Search Results (13 total)

The total cross sections for single ionization of helium and single and double ionization of argon by antiproton impact have been measured in the kinetic energy range from 3 to 25 keV using a new technique for the creation of intense slow antiproton beams. The new data provide benchmark results for the development of advanced descriptions of atomic collisions and we show that they can be used to judge, for the first time, the validity of the many recent theories.

Experimental results for the restricted energy loss of pairs created from 1–178 GeV photons in a thin Au target and subsequently passing a CCD detector are presented. It is shown that pairs—when detected close to the creation vertex—suffer a reduced energy loss due to the internal screening of the charges constituting the pair. Furthermore, the ability to measure directly the energy of the pair by calorimetry enables a comparison with theory as a function of energy. The observed phenomenon is in good qualitative agreement with general expectations from the Chudakov effect but indicates a quantitative disagreement with either of two mutually disagreeing theories.

The processes of coherent bremsstrahlung (CB) and coherent pair production (CPP) based on aligned crystal targets have been studied in the energy range 20–170 GeV. The experimental arrangement allowed for measurements of single photon properties of these phenomena including their polarization dependences. This is significant as the theoretical description of CB and CPP is an area of active debate and development. With the approach used in this paper, both the measured cross sections and polarization observables are predicted very well. This indicates a proper understanding of CB and CPP up to energies of 170 GeV. Birefringence in CPP on aligned crystals is applied to determine the polarization parameters in our measurements. New technologies for high-energy photon beam optics including phase plates and polarimeters for linear and circular polarization are demonstrated in this experiment. Coherent bremsstrahlung for the strings-on-strings (SOS) orientation yields a larger enhancement for hard photons than CB for the channeling orientations of the crystal. Our measurements and our calculations indicate low photon polarizations for the high-energy SOS photons.

Relativistic positronium (Ps) is of potential use to address fundamental questions in QED—e.g., through direct lifetime measurements of the “para” state of Ps, severe magnetic quenching of the ortho-Ps lifetime, so-called “superpenetration,” and possibly to measure Ps-atom cross sections. Existing schemes for the production of relativistic positronium have relied on pion production through, e.g., nuclear interactions. This yields a low-intensity beam with relatively poor characteristics in both longitudinal and transverse emittance. By use of positrons impinging on a thin carbon foil inside a high-frequency rf cavity, it is proposed to generate relativistic positronium ions (Ps⁻) by rapid acceleration. Relativistic Ps can be derived from the positronium negative ion by subsequent Lorentz stripping or photodetachment. Intensities of the order 100 per second and Lorentz factors ≃20 are feasible with present technology.

Experimental results for the radiative energy loss of 178 GeV positrons in Cu, Au, and W targets are presented. It is shown that for a few micron thick target, effects related to the formation zone disappear, in particular, the suppression due to the Landau-Pomeranchuk-Migdal and Ternovskii-Shul’ga-Fomin mechanisms. This disappearance may restrict the region of applicability of thin foils as a target for energy-selective production of high energy photons. Furthermore, transition radiation dominated by multiple scattering and structured target interference effects are shown to be likely ingredients for an accurate description of the data obtained at low photon energies.

We present experimental results of nuclear-charge changing interactions for 158A-GeV ultrarelativistic In ions in Si, Ge, Sn, W, and Pb targets. Calculations based on the abrasion-ablation model for hadronic interaction and the RELDIS model for electromagnetic dissociation are compared to the data.

Crystals present a uniquely simple environment for the investigation of strong electromagnetic fields. When energetic charged particles are incident on crystals close to major crystallographic directions, their electromagnetic interactions depend crucially on the kinematic conditions. The coherence of the crystalline field can produce very strong electric fields in the rest frame of the particle, exceeding the so-called Schwinger field or quantum critical field. In that domain, the radiation emission takes a substantial part of the electron energy and the “formation zone” changes character. In this review the theory appropriate to the different kinematics domains is described, concentrating on the effects occurring at extreme fields. Properties discussed include strong field synchrotron radiation, channeling radiation, bremsstrahlung, and photon interactions. Applications are given to radiation sources, bending of particle beams, and sources of polarized GeV photons.

