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The level density parameter and the back shift energy E1 are determined for nuclei with A-values across the whole periodic table from fits to complete level schemes at low excitation energy near the neutron binding energies. We find that the energy back shift E1 shows complicated behavior and depends on the type of the nucleus, even-even, odd mass, and odd-odd. The spin cut-off factor has also been investigated for nuclei mentioned above. The results are compared with the previous results and different experimental data on level densities.

The statistical properties of interacting fermions have been studied for various angular momentum with the inclusion of pairing interaction. The dependence of the critical temperature on angular momentum for several nuclei, have been studied. The yrast energy as a function of angular momentum for 28Si and 24Mg nuclei have been calculated up to 60.0 MeV of excitation energy. The computed limiting angular momenta are compared with the experimental results for 26Al produced by 12C+14N reaction. The relevant nuclear level densities for non-zero angular momentum have been computed for 44Ti and 136Ba nuclei. The results are compared with their corresponding values obtained from the approximate formulas.

Mass and charge identified ejectiles, spanning from {sup 11}B to {sup 22}Ne, have been detected in the 123 MeV {sup 19}F+{sup 56}Fe reaction. The coexistence of deep inelastic collision (DIC) and incomplete fusion (IF) mechanisms has been observed. The shape of the energy spectra and their behavior with angle allowed us to identify two components: The less dissipative one was dominating near the grazing angle. For both components experimental optimum {ital Q} values were derived. Two approaches based on the sum rule (SR) model of Wilczy{acute n}ski were used to calculate DIC and IF contributions to the complex fragment cross sections. Both prescriptions fit reasonably well experimental ejectile cross sections and {ital Q} optimum values. Results of the present investigation support the idea that the DIC can be treated on the same footing as IF in the SR model once the first process is confined in an inner angular momentum window, starting from the maximum fusion angular momentum, with respect to quasielastic processes. {copyright} {ital 1996 The American Physical Society.}

A detailed comparison between measured energy spectra and cross sections of {sup 1,2,3}H and {sup 4}He, evaporated from the composite system {sup 96}Ru ({ital E}{sup *}{congruent}113 MeV), and the predictions of the statistical model, has been carried out. Results obtained with transmission coefficients derived from optical model (OM), ingoing-wave boundary-condition model (IWBCM) and fusion systematics (FS) are presented. The best overall description of the data is obtained using IWBCM transmission coefficients, with reduced {ital s}-wave barriers, including, in the level density, deformations slightly larger than those predicted by the rotating liquid drop model (RLDM). {sup 2,3}H yields are well reproduced by IWBCM transmission coefficients, while they are largely overestimated by the OM ones; the failure is found to be related to the imaginary part of the optical potential which accounts for the absorption of nonfusion reactions, in peripheral collisions. No combination of phase space and transmission coefficients is able to reproduce, simultaneously, the low emission barrier and the cross section observed for protons.

Charged-particles in coincidence with fission fragments (FF) and evaporationresidues (ER) have been measured for the system 180-MeV 32S + 109Ag leading to the composite system 141Eu at the excitation energy Ex 90 MeV. Energy spectra and multiplicities in both channels have been compared to the predictions of the statistical model by using the code PACE2. The best overall agreement was obtained, by using af /aν=1.12, a fission delay 12 < τd < 14 ( x 10−21s) and reduced alpha barriers in the relevant J-region for the fission channel. In addition to pre-scission particle measurements, we stress the importance of measuring particles in the ER channel. These latter allow to impose further constraints on the statistical model input parameters and to extract a more reliable picture on the presence of dynamical effects.

Level densities and spin cut-off factors have been investigated within the microscopic approach based on the BCS Hamiltonian. In particular, the spin cut-off parameters have been calculated at neutron binding energies over a large range of nuclear mass using the BCS theory. The spin cut-off parameters {sigma}{sup 2}(E) have also been obtained from the Gilbert and Cameron expression and from rigid body calculations. The results were compared with their corresponding macroscopic values. It was found that the values of {sigma}{sup 2}(E) did not increase smoothly with A as expected based on macroscopic theory. Instead, the values of {sigma}{sup 2}(E) show structure reflecting the angular momentum of the shell model orbitals near the Fermi energy.

