123993434
Theoretical interpretation of the systematics of effective single-particle level densities from (n,p) reactions at 14.8 MeV energy
The universal experimental data for the energy-integrated angular distribution and the angle-integrated energy spectra for protons emitted from the (n,p) reactions at 14.8 MeV incident energy were analyzed earlier using the computer code PRECO-D2, developed on the basis of Kalbach’s semiempirical model for the preequilibrium reactions. The results of the analysis provided the semiempirical systematics of the single-particle level densities gR(exp) (effective in the residual nuclei) and gc(exp) (effective in the composite system). In order to interpret these results, we have carried out the theoretical calculations with Shlomo’s theory developed on the basis of the Green’s function approach. The theoretical values based on Shlomo’s theory for g Th (eRn )a ndg Th Rp (eRp) for the residual nucleus and g Th cn (ecn )a ndg Th cp (ecp ) for the composite systems, respectively, were calculated at various excitation energies by using a reasonable single-particle nuclear potential strength V0 ,a vailable from systematics in literature. Here, eRn and eRp are the single-particle excitation energies for single-particle level densities for the residual nucleus, and ecn and ecp are the single-particle excitation energies of effective single-particle level densities for the composite system for neutrons and protons, respectively. The Coulomb interaction potential Vc was included for protons over and above the nuclear potential V0. The total theoretical values were taken as g TTh (eR) = g Th Rn(eRn) + g Th Rp (eRp) for residual nuclei and g TTh c (ec) = g Th cn (ecn) + g TTh cp (ecp ) for the composite system. eR is the Fermi energy of the total effective single-particle level density for residual nuclei, and ec is the excitation energy of the effective single-particle level density of the composite system. Careful comparison of theoretical and experimental results shows that gR(exp) matches with g TTh R (ef )f orV i 0 = 45 MeV where (ef ) is the Fermi energy. The theoretical values g TTh (ef ) also nearly agree with the previously reported values by various groups, obtained through different approaches for a large number of cases. The values of gc(exp) always are found to be greater than gR(exp) and match with g TTh c (ec) at effective excitation energies ec, which are found to be invariably much higher than the respective ef and are positive and follow the nuclear shell model structure when plotted against A. This supports the concept that the values of effective gc(exp) are decided mainly by dominant transition, which occurs during initial stages of the multistep statistical direct preequilibrium process that involves unbound states. The values of gR(exp), on the other hand, are found to correspond to the effective eR around the Fermi energies of the bound states of residual nuclei involved in the decay processes. The ratios of gc(exp)/gR(exp) are found to follow the ( � e c� � ef � ) 1/2 f description and to match Shlomo’s theory.
124715654
Search for an effect of shell closure on nuclear dissipation via a neutron-multiplicity measurement
119414440
Cometary panspermia explains the red rain of Kerala
Red coloured rain occurred in many places of Kerala in India during July to September 2001 due to the mixing of huge quantity of microscopic red cells in the rainwater. Considering its correlation with a meteor airbust event, this phenomenon raised an extraordinary question whether the cells are extraterrestrial. Here we show how the observed features of the red rain phenomenon can be explained by considering the fragmentation and atmospheric disintegration of a fragile cometary body that presumably contains a dense collection of red cells. Slow settling of cells in the stratosphere explains the continuation of the phenomenon for two months. The red cells under study appear to be the resting spores of an extremophilic microorganism. Possible presence of these cells in the interstellar clouds is speculated from its similarity in UV absorption with the 217.5 nm UV extinction feature of interstellar clouds.
119347677
On the Shallow Decay of Some GRB Afterglows
Half of the radio afterglows for which there is a good temporal coverage exhibit, after 10 days from the burst, a decay which is shallower than at optical frequencies, contrary to what is expected within the simplest form of the standard model of relativistic fireballs or jets. We investigate possible ways to decouple the radio and optical decays. First, the radio and optical emissions are assumed to arise from the same electron population, and we allow for either a time‐varying slope of the power‐law distribution of electron energy or for time‐varying microphysical parameters. Then we consider two scenarios where the radio and optical emissions arise in distinct parts of the GRB outflow, either because the outflow has an angular structure or because there is a long‐lived reverse shock. We find that only the last scenario is compatible with the observations.
16519092
Analytic Light Curves of Gamma-Ray Burst Afterglows: Homogeneous versus Wind External Media
Assuming an adiabatic evolution of a gamma-ray burst (GRB) remnant interacting with an external medium, we calculate the injection, cooling, and absorption break frequencies and the afterglow flux for plausible orderings of the break and observing frequencies. The analytical calculations are restricted to a relativistic remnant and, in the case of collimated ejecta, to the phase where there is an insignificant lateral expansion. Results are given for both a homogeneous external medium and a wind ejected by the GRB progenitor. We compare the afterglow emission at different observing frequencies, for each type of external medium. It is found that observations at submillimeter frequencies during the first day provide the best way of discriminating between the two models. By taking into account the effect of inverse Compton (IC) scatterings on the electron cooling, a new possible time dependence of the cooling break is identified. The signature of the upscattering losses could be seen in the optical synchrotron emission from a GRB remnant interacting with a preejected wind, as a temporary mild flattening of the afterglow decay. The upscattered radiation itself should be detected in the soft X-ray emission from GRB remnants running into denser external media, starting a few hours after the main event.