Search Results (44 total)

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.

Intermediate-mass fragments from the interaction of ²⁷Al, ⁵⁹Co, and ¹⁹⁷Au with 200-MeV protons were measured in an angular range from 20^° to 120^° in the laboratory system. The fragments, ranging from isotopes of helium up to isotopes of carbon, were isotopically resolved. Double-differential cross sections, energy-differential cross sections, and total cross sections were extracted.

Plasmas offer the possibility of high acceleration gradients. An intriguing suggestion is to use the higher plasma densities possible in solids to get extremely high gradients. Although solid-state plasmas might produce high gradients they would pose daunting problems. Crystal channeling has been suggested as one mechanism to address these challenges. There is no experimental or theoretical guidance on channeling for intense electron beams. A high-density plasma in a crystal lattice could quench the channeling process. An experiment has been carried out at the Fermilab NICADD Photoinjector Laboratory to observe electron channeling radiation at high bunch charges. An electron beam with up to 8 nC per electron bunch was used to investigate the electron-crystal interaction. No evidence was found of quenching of channeling at charge densities two orders of magnitude larger than that in earlier experiments.

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.

Parametric X radiation (PXR) produced by electrons with an energy of E₀=4 MeV interacting with the atoms in the (220) plane of a 20-μm silicon and a 55-μm diamond crystal and observed at an angle of 44° by a Si(Li) detector has been investigated with respect to the interference with coherent bremsstrahlung (CB) that originates in the same interaction process between the incoming relativistic electron and the crystal. Since the energy of PXR and CB is identical, contributions of both types of radiation are indistinguishable. The newly derived analytical expressions describe the radiation which consists of a coherent superposition of PXR and CB. For the comparison of the experimental results with the theoretical predictions a Monte Carlo simulation taking into account all effects accompanying the radiation process has been performed. The comparison shows very good agreement between experiment and theory.

Parametric x rays (PXR) produced by bombarding silicon and diamond crystals with electrons of 30 to 87 MeV were detected at 180° relative to the direction of the electron beam. It was found that the dependence of the intensity on the orientation of the crystal agrees with the predictions of the kinematical theory of PXR. The absolute intensity is twice as large as predicted. These findings can be explained considering dynamical effects that govern the x-ray crystal interaction. Additionally, x rays caused by self-diffracted transition radiation have been observed.

The lattice sites of ⁵⁷Fe recoil implanted into diamond at low concentrations have been investigated in in-beam Mössbauer spectroscopy. ⁴⁰Ar ions incident at 110 MeV on an enriched ⁵⁷Fe foil were used to Coulomb excite and recoil implant ⁵⁷Fe nuclei into a pair of diamond targets. Conversion electron Mössbauer spectra were measured at sample temperatures of 300, 600, 700, and 800 K. The spectra were consistently resolved into three components: two symmetric doublets and a weak singlet. At 300 K the singlet had an isomer shift of δ=+0.16 mm/s: the doublets (D₁ and D₂) had quadrupole splittings of 2.33 mm/s and 2.10 mm/s, and isomer shifts of -0.51 mm/s and +0.04 mm/s, respectively. The line intensities and linewidths of the three components showed little change with temperature. Time-differential measurements showed no evidence of any dynamic rearrangement of the implanted Fe atoms within the time window of the measurements (100 ns). Comparison of the observed isomer shifts with theoretical calculations and similar measurements in Si allow us to attribute the singlet (f≈10%) to interstitial Fe. The isomer shift of doublet D₂ is in agreement with the calculations for substitutional Fe. However, the large electric-field gradients of D₁ and D₂ caution against any firm conclusions on these components, but indicate that two differently disturbed implantation sites are populated with considerable lattice damage in the neighborhood of the implanted ions.

Presented is the first experimental demonstration of a dramatic radiative redistribution in transverse states for multi-GeV electrons/positrons, interacting with strong crystalline fields. Detailed analysis of energy loss, photon multiplicities, and scattering distributions leads to new physical insight into open questions such as radiative cooling/heating of the beams in aligned crystals, validity of the constant-field approximation, radiative capture of random particles (“feed in”) into channeled states, and the Landau-Pomeranchuk effect in multiphoton radiation spectra.

We have measured the excitation functions of several reactions occurring in the fusion of ¹²C with ¹⁰³Rh at incident energies up to about 230 MeV. The data can be satisfactorily reproduced by considering the preequilibrium emission of particles during the thermalization of the composite nucleus. The energy evolution of the mean-field interaction is also discussed. © 1996 The American Physical Society.

