Fission-like Processes Research Articles

Isospin influence on fragments production in 78Kr + 40Ca and 86Kr + 48Ca collisions at 10 MeV/nucleon

In heavy ion collisions at low energy (E/A < 10–15 MeV/nucleon) the N/Z ratio in the entrance channel is closely related to the isospin degree of freedom and can influence the reaction mechanisms and consequently the production of the fragments in the exit channel. We analyse the fragment production cross sections in the reactions 86Kr + 48Ca and 78Kr + 40Ca , at the laboratory energy of 10 MeV/nucleon. The experiment was conducted at the INFN Laboratori Nazionali del Sud in Catania, by using the \( 4\pi\) detector CHIMERA. For the two reactions, elastic scattering measurements are performed to extract the normalization factor used to evaluate the absolute cross sections. Velocity and energy spectra, mass and charge distributions, as well as dynamic features of reaction products, are studied. The study of the characteristics of mass and charge distributions, energy and velocity spectra of the reaction products shows mainly a relaxed component, related to fusion reaction followed by evaporation or binary decay. Nevertheless signals ascribable to a non equilibrated component are present. Structure effects are evident in the staggered shape of emitted fragments cross sections and are more pronounced in the neutron poor system. The analysis highlights clear differences for the two systems in the contributions arising from different reaction mechanisms. Besides, a study of the overall influence from the entrance channel energy is performed. Comparisons to DNS (DiNuclear System) and GEMINI++ models are reported. The data analysis indicates a slightly higher fusion-evaporation cross section and a strongly pronounced probability of fission-like processes for the neutron poor system with respect to the neutron rich one. The neutron enrichment seems thus to limit the formation of the composite system and to inhibit the fission decay channel.

Intermediate Mass Fragments production at low energy: reaction mechanism and isospin influence

In this work, we present a recent study concerning the break-up of the Projectile-Like (PLF) into two fragments, following more violent deep-inelastic collisions in the reactions 78Kr + 40Ca and 86Kr + 48Ca at 10 A MeV, realized at INFN-Laboratori Nazionali del Sud by using the 4π multi detector CHIMERA. For these systems, we have already analyzed the fusion-evaporation and fission-like processes. A selection method has been developed, in order to discriminate PLF break up events from those due to other mechanisms which populate the same region of the phase- space. A preference for PLF aligned break-up, along the direction of the PLF-TLF separation axis with the light fragment emitted in the backward part, has been put in light, suggesting the presence of dynamical effects. Also a comparison between the neutron-rich 86Kr + 48Ca and neutron-poor 78Kr + 40Ca systems is presented, to put in evidence the isospin role in these processes.

