Target-projectile Systems Research Articles

Investigation of the influence of incomplete fusion on complete fusion of16O induced reactions at moderate excitation energies

An attempt has been made to investigate for the reaction dynamics leading to incomplete fusion (ICF) of heavy ions at moderate excitation energies, especially the influence of incomplete fusion on complete fusion (CF) of 16 O induced reactions at specific energies. Excitation functions (EFs) of various reaction products populated via CF and/or ICF of 16 O projectile with 45 Sc target were measured at energies ≈3-7 MeV/nucleon, using recoil catcher technique followed by offline γ-ray spectroscopy. The measured EFs were compared with theoretical values obtained using the statistical model code PACE4. The experimentally measured EFs were in general found to be in good agreement with the theoretical predictions for non α-emitting channels in the present target projectile system. However, for α-emitting channels the measured EFs were higher than the predictions of the theoretical model codes, which may be credited to incomplete fusion reactions at these energies.

Non-equilibrium processes in Heavy-ion induced Fission Reactions

Heavy ion induced reactions offer a wide range of possibilities in exploring the collective behavior of nuclei and nuclear processes. Recent years have seen a spurt in the experimental and theoretical studies to understand the mechanism of the fission process in heavy target projectile systems, arising from the interest to produce elements in the superheavy region. A number of new features are observed experimentally with respect to the kinetic energy, mass and angular distributions of fission fragments and their correlations, implying prevalence of non-equilibrium phenomena in the fission process in many target projectile systems. There are many theoretical attempts both in terms of static potential energy considerations and dynamics to understand the important degrees of freedom that govern the evolution of the nuclear system from initial interaction to the final fission stages. Depending on the degree and nature of equilibration, various processes such as fast-fission, quasi-fission and pre-equilibrium fission have been invoked that can compete with the fully equilibrated compound nuclear fission process. In what we call a C-T Fissility plot, we show the remarkable dependence of the onset of these non-equilibrium processes on the entrance channel parameters.

Nuclear level-density parameters of nuclei in theZ~70andA~180mid-shell regions

$\ensuremath{\alpha}$-particle evaporation energy spectra have been measured as a function of $\ensuremath{\gamma}$-ray fold for various target-projectile systems leading to residual nuclei in the range of $Z~70$ and $A~180$ with excitation energy of 30--40 MeV. The inverse level-density parameter $K$ was determined for various nuclei by comparing the high-energy part of the $\ensuremath{\alpha}$-particle evaporation spectra with PACE2 predictions. It is observed that the inverse level-density parameter remains constant for all systems studied in this work within statistical errors in the angular momentum range of 15--30 $\ensuremath{\hbar}$. The fold-gated $\ensuremath{\alpha}$-particle energy spectra in these systems are well reproduced by the PACE2 code with a level-density parameter value of $A/(8.2\ifmmode\pm\else\textpm\fi{}1.1)$ MeV${}^{\ensuremath{-}1}$.

Angular momentum dependence of the nuclear level-density parameter aroundZ~50

\ensuremath{\alpha}-particle evaporation spectra and $\ensuremath{\gamma}$-ray multiplicities were measured for various target-projectile systems corresponding to residual nuclei in the range of ${Z}_{R}=48\text{\ensuremath{-}}55$ and with excitation energy in the range of 30--40 MeV. The high-energy part of the evaporation spectra were analyzed using the statistical model code PACE2 to derive values of the inverse level-density parameter $(K)$. The $K$ values were found to be in the range of 9.0--10.5 for all systems. Angular momentum dependence of the inverse level-density parameter was investigated using the $\ensuremath{\gamma}$-ray multiplicity data. It is seen that there are strong variations in $K$ as a function of angular momentum for many systems. Present results provide important input information for a systematic understanding of the statistical properties of nuclei at moderate excitation energies and angular momenta.

Molecular Dynamics Study of Impact Velocity Dependence of Structure of Debris Cloud under Hyper Velocity Impact

A molecular dynamics simulation is carried out to study thehyper-velocity impact fracture at a space debris' attack to a man-madesatellite. A target-projectile system is modeled as aface-center-cubic system which consists of aluminum atoms. Morse potential is adopted as the interatomic potential. Impact velocity dependence of distribution of debris clouds is studied in the different velocity conditions. As a result, the structure of debriscloud of the case of 7km/s impact differs from that of 12km/s. Thesize of fragment of the debris clouds becomes smaller with increasing impact velocity. The axial component of debris cloud velocities areshown to decrease as target thickness-projectile diameter ratio t/D is increased. These results qualitatively agree well with anexperimental result.