The slowing-down process of pointlike charged particles in matter has been investigated by measuring the stopping power for antiprotons in materials of qualitatively very different nature. Whereas the velocity-proportional stopping power observed for metal-like targets such as aluminum over a wide energy range of 1–50 keV is in agreement with expectations, it is surprising that the same velocity dependence is seen for a large band-gap insulator such as LiF. The validity of these observations is supported by several measurements with protons and several checks of the target properties. The observations call for both a qualitative explanation and a quantitative theoretical model.

Experimental results for the bremsstrahlung energy loss of 149, 207, and 287 GeV electrons in thin Ir, Ta, and Cu targets are presented. For each target and energy, a comparison between simulated values based on the Landau-Pomeranchuk-Migdal (LPM) suppression of incoherent bremsstrahlung is shown. For the electron energies investigated, the LPM effect enters the quantum regime where the recoil imposed on the electron by the emitted photon becomes important. Good agreement between simulations based on Migdal’s theory and data from the experiment is found, indicating that the LPM suppression is well understood also in the quantum regime. Results from a comparison between simulations with the “threshold” energy E_LPM as a free parameter and the data are shown. This analysis reproduces the expected trend as a function of nominal radiation length, but yields values that tend to be low compared to Migdal’s theory.

Experimental results for the radiative energy loss of 149, 207, and 287 GeV electrons in a thin Ir target are presented. From the data we conclude that at high energies the radiation length increases in accordance with the Landau-Pomeranchuk-Migdal (LPM) theory and thus electrons become more penetrating the higher the energy. The increase of the radiation length as a result of the LPM effect has a significant impact on the behavior of high-energy electromagnetic showers.

There are several advantages in using a crystal for stripping of the H^- ion to obtain efficient injection of protons into a circular accelerator. First, the stripping efficiency of a crystal is at least as large as for an amorphous foil of the same substance and thickness. Second, the emittance increase imposed by the multiple Coulomb scattering of the protons on subsequent turns is drastically lower by a factor of up to ≃7. Third, the restricted energy loss of the protons is lower by a factor of up to ≃1.5—this, combined with the fact that the thermal conductivity of a single crystal of diamond is much higher than that of the amorphous material, will reduce the effect of heating of the stripping material. In high-power schemes based on amorphous foils heating of the electron stripping material is a limiting factor. Fourth, the reduced total energy loss is accompanied by a smaller energy loss straggling implying a smaller longitudinal emittance. Last, the so-called random orientation of the crystal can provide the option of stripping the H^- ions as in an amorphous foil while preserving the advantage of a high thermal conductivity, simply by changing the orientation of the crystal. A simulation using realistic parameters is presented, which reflects the efficient conservation of emittance using a diamond crystal. The phenomenon should in fact be applicable in general for the stripping of H^-, although the advantages depend on parameters such as the energy. A reasonable figure of merit is the ratio of the total transverse emittance increase of crystalline and amorphous foils in one turn and in the presented case this is as high as a factor 3.9.

The stopping power for antiprotons in various solid targets has been measured in the low-energy range of 1–100 keV. In agreement with most models, in particular free-electron gas models, the stopping power is found to be proportional to the projectile velocity below the stopping-power maximum. Although a stopping power proportional to velocity has also been observed for protons, the interpretation of such measurements is difficult due to the presence of charge exchange processes. Hence, the present measurements constitute the first unambiguous support for a velocity-proportional stopping power due to target excitations by a pointlike projectile.

Measurements of the energy loss and the energy-loss distributions of 160 GeV/amu fully stripped lead ions traversing a silicon single crystal are presented. The energy loss is measured using the silicon crystal as an intrinsic detector. Hence the measured energy loss is a restricted energy loss excluding very large energy transfers. For random incidence, the observed energy-loss distributions are very narrow and Gaussian-like. For well-channeled particles, the energy loss is strongly reduced as compared to so-called random particles. The observed energy loss is compared to calculations as well as simulations. Due to the small straggling, the energy-loss distributions are reflecting directly the distribution in transverse energy.