Generation of amorphous surface layers in LiNbO3 by ion-beam irradiation: thresholding and boundary propagation

The refractive-index profiles induced by high-energy (5 MeV, 7.5 MeV) silicon irradiation in LiNbO₃ have been systematically determined as a function of ion fluence in the range 10¹³–10¹⁵ cm^-2. At variance with irradiations at lower energies, an optically isotropic (‘amorphous’) homogeneous surface layer is generated whose thickness increases with fluence. These results have been associated with an electronic excitation mechanism. They are discussed in relation to the well-documented phenomenon of latent (amorphous) track generation under ion irradiation, requiring a threshold value S_e,th for the electronic stopping power S_e. Our optical data have yielded a value of ≈5 keV/nm for such a threshold, within the range reported by independent single-track measurements. The propagation of the amorphous boundary into the crystal during irradiation indicates that the threshold value decreases on increasing the fluence. Complementary Rutherford backscattering–channeling and micro-Raman (on samples irradiated at 30 MeV) experiments have been performed to monitor the induced structural changes. PACS 61.80.-x; 61.80.Az; 61.80.Jh     

Non-Gaussian range profiles in amorphous solids

Range profiles of boron in amorphous silicon exhibit pronounced deviations from Gaussian at energies above about 40 keV due to increasing electronic stopping. A detailed comparison with computed profiles allows a semiempirical determination of the electronic stopping cross section (S_e ≈ E^0.4).     

Quasimolecular KX-ray excitation by bombarding As,Zr, Nb,Mo and Rh target with Nb ions

By bombarding various targets with 65 MeV and 40 MeV Nb ions continuous X-ray distributions have been obtained, which range up to the KX-ray energies of quasiatoms woth Z=Z₁+Z₂. The hogh-energy parts of these continua are interpreted as KX-radiation of quasimolecules transiently formed during the adiabatic heavy-ion atomic collisions.     

Atomic mixing of Ni2O3/SiO2, NiO/SiO2, and Ni/SiO2 interfaces induced by swift heavy ion irradiation

We have investigated the interface mixing of Ni₂O₃/SiO₂, NiO/SiO₂, and Ni/SiO₂ induced by the irradiation with Ar, Kr and Xe ions of energies ranging from 90 MeV to 260 MeV. Since these energies are in the electronic stopping regime, atomic transport processes will not be directly initiated by elastic ion–target collisions, but need to be excited by secondary processes like electron–phonon coupling or Coulomb explosion. Nevertheless, we have observed a strong mixing effect in the ceramic systems if the electronic energy loss exceeds a certain threshold value. Estimation of an effective diffusion constant indicates that diffusion takes place in the molten ion track. In contrast to the ceramics, the metallic Ni layer is still insensitive even for the highest electronic stopping power used (S_e=28 keV/nm) and does not exhibit mixing with its SiO₂ substrate. In addition, NiO/SiO₂ and Ni/SiO₂ were irradiated in the nuclear stopping regime with 600 keV Kr and 900 keV Xe–ions. Here the intermixing effect is in good agreement with the assumption of ballistic atomic transport. Received: 5 February 2002 / Accepted: 11 February 2002 / Published online: 3 May 2002 RID="*" ID="*"Corresponding author. Fax: +49-711/685-3866, E-mail: bolse@ifs.physik.uni-stuttgart.de     

High-resolution (e,e') study of isovector M1 and M2 transitions in the oxygen isotopes: (I). 16O

The M1 and M2 transition strength distribution for ¹⁶O in the excitation energy range from 16 to 20 MeV has been measured in a high-resolution electron scattering experiment. The M1 strength is concentrated in three sharp states at E_x = 16.22, 17.14 and 18.79 MeV (± 0.01 MeV) with B(M1, k)↑ = 0.20 ± 0.02, 0.32 ± 0.03 and 0.13 ± 0.03 μ_N², respectively. An additional strength of 0.35 ± 0.09 μ_N², distributed over eight weakly excited states with excitation energies E_x = 17.4 to 18.0 MeV, brings the total measured M1 strength to B(M1, k)↑ = 1.0 ± 0.1 μ_N². The experimental M2 strength is distributed over states at E_x = 16.82, 17.78, 18.50 and 19.0 MeV (± 0.01 MeV) with B(M2, k)↑ = 19 ± 2, 13 ± 2, 59 ± 7 and 341 ± 51 μ_N² · fm², respectively. Electric transitions were also measured to states at E_x = 16.45 MeV (2⁺, E2), 17.30 MeV (1⁺, E1) and 18.20 MeV (2⁺, E2). Calculations were performed using the modified surface delta interaction in a 2p-2h shell model for the M1 transitions and the random phase approximation for the M2 transitions. The results show the sensitivity of the M1 strength as a measure of ground-state correlations and compare well with results from the ¹⁵N($\text{p}$, γ) reaction.     

