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1.
Using a dynamical Langevin equation coupled with a statistical decay model, we calculate the excess of the pre-scission neutron multiplicities over its standard statistical-model values as a function of the nuclear dissipation strength for the three nuclei 190Os, 200Hg, and 210Po which have the same neutron-to-proton ratio N/Z. We find that by decreasing the size of the fissioning nuclei, the effects of nuclear dissipation on the excess of the pre-scission neutron multiplicity are substantially amplified, and that the sensitivity of this excess to the nuclear friction strength is considerably increased as well. We suggest that for those fissioning systems with the same N/Z that are populated in fusion reactions, to obtain a more accurate information of the nuclear dissipation strength by measuring the pre-scission neutron multiplicity, it is best to choose a system with a small size. 相似文献
2.
HADI ESLAMIZADEH 《Pramana》2016,87(1):2
A stochastic approach to fission dynamics based on two-dimensional Langevin equations was applied to calculate the anisotropy of the fission fragments angular distribution and average pre-scission neutron multiplicities for the compound nucleus 248Cf formed in the 16O+232Th reactions. Postsaddle nuclear dissipation strength of (12–14) × 1021 s?1 was extracted for Cf nucleus by fitting the results of calculations with the experimental data. Furthermore, it was found that the results of calculations for the anisotropy of the fission fragments angular distribution and pre-scission neutron multiplicities are very sensitive to the magnitude of post-saddle nuclear dissipation. 相似文献
3.
WU Feng YE Wei CHEN Na 《理论物理通讯》2007,48(4):710-712
Using a Langevin equation coupled with a statistical model, we calculate pre-scission giant dipole resonance (GDR) γ-ray multiplicity of nuclei 194 pb, 200Pb, 206Pb, and 200 Os. It is demonstrated that with increasing the isospin asymmetry of these fissioning nuclei the sensitivity of the emitted γ multiplicity to the nuclear viscosity coefficient is decreased significantly. For 200Os nuc/eus, this γ-ray emission is no longer sensitive to the magnitude of the viscosity coefficient. In addition, the effect of the isospin asymmetry on the γ rays as a probe of nuclear dissipation is reduced with increasing angular momentum. These results suggest that to obtain a more accurate information of the viscosity coemfficient by the measurement of pre-scission GDR γ-ray multiplicity it is better to choose those compound systems with small isospin asymmetry and low spin. 相似文献
4.
《中国物理C(英文版)》2015,(5)
The evaporation residue cross section anisotropy of the fission fragment angular distribution, pre-scission neutron multiplicity and the pre-saddle and post-saddle contributions of the pre-scission neutron multiplicity were analyzed within a stochastic approach based on one-, two- and three-dimensional Langevin equations for the compound nucleus224 Th formed via a complete fusion. In these calculations, dissipation was generated through the chaos weighted wall and window friction formula. Comparison of the theoretical results with the experimental data showed that three-dimensional Langevin equations with dissipation generated through the chaos weighted wall and window friction formula make it possible to reproduce satisfactorily the above-mentioned experimental data. 相似文献
5.
A dynamical Langevin model is employed to evaluate the excess of the neutron emission in the fission of heavy nuclei 240Cf, 246Cf, 254Cf, 240U relative to the standard statistical-model prediction at various saddle-to-scission friction strengths. It is shown that when the neutron-to-proton ratio N/Z of the system increases, the sensitivity of the excess to the friction decreases substantially, and it almostdisappears for 240U. We suggest that using those compound systems with low N/Z favors an accurate determination for the saddle-to-scission friction strength based on the measurement of the pre-scission neutron multiplicity. 相似文献
6.
Based on a dynamical Langevin equation coupled with a statistical decay model, we calculate the variation of the post-saddle giant dipole resonance (GDR) q-ray multiplicity of the heavy nuclei 24^240Cf, ^246Cf, ^252Cf and ^240U with the post-saddle friction strength (13). We find that the sensitivity of the post-saddle γ emission to β decreases considerably with increasing the neutron-to-proton ratio (N/Z) of the system. Moreover, for 240 U, the γ emission is no longer sensitive to 13. We suggest that to accurately obtain information of the post-saddle friction strength by measuring pre-scission GDR γ-ray multiplicities, it is optimal to choose among the various compound systems those with low N/Z. 相似文献
7.
A dynamical Langevin model is employed to evaluate the excess of the neutron emission in the fission of heavy nuclei ^240Cf, ^246 Cf, ^254Cf, ^240U relative to the standard statistical-model prediction at various saddle-to-scission friction strengths. It is shown that when the neutron-to-proton ratio N/Z of the system increases, the sensitivity of the excess to the friction decreases substantially, and it almost disappears for ^240U. We suggest that using those compound systems with low N/Z favors an accurate determination for the saddle-to-scission friction strength based on the measurement of the pre-scission neutron multiplicity. 相似文献
8.