Anomalous increase in width of fission-fragment mass distribution as a probe for onset of quasifission reactions in deformed target-projectile system at near and sub-barrier energies

Fission-fragment mass distribution has been studied for the $^{16}\mathrm{O}+^{232}\mathrm{Th},^{209}\mathrm{Bi}$ systems over an energy range of $102.8--78.6\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ and $81.6--72.6\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$, respectively, in a laboratory frame. The variance of the mass distribution $({\ensuremath{\sigma}}_{m}^{2})$ for the $^{16}\mathrm{O}+^{209}\mathrm{Bi}$ system varies linearly with center of mass energy, while a significant anomalous behavior is found for the system $^{16}\mathrm{O}+^{232}\mathrm{Th}$. Coupled with our earlier observation for the system $^{19}\mathrm{F}+^{232}\mathrm{Th}$ [T. K. Ghosh et al., Phys. Rev. C 69, 031603(R) (2004)], we propose that the accurate measurement of mass distribution is a powerful tool to look for the onset of a nonstatistical reaction mechanism in heavy-ion-induced fission of deformed heavy nuclei.

Some aspects of heavy ion fusion-fission dynamics

Study of heavy ion induced fusion-fission reactions at near and below barrier energies has attracted a great deal of attention in recent years, due to the observations of anomalous features in the fragment angular distributions for many target-projectile systems. Additionally there are also measurements of the fragment spin distributions and time-scales of the fusion-fission reactions, which have provided important information on the dynamics of these processes. In the present paper, the emphasis would be to highlight some of the recent experimental findings and their implications on the dynamics of the fusion-fission reactions in heavy ion collisions at near and above barrier energies.

Exclusive measurements of light fragment production at forward angles in NePb and NeNaF collisions at [formula omitted

Emission of light fragments at small angles is studied in relativistic heavy ion collisions using the Diogene plastic wall for both symmetrical and non-symmetrical target-projectile systems with 400 MeV per nucleon and 800 MeV per nucleon incident neon nuclei. Efficiency of multiplicity measurements in the small angle range for the selection of central or peripheral collisions is confirmed for asymmetric systems. Differential production cross sections of Z = 1 fragments show evidence for the existence of two emitting sources. The apparent temperature of each source is obtained from comparison with a thermodynamical model.

Geometric scaling and nuclear interaction cross sections at ultrarelativistic energies

view Abstract Citations (4) References (17) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Geometric Scaling of Nuclear Interaction Cross Sections at Ultrarelativistic Energies Karol, P. J. Abstract Use is made of recent experimental results that have shown the elementary nucleon-nucleon cross section at ultrarelativistic energies increasing as log-squared E. The principle of geometric scaling implies that the effective size of the nucleon scales to the same degree. When geometric scaling is combined with the 'soft spheres model' for nuclear interaction cross sections, one may calculate the interaction probabilities of target-projectile systems at energies corresponding to the highest seen for cosmic rays. The model is readily applied to strongly interacting elementary particles including antiparticles as well as to complex nuclei. Cosmic ray interaction cross sections calculated at ultrarelativistic energies are found to increase substantially. Publication: The Astrophysical Journal Pub Date: September 1988 DOI: 10.1086/166680 Bibcode: 1988ApJ...332..615K Keywords: High Energy Interactions; Nuclear Astrophysics; Nuclear Interactions; Relativistic Particles; Relic Radiation; Astronomical Models; Hadrons; Red Shift; Space Radiation; COSMIC RAYS: GENERAL; NUCLEAR REACTIONS full text sources ADS |

A study of the friction force dependence of the heavy-ion fusion cross sections

A systematic study of the dependence of fusion cross sections on the strength of the friction force has been performed by carrying out classical dynamical trajectory calculations for a number of heavy-ion target-projectile systems. On comparing the calculations with the available experimental data on fusion excitation functions for few typical light-heavy-ion systems, it was seen that in the high-energy region, where the fusion cross sections depend sensitively on the strength of the friction force, there appears to be a trend in the data, that is not easily explained by a simple linear velocity-dependent friction force. However, even in this region, the paucity of experimental data prevents one from drawing any definite conclusions.