Measurements have been performed at the superconducting Darmstadt electron linear accelerator (S-DALINAC) to investigate systematically channeling radiation produced by bombarding natural diamond crystals with thicknesses of 13, 20, 30, and 55 μm with electrons at 5.2 and 9.0 MeV. Planar channeling from the (110) and (111) planes was studied for a variety of transitions with respect to their energy, intensity, and linewidth. Axial channeling from the 〈110〉 axis could be detected as well. It was found that the intensity increases as a function of the crystal thickness, and values up to 7.7×10^-2 photons/esr could be obtained, which is the highest intensity at low electron energies achieved so far. The intensity increases with electron energy as γ^5/2. The 1/e occupation length deduced from the photon yield as a function of the crystal thickness was found to be l_occ≈29 and 85 μm for planar and for axial channeling, respectively. These values are by far the largest ever observed. Comparison with a quantum mechanical theory of channeling radiation exhibits fairly good agreement for the intensity and linewidth provided that contributions caused by electronic scattering and Bloch wave broadening, which actually are largest for diamond, are properly taken into account. It turns out that multiple scattering dominates in the planar case and single scattering for the axial channeling. The coherence length could be deduced to be of the order of 0.7 μm, which is about a factor of 2 larger than observed before in silicon.

Utilizing 90° Compton scattering the linear polarization of channeling radiation produced at the superconducting accelerator S-DALINAC with 62 MeV electrons in silicon and diamond has been measured in the energy range between 50 and 400 keV. Planar channeling radiation due to transitions involving transversal bound as well as unbound states is completely linearly polarized perpendicular to the channeling plane. Axial channeling radiation does not show linear polarization.

Parametric x-ray radiation of type B has been produced with an electron beam of energies between 3.5 and 9.1 MeV from the superconducting accelerator S-DALINAC and diamond of thickness 55 μm. The photon intensity and its energy dependence were determined as a function of the tilt angle of the crystal. The intensity maximum varies with γ² and is about 3 orders of magnitude smaller than channeling radiation. Comparison with theoretical predictions exhibits very good agreement after taking into account effects caused by multiple scattering of the electrons in the crystal.

NMR measurements on single crystals of pure ¹³C diamond are reported. The line shape is very sensitive to the orientation of the external magnetic field relative to the crystallographic axes, typical of dipolar line broadening. With the magnetic field oriented along the [110] direction, the line is broad and flat, whereas a more narrow, Gaussian shape is seen along [001]. For the [111] direction, a spectacular line splitting of 8.5 kHz is observed. For the sample studied, the spin-lattice relaxation time was about 15 s at room temperature and 140 s at 140 K. The spectra are interpreted by a simple model using dipolar interactions.

The intensity of channeling radiation from natural diamonds of thicknesses of 13 to 55 μm has been investigated at electron energies of 5.2 and 9.0 MeV. The intensities scale with electron energy according to γ^5/2 and vary with the crystal thickness d as (1-e^-λd), from which the occupation length λ^-1 of the electronic states in the crystal could be deduced. It amounts to 18 and 29 μm for 5.2 and 9.0 MeV electrons, respectively. The strongest transition has an intensity of 0.1 photon per e^- per sr.

The nuclear hyperfine structure of bond-centered muonium in ¹³C enriched diamond has been resolved using time-differential transverse-field muon spin rotation. The measured nearest-neighbor ¹³C hyperfine parameters are compared to theoretical values from a recent ab initio Hartree-Fock type cluster calculation. The ratios of the measured ¹³C hyperfine parameters and free atom values indicate that 92% of the unpaired electron spin density resides on the two nearest-neighbor atoms, in marked contrast with the situation for the analogous muonium center in Si.

Gamma-ray emission from the recoil particle can have a large effect on the blocking dip observed in crystal blocking experiments. We demonstrate this effect experimentally, and give a theoretical description based on standard chaneling theory that is in good agreement with the measurement. We discuss the implications for lifetime measurements using the crystal blocking technique.

The minimum yield has been measured for the channeling of 1.0-MeV protons in diamond overlaid with a wide range of thicknesses of amorphous carbon, aluminum, or gold layers. The data have been compared with the predictions of multiple-scattering theory, and good agreement is found. A quartic approximation is introduced to describe the angular yield, which is needed to evaluate the theoretical minimum yield. The use of power-law scaling in the relationship between scattering in different materials is investigated and shown to be useful in the treatment of mixed layers. An approximate expression for the effect on the yield of thin layers is derived and investigated. This expression is useful in the evaluation of the effect of thin contamination layers on crystals.