Shell effects in particle emission for two doubly magic nuclei 132 Sn and 208 Pb were studied in the framework of Smoluchowski equation taking into account temperature and spin-dependent shell correction. It is shown that the shell effects in the enission of pre-scission neutrons are sensitive to the spin dependence of the shell correction at a moderate excitation energy. Therefore, we propose to use neutron multiplicity as an observable to probe the shell correction at high spins. 相似文献
9.
Shell effects in particle emission for two doubly magic nuclei ^132Sn and ^208pb were studied in the framework of Smoluchowski equation taking into account temperature and spin-dependent shell correction. It is shown that the shelle ffects in the emission of pre-scission neutrons are sensitive to the spin dependence of the shell correction at a moderate excitation energy. Therefore, we propose to use neutron multiplicity as an observable to probe the shell correction at high spins. 相似文献
10.
S. M. Lenzi C. H. Dasso M. A. Nagarajan H. M. Sofia A. Vitturi 《Acta Physica Hungarica A》1997,6(1-4):95-98
The existence of a neutron skin in neutron-rich nuclei is discussed in connection with the excitation of isovector dipole and quadrupole giant modes via isoscalar nuclear probes. In the case of large neutron excess, important contributions are obtained from the nuclear excitation, which may even become predominant according to proper kinematical conditions. At variance with the usual situation encountered in inelastic processes, constructive interference can be found between nuclear and Coulomb contributions. 相似文献
11.
Bivash R Behera Subinit Roy P Basu M K Sharan S Jena M Satpathy M L Chatter Jee S K Datta 《Pramana》2001,57(1):199-202
Anisotropies in fission fragment angular distributions measured for the system 16O + 181Ta over a range of bombarding energies from 83 MeV to 120 MeV have been analysed. It is shown that statistical transition
state model (TSM) with pre-scission neutron correction described adequately the measured anisotropy data. Strong friction
parameter is found to be necessary to estimate the pre-saddle to pre-scission neutron ratio. 相似文献
12.
Isospin effects on particle emission of fissioning isobaric sources
202Fr, 202Po, 202Tl and isotopic sources
189,202,212Po, and its dependence on the excitation energy are studied via Smoluchowski equations. It is shown that with increasing the
isospin of fissioning systems, charged-particle emission is not
sensitive to the strength of nuclear dissipation. In addition, we
have found that increasing the excitation energy not only
increases the influence of nuclear dissipation on particle
emission but also greatly enhances the sensitivity of the emission
of pre-scission neutrons or charged particles to the isospin of
the system. Therefore, in order to extract dissipation strength
more accurately by taking light particle multiplicities it is
important to choose both a highly excited compound nucleus and a
proper kind of particles for systems with different isospins. 相似文献
13.
A. Matthies R. Kotte W. Seidel F. Stary D. Wohlfarth 《Zeitschrift für Physik A Hadrons and Nuclei》1990,337(4):439-446
The population of the three fission modes predicted by Brosa's multi-channel fission model for the uranium region was studied in different fissioning systems. They were produced bombarding232Th and238U targets by light charged particles with energies slightly above the Coulomb barrier. Though the maximum excitation energy of the compound nucleus amounted to about 22 MeV, the influences of various spherical and deformed nuclear shells on the mass and total kinetic energy distributions of fission fragments are still pronounced. The larger variances of the total kinetic energy distributions compared to those of thermal neutron induced fission were explained by temperature dependent fluctuations of the amount and velocity of alteration of the scission point elongation of the fissioning system. From the ratio of these variances the portion of the potential energy dissipated among intrinsic degrees of freedom before scission was deduced for the different fission channels. It was found that the excitation remaining after pre-scission neutron emission is mainly transferred into intrinsic heat and less into pre-scission kinetic energy. 相似文献
14.
THE NEUTRON EXCESS DEPENDENCE OF TOTAL REACTION CROSS SECTION IN THE INTERMEDIATE ENERGY DOMAIN
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A modified microscopic model for calculating the total reaction cross section in the intermediate energy domain is given,the effects of neutron skin,ground state deformation and surface diffusion of the nuclear matter distribution is discussed quantitatively.The experimental data and the calculated results show that the surface diffusion of the nuclear matter distribution which was assumd to be dependent on the neutron excess is a important factor for the rapid increase of the total reaction cross section induced by neutron-rich nuclei. 相似文献
15.
