Origin of projectile-like fragments from40Ar +68Zn collisions between 15 and 35 MeV/nucleon: Direct transfer of nucleons and projectile fragmentation

The projectile-like fragments emitted in the⁴⁰Ar +⁶⁸Zn reaction performed at 14.6, 19.6, 27.6 and 35 MeV/nucleon are studied. Their energy spectra and angular distributions have been measured. Velocities, widths of the linear momentum distributions and cross sections have been deduced. The results are discussedi) in terms of transfer of a few nucleons and analysed with a diffractional model. They are consistent, for stripping reactions, with a direct transfer of nucleons and a target excitation yielding multiparticlemultihole configurations,ii) In terms of projectile fragmentation-like process with a modified abrasion model taking into account the energy separation of the participant nucleons. The projectile fragmentation-like process appears above 20 MeV/nucleon and strongly competes with transfer of nucleons at 35 MeV/nucleon. The evolution of the mechanisms with incident energy is discussed on the basis of the reduced widths of the linear momentum distributions and on those of velocities and cross sections.     

Fast neutron emission from partial fusion heavy ion reactions

The Boltzmann master equation theory which succesfully reproduces the spectra of fast nucleons emitted in fusion and quasi-fusion heavy ion reactions is used to predict the spectra of fast nucleons in coincidence with a projectile-like fragment in the interaction of 35 MeV/ nucleon¹⁴N with¹⁶⁵Ho. In particular it is shown that the spectra of neutrons in coincidence with a high energy projectile-like fragment emitted at a very forward angle with energy corresponding to the beam velocity may be reproduced satisfactorily assuming an elastic breakup of the projectile followed by fusion of one of the fragments with the target nucleus.     

I. The properties of hot Ca-like fragments from the 40Ca + 40Ca reaction at 35AMeV

The creation of hot Ca-like fragments was investigated in the ⁴⁰Ca + ⁴⁰Ca reaction at 35 AMeV. Using the AMPHORA 4π detector system, the primary projectile-like fragment was reconstructed and its properties were determined. Both primary and secondary distributions are compared with the predictions of a Monte Carlo code describing a heavy-ion collision as a two-step process. Some of the nucleons which are identified as participants in the first step are transferred in the second step to these final states, which correspond on the average to the maximum value of entropy (thermodynamic probability). The model allows for competition between mean-field effects and nucleon-nucleon interactions in the overlap zone of the interacting nuclei. The analysis presented here suggests a thermalized source picture of the decay of the projectile-like fragment. The validity of the reconstruction procedure for projectile-like fragments is discussed. Received: 12 March 2001 / Accepted: 20 June 2001     

Recoil effects in peripheral nucleus-nucleus collisions at E/A=84 MeV

Azimuthal angular correlations between projectile fragments and light particles have been measured in ¹⁸O induced reactions on ^58.64Ni and ¹⁹⁷Au targets at E/A=84 MeV. Neither sequential projectile decay nor evaporation from an equilibrated target-like recoil can explain the observed correlations. The data are well described in terms of a sideways-moving source suggesting emission of midrapidity light particles from a subset of nucleons which carries a major part of the transverse recoil momentum imparted by the projectile fragment.     

Fast emission ofα-particles in deep-inelastic36Ar+197Au collisions

Angular and energy correlations betweenα-particles and deep-inelastic projectile-like fragments were measured for the system³⁶Ar+¹⁹⁷Au atE _Lab=380 MeV. At most half of the coincident events can be attributed to statistical emission ofα-particles from the fully accelerated projectile-like fragments. The remainder of the events may be due to direct emission during the first 10^?22 s of the scattering process and to preequilibrium processes taking place within some 10^?21 s.     

Neutron emission from 14N + 165Ho collisions at 35 MeV/u

Neutron emission from ¹⁴N + ¹⁶⁵Ho collisions has been studied at 25 MeV/u incident energy. Energy and angular distributions of the neutrons were measured in coincidence with projectile-like fragments (Li, Be, B, and C) emitted at angles of 10° and 30°. The spectra of neutrons at angles far from the angle of a coincident fragment have been satisfactorily parameterized in terms of a slowly moving, target-like source of temperature 2–3 MeV and a half-beam-velocity source of temperature about 7 MeV. The latter source accounts for about 20% of the detected neutrons for in-plane measurements. The out-of-plane cross sections are smaller. The relevant parameters of the moving-sources parametrization suggest a simple model which qualitatively explains the data in terms of the development of a hot participant zone and its subsequent mass exchange interactions with spectators in the projectile and target nuclei.     

Momentum distributions in A = 3 and 4 nuclei

We develop a general Monte Carlo method to study momentum distributions of nucleons and nucleon clusters in nuclei. The method is used to calculate the momentum distributions of protons and neutrons in A = 3 and 4, d + p amplitudes in ³He, and t + p and d + d amplitudes in ⁴He nuclei, with improved variational wave functions. The nucleon and d + p momentum distributions in ³He are also calculated from a five-channel Faddeev wave function. The calculations are based on realistic hamiltonians that include the three-nucleon interaction, and give reasonable binding energies and densities for light nuclei and nuclear matter. The calculated proton and d + p momentum distributions in ³He at low k are in good agreement with the results obtained by the Saclay analysis of the electron scattering data in the plane-wave impulse approximation; however, at higher values of k, the calculated momentum distributions are larger. The calculated values of the asymptotic D- to S-wave ratio of the d+n and d+d amplitudes are also in agreement with the values obtained from (d, t) and (d, α) reactions. The number of deuterons is found to be 1.38 and ∼2.4 in A = 3 and 4 nuclei, while the number of tritons in ⁴He is ∼1.6. This large value of the triton number reflects the large contribution (more than 90% at small k) of the t+p state to the total proton momentum distribution in ⁴He.     