Fission fragment angular distributions in pre-actinide nuclei

Background: Complete fusion of two nuclei leading to formation of a heavy compound nucleus (CN) is known to be hindered by various fission-like processes, in which the composite system reseparates after capture of the target and the projectile inside the potential barrier. As a consequence of these non-CN fission (NCNF) processes, fusion probability $({P}_{\mathrm{CN}})$ starts deviating from unity. Despite substantial progress in understanding, the onset and the experimental signatures of NCNF and the degree of its influence on fusion have not yet been unambiguously identified.Purpose: This work aims to investigate the presence of NCNF, if any, in pre-actinide nuclei by systematic study of fission angular anisotropies and fission cross sections $({\ensuremath{\sigma}}_{\mathrm{fis}})$ in a number of nuclear reactions carried out at and above the Coulomb barrier $({V}_{\mathrm{B}})$.Method: Fission fragment angular distributions were measured for six $^{28}\mathrm{Si}\text{-induced}$ reactions involving isotopically enriched targets of $^{169}\mathrm{Tm},\phantom{\rule{0.16em}{0ex}}^{176}\mathrm{Yb},\phantom{\rule{0.16em}{0ex}}^{175}\mathrm{Lu},\phantom{\rule{0.16em}{0ex}}^{180}\mathrm{Hf},\phantom{\rule{0.16em}{0ex}}^{181}\mathrm{Ta}$, and $^{182}\mathrm{W}$ leading to probable formation of CN in the pre-actinide region, at a laboratory energy $({E}_{\mathrm{lab}})$ range of 129--146 MeV. Measurements were performed with large angular coverage $({\ensuremath{\theta}}_{\mathrm{lab}}={41}^{\ensuremath{\circ}}$--${170}^{\ensuremath{\circ}})$ in which fission fragments (FFs) were detected by nine hybrid telescope $(E\text{\ensuremath{-}}\mathrm{\ensuremath{\Delta}}E)$ detectors. Extracted fission angular anisotropies and ${\ensuremath{\sigma}}_{\mathrm{fis}}$ were compared with statistical model (SM) predictions.Results: Barring two reactions involving targets with large non-zero ground state spin $(\mathcal{J})$, viz., $^{175}\mathrm{Lu}\left({\frac{7}{2}}^{+}\right)$ and $^{181}\mathrm{Ta}\phantom{\rule{4pt}{0ex}}\left({\frac{7}{2}}^{+}\right)$, experimental fission angular anisotropies were found to be higher in comparison with predictions of the statistical saddle point model (SSPM), at ${E}_{\mathrm{c}.\mathrm{m}.}$ near ${V}_{\mathrm{B}}$. Comparison of present results with those from neighboring systems revealed that experimental anisotropies increasingly deviated from SSPM predictions as one moved from pre-actinide to actinide nuclei. For reactions involving targets with large nonzero $\mathcal{J}$, this deviation was subdued. Comparison between measured ${\ensuremath{\sigma}}_{\mathrm{fis}}$ and predictions of SM indicated the presence of NCNF in at least four systems, when shell effects, both in the level density and the fission barrier, were included in the calculation.Conclusions: Systematic SM analysis of measured fission angular anisotropies and ${\ensuremath{\sigma}}_{\mathrm{fis}}$ confirmed the onset of NCNF in pre-actinide nuclei. Discrepancies between results about the degree of its influence on complete fusion, as deduced from various experimental probes, remain challenges to be solved. Complete measurement of all signatures of NCNF for many systems and preferably a dynamical description of the collisions between projectile and target nuclei are warranted for a deeper understanding.

Probing fusion-fission dynamics inBi203

Background: Complete fusion between two massive nuclei after capture inside the potential barrier is inhibited by competing fission-like processes. The target-projectile composite system may reseparate after capture without proceeding towards formation of the compound nucleus (CN), which is equilibrated in all degrees of freedom. The nature of these non-CN fission (NCNF) processes and factors that affect them are not completely known yet.Purpose: The nuclear mass regions from where NCNF processes begin to manifest themselves are not clearly demarcated. This work aims to study the onset of NCNF, if any, in the mass region $\ensuremath{\sim}200$.Methods: Fission fragment (FF) mass and angular distribution (MAD) and pre-scission and post-scission neutron multiplicities were measured for the reaction $^{19}\text{F}+^{184}\text{W}$ at a laboratory energy $({E}_{\mathrm{lab}})$ range of 84--125 MeV. The measurements were carried out using two multiwire proportional counters (MWPC) to detect the FFs in coincidence and four neutron detectors to measure neutron time of flight (TOF). Statistical model (SM) calculation was performed.Results: No significant mass-angle correlation was observed in the MAD plots. Extracted mass ratio distributions were single-peaked and of Gaussian shape. Measured pre-scission neutron multiplicity values indicated dissipative nature of CN decay for this reaction.Conclusions: No clear signatures of NCNF were observed in the studied reaction, indicating that the target-projectile composite system predominantly proceeds towards formation of the CN after capture.

The Dynamics of Non-Convergent Learning with a Conflicting Other: Internally Persuasive Discourse as a Framework for Articulating Successful Collaborative Learning

Successful collaborative learning is often conceptualized in terms of convergence, a process through which participants' shared understanding increases. This conceptualization does not capture certain successful collaborative learning processes, especially in the humanities, where multiple perspectives are often celebrated. Such is the context of the current study, where 52 Israeli Jewish and 52 Israeli Arab high-school students collaboratively e-investigated a historical event related to the Israeli–Palestinian conflict. Utilizing a bi-dimensional qualitative analysis, we identified four discussion types, including fission, a non-convergent and disputatious process leading to successful collaborative learning outcomes Bakhtin's notion of Internally Persuasive Discourse highlights the quality of collaboration as manifested in the dialogic agency developed by discussants, regardless of whether or not their knowledge converged. It captures the essence of fission-like processes as well as convergence processes, and is hence useful in explaining a wider variety of learning situations, especially in multicultural and/or disputatious contexts.