Mass and charge identified ejectiles, spanning from B-11 to Ne-22, have been detected in the 123 MeV F-19+ Fe-56 reaction. The coexistence of deep inelastic collision (DIC) and incomplete fusion (IF) mechanisms has been observed. The shape of the energy spectra and their behavior with angle allowed us to identify two components: The less dissipative one was dominating near the grazing angle. For both components experimental optimum Q values were derived. Two approaches based on the sum rule (SR) model of Wilczynski were used to calculate DIC and IF contributions to the complex fragment cross sections. Both prescriptions fit reasonably well experimental ejectile cross sections and Q optimum values. Results of the present investigation support the idea that the DIC can be treated on, the same footing as IF in the SR model once the first process is confined in an inner angular momentum window, starting from the maximum fusion angular momentum, with respect to quasielastic processes.

The multimodal random neck rupture model has been employed to study the energy distribution in nuclear fission. Monte Carlo calculations have been carried out on the de-excitation of fission fragments of {sup 252}Cf(sf) using this model. In this study special attention has been paid to the law of conservation of energy. Results for the neutron multiplicities and kinetic energies, average gamma energies are in good agreement with the experimental data. Mass and charge distributions of secondary fission fragments are also well reproduced in the calculations.

Evaporative [sup 1,2,3]H and [sup 4]He yields and energy spectra have been analyzed in the framework of the statistical model in ten heavy-ion reactions, spanning composite systems with 20[lt][ital Z][lt]56. Transmission coefficients derived from the ingoing-wave boundary condition (IWBCM) or optical models (OM) have been used in the calculations. The effect of using different values of deformation parameters, level-density parameter [ital a] and radii of the real part of the transmission coefficient potentials has been investigated. Except for the low-energy side of the proton spectra, the shape of energy spectra are reasonably well reproduced by both sets of transmission coefficients. While the two models are essentially equivalent for [sup 1]H and [sup 4]He cross sections, they differ significantly in the prediction of [sup 2]H and [sup 3]H cross sections. OM transmission coefficients fail in reproducing [sup 2,3]H cross sections, unless no standard parameters are used, while IWBCM transmission coefficients provide a good overall agreement. The failure of the OM is ascribed to the inclusion of nonfusion reactions in the inverse process.

Isomer yield ratio measurements in fission are important in understanding the fission process. With the development of new instrumental techniques, a large number of yield data are now available. The experimental data on isomer yield ratios in the thermal neutron induced fission of235U are compared with those calculated from the simple statistical model by Madland and England. The method of calculation has been extended to the isotopes having more than one isomeric state. The results may be explained according to the multi-exit-channel model of fission. © 1997 Akadémiai Kiadó.

Two fissioning systems [U-235(n(th), f) and Cf-252(sf)] have been studied to see the consequence of prompt neutron emission on odd-even effects. Monte Carlo calculations have been carried out for prompt neutron emission. Fragment and product yields have been analyzed by the Wahl Z(p), Model the Method of Differences and Model Independent Calculations. The result of the wash-out effect of prompt neutron emission on the neutron odd-even effect has been shown: however, this was not seen with the protons.

Emission of 4 He and 1 H has been studied in reactions of 485 MeV 56 Fe + 197 Au, using low-threshold detector arrays for light charged particles, and two heavy-fragment trigger detectors placed at symmetrically opposite angles with respect to the beam direction. The light charged particles were measured both in singles and in coincidence with heavy products of deeply inelastic and fission reactions detected near the grazing angle. Statistical model analyses of the data show that most of the 4 He/ 1 H intensity is due to evaporation from energy equilibrated emitters both in deeply inelastic and fission reactions. In deeply inelastic reactions, the observed 4 He/ 1 H emission can be attributed to evaporation from the post-scission reactant-like products, while in fusion-like fission reactions evaporation from pre- and post-scission sources are observed in comparable amounts. Angle-integrated multiplicities for 4 He and 1 H are deduced for each source of emission, and are compared with results from similar systems. The experimental 4 He/ 1 H multiplicity ratios indicate roughly equal probability for 4 He and 1 H evaporation from a given excited source. The observation of pre-scission 4 He/ 1 H in fusion-like fission reactions supports the notion that thermal equilibration and subsequent particle evaporation proceed more rapidly than the collective motions that drive the system toward fission. Conversely, the lack of appreciable 4 He/ 1 H emission from pre-scission sources in deeply inelastic reactions implies that the interaction times are short compared to the time required for particle evaporation.

