Three-Body Spectrum of 18C and its Relevance to r-Process Nucleosynthesis

The ¹⁸C spectrum has been studied in a three body n + n +¹⁶C model that includes deformation and the 2⁺ excitation of the ¹⁶C core as well as Pauli projection of forbidden states. The ¹⁶C – n interaction employed in this study has been fitted to reproduce the experimental spectrum of ¹⁷C. The calculations show that two neutron separation energy in ¹⁸C in consistent with three-body structure of this nucleus and predict more states bound with respect to three-body decay. The comparison of their position to known excited states in ¹⁸C is discussed. These calculations suggest also that a few states may exist in astrophysically relevant region between the ¹⁷C+n and ¹⁶C + 2n decay thresholds. The most important of them is 1⁻ as it can give a large E1 resonant contribution to ¹⁷C(n, γ)¹⁸C neutron capture. The calculations also suggest that a virtual s-wave state may exist above the ¹⁷C + n threshold that can give rise to non-negligible M1 contributions to the ¹⁷C(n, γ)¹⁸C reaction rate. The presence of these states in the ¹⁸C spectrum can lead to an increased ¹⁷C(n, γ)¹⁸C reaction rate, which can significantly influence the abundances of uranium and thorium synthesized in the r-process in the supernovae explosions.     

Computer simulation of proton channelling in silicon

The channelling of 3 MeV protons in the 〈110〉 direction of silicon has been simulated using Vineyard model taking into account thermally vibrating nuclei and energy loss due to ion-electron interactions. A beam made up of constant energy particles but with spatial divergence has been simulated for the purpose. The values of the minimum scattering yield and half width of the channelling dip are shown to be depth sensitive and agree well with the measured values. The dependence of yield on the angle of incidence has been found to give information of all three types of channelling. The critical angles for the three types of channelling and wavelength of planar oscillations are consistent with the previous calculations.     

Energy dependence of multiplicity in proton-nucleus collisions and models of multiparticle production

This is a continuation of our earlier investigation (Gurtuet al 1974Phys. Lett. 50 B 391) on multiparticle production in proton-nucleus collisions based on an exposure of emulsion stack to 200 GeV/c beam at the NAL. It is found that the ratioR _em = 〈n _s〉/〈n _ch〉, where 〈n _ch〉 is the charged particle multiplicity in pp-collisions, increases slowly from about 1 at 10 GeV/c to 1·6 at 68 GeV/c and attains a constant value of 1·71 ± 0·04 in the region 200 to 8000 GeV/c. Furthermore,R _em = 1·71 implies an effectiveA-dependence ofR _A =A ^0.18,i.e., a very weak dependence. Predictions ofR _em on various models are discussed and compared with the emulsion data. Data seem to favour models of hadron-nucleon collisions in which production of particles takes place through adouble step mechanism,e.g., diffractive excitation, hydrodynamical and energy flux cascade as opposed to models which envisage instantaneous production.     

15C-15F Charge symmetry and the 14C(n,gamma)15C reaction puzzle

The low-energy reaction 14C(n,gamma)15C provides a rare opportunity to test indirect methods for the determination of neutron capture cross sections by radioactive isotopes versus direct measurements. It is also important for various astrophysical scenarios. Currently, puzzling disagreements exist between the 14C(n,gamma)15C cross sections measured directly, determined indirectly, and calculated theoretically. To solve this puzzle, we offer a strong test based on a novel idea that the amplitudes for the virtual 15C-->14C + n and the real 15F -->14O + p decays are related. Our study of this relation, performed in a microscopic model, shows that existing direct and some indirect measurements strongly contradict charge symmetry in the 15C and 15F mirror pair. This brings into question the experimental determinations of the astrophysically important (n,gamma) cross sections for short-lived radioactive targets.     

Efimov Spectrum in Bosonic Systems with Increasing Number of Particles

It is well-known that three-boson systems show the Efimov effect when the two-body scattering length a is large with respect to the range of the two-body interaction. This effect is a manifestation of a discrete scaling invariance (DSI). In this work we study DSI in the N-body system by analysing the spectrum of N identical bosons obtained with a pairwise gaussian interaction close to the unitary limit. We consider different universal ratios such as \({E_N^0/E_3^0}\) and \({E_N^1/E_N^0}\) , with \({E_N^i}\) being the energy of the ground (i = 0) and first-excited (i = 1) state of the system, for \({N \leq16}\) . We discuss the extension of the Efimov radial law, derived by Efimov for N = 3, to general N.