Fission fragment mass and angular distributions: Probes to study non-equilibrium fission

Synthesis of heavy and superheavy elements is severely hindered by fission and fission-like processes. The probability of these fission-like, non-equilibrium processes strongly depends on the entrance channel parameters. This article attempts to summarize the recent experimental findings and classify the signatures of these non-equilibrium processes based on macroscopic variables. The importance of the sticking time of the dinuclear complex with respect to the equilibration times of various degrees of freedom is emphasized.

Evolution of the excited electron bubble in liquidH4eand the appearance of fission-like processes

We have studied the evolution of an excited electron bubble in superfluid 4He for several tens of picoseconds combining the dynamics of the liquid with an adiabatic evolution for the electron. The path followed by the excited bubble in its decay to the ground state is shown to strongly depend on pressure. While for pressures below 1 bar the 1P excited electron bubble has allowance for radiatively decay to the deformed ground state, evolving then non-radiatively towards the ground state of the spherical electron bubble, we have found that above 1 bar two distinct baby bubbles appear in the course of the dynamical evolution, pointing to a different relaxation path in which the electron may be localized in one of the baby bubbles while the other collapses, allowing for a pure radiationless de-excitation. Our calculations are in agreement with experiments indicating that relaxed 1P bubbles are only observed for pressures smaller than a critical one, of the order of 1 bar, and that above this value the decay of the excited bubble has to proceed differently. A similar analysis carried out for the 2P bubble shows that the adiabatic approximation fails at an early stage of its dynamical evolution due to the crossing of the 2P and 1F states.

Deep-inelastic scattering in124,136Xe+58,64Niat energies near the Coulomb barrier

Cross sections, angular distributions, and mass distributions have been measured for deep-inelastic scattering in {sup 124}Xe+{sup 58}Ni and {sup 136}Xe+{sup 64}Ni at laboratory energies in the vicinity of the Coulomb barrier. The mass distributions show distinct components due to deep-inelastic and fissionlike processes. The strength of deep-inelastic scattering is similar in the two systems measured and comparable to previous measurements in {sup 58}Ni+{sup 112,124}Sn. {copyright} {ital 1997} {ital The American Physical Society}

Coincidence correlations between light charged particles in the matched reactions 905 and 1030 MeV and 550 and 750 MeV

Abstract Coincidence measurements have been made between 4 He 1 H and a second particle ( 4 He 1 H ) produced in the reactions 905 and 1030 MeV 121 Sb + 27 Al and 550 and 750 MeV 86 Kr + 63 Cu . All four of the reactions lead to composite systems of 149Tb∗ or 148Gd∗ which facilitates comparisons of matched reactions. In the two 121 Sb + 27 Al reactions and the 550 MeV 86 Kr + 63 Cu reaction, the particle-particle coincidences are dominated by processes that lead to evaporation residues. In the 750 MeV 86 Kr + 63 Cu reaction, about half of the 1H and 4He are associated with evaporation residue production. These particle-particle coincidences arise from cross sections that are subsets of the total evaporation residue cross sections. The derived individual particle chain lengths are ∼ 2 for both protons and alphas in each of the four reactions. The particle multiplicities and subset cross sections associated with the evaporation residues are quite similar for the matched reactions, in contrast to fission-like processes where spin-dependent entrance channel effects were observed.

Tracking fission-like processes in central collisions of 40Ar+ 232Th; E = 15–115 A MeV

Fission-like fragments and coincident charged particles have been measured in a 4π geometry over a wide energy range (15–115 A MeV) for the reaction 40Ar+ 232Th. The exclusive folding angle distribution data provide direct evidence that fission-like processes following incomplete-fusion are still an appreciable exit channel for beam energies as high as 115 A MeV.