Particle-particle coincidence measurements have been made between light charged particles (/sup 1,2,3/H and /sup 4/He) produced in the reaction (8.5 MeV/nucleon) /sup 56/Fe+/sup nat/Ag. This coincidence requirement emphasizes central collisions leading to evaporation residues. Average multiplicities are found to be approximately three alphas and approximately four protons per evaporation residue; these values imply that approximately (1/2 the initial excitation energy of about 211 MeV is evacuated from the evaporation residues by these particles. This fractional energy evacuation by charged particles is considerably larger than that found for the similar reaction (8.5 MeV/nucleon) /sup 40/Ar+/sup nat/Ag. In both reactions the effective emission barriers for /sup 1/H and /sup 4/He (as deduced from the energy spectra) are found to be significantly lower than corresponding empirical fusion barriers between cold nuclei. This is particularly surprising for emitter nuclei that survive fission.

Light charged particles from the reactions 247 and 337 MeV $^{40}\mathrm{Ar}$${+}^{\mathrm{nat}}$Ag have been measured in singles and in coincidence with fission-like products. Most of the $^{1}\mathrm{H}$ and $^{4}\mathrm{He}$ in coincidence with fusion-fission and quasifission can be accounted for by evaporation from the composite nucleus or by the fragments. Multiplicities are determined for $^{1}\mathrm{H}$ and $^{4}\mathrm{He}$ in association with evaporation residues, fusion-fission, and quasifission. The probability for composite nucleus emission decreases with increasing spin zone of the emitter and gives a view of the role of emitter lifetime. The ratio of composite-nucleus multiplicities for quasifission/fusion-fission is about 1/2 ((1/4) for 337 MeV $^{40}\mathrm{Ar}$ (247 MeV $^{40}\mathrm{Ar}$) which implies a similar ratio for the lifetimes of the respective composite nuclei between thermalization and scission. Energy spectra and angular distributions of particles (in coincidence with fusion-fission and quasifission) are consistent with emission from a strongly distorted nuclear system en route toward scission. Prethermalization emission of $^{4}\mathrm{He}$ is observed at 10\ifmmode^\circ\else\textdegree\fi{} in comparable amounts for peripheral collisions (quasifission and fusion-fission) and for central collisions.

The emission of4He and1H has been measured in coincidence with fission for reactions of 469-MeV56Fe+238U. By using a gas-ionization telescope in kinematic coincidence with a position-sensitive avalanche detector, the folding angle between two fission fragments was determined in order to distinguish fusion reactions from fission following smaller-momentum-transfer collisions. In both fusion fission and sequential fission reactions, the4He/1H energy spectra are relatively narrow with relatively flat angular distributions at backward angles and become broader in energy with enhanced cross-sections at forward angles. The extent of forward peaking is significantly greater for peripheral collisions than for central collisions. The light-charged-particle multiplicities are quite similar for4He and1H, being much larger for fusion fission than for sequential fission. Detailed comparisons of the spectral shapes with Monte Carlo simulations of reaction kinematics impose strong constraints on the participation of different emission sources. We find important contributions to the observed4He/1H emission both from accelerated fragments (FE) and from the composite system prior to fission (CE). For4He emission, the multiplicity of CE is much larger for fusion fission than for sequential fission, possibly as a consequence of the higher spins and shorter reaction times associated with deeply inelastic and quasi-elastic processes. For1H emission, a corresponding but somewhat smaller difference is observed for the CE multiplicities. An excess of4He/1H particles, found at forward angles in both fusion and sequential fission processes, cannot be attributed to evaporative emission from any fragments and therefore must originate in pre-thermalization emission.

