle energies up to 300 GeV is presented. It is based on the well known EGS4-code for amorphous materials and accounts for coherent radiation and pair production which arise in a crystal. Channeling radiation and crystal field assisted pair production are calculated within the uniform field approximation. The increase of energy loss of channeled electrons due to radiation cooling is also included. Good agreement with the available experimental data can be reported.

ISSN:1042-0150    

DOI:10.1080/10420159108211487

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Channeling radiation from 39 and 45 MeV electrons channeled along the <0001> axis, the (0110) plane and the (1210) plane of a 30μmthickLiNbO3crystal has been measured. Calculations of the planar crystal potentials were performed by means of the many-beam formalism. Good agreement between theory and experiment is obtained for the planar channeling radiation. Associated with channeling additional radiation lines have been observed, which may be explained by a periodic perturbation of the continuum potential.

ISSN:1042-0150    

DOI:10.1080/10420159108211488

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Within the framework of the multistring model of axial channeling of electrons the method for calculating the losses to radiation in channeling is proposed. The physical model includes the multiple scattering, the beam and target characteristics. The simulated results are compared with the experimental data.

ISSN:1042-0150    

DOI:10.1080/10420159108211489

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

ISSN:1042-0150    

DOI:10.1080/10420159108211490

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

ISSN:1042-0150    

DOI:10.1080/10420159108211491

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor

摘要:

Motivated mainly by yet unexplained data from heavy ion collisions, various attempts have been recently made to search for new light, short-lived neutral particles. Channeling of very high energy electrons can provide new ways to search for particles whose peculiar properties require for their production the presence of strong fields, such as particles with internal structure. We also discuss the possibility of enhancing the rate of Higgs boson production at high energy electron linear colliders by crystal channeling.

ISSN:1042-0150    

DOI:10.1080/10420159108211492

出版商:Taylor & Francis Group    

年代:1991

数据来源: Taylor