Complex Fragment Emission Research Articles

Signature of fusion suppression in complex fragment emission

The effect of weak binding of $^{9}\mathrm{Be}$ on complete fusion has been explored through the study of complex fragment emission in $^{20}\mathrm{Ne}+^{9}\mathrm{Be}$ reaction. The yields of the fragments $^{6,7}\mathrm{Li}$ and $^{7,9}\mathrm{Be}$ emitted from the excited compound nucleus $^{29}\mathrm{Si}^{*}$ have been compared with the respective statistical model predictions. Emission of same fragments from another close-by compound nucleus $^{28}\mathrm{Si}^{*}$ at similar excitation energy, formed by the fusion of two strongly bound nuclei, $^{16}\mathrm{O}+^{12}\mathrm{C}$, has been studied for comparison. It has been observed that for the system $^{16}\mathrm{O}+^{12}\mathrm{C}$, the yields of $^{6,7}\mathrm{Li}$ and $^{7,9}\mathrm{Be}$ fragments are close to the predictions of the statistical model. However, for the $^{20}\mathrm{Ne}+^{9}\mathrm{Be}$ system, although the experimental yield pattern follows the statistical model prediction, there is substantial reduction in yield for all detected fragments. These observations have been attributed to the suppression of complete fusion in $^{20}\mathrm{Ne}+^{9}\mathrm{Be}$ system due to the weak binding of $^{9}\mathrm{Be}$, a dynamical effect which is not incorporated in the conventional statistical models. It is the first time that a clear signature of the suppression of complete fusion in light systems involving weakly bound nucleus has been observed in complex fragment emission from fully equilibrated composite produced in fusion well above the barrier.

Complex fragment emission in dissipative binary decay of Kr74,76

The fragment emission mechanism in the binary decay of composites formed in the reactions $^{20}\mathrm{Ne}+^{56}\mathrm{Fe}$ and $^{16}\mathrm{O}+^{58}\mathrm{Ni}$ has been studied at two different excitation energies. The inclusive energy and angular distributions of the emitted fragments $^{6,7}\text{Li}$, $^{7,8,9}\text{Be}$, $^{10,11}\text{B}$, and $^{11,12,13,14}\text{C}$ have been measured in the laboratory angles ranging from ${15}^{\ensuremath{\circ}}$ to ${35}^{\ensuremath{\circ}}$ (corresponding angles in center of mass ranging from ${20}^{\ensuremath{\circ}}$ to ${50}^{\ensuremath{\circ}}$). The energy distributions of the fragments are found to be Gaussian and peaked at energies higher than those expected from fusion-fission-type reactions. The center-of-mass angular distributions of all the fragments have been found to fall faster than $\ensuremath{\approx}1/\mathrm{sin}{\ensuremath{\theta}}_{\mathrm{c}.\mathrm{m}.}$-like dependence and the average $Q$ values of the fragments are found to decrease with increasing the center-of-mass angle of the emitted fragment for both the systems. The above characteristics of fragments signify that they were emitted from nonequilibrium sources, produced in a highly energy damped deep-inelastic-type reaction. The lifetimes of the dinuclear composites estimated from the angular distributions of these fragments are found to be in the range of $\ensuremath{\approx}(0.5--2.7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}22}$ s, which are smaller than the respective compound-nuclear lifetimes $[\ensuremath{\approx}(1.0--2.0)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}21} \mathrm{s}]$. The angular-momentum dissipations estimated from the average kinetic energies of the fragments are found to be, for lighter fragments in particular, greater than those predicted by the empirical sticking limit.

Emission of charged particles from excited compound nuclei 90,94,98Mo

The production and decay processes of compound nuclei are studied in the framework of the dinuclear system model, using proximity potentials. Cross-sections for the emission of complex fragments are computed and compared against the experimental data for the reactions 78,82,86Kr + 12C. A good consistency between the results of this work and the experimental data is obtained.

Emission of heavy clusters in nuclear reactions at low collision energies

A survey of theoretical and experimental investigations of the process involving the emission of heavy clusters from excited nuclear systems produced in heavy-ion reactions at low collision energies is given. The dinuclear system (DNS) model for calculating cross sections for the formation of heavy clusters in complete-fusion and quasifission reactions is described in detail. The results of respective calculations are compared with relevant experimental data and with the results obtained on the basis of different models. The role of the angular momentum, the asymmetry of the entrance channel, the N/Z ratio, and the excitation energy in the formation of final reaction products is studied within the proposed approach. A method is developed for calculating cross sections for evaporation-residue formation. This method takes into account both channels of light-particle emission and channels of heavy-cluster emission. The possibility for the formation of Rn, Fr, and Ra isotopes in channels of heavy-cluster emission from the excited compound nucleus of Pu is demonstrated for the first time. The calculated cross sections and isotopic distributions for residual nuclei arising upon the emission of heavy clusters from an excited compound nucleus of Pu are in good agreement with experimental data. The model developed in the present study permits finding optimum experimental conditions (projectile-target combination and bombarding energy) for studying processes involving the emission of specific complex fragments.