The axial dechanneling of protons in natural diamond crystals selected for low defect levels has been studied for beam energies of 1.0–8.9 MeV and crystal temperatures from 20–600 °C. Measurements of the dechanneled-ion yield were taken along the three major axes 〈110〉, 〈111〉, and 〈100〉. The data have been analyzed in terms of the diffusion model of dechanneling and generally good agreement between experiment and theory is obtained. The theory indicates that the predominant contribution to the dechanneling is from the electronic scattering. A scaling of the energy-dependent data for a given axis with a distance characteristic of the electronic scattering is observed. This scaling holds approximately for all data in the three axes considered. Measurements of the yield as a function of temperature indicate that the theory underestimates the nuclear scattering.

Observations with the muon-spin-rotation (μSR) technique of the thermally activated transition from the isotropic muonium state Mu to the anisotropic muonium state Mu^* in diamond are used to determine the sign of the Mu^* hyperfine constants. It is found that the isotropic part of the Mu^* hyperfine interaction is negative, indicating the importance of exchange polarization. The transition rate from Mu to Mu^* follows an Arrhenius law over more than four decades. Finally, data are presented on the temperature dependence of the hyperfine interaction of isotropic Mu in diamond.

We present the results of a search for resonantlike structures in the two-photon decay channel of the e⁺-e^- system at energies corresponding to the anomalous peaks seen at Gesellschaft für Schwerionen-forschung in heavy-ion reactions. Within our sensitivity, no evidence for resonances at the 3% level was found. This enables us to exclude the existence of a Wong-Becker-type particle and to set an upper limit on the ratio Γ_γγ / Γ_e⁺e^-≤0.7 at 810 keV.

A study was made of the volume expansion obtained during ion implantation of 170-keV fluorine ions into diamond under ambient temperature. By concluding that the expansion is not related to the onset of amorphization or graphitization of the ion-damaged layer, a simple theoretical description of the phenomenon could be developed taking into account only the creation and interaction of the point defects resulting from the collision cascades. The results support the view that the so-called ‘‘amorphous’’ layers obtained in diamond under these implantation conditions are primarily vacancy-rich regions which to a large extent retain their diamond characteristics in skeletal form.

The crystal-blocking technique was used to measure the deexcitation times of the evaporation residues of 120-MeV ¹⁶O on ¹²C, emerging along the 〈110〉 axis of a 12-μm-thick diamond crystal. The extracted times ranged from 4×10^-19 sec for Mg to 4×10^-18 sec for N, and they are consistent with statistical-model predictions.

New observations of electron-energy-loss spectra for diamond have been made by exploiting the direction dependence of the spectra. The loss peaks known from other workers were present with high intensities. The 23-eV loss peak is attributed mainly to interband transitions, in contrast to other earlier assignments as being mainly due to surface plasmons. The dependence of the loss spectra on direction is interpreted as being due to LEED (low-energy electron diffraction). We suggest that this effect, namely LEED-modulated loss spectroscopy, may be used as a complementary tool for the study of crystalline levels.

Axially channeled electrons are captured into bound states of the "atomic-string" potential. When two rows lie in close proximity as in the 〈110〉 direction of diamond, the potentials overlap, forming a saddle point between the rows. For 4-MeV electrons in the 〈110〉 diamond potential, the 2p level lies above the saddle point, and molecular states are formed. The observation of radiation arising from transitions of these states is reported. Spectral information is used to deduce electron-density enhancement in the C-C bond.

In this paper the critical angles for ions channeling in diamond single crystals are presented and considered. Careful attention was paid to experimental precautions to ensure reliable results. In diamond the thermal vibration amplitude is much less than the Thomas-Fermi screening distance even at ordinary temperatures, and this crystal can be expected to furnish a stringent test of theoretical critical-angle expressions which depend on the ratio of these two quantities. Data were taken for different axes and planes, for different ions and ion energies, and for different crystal temperatures. It is shown that most of the diamond data are in fact well represented by the established semiempirical expressions due to Barrett and can also be predicted accurately by the procedure of Varelas and Sizmann. This provides fresh confirmation of the continuum model for channeling and of some of the physical considerations underlying these theoretical approaches. The areas of disagreement serve to highlight important second-order effects of crystal geometry and can be understood within the terms of current theory.