The environment in which bremsstrahlung and pair creation occurs can strongly affect cross sections for these processes. Because ultrarelativistic electromagnetic interactions involve very small longitudinal momentum transfers, the reactions occur gradually, spread over long distances. During this time, even relatively weak factors can accumulate enough to disrupt the interaction. In the Landau-Pomeranchuk-Migdal effect, multiple scattering reduces the bremsstrahlung and pair production cross section. This review will discuss this and a variety of other factors that can suppress bremsstrahlung and pair production, as well as related effects involving beamstrahlung and QCD processes. After surveying different theoretical approaches, experimental measurements will be covered. Recent accurate measurements by the SLAC E-146 Collaboration will be highlighted, along with several recent theoretical works relating to the experiment.

/pdf/suppression-of-bremsstrahlung-and-pair-production-due-to-zpdpblmqea.pdf

Suppression of bremsstrahlung and pair production due to environmental factors

The coherent effects in the high energy particle interaction with crystals open up new opportunities for accelerating and detecting techniques. The effective electromagnetic fields that arise in this case can exceed a thousand times the fields that are now attainable at experimental installations. The first part of the thesis is devoted to the theoretical study of the multiple scattering effects on the dynamics and radiation of high energy charged particles in crystals. One of the goals of this study is to find optimal conditions to carry out the measurement of the anomalous magnetic moment of the charmed Lambda Baryon at the LHC. Due to the short lifetime of this particle the only way to provide such a measurement is to use a bent crystal, which can imitate the magnetic field of order of thousand Tesla. The optimization of the parameters for an experimental setup was carried out on the basis of computer simulation of the Lamda Baryon passage through a bent crystal using the binary collisions model, taking in to account incoherent scattering on thermal vibrations of atoms at lattice nodes, and scattering on an electronic subsystem of a crystal. The results of the research conducted in the thesis give an optimistic forecast for the possibility of carrying out such an experiment at the LHC and became the basis of the corresponding proposal. The multiple scattering effects of the thesis. This study is performed in the framework of classical electrodynamics approach for describing the relativistic particle radiation and the computer simulation of fast charged particles passing through a crystal using the above-mentioned model. It is shown, that the spectral-angular distribution and the polarization characteristics of radiation essentially differ from the results of the Bethe–Heitler theory. The conditions for the experimental observation of these effects are presented. The second part of the thesis is devoted to the study of transient processes in advanced fast reactor breeder working in a self-sustained nuclear burning wave (NBW) regime. It is a new concept of nuclear fission reactors with the so-called “intrinsic safety”, in which the development of uncontrolled chain nuclear reaction is impossible due to the physical principles of reactor operation. The promising concepts of NBW reactor in the case of its implementation allows to utilize the depleted Uranium and provides the treatment of long-lived radioactive waste. This study is based on numerical solving the non-stationary non-linear diffusion equation of neutron transport together with a set of the burn-up equations for fuel components and the equations of nuclear kinetics for precursor nuclei of delayed neutrons using effective multi-group approach. The model of cylindrical homogeneous reactor with metallic fuel of U–Pu, Th–U and mixed Th–U–Pufuel cycles is considered in the framework of the buckling concept. As a result of these studies the existence of the NBW mode in the case of mixed Th–U–Pu fuel and its essential advantages are demonstrated; the detailed analysis of a special kind of the negative reactivity feedback that is inherent to the NBW regime and underlies the “intrinsic safety” of such a reactor is performed; the scenarios of a smooth start-up, forced shutdown and subsequent restart of the NBW reactor are developed.

https://tel.archives-ouvertes.fr/tel-01777441/document

Multiple Scattering Effects on the Dynamics and Radiation of Fast Charged Particles in Crystals. Transients in the Nuclear Burning Wave Reactor

The condition and specific features of non-dipole regime of radiation is discussed in connection with the results of recent CERN experiment NA63 on measurement of radiation power spectrum of $149\,$GeV electrons in thin tantalum targets. The first experimental detection of logarithmic dependence of radiation yield from the target thickness is the conclusive evidence of the effect of radiation suppression in a thin layer of matter, which was predicted many years ago, and which is the direct manifestation of radiation of relativistic electron with non-equilibrium own Coulomb field. The special features of angular distribution of radiation and its polarization in a thin target at non-dipole regime are proposed for a new experimental study.

Radiation of a relativistic electron with non-equilibrium own Coulomb field

We study the effect of the photon formation length on the interference of two bremsstrahlung photons in the simplest case of radiation emitted by only two charged particles. Most of interference patterns obtained can be interpreted in terms of the photon formation length. After having analyzed the simplest situations we discuss the interference of bremsstrahlung photons in more realistic and more complicated situations. A possibility of studying the effect in synchrotron radiation is briefly sketched.

