Resonant Bhabha scattering at MeV energies

We discuss, on a phenomenological level, the possible appearance of resonances ine ⁺+e ^? scattering at energies in the MeV region. The expected resonance cross section and angular distribution are examined. The observability depends crucially on the attainable energy resolution which is limit by the momentum distribution of the electrons contained in the target.     

From quasi-elastic to quasi-compound nucleus reactions induced by a 140 MeV 19F beam bombarding 89Y target nuclei

Various exit channels ranging from ⁶Li to ²⁷Al for the ⁸⁹Y(¹⁹F, x)y reaction have been studied at 140 MeV incident energy. Energy spectra and angular distributions were measured between 10° and 160° laboratory angles. Three different reaction mechanisms were observed: quasi-elastic collisions, deep-inelastic collisions and, with weaker cross sections and longer lifetimes, compound nucleus-like processes.     

Resonant scattering of an X-ray photon by a heavy atom

The influence of many-body and relativistic effects on the absolute values and shape of the double differential cross section for the resonant scattering of a linearly polarized X-ray photon by a free xenon atom near the K-shell ionization threshold has been theoretically analyzed. The evolution of the spatially extended structure of the scattering cross section to the K _{α, β} structure of the X-ray spectrum of the xenon atom emission has been demonstrated. The calculations have been performed in the dipole approximation for the anomalous dispersion component of the total inelastic scattering amplitude and in the impulse approximation for the contact component of this amplitude. The contribution of the Rayleigh (elastic) scattering component is taken into account using the methods developed in Hopersky et al., J. Phys. B 30, 5131 (1997). The effects of the radial relaxation of the electron shells, spin-orbit splitting, double excitation/ionization of the atomic ground state, as well as the Auger and radiative decays of the produced main vacancies, are considered. Using the results obtained by Tulkki, Phys. Rev. A 32, 3153 (1985) and Biggs et al., At. Data Nucl. Data Tables 16, 201 (1975), the nonrelativistic Hartree-Fock wavefunctions are changed to the relativistic Dirac-Hartree-Fock wavefunctions of the single-particle scattering states when constructing the process probability amplitude. The calculations are predicting and are in good agreement with the synchrotron experiment on the measurement of the absolute values and shape of the double differential cross section for the resonant scattering of an X-ray photon by a free xenon atom reported by Czerwinski et al., Z. Phys. A 322, 183 (1985).     

Three-body calculation of neutron-deuteron elastic scattering in the energy region from 2.5 MeV up to 50 MeV

The three-body Faddeev equations for neutron-deuteron scattering are solved in the energy region from 2.5 MeV to 50 MeV of the incident neutron energy with small energy steps. Higher-rank separable potentials are used in the¹ s ₀ wave and in the³ s ₁—³ d ₁ waves, while rank-1 separable potentials are used in¹ p ₁,³ p _0,1,2 ¹ d ₂ ³ d _2,3 waves. The calculation is compared with experiments for the total cross section, the total break-up cross section, the differential cross section and the analyzing power of neutron-deuteron scattering. The improvements in the agreement as compared to previous calculations are impressive in many cases. Especially, the calculated total cross section agrees with the experiment below 30 MeV within the error bars, which are as small as 1%. A discussion on the numerical accuracy is given. General aspects of the calculated cross section are discussed. It is pointed out that thes-wave asymptotic normalization of the deuteron wave function (A _s ) is important.     

Resonant inelastic scattering of an X-ray photon by a linear molecule

The double differential cross section of resonant inelastic scattering of a linearly polarized X-ray photon by a spatially oriented HF molecule in a gas phase is theoretically described in the energy range of the ionization threshold of the deep molecular orbital 1σ. The effects of radial relaxation of the wave functions of the core and excited scattering states in fields of the formed core vacancies, the vibronic effects, and the effects of Auger and radiative decays of the vacancies are taken into account in the nonrelativistic Hartree-Fock approximation. The results of the calculations are predictive and agree well with the results of the experimental measurements of the Kα emission spectrum of the HF molecule at an incident photon energy of 2 keV, which considerably exceeds the energy of the 1σ ionization threshold.     

Resonant scattering of photon by electron in the presence of the pulsed laser field

The scattering of photon by electron in the presence of the field of the low-intensity circularly-polarized pulsed laser wave is analyzed at a laser pulse duration that is significantly greater than the characteristic oscillation time. The conditions for the resonance process in which the intermediate particle is located in the vicinity of the mass surface are determined. The existence of three resonance ranges is demonstrated. The first one corresponds to the resonance of direct diagram, and two ranges correspond to the resonance of exchange diagram via electron and positron intermediate states. The conditions under which only the resonance of the direct diagram takes place are determined, and the probability of such process is calculated. It is demonstrated that the resonance probability can be significantly greater than the probability of the Compton effect in the absence of the external field.     

Quasi-elastic scattering of the proton drip line nucleus 17F on 12C at 60 MeV

The quasi-elastic scattering angular distribution of the proton drip line nucleus ¹⁷F on a ¹²C target was measured at 60 MeV. The experimental data have been compared with the theoretical analysis based onto optical model and continuum discretized coupled channels (CDCC). The couplings between breakup and elastic scattering channels, and between inelastic and elastic scattering channels resulted very weak. In order to explore the breakup effects the total reaction cross-section was deduced from the angular distribution of the quasi-elastic scattering data, and then compared with the existing data for the other weakly and tightly bound nuclei on ¹²C target using a universal function. From this comparison, we concluded that the breakup effect is not important for weakly bound projectiles on the light target as obtained also with the CDCC analysis.     