Messung des Energieverlustes nicht-kanalisierter α-Teilchen in Transmission und Deutung mit Hilfe eines Durchschnittspotentials

The anisotropies of the MO-2pσ-radiation have been measured for five symmetric systems with a united atom numberZ _u between 52 and 94 at beam energies in the range from 7 up to 66 MeV. A comparison with the available theoretical data suggests that the anisotropic part of the 2pσ-radiation is mainly due to transitions from near continuum initial states into the minimum of the molecular 2pσ-orbital.     

Calculations of implanted-ion range and energy-deposition distributions:11B in Si

The integro-differential equations for moments of range and energy-deposition distributions of heavy ions implanted into amorphous targets are solved by an improved method, allowing accuracy to be retained to higher energies. Correlation of electronic stopping with scattering is found to have negligible effects on the calculated distributions for those scattering cross sections for which uncorrelated-stopping calculations are meaningful; however inclusion of correlation allows a wider range of scattering potentials to be used in the calculations. Effects of varying this potential are explored and it is indicated that a careful study of the collision cascade could provide information about the potential.

Computation has been done for ¹¹B implanted into Si at energies from 1 keV to 10 MeV. Some comparison of the range calculations with experiment has been made.     

Das elektronische Bremsvermögen von Stickstoff und Sauerstoff für niederenergetische Protonen

The electronic stopping power of molecular oxygen and nitrogen for protons with energies between 1 keV and 30 keV has been measured using a differentially pumped stopping cell. Our results give a surprisingly good confirmation of the Lindhard-Scharff statistical theory which predicts a linear velocity dependence of the electronic stopping power at low projectile energies. Moreover our data are in fair agreement with earlier high energy (E ≧ 20 keV) measurements in other laboratories. The combination of the present measurements with theoretically calculated nuclear stopping powers yields an estimate of the atomic stopping power. This estimate leads to substantially lower atomic stopping powers at low energies compared to values derived from range measurements. This result may have interesting implications on auroral hydrogen emissions.     

Elastische Streuung von Deuteronen an10B bei niedrigen Energien

The excitation curve for the elastic scattering of deuterons from¹⁰B in the energy range from 1.0 to 2.0 MeV has been measured atθ _L =135°. Strong indications for compound nucleus resonances in¹²C at bombarding energies of approximately 1.0 and 1.9 MeV were observed. Angular distributions from 80 to 175° were determined in 200 keV steps from 1.0 to 2.0 MeV. Optical model analysis were made both for volume absorption and surface absorption potentials. Reasonable fits were obtained at energies up to 1.6 MeV with one set of parameters in both cases, whilst for the higher energies strong deviations were found with the same parameters.     

Nonlinear model-Boltzmann equations and exact solutions

The present paper is devoted to the subject of photonuclear reactions of the type (γ, p), (γ, n), (γ, np), (γ, nn), (γ, pp) as well as nucle on capture reactions as (p, γ) at intermediate photon energies. The theoretical part of this paper reviews and extends earlier calculations of the authors up t gamma energies of E_γ ≈ 400 MeV. The paper is not intended to give a review on different theoretical approaches which have been underta ken in the past. Instead, emphasis is put on the dynamical aspects of the photonuclear reactions in order to see which elementary processes govern the reactions at different photon energies. In view of the success of the present treatment up to E_γ ≈ 120 MeV photon energy, we tempt the predictive power of the model up to photon energies of E_γ ≈ 400 MeV for the reactions in ⁴He and ¹⁶O.     

XRD and AFM study of radiation damage induced by swift heavy ions in Y3Al5O12 single crystals

Defects induced in Y₃Al₅O₁₂ single crystals by swift heavy ions are investigated by X-ray diffraction (XRD) and atomic force microscopy. The irradiation was performed at GANIL with 561 MeV ⁵¹Cr, 466 MeV ¹²⁸Te, and 957 MeV ²⁰⁸Pb ions. The XRD data reveal that the lattice strain increases with increasing electronic stopping power, whereas the hillock parameters (height and diameter) are not influenced by the electronic stopping power. According to our experimental data, for the same mean electronic stopping power, the hillock parameters are more pronounced for the lower range in contrast to swelling measurements. The experimental data show a strong increase in the hillock parameter at higher fluence, indicating the amorphization of Y₃Al₅O₁₂ single crystals.     