Complex-fragment emission in low-energy light-ion reactions

Inclusive energy spectra of the complex fragments (3 $\leq$ Z $\leq$ 5) emitted in the reactions $^{12}$C (77 MeV)+ $^{28}$Si, $^{11}$B (64 MeV)+ $^{28}$Si and $^{12}$C (73 MeV)+ $^{27}$Al (all having the same excitation energy of $ \sim$ 67 MeV), have been measured in the angular range of 10$^\circ$ $\lesssim \theta_{lab} \lesssim$ 60$^\circ$. The fully energy damped (fusion-fission) and the partially energy damped (deep inelastic) components of the fragment energy spectra have been extracted. It has been found that the yields of the fully energy damped fragments for all the above reactions are in conformity with the respective statistical model predictions. The time scales of various deep inelastic fragment emissions have been extracted from the angular distribution data. The angular momentum dissipation in deep inelastic collisions has been estimated from the data and it has been found to be close to the corresponding sticking limit value.

Emission of charged particles from excited compound nuclei

The formation of excited compound nucleus (CN) and its statistical decay is investigated within the dinuclear system (DNS) model.The initial DNS is formed in the entrance channel when the projectile is captured by a target, and then the evolution of DNS in mass asymmetry coordinate leads to formation of the hot CN. The emission barriers for complex fragments were calculated within the DNS model by using the double folding procedure for the interaction potential. It is shown that cross sections for complex fragment emission become larger when excited CN is more neutron deficient. This approach gives also an opportunity to calculate the new neutron deficient isotopes production cross sections and can be applied to describe the hot fission of heavy systems.The model was tested by comparison of calculated results with experimental data

Evidence of large nuclear deformation ofS32*formed in theNe20+C12reaction

Deformations of hot composite $^{32}$S$^{*}$ formed in the reaction $^{20}$Ne ($\sim$ 7 -- 10 MeV/nucleon) + $^{12}$C have been estimated from the respective inclusive $\alpha$-particle evaporation spectra. The estimated deformations for $^{32}$S$^{*}$ have been found to be much larger than the `normal' deformations of hot, rotating composites at similar excitations. This further confirms the formation of highly deformed long-lived configuration of $^{20}$Ne + $^{12}$C at high excitations ($\sim$ 70 -- 100 MeV) -- which was recently indicated from the analysis of the complex fragment emission data for the same system. Exclusive $\alpha$-particle evaporation spectra from the decay of hot composite $^{32}$S$^{*}$ also show similar behaviour.

Extended Hauser-Feshbach method for statistical binary decay of light-mass systems

An Extended Hauser-Feshbach Method (EHFM) is developed for light heavy-ion fusion reactions in order to provide a detailed analysis of all the possible decay channels by including explicitly the fusion-fission phase-space in the description of the cascade chain. The mass-asymmetric fission component is considered as a complex-fragment binary-decay which can be treated in the same way as the light-particle evaporation from the compound nucleus in statistical-model calculations. The method of the phase-space integrations for the binary-decay is an extension of the usual Hauser-Feshbach formalism to be applied to the mass-symmetric fission part. The EHFM calculations include ground-state binding energies and discrete levels in the low excitation-energy regions which are essential for an accurate evaluation of the phase-space integrations of the complex-fragment emission (fission). In the present calculations, EHFM is applied to the first-chance binary-decay by assuming that the second-chance fission decay is negligible. In a similar manner to the description of the fusion-evaporation process, the usual cascade calculation of light-particle emission from the highly excited complex fragments is applied. This complete calculation is then defined as EHFM+CASCADE. Calculated quantities such as charge-, mass- and kinetic-energy distributions are compared with inclusive and/or exclusive data for the $^{32}$S+$^{24}$Mg and $^{35}$Cl+$^{12}$C reactions which have been selected as typical examples. Finally, the missing charge distributions extracted from exclusive measurements are also successfully compared with the EHFM+CASCADE predictions.