Reaction mechanism studies with 7-35 MeV/nucleonHe4ions incident on heavy target nuclei

Reaction mechanism studies have been performed on the $^{4}\mathrm{He}$ + $^{233}\mathrm{U}$ system at energies of 7-35 MeV/nucleon and on the $^{4}\mathrm{He}$ + $^{209}\mathrm{Bi}$ system at 12-35 MeV/nucleon. Measurements included (1) forward-angle elastic scattering, (2) angular distributions and total cross sections for fission, (3) fission-fragment angular correlations, and (4) fragment charge, mass, and energy distributions for $^{233}\mathrm{U}$($\ensuremath{\alpha}$,$f$) at 140 MeV. Total inelastic cross sections and interaction radii were derived from the elastic scattering data. These values were found to be in good agreement with the total cross section for fission for the $^{233}\mathrm{U}$ + $^{4}\mathrm{He}$ system, confirming the assumption that ${\ensuremath{\sigma}}_{R}\ensuremath{\simeq}{\ensuremath{\sigma}}_{f}$. Fission-fragment angular-correlation measurements were performed in order to deduce the distribution of linear momenta which characterize the residual heavy nuclei formed in these collisions. Complete linear momentum transfer probabilities (complete fusion) were also derived for both target-projectile systems as a function of energy. The data are compared with predictions of the proximity potential with one-body energy dissipation and with the intranuclear cascade code. The results of these comparisons are consistent with a picture in which complete fusion dominates the reaction mechanism below 10 MeV/nucleon, but pre-equilibrium processes assume increasing importance above this energy at the expense of fusion. In addition, evidence for a possible reduction in the interaction radius above 20 MeV/nucleon is reported.NUCLEAR REACTIONS $^{209}\mathrm{Bi}$($\ensuremath{\alpha}$,$\ensuremath{\alpha}$), $^{233}\mathrm{U}$($\ensuremath{\alpha}$,$\ensuremath{\alpha}$), $E=69.2, 140$ MeV, ${\ensuremath{\sigma}}_{R}(E)$; $^{209}\mathrm{Bi}$($\ensuremath{\alpha}$,$f$), $E=69.2, 140$ MeV, $^{233}\mathrm{U}$($\ensuremath{\alpha}$,$f$), $E=28\ensuremath{-}140$ MeV, $\ensuremath{\sigma}(\ensuremath{\theta})$, $\ensuremath{\sigma}(E)$, fission-fragment angular correlations, $Z$ and $A$ distributions at 140 MeV; linear momentum transfer to residual nucleus deduced; results compared with ion-ion potential models and intranuclear cascade code.

A model for high-energy heavy-ion collisions

A model is developed for high-energy heavy-ion collisions that treats the variation across the overlap region of the target and projectile in the amount of energy and momentum that is deposited. The expression for calculating any observable takes the form of a sum over a series of terms, each one of which consists of a geometric, a kinematic, and a statistical factor. The geometrical factors for a number of target projectile systems are tabulated.

Experimental study of Yb nuclei at high angular momentum

Methods have been developed to study the continuum γ-ray spectrum following heavy-ion compound-nucleus reactions. In the Yb nuclei studied these spectra were found to depend mainly on the input angular momentum, and not on the target-projectile system. The gross features of the spectra are a high-energy tail of ≈ 4 transitions corresponding to the statistical cascade and a lower-energy bump composed of many unresolved collective E2 transitions. Several methods were devised to obtain moment-of-inertia values from the detailed features of this bump for angular momenta as high as 50 ħ.