/pdf/photon-formation-length-in-hanbury-brown-and-twiss-1xaqbhi9du.pdf

Photon formation length in Hanbury-Brown and Twiss interferometry of bremsstrahlung photons

In this letter the high-energy expansion for scattering from extended targets, which the authors previously applied to beamstrahlung radiation and pair production, is applied to the problem of radiation in a medium with multiple scattering. The suppression of the emission of long wave-length photons, the Landau-Pomeranchuk-Migdal effect, is treated and explained in physical terms. This extends previous classical treatments of the problem to the quantum domain and corrects certain approximations made in these earlier works; for example, the effects of finite target thickness can be treated. A model of a random scattering medium is defined that allows a quantum treatment of multiple scattering and the resultant suppression of bremsstrahlung radiation. Submitted to Physical Review Letters ∗Work supported by Department of Energy contract DE–AC03–76SF00515. Perhaps the most ubiquitous process occurring in high-energy physics is the bremsstrahlung of photons by a charged particle in the field of an atom first described by Bethe and Heitler [1]. Following experimental confirmation in 1993 of the Landau–Pomeranchuk–Migdal (LPM) effect [2-6], there is renewed interest in extensions of this process, as well as in its strong interaction analogue, gluon radiation at very high energies in heavy nuclei. We describe here the application of eikonal techniques developed for the beamstrahlung process [7] that lead to a simpler, more straightforward, and physically transparent quantum mechanical derivation of the LPM suppression of soft photon radiation from high-energy electrons in dense matter. This effect was first described by Landau and Pomeranchuk [8] who treated the classical radiation of a high-energy particle in the fluctuating and ‘random’ field inside an infinitely thick medium. The longitudinal momentum transfer q|| of a high-energy electron of momentum p and mass m, radiating a photon of momentum k ≡ (1 − x)p, has a minimum value given by qmin || = m2(1 − x)/2xp. The uncertainty principle can be used to define the formation length lf = (1/q min || ) which at high energies (p >> m) and soft photon emission (1− x) << 1 can grow quite large relative to the interatomic spacing. In their classical derivation, which is appropriate to this kinematics limit, Landau and Pomeranchuk were the first to show that the familiar Bethe-Heitler radiated photon spectrum dN ∼ dk/k is modified by the multiple scattering of the electron as it traverses the rapidly varying electric fields of the medium. When the mean free path of the electron αL is comparable or less than the formation length lf , they found that the spectrum is suppressed, ultimately achieving the form dN ∼ dk/(p √ Lk) . (1)

/pdf/a-quantum-treatment-of-the-landau-pomeranchuk-migdal-effect-50vmi74a3s.pdf

A Quantum Treatment of the Landau-Pomeranchuk-Migdal Effect

We have performed an experiment to measure accurately the Landau‐Pomeranchuk‐Migdal (LPM) effect in the production of 5 to 500 MeV photons due to bremsstrahlung of 8 and 25 Gev electron beams traversing thin (2 to 6% X0) targets of varying densities. Our measurements confirm that the LPM effect exists and that the Migdal calculations are accurate. We see that, for thin targets, LPM suppression disappears leaving a Bethe‐Heitler spectrum, as predicted by theory. For intermediate target thicknesses, we lack an acceptable theory, but have measured energy spectra for targets of differing thickness.We have also measured the production rate of 500 keV to 5 MeV photons at the same electron energies, to study dielectric suppression. We see qualitative agreement with the theory of Ter‐Mikaelian; more work is needed before we can make quantitative comparisons.

A measurement of the LPM effect

We have developed a technique that allows SLAC to provide parasitic low-intensity secondary e/sup +/ and e/sup -/ beams up to 25 GeV to End Stations A and B and to the FFTB during SLC operation. This beam was successfully used for a one-month-long experimental run in End Station A by the SLAC E146 collaboration. The experiment used 400 MeV to 25 GeV electron beams at intensities averaging one electron per pulse and 120 pulses per second. The method for producing such a beam without the need for dedicated beam time was to operate parasitically from SLC: photons produced in the SLC beam scrapers in linac sectors 28, 29 and 30 were converted to positrons and electrons in a target downstream from the SLC splitter magnet. The secondary electrons or positrons were then transported to ESA with the A-line. >

/pdf/a-method-of-obtaining-parasitic-e-sup-or-e-sup-beams-during-1clvmohs6k.pdf

A method of obtaining parasitic e/sup +/ or e/sup -/ beams during SLAC linear collider operation

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A measurement of the LPM effect

A method of obtaining parasitic e/sup +/ or e/sup -/ beams during SLAC linear collider operation