Origin of complex fragments from32S+natAg reaction at 37.5 A·MeV

Fragment emission from collisions of32S withnatAg at 37.5 A·MeV has been studied with the 4π multi-detector AMPHORA. Production of intermediate mass and heavy fragments as well as of light charged particles has been measured. The total charged particle multiplicity and polar angular distributions have been used to select various classes of collisions. Analysis of angular and energy distributions of fragments and light particles in central collisions indicates the formation of a hot source (excitation energy of≈4.4 A·MeV) with an additional contribution from a preequilibrium process at more forward angles. Azimuthal angle correlations of He-Li, Li-Li, B-B, and C-C pairs have been used as a tool to study the origin of complex fragments. Data at backward angles are well described by considering a thermalized emitter with an angular momentum around 70ℏ and a fragment emission time of the order of 200 fm/c. A microscopic approach of BNV type confirms these emission times and angular momenta indicating the persistence of an incomplete fusion process responsible for the emission of complex fragments at backward angles.

Complex fragment emission from low energy compound nucleus decay to multifragmentation

In the first of these lectures, the experimental emission probabilities of complex fragments by low energy compound nuclei and their dependence upon energy and Z value are compared to the transtion state rates. In the second part, the high energy multi-fragment emission probabilities are shown to be reducible to the single fragment emission probability through the binomial distribution. The extracted one-fragment emission probabilities have a thermal dependence of the formp=e−B/T. This suggests that multifragmentation is a sequence of thermal binary decays.

Transition state rates and complex fragment decay widths.

Experimental excitation functions for the emission of complex fragments from compound nuclei are analyzed to search for atomic number {ital Z} and energy {ital E} dependent deviations from transition-state-method predictions. No {ital Z} and/or {ital E} dependent effects that could be attributed to an increased collectivity with increasing mass (charge) of the emitted fragment and associated with transient or stationary solutions of Kramers` diffusion equation are visible. Over seventy excitation functions, for complex fragments from four different compound nuclei, can be collapsed into a single universal straight line that is rigorously consistent with the transitions-state predictions.

Mass-Asymmetric Breakup in the Extended Finite-Range Macroscopic Model

Using the Cassinian ovaloid as fission shape both in symmetry and asymmetry, including Yukawa-plus-exponential finite range interaction and diffused surface, the rotating finite-range Cassinian ovaloid liquid drop model is developed. The dependence of the fission barrier heights on mass-asymmetry and on angular momentum is studied for the compound nuclei 112In, 159Tb and 209Bi. The predicted asymmetric fission barriers and mass yields are compared with experimental data obtained from the deexcitation of hot composite nuclei by complex fragment emission.

Emission of complex fragments in the reaction Ar + Au at 44 and 77 A · MeV

Complex-fragment emission from the 44 and 77 A · MeV 40 Ar+ 197 Au reaction has been investigated. Equilibrium and non-equilibrium components have been identified which are discussed in terms of statistical emission from the hot target-like fragment and of a possible persistence of a deep-inelastic process.

A multi-element detector array for heavy fragments emitted in intermediate energy nuclear reactions

To investigate complex fragment emission in intermediate energy nucleus-nucleus collisions, an array of three-element telescopes has been constructed. The array is designed to measure energy, charge and emission angles of fragments with Z values from 2 up to the projectile atomic number when studying reactions in reverse kinematics. In this case it has a good efficiency even for events with complex fragment high multiplicity. Each telescope is made of an ionization chamber (filled with CF 4 at a pressure up to 300 mbar), a Si detector (500 μm thick, position sensitive in two dimensions) and a CsI(Tl) scintillator with photodiode readout.

Complex fragment production and multifragmentation in 139La-induced reactions at 35, 40, 45 and 55 MeV/u

Complex fragment emission ( Z>3) has been studied in the reactions of 35, 40, 45 and 55 MeV/u 139La+X. Charge, angular, and energy distributions were measured inclusively and in coincidence with other complex fragments, and were used to extract source rapidities, velocity distributions, and cross sectins. Multifragment events increase with both bombarding energy and entrance-channel mass asymmetry. The excitation functions for multifragment events rise strongly with excitation energy. These excitation functions are independent of the target-projectile combination and bombarding energy suggesting, the formation of an intermediate nuclear system, whose decay properties depend mainly on its excitation energy and angular momentum.

Excitation functions for complex fragment emission in the E/A =20-100 MeV 14N+natAg, 197Au reactions.