Nucleus-nucleus optical potential

The real part of the nucleus-nucleus potential is calculated by using a density-dependent effective interaction where the local density takes into account the contribution of both the projectile and the target densities. The results indicate that the repulsion produced by the density-dependent part of the interaction is important when the densities of the two nuclei overlap significantly but not so important at a distance much greater than the touching radius. For a quite heavy target-projectile system the repulsive term is important even in the tail region. The exchange effects have been found to be be not so important. The imaginary potential is approximately estimated by using a forward scattering amplitude approximation. The preliminary results agree with phenomenological predictions. The parameter ${\mathcal{r}}_{C}$ which determines the critical distance of approach both for the real and the imaginary potential, remains constant [${\mathcal{r}}_{C0}\ensuremath{\simeq}1.0=\frac{{R}_{C0}}{({{A}_{P}}^{\frac{1}{3}}+{{A}_{T}}^{\frac{1}{3}})}$] throughout the Periodic Table.NUCLEAR REACTIONS Calculated optical potential for two colliding nuclei using a density-dependent effective interaction and the forward scattering amplitude approximation for the imaginary potential.

Neutron Emission from theZn64Compound Nucleus Formed in Two Ways:p+Cu63andα+Ni60

The ${\mathrm{Zn}}^{64}$ compound nucleus was formed at an excitation energy of 33.3 MeV using tow different target-projectile systems: 26.0-MeV protons on ${\mathrm{Cu}}^{63}$ and 31.3-MeV $\ensuremath{\alpha}$ particles on ${\mathrm{Ni}}^{60}$. Neutrons were detected with nuclear emulsions using the internal-radiator method. Exposures were made simultaneously at the following angles: 18, 33, 90, 147, and 162\ifmmode^\circ\else\textdegree\fi{}. The c.m. energy spectra (2-15 MeV) of the two systems are very nearly the same. For both systems symmetry about 90\ifmmode^\circ\else\textdegree\fi{} in the c.m. angular distributions is seen up to about 6 MeV followed by a forward peaking at higher energies. The symmetric (compound-nuclear) parts of the angular distributions differ significantly in anisotropy. For the $p+{\mathrm{Cu}}^{63}$ system the distribution is essentially isotropic while for the $\ensuremath{\alpha}+{\mathrm{Ni}}^{60}$ system we have an average anisotropy of about 1.4. We also observe for the $\ensuremath{\alpha}+{\mathrm{Ni}}^{60}$ system a slight but significant increase in anisotropy with energy of the emitted neutron (1.290 \ifmmode\pm\else\textpm\fi{} 0.047 at 2 MeV to 1.649 \ifmmode\pm\else\textpm\fi{} 0.119 at 6 MeV). The changes in average anisotropy, as we change target-projectile systems, are basically consistent with multiple-emission calculations using a rigid-sphere moment of inertia (${r}_{0}=1.22$ F). The change in anisotropy with emission energy is not seen in the calculations. It is estimated that, for the $p+{\mathrm{Cu}}^{63}$ and $\ensuremath{\alpha}+{\mathrm{Ni}}^{60}$ systems, direct-reaction neutrons constitute 5 and 3% of the total observed neutron spectra.

Formation and decay of the compound nucleus 68Ge (II). Calculation; competition between gamma-ray and particle emission

The statistical theory of nuclear reactions has been used to calculate cross sections for a number of nuclear reactions proceeding through the compound nucleus 68Ge. The target projectile pairs were 4He + 64Zn, 12C + 56Fe and 16O + 52Cr; a maximum of two emitted particles was considered. The calculations were performed with an IBM 360/40 computer according to a formalism containing the explicit dependence of particle-emission probabilities on nuclear angular momentum. Probabilities for γ-ray emission were calculated according to the single-particle model modified to reflect experimentally observed collective effects. Agreement between experiment and calculation was found to be good for 64Zn + 4He excitation functions over the entire energy range studied experimentally. Agreement in the 56Fe + 12C case was encouraging considering the complexity of the target-projectile system and attendant theoretical difficulties.

