S. Kailas

Turbulent flux events in a nearly neutral atmospheric boundary layer

We propose here a novel method of analysing turbulent momentum flux signals. The data for the analysis come from a nearly neutral atmospheric boundary layer and are taken at a height of 4 m above ground corresponding to 1.1×105 wall units, within the log layer for the mean velocity. The method of analysis involves examining the instantaneous flux profiles that exceed a given threshold, for which an optimum value is found to be 1 s.d. of the flux signal. It is found feasible to identify normalized flux variation signatures separately for positive and negative ‘flux events’—the sign being determined by that of the flux itself. Using these signatures, the flux signal is transformed to one of events characterized by the time of occurrence, duration and intensity. It is also found that both the average duration and the average time-interval between successive events are of order 1 s, about four orders of magnitude higher than a wall unit in time. This episodic description of the turbulence flux in the time domain enables us to identify separately productive, counter-productive and idle periods (accounting, respectively, for 36, 15 and 49% of the time), taking as criterion the generation of the momentum flux. A ‘burstiness’ index of 0.72 is found for the data. Comparison with laboratory data indicates higher (/lower) ejection (/sweep) quadrant occupancy but lower (/higher) contributions to flux from the ejection (/sweep) quadrant at the high Reynolds numbers of the atmospheric boundary layer. Possible connections with the concept of active and passive motion in a turbulent boundary layer are briefly discussed.

HEAVY-ION INDUCED FISSION AT NEAR-BARRIER ENERGIES

The study of heavy-ion induced fission fragment angular distributions continues to be a source of rich information as regards fission process in general and fission dynamics in particular. Considerable progress has been made towards understanding many features of the fission phenomenon. While some of the new sets of data measured in the last few years have confirmed the theoretical expectations, the others have provided surprises not quite anticipated. In the present review article the emphasis will be mainly on the recent experimental results of heavy-ion-induced fission fragment angular distributions at energies near the fusion barrier, their implications and new puzzles in this area which require not only suitable explanation but also additional measurements.

Influence of projectile α-breakup threshold on complete fusion

Abstract Complete fusion excitation functions for B 11 , 10 + Tb 159 have been measured at energies around the respective Coulomb barriers, and the existing complete fusion measurements for Li 7 + Tb 159 have been extended to higher energies. The measurements show significant reduction of complete fusion cross sections at above-barrier energies for both the reactions, B 10 + Tb 159 and Li 7 + Tb 159 , when compared to those for B 11 + Tb 159 . The comparison shows that the extent of suppression of complete fusion cross sections is correlated with the α-separation energies of the projectiles. Also, the two reactions, B 10 + Tb 159 and Li 7 + Tb 159 were found to produce incomplete fusion products at energies near the respective Coulomb barriers, with the α-particle emitting channel being the favoured incomplete fusion process in both the cases.

Backscattering spectrometry studies on metal ion distribution in polymer inclusion membranes

Abstract Polymer inclusion membranes based on cellulose triacetate polymer, containing 2-nitrophenyloctyl ether as plasticizer and dinonyl napthalene sulfonic acid as cation exchanger were loaded with Cs+ and Ag+ and were studied by backscattering spectrometry technique to measure the distribution of the metal ions across the membrane using a proton beam. The study showed that the distribution of the metal ions across the membrane is uniform, thereby suggesting the uniform distribution of the cation exchanger. This indicates that the hydrophilic nature of the sulfonic acid group in the cation exchanger does not significantly influence its distribution in the predominantly hydrophobic matrix of cellulose triacetate. However, the microstructure of the membrane, obtained by atomic force microscopy, shows the formation of blobs on the surface of the PIM indicating the inhomogeneity at the sub-micron scale.

Fusion of 19F with 188, 192Os

Fission fragment angular distributions and evaporation residue excitation functions have been measured in the bombarding energy range from 80 to 105 MeV for the 19F+188, 192Os systems. The measured fission and evaporation residue excitation functions are compared with the predictions of the statistical model code PACE. The fission and the evaporation residue excitation functions alone are not sufficient to constrain the statistical model parameters. Hence, the pre-fission neutron multiplicity, multi-chance behaviour of fission and the fission fragment angular distribution, which are very sensitive to statistical model parameters, have also been considered in carrying out a comprehensive statistical model analysis. Fission fragment anisotropies calculated in the statistical saddle point model (SSPM) framework using rotating finite range model (RFRM) effective moment of inertia values were found to over-predict the experimental data. Using scaled up values of RFRM effective moment of inertia, a good agreement between the fission anisotropy data and the SSPM calculations has been obtained over the entire energy range except at the highest energy.

Heavy-ion total reaction cross sections

A one-parameter expression has been found which predicts with a smaller χ2 than previous expressions, the total reaction cross sections for a large number of heavy-ion systems covering a wide energy range. The expression uses an interaction radius which is a sum of the equivalent uniform matter radii of the interacting nuclei plus and energy-dependent term.

Total (p, n) reaction cross section study on51V over the incident energy range 1·56 to 5·53 MeV

The total (p, n) reaction cross section for51V has been measured as a function of proton energy in the energy range 1·56 to 5·53 MeV with thick and thin targets. The fluctuations in the fine resolution excitation functions were analysed, to extract 〈Γ〉, the coherence width. The thick target excitation function suitably averaged over appropriate energy intervals has been compared with the optical model, Hauser-Feshbach and Hauser-Feshbach-Moldauer calculations. The strong isobaric analog resonance atEp ∼ 2·340 has been shape analysed to extract the proton width Γp, the spreading withW and the spectroscopic factor.

Exploring contributions from incomplete fusion inLi6,7+Bi209andLi6,7+Pt198reactions

We use the breakup absorption model to simultaneously describe the measured cross-sections of the Complete fusion (CF), Incomplete fusion (ICF), and Total fusion (TF) in nuclear reactions induced by weakly bound nuclei

Measurement of 42 MeV 7Li projectile breakup on 58Ni target beyond grazing incidence

^{6,7}

Exploring the breakup and transfer coupling effects in 9 Be elastic scattering

Li on