Excitation functions have been measured for complex fragments with atomic number Z=3--15 emitted in collisions of E/A=20--100 MeV $^{14}\mathrm{N}$ ions with targets of $^{\mathrm{nat}}\mathrm{Ag}$ and $^{197}\mathrm{Au}$. The results are analyzed in terms of a three-source model which includes: projectilelike fragments from peripheral processes, nonequilibrium emission, and statistical decay of fully equilibrated compound nuclei. Nonequilibrium emission of complex fragments is found to increase rapidly relative to equilibrium emission up to E/A\ensuremath{\approxeq}60 MeV and then remain approximately constant thereafter. Evidence for enhanced production of heavier intermediate-mass fragments is found in the data for reactions with the $^{\mathrm{nat}}\mathrm{Ag}$ target at energies above E/A\ensuremath{\approxeq}60 MeV. Fit parameters for the equilibrium and nonequilibrium sources are examined. In particular, it is observed that the temperature parameter for nonequilibrium emission of complex fragments is independent of bombarding energy up to E/A=100 MeV.

Complex fragment emission in the , 197Au reactions

Abstract Excitation functions have been measured for complex fragments with Z=3–15 emitted in collisions of E/A=60, 80 and 100 MeV 14 N ions with targets of natAg and 197Au. The results are analyzed in terms of a three-source model: projectile fragmentation, non-equilibrium emission from the composite system, and statistical decay of an equilibrated heavy residue. Enhanced heavy fragment production is found for reactions with the natAg target above E/A⩾60 MeV. Above E/A≅50 MeV the ratio of equilibrium to non-equilibrium cross sections is found to be nearly independent of bombarding energy. Fit-parameter systematics are presented.

Populations of excited states and reaction mechanisms in the emission of complex fragments for collisions of 58Ni+58Ni at 11 MeV/nucleon.

Cross sections for emission of complex fragments ({ital Z}{gt}2) in their ground and excited states are presented for the reaction {sup 58}Ni+{sup 58}Ni at a bombarding energy of 630 MeV. Measurements of energy spectra, angular distributions, and angle-integrated cross sections were made for fragments of {ital Z}=4 to 20. {gamma} rays emitted in coincidence with the fragments were measured in a 4{pi} geometry using 62 NaI detectors and 8 Compton-suppressed germanium detectors. Data presented are mostly for the cross sections extracted by {gamma}-ray techniques. A simple statistical approach to associate the ratio of cross sections for excited states and ground states to the temperature of the emitter fails to give the expected temperatures. However, it is shown that this is mostly due to the fact that the fragments that {gamma} decay are secondary fragments produced by particle decay of the primary fragments. A Hauser-Feshbach analysis accounts well for the fragment cross sections, energy spectra, and extracted temperatures, provided that evaporation from the primary fragments is included.

A complete ridge-line potential for complex fragment emission

Cross sections were measured for fragments (4<Z<27) from the 5.0,6.2,6.9,8.0,10.2 and 12.7 MeV/N63Cu+12C reactions. Excitation functions were constructed for each Z value, and a nearly complete set of mass-asymmetric barriers has been obtained for75Br. There is excellent agreement between the experimentally determined barriers and the finite-range model calculations, while there is strong disagreement with the liquid-drop model calculations.

Equilibrium and non-equilibrium complex fragment emission in 50–100 MeV/u 139La + 12C reactions

Abstract Complex fragment emission ( Z > 2) has been studied in the reactions of 50, 80, and 100 MeV/u 139 La + 12 C. Charge, angle, and energy distributions were measured inclusively and in coincidence with other complex fragments, and were used to extract source rapidities, velocity distributions, and cross sections. The binary signature of the coincidence events and the sharpness of the velocity distributions illustrate the primarily 2-body nature of these reactions. Calculations based on statistical compound nucleus decay have been compared with the experimental data. The emission velocities, angular distributions, and absolute cross sections of fragments of 20 ⩽ Z ⩽ 35 at 50 MeV/u, 19 ⩽ Z ⩽ 28 at 80 MeV/u, and 17 ⩽ Z ⩽ 21 at 100 MeV/u are consistent with the binary decay of compound nuclei formed in incomplete fusion reactions in which the 139 La projectile picks up about one-half of the 12 C target. At 80 and 100 MeV/u, statistical model calculations are also able to reproduce the isotropic portion of the cross section for lighter and heavier fragments. However, a significant fraction of the total cross section for these fragments is due to non-equilibrium emission. Although the emission process is still mainly binary, and the relative velocity between the fragments is determined by their mutual Coulomb repulsion, the anisotropic angular distributions and the magnitudes of the absolute yields are incompatible with standard compound-nucleus statistical decay.