Reactions Induced inFe54with 21-63 MeVLi6Ions

Excitation functions have been measured for the production of ${\mathrm{Ni}}^{56}$, ${\mathrm{Ni}}^{57}$, ${\mathrm{Co}}^{55}$, ${\mathrm{Co}}^{56}$, ${\mathrm{Co}}^{57}$, ${\mathrm{Co}}^{58}$, ${\mathrm{Fe}}^{52}$, ${\mathrm{Fe}}^{55}$, and ${\mathrm{Mn}}^{54}$ from ${\mathrm{Fe}}^{54}$ bombarded with ${\mathrm{Li}}^{6}$ ions of 21-63-MeV kinetic energy. The targets were enriched to 95% ${\mathrm{Fe}}^{54}$. Excitation functions for the production of ${\mathrm{Ni}}^{57}$, ${\mathrm{Co}}^{55}$, ${\mathrm{Co}}^{56}$, ${\mathrm{Co}}^{57}$, ${\mathrm{Fe}}^{55}$, and ${\mathrm{Mn}}^{54}$ resulting from ${\mathrm{Li}}^{6}$ bombardment of ${\mathrm{Fe}}^{54}$ and deuteron bombardment of ${\mathrm{Ni}}^{58}$ are compared. Of these, excitation functions for the production of ${\mathrm{Ni}}^{57}$, ${\mathrm{Co}}^{57}$, and ${\mathrm{Mn}}^{54}$ are mutually consistent with decay of a ${\mathrm{Cu}}^{60}$ compound nucleus. Excitation functions for production of ${\mathrm{Co}}^{55}$ and ${\mathrm{Fe}}^{55}$ with the two projectiles appear to proceed by different mechanisms. The ${\mathrm{Co}}^{56}$ excitation functions in the two target-projectile systems are not directly comparable, since the probable reactions producing ${\mathrm{Co}}^{56}$ are ${\mathrm{Ni}}^{58}(d,\ensuremath{\alpha}){\mathrm{Co}}^{56}$ and ${\mathrm{Fe}}^{54}({\mathrm{Li}}^{6},2p2n){\mathrm{Co}}^{56}$. All excitation functions studied in the ${\mathrm{Fe}}^{54}$+${\mathrm{Li}}^{6}$ system show the competitive behavior of compound-nucleus reactions with the exception of the high-energy tail of the ${\mathrm{Co}}^{58}$ excitation function (apparently due to 5% ${\mathrm{Fe}}^{56}$ impurity in the targets) and the low-energy portions of the ${\mathrm{Co}}^{55}$ and ${\mathrm{Fe}}^{55}$ excitation functions. The very low yields observed for the production of ${\mathrm{Ni}}^{56}$ and ${\mathrm{Ni}}^{57}$ are attributed to the effect of the 28-nucleon shells on nuclear level densities. The sum of measured cross sections from ${\mathrm{Fe}}^{54}$+${\mathrm{Li}}^{6}$ reactions is compared with calculated optical-model nonelastic cross sections.

Coincident Time-of-Flight Measurements of the Velocities of Fission Fragments from Charged-Particle-Induced Fission

Measurements have been made of the velocities of the coincident fission fragment pairs emitted in the 25.7- and 29.5-MeV ${\mathrm{He}}^{4}$-induced fission of ${\mathrm{U}}^{233}$ and ${\mathrm{Th}}^{230}$, the 25.7- and 29.5-MeV ${\mathrm{He}}^{4}$-induced fission of ${\mathrm{Th}}^{232}$, and the 12.0- and 14.0-MeV ${\mathrm{H}}^{2}$-induced fission of ${\mathrm{Th}}^{230}$. Previously unpublished data for the 21.8-MeV ${\mathrm{He}}^{4}$-induced fission of ${\mathrm{U}}^{233}$ and ${\mathrm{Th}}^{232}$ are also included. A decrease in average total fragment kinetic energy for symmetric mass division is observed in each case. The relatively large yield of symmetric mass divisions in this work permits a reliable measurement of this effect. The distributions of fragment velocities, masses, and kinetic energies are consistent with the hypothesis of two competing modes of fission. A comparison is made of the primary mass yields obtained in the present work with the yields obtained by radiochemical methods to determine the dependence of prompt neutron emission on fragment mass. The neutron emission found, although subject to poorly known and possibly large uncertainties in the radiochemical data, appears to show a rather pronounced structure of just such a nature as might be attributed to fragment-shell effects that persist to these high excitation energies. Such a structure is not observed, however, when the present primary mass data are compared with mass distributions derived from recent fragment double-energy measurements. The time-of-flight mass distributions were corrected for the influence of the prefission neutron emission and the detection solid angle. As a part of the correction, the dependences of the yield of symmetric mass divisions on the excitation energy of the fissioning nuclei were estimated for each of the target-projectile systems.