[en] The Barkas effect is the difference in stopping of swift, positive and negative particles. This qualitative discussion of it concerns the theory of higher Z₁-corrections to the Bethe formula. It is shown that simple classical concepts suffice for obtaining main features of stopping formulae. Thereby, a simple conception of the Z₁³-correction is achieved, the magnitude being about twice that in previous work. In comparisons between α-particles and protons one must in addition apply the Z₁⁴-correction of Bloch. (Auth.)

[en] Short communication

[en] It is attempted to construct a theory of static gravitation in empty space on the basis of only special relativity and equivalence. The derivation contains three steps: fixation of time coordinate, derivation of field equation, and fixation of space coordinates. First, by conservation of energy it is shown that static gravitation is governed by one function, 2chi(r) = log g₀₀. A universal time coordinate is found, and the redshift obtained. The components of the line element are fixed, except for its spatial part. Second, the field equation for chi is derived by equivalence with the accelerated Moeller box. These two steps remain in accordance with general relativity. Third, universal spatial coordinates are found by common measurements by all possible test particles, leading to harmonic and isotropic coordinates. The result is in agreement with the Schwarzschild solution to first order beyond Newtonian gravitation. The second-order disagreement with general relativity is discussed. (Auth.)

[en] In a recent experiment, Simpson found an excess intensity in the measured tritium beta spectrum below 1.5 keV. A detailed account of the decay energy and Coulomb screening effects raises the theoretical curve in precisely this energy range, so that little, if any, of the excess remains. This correction is shown to be approximately given by a simple analytical expression

[en] It is known that ultrarelativistic electrons, when moving nearly parallel to an axis in a single crystal, will emit very intense radiation containing high-energy quanta. The radiation spectra are connected to synchrotron spectra in excessively strong magnetic fields. The latter spectra have previously been calculated by quantum perturbation theory, and are much reduced compared to the corresponding classical spectra. By application of the correspondence principle, it is shown that, for spin-zero particles, the complete quantum spectrum is obtained exactly from the classical spectrum by a simple modification of the frequency variable. The result holds for arbitrary external fields and with respect to angular distribution, polarization, and frequency distribution. Agreement with previous calculations of the synchrotron case is verified. For spin-1/2 particles the quantum frequency spectrum can be expressed with satisfactory accuracy by the classical radiation of spin-zero particles. These results may be of assistance in our understanding of radiation by ultrarelativistic particles

[en] The discovery of the exceptional stability of C₆₀ (Buckminsterfullerene) and its ions has prompted a large number of experimental and theoretical investigations concerning this soccerball-shaped molecule. After the discovery of a method, by which macroscopic quantities of C₆₀ are available, this molecule has become an obvious candidate for structural and dynamic investigations by atomic-collision experiments. In our experiments, the fragment distributions suggest a mechanism where fragmentation occurs by sequential loss of 'pairs' of carbon atoms. We have continued the C⁺₆₀ fragmentation studies and also included negative and multiply charged as well as other fullerene ions besides C₆₀. In the case of positive fullerenes, only even-numbered molecules were observed, but for the negative fullerenes, also odd-numbered molecules were recorded on the ∼ 0.5% level relative to the even-numbered components. This finding indicates that the electron structure also plays an important role for the stability of fullerene ions, or perhaps that negative molecules can exist in some isomeric form. The total destruction cross sections for fullerene ions have been measured in several target gases. We are planning to continue with collisional studies of various fullerene ions with special emphasis on a better understanding of the collisionally induced fragmentation for ions in well prepared states, and also endohedral complexes and fusion will be investigated in collision studies. (EG) (8 refs.)

[en] The authors study dynamical aspects of equivalence between mass and energy, for systems of interacting particles. The starting-point consists in the classical formulae for electromagnetic self-momentum, self-energy and self-force. These formulae possess puzzling terms which have been subject to various explanations, like compensating Poincare stresses, or were bypassed through attempts of redefinition of the classical electron model. By means of a comprehensive study of acceleration processes it is shown that there is a crucial error in the usual derivations of self-force. A basic acceleration equation is derived for a point-like system, with detailed equivalence. It also follows that the standard formulae for self-energy and self-momentum are, at best, misleading. Next the authors study how systems are to be described in an accelerated, rigid coordinate frame - the Moeller box. Classical and quantal equations of motion for fields and for particles in the Moeller box, including the Dirac equation for the hydrogen atom, are investigated in great detail, arriving at equivalence. Finally, properties of composite systems as compared with properties of particles are discussed. (Auth.)

[en] We calculate the electronic stopping power and the corresponding straggling for ions of arbitrary charge number, penetrating matter at any relativistic energy. The stopping powers are calculated by a simple method. Its starting point is the deviation of the precise theory from first-order quantum perturbation. We show that this deviation can be expressed in terms of the transport cross section, σ_tr, for scattering of a free electron by the ion. In the nonrelativistic case the deviation is precisely the Bloch correction to Bethe close-quote s formula; we look into the nonrelativistic case in order to clarify both some features of our method and a seeming paradox in Rutherford scattering. The corresponding relativistic correction is obtained from σ_tr for scattering of a Dirac electron in the ion potential. Here, the major practical advantage of the method shows up; we need not find the scattering distribution, but merely a single quantity, σ_tr, determined by differences of successive phase shifts. For a point nucleus our results improve and extend those of Ahlen. Our final results, however, are based on atomic nuclei with standard radii. Thereby, the stopping is changed substantially already for moderate values of γ=(1-v²/c²)^-1/2. An asymptotic saturation in stopping is obtained. Because of finite nuclear size, recoil corrections remain negligible at all energies. The average square fluctuation in energy loss is calculated as a simple fluctuation cross section for a free electron. The fluctuation in the relativistic case is generally larger than that of the perturbation formula, by a factor of ∼2 endash 3 for heavy ions. But the finite nuclear radius leads to a strong reduction at high energies and the elimination of the factor γ² belonging to point nuclei. copyright 1996 The American Physical Society

[en] The distribution of fragments resulting from collisions between 50--200-keV C₆₀⁺ ions and H₂ and He is found to follow approximately a simple power law I(m)=cp^m where p is a constant depending on both energy and target gas, and m is the number of missing ''pairs'' of carbon atoms. Based on this observation, a new dynamical fragmentation model involving the ratio of two characteristic times is proposed. In collisions by 300-keV C₆₀⁺⁺ ions, the singly charged products are distributed quite differently, which implies the first evidence of the presence of charge-separation reactions