Chubarian GG Itkis MG Kondratiev NA Kozulin EM Pashkevich VV Pokrovsky IV Rusanov AY Salamatin VS Schmitt RP 《Physical review letters》2001,87(5):052701
Two components in the M(gamma)(M) distribution were established in detailed measurements of mean gamma-ray multiplicities from fission fragments of (226)Th. For the first time in the M(gamma)(M) dependencies we were able to distinguish two components associated with primary and the final (after the neutron evaporation) fission fragments, and show that at the scission point M(gamma) is extremely sensitive to symmetric and asymmetric modes of fission. Theoretical calculations of the pre-scission shapes of the fissioning nuclei confirm our conclusions. 相似文献
16.
Heavy-ion reactions are investigated by statistical methods in the framework of a dinuclear system model (DNS). Synthesis
of superheavy elements in cold and hot fusion reactions on Pb and Bi targets, as well as in incomplete fusion reactions, is
considered. We also take a look at production of neutron-deficient isotopes and pre-scission neutron emission in quasifission
reactions. The results of calculations are compared with the available experimental data. The sensitivity of these results
to the method of calculation of the level density and to various theoretical predictions of nuclear properties is analyzed. 相似文献
17.
《Nuclear Physics A》1987,472(2):318-332
Pre- and post-scission neutron multiplicities have been measured as a function of fission mass-split for the reactions of 105 and 120 MeV 19F with 232Th. The post-scission multiplicities show no evidence for persistence of the sawtooth yield observed for spontaneous fission of 252Cf; the dependence on mass-split is not consistent with the random neck rupture model. The pre-scission multiplicities show no evidence of quasi-fission for asymmetric mass-splits. It is proposed that pre-scission neutron multiplicity measurements should be a useful tool in studying quasi-fission produced by reactions with heavier projectiles. 相似文献
18.
《Nuclear Physics A》1997,624(3):449-458
The evolution of the dipole response in nuclei with strong neutron excess is studied in the Hartree-Fock plus random phase approximation with Skyrme forces. We find that the neutron excess increases the fragmentation of the isovector giant dipole resonance, while pushing the centroid of the distribution to lower energies beyond the mass dependence predicted by the collective models. The radial separation of proton and neutron densities associated with a large neutron excess leads to non-vanishing isoscalar transition densities to the GDR states, which are therefore predicted to be excited also by isoscalar nuclear probes. The evolution of the isoscalar compression dipole mode as a function of the neutron excess is finally studied. We find that the large neutron excess leads to a strong concentration of the strength associated with the isoscalar dipole operator ∑iri3Y10, that mainly originates from uncorrelated excitations of the neutrons of the skin. 相似文献
19.
A. Saxena D. Fabris G. Prete D. V. Shetty G. Viesti B. K. Nayak D. C. Biswas R. K. Choudhury S. S. Kapoor M. Lunardon S. Moretto G. Nebbia S. Pesente V. Rizzi A. M. Samant M. Barbui E. Fioretto M. Cinausero A. Brondi G. La Rana R. Moro E. Vardaci N. Gelli F. Lucarelli 《Nuclear Physics A》2004,730(3-4):299-315
Pre-scission and post-scission multiplicities of neutrons and alpha particles have been simultaneously measured for the fission-like reactions of 340 MeV 28Si on 232Th. Dynamical model calculations using HICOL code predict that about 90% of the observed events are of quasi-fission type while the remaining 10% are from compound nucleus fission decay. Moving source fits were carried out to the observed neutron and alpha particle spectra, measured at different angles with respect to the fragment directions. The pre-scission and post-scission neutron multiplicities are deduced to be 8.7±2.0 and 9.4±2.0, respectively. The corresponding multiplicity values for alpha particles are found to be 0.22±0.08 and 0.1±0.03. From the measured post-scission neutron multiplicity, it is inferred that about 65±20 MeV of the initial excitation energy remains at scission. This may be compared to the value of 85±30 MeV estimated from PACE2 statistical model calculations, adjusted to reproduce the measured pre-scission neutron multiplicity. From a comparison of the Statistical Model predictions with the measured pre-scission neutron multiplicity, the fission delay is estimated to be of 5+7−3×10−20 s which overlaps with the average duration of fission-like process from the contact to the scission point (2×10−20 s) as determined from HICOL-based dynamical calculations. For the delay time deduced as above, the pre-scission alpha particle multiplicity calculated by the PACE2 code is about a factor two larger than the experimental one, demonstrating the difficulties in modelling the alpha particle emission from highly elongated shapes that characterize the fissioning system from the contact point to scission. 相似文献