Proton-proton scattering from 0.35 to 1.0 MeV

Differential cross sections for the scattering of protons by protons have been measured at energies from 0.35 to 1.0 MeV over an angular range from 24° to 110° in the c.m. system with an absolute accuracy of 0.2 %. Special care was taken at energies around the interference minimum where, due to the extremely small values of the cross section, the accuracy has been 0.8 % at θ_{c.m. = 90°}. The resulting value for the minimum energy is E_IM = 382.48 ± 0.10 keV. The high accuracy could be achieved mainly by using a new double compensated calorimeter for the current integration.     

Differential cross sections of proton compton scattering at photon laboratory energies between 700 and 1000 MeV

Differential cross sections of proton Compton scattering have been measured at the Bonn 2.5 GeV synchrotron. 78 data points are presented as angular distributions at photon lab energies of 700, 750, 800, 850, 900, and 950MeV. The c.m. scattering angle ranges from 40°–130°, corresponding to a variation of the four momentum transfer squared betweent=?0.10 tot=?0.96 GeV² at 700 and 950 MeV, respectively. Two additional differential cross sections have been measured at 1000MeV, 35.6° and 47.4°. The angular distributions show forward peaks whose extrapolations to 0° are consistent with calculated forward cross sections taken from literature. The small angle data (|t| ?0.2 GeV²) together with the calculated cross sections at 0° are also consistent with the assumption of a slope parameterB of 5 GeV^?2. For the first time a rerise of the angular distributions towards backward angles has been observed. It becomes less steep with increasing energy. The most interesting feature of the angular distributions is a sharp structure which appears betweent=?0.55 GeV² at 700MeV andt=?0.72 GeV² at 950 MeV. Such a rapid varation of the differential cross section witht has never been ovserved in elastic hadron-hadron scattering or photoproduction processes. It indicates the existence of a dynamical mechanism which could be a peculiarity of Compton scattering.     

Small-angle photon scattering on complex nuclei

Photons of 3 GeV and 5 GeV were scattered on 7 different elements, ranging from Be to Au, and detected with a pair spectrometer. The angular distributions show diffractive patterns consistent with known nuclear sizes. Forward cross sections are 20–30% lower than expected from an A² dependence. This shadowing effect is qualitatively explained by photon interactions via intermediate hadronic states.     

7Li quasi-free scattering off the α-cluster in 9Be nucleus

Spectra of coincident charged particles from the reactions induced by a 52 MeV ⁷Li beam incident on a beryllium target were measured. Strong contributions of the ⁷Li quasi-free scattering off the α-cluster in ⁹Be nucleus were observed. This observation supports the conclusions from the study of complete fusion of weakly bound light nuclei at low energies that the “fragility” of the nuclei makes their fusion less probable. Received: 1 June 1998     

Radiative corrections to pion nucleus scattering

The radiative corrections to elastic pion-nucleus scattering are evaluated for kinematical conditions pertaining to the pion spectrometer facilities at the newer meson factories, and are found to be experimentally relevant. A reasonably simple, experimentally useful approximation to the exact expression for the radiative correction is given.     

Real and virtual photon contributions to inelasticep scattering

We investigate direct and resolved photon contributions to hardep scattering at high energies. Some terms of the direct contribution are already included in the resolved photon one, described in terms of the photon structure functions. A suitable subtraction mechanism has to be found in order to avoid double counting. Inelasticep scattering requires the knowledge of photon structure functions over a wide range in photon virtuality, in particular from approximately zero up to almost 10⁵ GeV² at HERA. We analyse the real and virtual photon structure function and we comment on the structure of the photon in the intermediate region of low virtuality. We finally describe howep cross sections can be obtained as functions of the virtuality of the exchanged photon.     

R-matrix formulation and phase shifts for n-He and p-He scattering for energies up to 20 MeV

An R-matrix formulation for a single level with a background approximation has been used to analyse n-⁴He and p-⁴He elastic scattering data below the inelastic thresholds near 22 MeV. For every partial wave a single level and a distant pole contribution constant with energy has been employed. Simple relations between neutron and proton scattering parameters have been sought to possibly derive more dependable values for neutron phase shifts and analysing powers at energies where little experimental information exists. The R-matrix parameters corresponding to ⁵He and ⁵Li ground and 1st excited states, phase shifts up to 20 MeV and neutron analysing powers up to 15 MeV are given in tabular form and are compared to results of earlier analyses. Differences in n-⁴He phase shifts are discussed and shown to be due to a scarcity of accurate neutron scattering data above 11 MeV. Some typical fits to n-⁴He scattering data are shown.     

Probing scattering wave functions close to the nucleus

Recently, three-dimensional imaging of the ejected electrons following 100 MeV/amu C6+ single ionization of helium led to the observation of a new structure not predicted by theory [Nature (London) 422, 48 (2003)]]. Instead of the usual "recoil lobe" centered on the momentum-transfer axis, a ring-shaped structure centered on the beam axis was observed. New measurements at 2 MeV/amu exhibit a similar structure, which is now predicted by theory. We argue that the same theory failed at 100 MeV/amu because the faster projectiles probe distances much closer to the nucleus, where our multiple-scattering model is expected to break down.     

Microscopic interpretation of 7Li+24Mg inelastic scattering at 34 MeV

Theoretical differential cross sections for ⁷Li+²⁴Mg inelastic scattering have been calculated in the DWA using a double folding model. The projectile and target transition densities are matched to electron scattering data and the nucleon-nucleon interaction is chosen to reproduce the real part of the optical potential at the distance $\text{D}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ where there is 50 % transmission in the elastic channel. There are no free parameters in the calculations. The agreement between the theoretical results and the available experimental data is good which serves to establish a consistency between elastic and inelastic scattering. The cross section for a mutual transition which leaves both the projectile and target in their first excited states is found to come mainly from relative L_r = 4 transfer. It is speculated that a quadrupole term in the optical potential might make important contributions at backward angles in the elastic cross section.