Inclusive and exclusive measurements of light-charged particles (/sup 1,2,3/H,/sup 4/He) and heavy fragments have been made for the reaction 480 MeV /sup 56/Fe+/sup nat/Ag. The backward hemisphere emission of /sup 4/He and /sup 1/H in coincidence with a heavy fragment can be well described by evaporative emission from a combination of three sources: the detected fragment, the undetected fragment, and the composite system prior to scission. Multiplicities for each of these sources are determined for two coincident fragment groups: a fusion-fission-like group and a deeply-inelastic-reaction group. These multiplicities have also been studied at two angles for the trigger fragment (theta/sub TR/ = 26/sup 0/ and 50/sup 0/). For theta/sub TR/ = 26/sup 0/ the multiplicity of the light-charged particles (/sup 1/H or /sup 4/He) emitted from the composite nuclear system (i.e., prior to scission) is approx. =(2/3 for the fusion-fission-like fragments, compared to approx. =(1/3 for the deeply inelastic reactions. This decrease implies that the deeply inelastic reactions occur in approx. =(1/2 the time required for fission reactions. For the deeply inelastic reactions, these multiplicities change with trigger angle. This result also suggests that the lifetime of the composite system exerts some control on the extent of evaporative particle emission.more » Energy and angular distributions of the coincident light particles suggest extensive emission from a strongly deformed composite system, such as that of a system en route to scission. Low-energy /sup 4/He particles at forward angles, which cannot be attributed to evaporation, exhibit a dependence on the angle with respect to the scission axis. This result is attributed to emission that occurs very near to the instant of scission.« less

The excitation functions of 29 Si(p, α 0 ) 26 Al, 29 Si(p, α 1 ) 26 Al and 29 Si(p, α 2 ) 26 Al were measured with high beam energy resolution in order to determine the isospin dependence of the nuclear level width of the composite nucleus, 30 P, at an average excitation energy of 19.6 MeV. From an auto-correlation analysis of these excitation functions, the level widths of the T = 0 and T > = 1 isospin states are determined as 81 ± 17 keV and 104 ± 35 keV, respectively. An analysis including isospin mixing is also performed. With the aid of the statistical theory of nuclear reactions, the coherence energies are used to deduce the relative densities of two isospin states. The predictions of the Fermi gas model of level densities including isospin are in good agreement with the obtained results.

We have measured coincidences between $^{4}\mathrm{He}$ and $^{1}$,2,3H or a second $^{4}\mathrm{He}$ (large-angle correlations) in the reactions 247 and 337 MeV $^{40}$Ar${+}^{\mathrm{nat}}$Ag. These coincidences are dominated by reactions leading to evaporation residues with spins of about 0--70\ensuremath{\Elzxh}. Large values of the angle-integrated double and triple coincidence cross sections indicate that many particle evaporation chains involve 2 or 3 protons and/or 2 or 3 alphas. The angular and energy distributions are well described by an evaporation model with Hauser-Feshbach coupling. However, one must employ effective evaporation barriers for $^{1}\mathrm{H}$ and $^{4}\mathrm{He}$ that are strongly reduced compared to empirical fusion barriers. The barrier reduction required for $^{2}\mathrm{H}$ is rather small, and for $^{3}\mathrm{H}$ no barrier reduction is needed. Evaporation calculations that include emitter deformation do not give a satisfactory explanation for these results. We suggest a new aspect of composite nucleus excitation and evaporation-like decay. A hot, very diffuse nuclear surface may be formed that promotes evaporative decay prior to its relaxation. The low barriers for $^{1}\mathrm{H}$ and $^{4}\mathrm{He}$ reflect the radial extent of this transitory, diffuse surface.

Inclusive1H/4He emission has been studied for three reactions in “reversed kinematics”,40Ar/56Fe/86Kr (8.5MeV/u)+12C. The4He spectra include the heretofore unobserved region of low c.m. energies in the direction of the heavy reactant. A test of forward-backward symmetry is made for the hot-spot, moving source model; the data do not support the model. Effective emission barriers for4He evaporation are found to be within ∼10% of the corresponding fusion barriers, while for1H evaporation the emission barriers are more than 25% lower. It is possible that deformations predicted by the rotating liquid drop model can account for the4He evaporation spectra, but substantial change in the standard evaporation treatment is required to explain the spectra for1H.

Inclusive4He and4H energy spectra and heavy fragment coincidence correlations have been measured for reactions of 7.31 MeV/u238U with238U and−197Au targets. The H/He production cross sections are in the range 15–26 mb, and their emission spectra are very similar for the two systems. The observed strong kinematic shifts with angle are reproduced in shape and magnitude by Monte Carlo simulations of particle evaporation from projectile-like and target-like fragments, indicating competition between charged particle emission and sequential fission. No evidence is found for high energy charged particle emission associated with ultra-highZ composite systems. Heavy fragment measurements indicate an abundance of quasielastic and deeply inelastic reaction fragments, as well as sequential fission of target and projectile nuclei. For238U nuclei, the fission occurs predominantly in an asymmetric mode, reminiscent of fission at low excitation energy. For238+238U reactions in the vicinity of the grazing angle, the frequency of single sequential fission (with survival of the partner fragment) is twice as large as double sequential fission in which both the target and projectile undergo fission. In238U+197Au reactions, the survival probability of the heavy fragments is even greater. The surprisingly high survival probabilities of high-Z fragments imply a preponderance of very soft collisions in these very-heavy-ion reactions, at least at energies not very far over the Coulomb barrier.