Investigation of 89Y and 87Rb by (p,p′) reactions

The (p, p′) reaction on ⁸⁹Y at incident energies of 20.51 MeV and 14.33 MeV and on ⁸⁷Rb at 20.89 MeV has been studied. In ⁸⁹Y 28 levels with E_ex between 2 and 4.2 MeV and 79 levels with E_ex between 4.2 and 6 MeV have been identified. In ⁸⁷Rb 45 levels with E_ex up to 4.2 MeV have been found. Transferred orbital angular momenta and deformation parameters have been deduced from a macroscopic DWBA analysis of the differential cross sections. The experimental results are not consistent with the interpretation of states in ⁸⁹Y and ⁸⁷Rb as resulting from the weak-coupling of a proton or proton-hole to excited states of the ⁸⁸Sr core. Simple shell model arguments are able to yield at least a qualitative agreement with the level scheme as found for ⁸⁹Y.     

The 10,11B(n, p)10,11Be reactions at En=96 MeV

Double-differential cross sections of the ^10,11B(n, p)^10,11Be reactions have been measured at 96 MeV in the angular range 0°–30° for excitation energies up to 35 MeV. The spectra have been decomposed into different multipolarities using sample angular distributions calculated within the distorted-wave Born approximation. From the identified Gamow–Teller strength, S_β⁺ values were obtained for ¹⁰B and ¹¹B. At higher excitation energies, the spectra are dominated by L=1 strength in broad distributions with maxima around 22 and 12 MeV in ¹⁰B and ¹¹B, respectively.     

Excitation of the even-even nuclei 24Mg, 28Si and 32S by neutron scattering at incident energies between 7 and 14 MeV

Neutron scattering on ²⁴Mg, ²⁸Si and ³²S was studied in the incident energy range from 7 to 14 MeV. In the experiment natural samples and the time-of-flight technique were used. The theoretical analysis was extended including other experiments to bombarding energies around 14 MeV. The excitation of the first 2⁺ levels shows for these nuclei, despite their quite different nulcear structure, some similarities. For ²⁴Mg also higher-lying states up to E_x = 6.01 MeV have been investigated. The sum of compound-reaction and collective-model contributions reasonably reproduces all the experimental data.     

The tensor analyzing power AzzATθ = 0° for the 3He(d, p)4He reaction

The tensor analyzing power f_zz has been measured for the ³He($\text{d}$, p)⁴He reaction at 0 = 0° over an incident deuteron energy range E_d = 6.6–15.8 MeV in steps of 0.5 MeV. The present results agree with and extend the previous measurements of Grüebler et al. The present results indicate that this reaction is a very good tensor analyzer for polarized deuteron beams with energies up to 15.8 MeV.     

Detailed energy dependence of the elastic and inelastic12C +12C scattering between 5 and 10 MeV/nucleon

Excitation functions for¹²C+¹²C elastic and inelastic scattering to 2⁺ level have been measured over the energy range 30–60 MeV (cm) by 250 keV steps using the kinematical coincidence method. The intermediate structure resonances disappear aboveE _cm=35 MeV while the broad and irregular structure becomes a general feature of the interaction at higher energies.     

Multipole analysis of π+ and π0 photoproduction from threshold to Eγ = 330 MeV

We present results of an energy independent multipole analysis of π⁺ and π⁰ photoproduction on protons for photon energies between 160 MeV and 330 MeV. Each isospin part of the l ? 1 amplitudes has been parametrized; the 1 ? 2 amplitudes have been approximated by the Born terms. Unambiguous solutions for all the multipoles in the whole range of E_γ have been obtained. The continuity of the solutions as functions of the energy is very good.     

On the nuclear curvature energy

Volume integrals of the imaginary part of the proton and the neutron optical potentials obtained from the existing phenomenological analyses for the mass number range of 12–209 and for a range of nucleon energies (E _p =10–180 MeV,E _n =8–150 MeV) have been fitted with an empirical expression. The isoscalar, the isovector and the Coulomb components determined empirically have been compared with those obtained from the microscopic approach. Though there is qualitative agreement between the two predictions, there are differences when compared quantitatively.     

Shell-model calculations on Ni and Cu isotopes

Binding energies, excitation energies and spectroscopic factors have been calculated for^57–67Ni and^58–68Cu in an unrestricted (2p_3/2, lf _5/2,2p_1/2) shell-model space. The effective two-body matrix elements are obtained from the modified surface delta interaction (MSDI) and from a least-squares fit to experimental binding and excitation energies (ASDI). The average deviation between about 100 experimental and calculated energies is 0.14MeV for MSDI and 0.08 MeV for ASDI. Excitation energies of high-spin states are given also. Spectroscopic factors have been calculated for all single-nucleon transfer reactions on stable Ni or Cu targets leading to Ni or Cu isotopes. For spectroscopic factors larger than 0.4 the average deviation between theory and experiment is about 30%. The experimentally observed and calculated spectroscopic strengths are compared by using sum rules and are found to be consistent. An extensive compilation has been made of experimental data on energies,J ^π assignments and spectroscopic factors.