Studies in nuclear macrophysics through fission and fission-like reactions

Recent developments in the study of fission and fission-like reactions are briefly reviewed. After a brief introduction of some of the important features of the fission process, binary fission and fission-like processes in heavy ion-induced reactions are discussed. It is shown that studies of the fission fragment angular distributions which provide a way to determine relative contributions of compound nucleus fission and non-equilibrium fission-like events in heavy ion-induced fission have proved to be quite valuable in investigating the very shortK-equilibration times of the order of 10−20 s involved in the nuclear dynamics of the dinuclear complex on its way to compound nucleus formation following nucleus-nucleus collision.

Strong change in the mass distribution of fission-like processes around ACN = 100

The completely relaxed reaction products resulting from bombardment of 59Co, 65Cu, 74Ge, 79Br, 85Rb and 89Y with 32S have been studied. At incident energies from 153 MeV to 184 MeV, compound of composite nuclei with masses, A CN, between 91 and 121 are formed with excitation energies of 80–100 MeV. Their spins might be as high as 60–70 h ̷ which allows for fission decay. The mass distribution from the reaction 32S on 89Y exhibits a sharp peak corresponding to symmetric fragmentation and consistent with a fission mechanism. The mass distribution over the above range range of decaying composite nuclei continuously broadens with decreasing mass, A CN. Maxima around the projectile and target mass gradually become more predominant and finally dominate the mass spectrum obtained in the reaction 32S on 59Co. These changes in the overall character of the mass distribution are discussed in terms of various mechanisms.

Two-body final-state coincidence measurements for fissionlike processes followingCl35+Al27fusion

A two-body final state with a mass distribution centered about half the mass of the compound system and with an average kinetic energy distribution equal to the Coulomb energy of the final system is observed following the fusion of $^{35}\mathrm{Cl}$ with $^{27}\mathrm{Al}$ at 170 MeV incident energy. Such a process is consistent with the onset of a fissionlike process resulting from the lowering of the rotating liquid drop fission barrier as a function of angular momentum.NUCLEAR REACTIONS, FISSION $^{35}\mathrm{Cl}$ + $^{27}\mathrm{Al}$ fusion, $E=170$ MeV; measured fragment-fragment ($E$; $A$).

Critical asymmetry and the systematics of fission-like processes in the reactions ofC12projectiles with medium mass nuclei

The systematics of fission-like processes observed in reactions induced by 107- and 197-MeV $^{12}\mathrm{C}$ projectiles have been investigated. Counter telescopes were employed to detect and identify the products of reactions with $^{89}\mathrm{Y}$, $^{98}\mathrm{Mo}$, Ag, and $^{116}\mathrm{Sn}$. Limited experiments using Ni and Au targets were also performed. For the medium mass targets, cross sections, energy distributions, and angular distribution of the reaction products are reported. Limiting angular momenta for the fusion of $^{12}\mathrm{C}$ with Y, Mo, and Ag have been determined and are used to estimate the angular momenta of the collisions leading to fission-like reactions. The probability of occurrence of such reactions is compared with predictions based upon the concept of an angular momentum dependent critical asymmetry. This concept is found to be very useful in discussing the reaction mechanisms involved.NUCLEAR REACTIONS Ni, $^{89}\mathrm{Y}$, $^{98}\mathrm{Mo}$, Ag, $^{116}\mathrm{Sn}$, $^{197}\mathrm{Au}(^{12}\mathrm{C},x)$, $E=107, 197$ MeV; measured absolute $\ensuremath{\sigma}(\ensuremath{\theta})$, $\ensuremath{\sigma}$ and $\overline{E}$ for fission-like products.

Ratio of Photodisintegration to Electrodisintegration of Al, Fe, and Ta Bombarded with 5.0-GeV Electrons

The cross-section ratio for photodisintegration to electrodisintegration was measured for targets of aluminum, iron, and tantalum bombarded with 5.0-GeV electrons. These ratios (or $F$ values) were determined for a total of six cascade-evaporation reactions in which as many as 12 nucleons are emitted from the target nucleus. The $F$ values for the six reactions average 2.1, and vary little with target mass and with the number of emitted nucleons. In addition, $F$ values were also measured for two other reactions whose mechanisms involve fragmentation and/or fissionlike processes. The average of these two $F$ values is 3.25 or about 50% larger than for the cascade-evaporation reactions.