Emission of4He in the reaction 334 MeV40Ar+238U has been studied by triple coincidence measurements that allow the separate identification of fusion fission and sequential fission. For the4He evaporative spectra from fusion fission the composite system is shown to be the predominant contributor; whereas, for sequential fission the dominant emission is from the fragments. This result demonstrates a correlation between evaporative emission probability and lifetime expectancy of the composite system. To account for the observed4He spectra two other mechanisms are necessary in addition to nuclear evaporation. At forward angles, the4He spectra from both fusion fission and sequential fission exhibit higher intensities and larger energies than those expected from purely evaporative processes. This forward-peaked component must be related to a very rapid or pre-thermalization stage of the reaction. At backward angles yet another component is observed for fusion fission. As it is sensitive to the fragment masses but does not carry the kinematic shift characteristic of their full acceleration, this component must originate near to the time of scission. The average4He energy for this component is approximately 17 MeV (c.m.), and its intensity is correlated with a plane perpendicular to the fission fragment separation axis. These signatures are similar to those for long range alpha particle emission in low energy fission. Alpha particles evaporated from the composite nuclei in fusion-fission reactions are shown to be preferentially associated with fission events which result in the more symmetric masses. This result is consistent with the notion that mass asymmetric fission is a faster process than symmetric fission. Such a correlation between mass asymmetry and lifetime is an essential part of the “fast fission” or “quasifission” idea, which has attracted much current attention.

Multiplicities are compared for4He evaporation in reactions of40Ar and56Fe projectiles (8.5 MeV/u) with withnatAg and238U targets. Coincidence requirements involving fusion-fission and projectile-like fragments have been used to select separate entrance-channel spin zones. Evaporation-like4He emission from the composite nuclei is shown to be large for mean spins ≦100ℏ, and to decrease dramatically with increasing spin. Coincidence measurements between two4He particles have also been studied for40Ar+natAg. This requirement is shown to select reactions that lead to evaporation residues and therefore to a zone of relatively low spin. The shapes of the4He spectra provide an interesting probe of the mechanism and also of the thermodynamic properties of hot nuclei.

The consequences of the evaporation of prompt neutrons and ..gamma.. rays from primary fission fragments in the spontaneous fission of /sup 252/Cf have been examined by a Monte Carlo method. A semiempirical mass equation and experimental values of total kinetic energies are used to obtain the sum of excitation energies of complementary fragments. The division of this energy between complementary fragments is determined by the two-spheroid model for the scission configuration. The prompt neutron multiplicity distribution gives a value of 3.68 for the average number of prompt neutrons in /sup 252/Cf (sf). Calculated values of the average number of prompt neutrons as a function of fragments mass are in good agreement with the experimental values. Calculated results for the charge dependence of total ..gamma..-ray energies show an odd-even effect, whereas no such effect is apparent in the charge dependence of the average number of neutrons. Mass and charge distributions of secondary fission fragments are also well reproduced in the calculations.

Statistical fluctuation analyses of /sup 31/P(p, alpha ) and /sup 31/ P(p, p') ex citation functions are used to deduce level widths for the T isospin states in /sup 32/S at 17.8 MeV of 38.7 plus or minus 2.7 and 26.2 plus or minus 3.5 keV, respectively. These two widths in /sup 55/Mn at 17.6 MeV are reported also. Relative level densities of the two isospins are calculated for each nucleus. (auth)

We have investigated the emission of 4 He in coincidence with mass-selected fragments from fission for 334 MeV 40 Ar + 238 U. Upper-limit multiplicities are assigned for fragment evaporation and composite-nucleus evaporation, but additional mechanisms are also required. A new low-energy component is found at backward angles; it is sensitive to the fragment masses but does not reflect their full kinematic shifts. Thus it must be due to near-scission emission (prior to full fragment acceleration).

A Monte Carlo simulation is used to study the ..cap alpha..-particle angular distribution from the fission reactions /sup 56Fe/Fe + /sup 165/ Ho.(AIP)