Unexpected behavior of the near-threshold double-photoionization cross section of beryllium

The threshold region of the double-photoionization cross section of atomic beryllium was investigated using monochromatized synchrotron radiation. The photon energy dependence of the double-photoionization cross section can be described by the Wannier power law up to 1.7 eV above threshold. However, we unexpectedly find oscillations in the cross section, which are in excellent agreement with a modulated threshold law based on the Coulomb-dipole theory [Phys. Rev. Lett. 49, 365 (1982)]]. This new finding casts some doubts on the general applicability of the Wannier power law.     

Experimental evidence for modulations in the relative double-photoionization cross section of C60 from threshold up to 280 eV

The relative double-photoionization cross section of neutral C60 clusters was investigated using monochromatized synchrotron radiation between 18 and 283 eV. Our measurement of the double-to-single photoionization ratio reveals two modulating components that are superimposed on a smooth ratio curve from threshold (19.0 eV) up to 280 eV, when inner-shell excitations become possible. The maxima in the modulation can be related to geometrical dimensions of the C60 cluster.     

The total cross section for π+ electroproduction on hydrogen near threshold

The total cross section for the reaction ep → enπ⁺ at momentum transfers |k²| between 0.08 and 0.32 ($\text{GeV}\text{c}$)² and for centre of mass energies between threshold, at W = 1.078 GeV, and W = 1.106 GeV has been measured by detecting the electron and neutron in coincidence. Comparisons are made with both current algebra and dispersion relation theoretical models.     

Near threshold neutron-fission cross section

The formal theory of intermediate structure in nuclear reactions is applied to the problem of sub-threshold fission processes induced by a neutron beam in a region of overlapping resonances. An explicit expression is obtained for the energy-averaged cross section also in the presence of a fission vibration in the second well. A method is shown for computing heights and curvatures of the two fission barriers from a knowledge of the slope of the cross section near the threshold, using also information from low-energy intermediate structures and from g.s. isomers. The method is applied to ²³⁵U, ²³⁸Np and ²⁴³Pu. The existence of a new isomer in ²³⁸Np is also predicted.     

The d(7Li,p)8Li cross section near E cm = 0.61 MeV

The absolute cross section σ of the d(⁷Li, p)⁸Li reaction near the E_cm = 0.61 MeV resonance has been measured using a ⁷Li ion beam and a windowless gas target system filled with D₂ gas. The proton yield of the reaction and the β-delayed α-activity of the residual nuclides ⁸Li were observed both concurrently with the elastic scattering yield, relating σ to the Rutherford scattering cross section σ_R. The resulting values, σ (from p) = 143.6 ± 8.9 mb and σ (from ⁸Li) = 151 ± 20 mb, lead to a weighted mean value of σ = 153 ± 6 mb (χ² = 2.26) including all available values and σ = 146 ± 5 mb (χ² = 0.05) removing some values from the data set. The consequences for the expected flux of high-energy solar neutrinos are discussed.     

Absolute pair production cross section measurements in Z = 92 at energies near threshold

Absolute cross sections for the production of electron-positron pairs by photons on uranium were measured using the internal source method with γ-rays of 1.119 and 2.6145 MeV, and for the mixed γ-rays in the decay of ⁶⁰Co.     

Energy dependence of11Li dissociation cross section

The Coulomb breakup cross section of¹¹Li is calculated as a function of its bombarding energy. Comparison is made to cross sections at 790 MeV/nucleon and 30 MeV/nucleon. Low energy reactions on a high-Z target show a greatly enhanced Coulomb breakup cross section that is more sensitive to the distribution of dipole response strength than high energy reactions thus providing more structure information.     

Charm and bottom quark production cross sections near threshold

The cross sections for charm and bottom quark production in the threshold region are discussed. We consider the effects of an all order resummation of initial state soft-plus-virtual gluon radiation on the total cross sections compared to the order $←pha_{s}^{3}$ results.     

Near threshold photodetachment cross section of negative atomic oxygen ions

A 40-target state close-coupling calculation for the photodetachment cross section of negative atomic oxygen near threshold is carried out with core－valence electron correlation by using the R-matrix method. It was shown that after considering the excitations of two electrons from the 2s shell, the electron affinity of O^- (2s^22p^5{}^2P°) agrees with the experimental result much better than that just considering the excitations of electrons only from the 2p shell as well as only one electron from the 2s shell. Total cross section as well as the main contribution of the ionization channels to the partial cross section are illustrated to show the structure near threshold clearly.     

Inelastic electron scattering from 6Li near the α-d threshold

The form factor for the alpha-deuteron continuum in ⁶Li has been measured in the excitation region within 2 MeV of threshold, for momentum transfers q < 1.2 fm^?1. The results are interpreted in terms of an α-d cluster model. The monopole breakup appears to be predominant near threshold, but the influence of the J = 2⁺ (4.31 MeV) quadrupole resonance becomes important a few MeV above threshold. The form factor for the C2 transition to the 3⁺ (2.185 MeV) state is also presented.