Neutron emission in deep inelastic collisions of28Si on64Ni at 170 MeV

Energy and angular distributions of neutrons were measured in coincidence with the projectile-like fragments in deep inelastic collisions of²⁸Si on⁶⁴Ni at 170 MeV. Our data are consistent with the assumption of isotropic emission of neutrons in the rest frame of the fully accelerated fragments. Neutron multiplicities suggest that thermal equilibrium is not reached at least for energy losses of the system around 50 MeV.     

Light-particle emission in the reactions of 16O with Ti

Inclusive energy spectra and angular distributions for heavy ions $\text{(Z ≧ 3)}$ produced in the reactions of 227 MeV and 310 MeV ¹⁶O with Ti were measured. Also measured at the projectile energy of 310 MeV were energy and angular correlations between light charged particles (Z ≦ 2) and heavy ions. From comparisons with statistical model calculations upper limits to the complete fusion cross sections of 647 mb and 265 mb were derived for projectile energies of 227 MeV and 310 MeV, respectively. At 310 MeV the cross section of incomplete fusion processes was estimated to be over 505 mb. Emission of fast, high-energy α-particles and protons was observed to be a characteristic feature of quasi-elastic, deep-inelastic and fusion-like reactions. Average multiplicities of fast light particles in coincidence with heavy ions at +20° and +40° were estimated to be of the order of 1. The prompt emission of light appears to be the principal mechanism which limits complete fusion. A second component of α-particles observed in coincidence with deep-inelastic projectile-like fragments and having an energy comparable to Coulomb energies of particles emitted from target-like and projectile-like fragments appears as an excess yield in the direction of the recoiling target-like fragments. This component cannot be accounted for in terms of sequential emission processes and may result from a mechanism other than the one which leads to fast particle emission.     

The interaction of K mesons with nuclei

The experimental data for K⁻ capture by nuclei is interpreted in terms of K⁻ nucleon scattering amplitudes in a region of negative relative kinetic energy. It is shown that constant scattering length solutions which represent the data in the region of positive relative kinetic energy up to 100 MeV do not satisfactorily describe the interaction with bound nucleons and a modified scattering length solution is proposed. Measured values of the widths of K-mesic X-ray lines which require a strongly attractive K⁻ nucleons optical potential are successfully interpreted in terms of K⁻N scattering lengths appropriate for bound nucleons. The information obtainable about relative neutron and proton density distributions in the nuclear periphery from studies of this kind is discussed. The need for more data on interaction of K⁻ in hydrogen at low kinetic energies and at rest and in other elements at rest is emphasised.     

Deep processes in nuclei and in-medium dressing of the nucleons and mesons

K⁺ scattering and quasielastic electron scattering from nuclei are expected to provide information about the nucleons and mesons in the inner regions of nuclei. The K⁺- nucleus cross sections and the quasielastic electron-nucleus response functions have been calculated taking into account the same in-medium dressing of the nucleons and the same coupling of the σ and ω mesons to the polarization of nuclear matter. We obtain a good agreement with experimental data for the two processes.     

Two source emission behavior of projectile fragments alpha in 84Kr interactions at around 1 GeV per nucleon

The emission of projectile fragments alpha has been studied in ⁸⁴Kr interactions with nuclei of the nuclear emulsion detector composition at relativistic energy below 2 GeV per nucleon. The angular distribution of projectile fragments alpha in terms of transverse momentum could not be explained by a straight and clean-cut collision geometry hypothesis of Participant — Spectator (PS) Model. Therefore, it is assumed that projectile fragments alpha were produced from two separate sources that belong to the projectile spectator region differing drastically in their temperatures. It has been clearly observed that the emission of projectile fragments alpha are from two different sources. The contribution of projectile fragments alpha from contact layer or hot source is a few percent of the total emission of projectile fragments alphas. Most of the projectile fragments alphas are emitted from the cold source.     

Light charged particle emission and fission in238U+238U collisions at 1,740 MeV

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.     

Magnitude and alignment of transferred angular momentum in both quasi and deeply inelastic scattering

In-plane and out-of-plane angular correlations of fission fragments in coincidence with projectile-like fragments have been measured for the ⁸⁶Kr + ²³⁸U reaction at 730 MeV. The dependence of the magnitude and alignment of the angular momentum transferred to the fissioning heavy reaction product has been determined. Both quantities decrease strongly with decreasing energy loss in the quasi elastic region, in agreement with the predictions of a transport model.     

Sequential decay of light fragments in heavy ion reactions

A new interpretation of the production of light nuclei in heavy-ion reactions is presented. Yields and kinetic energies result from sequential decay by nucleons and α-particles of excited primary fragments formed by inelastic scattering or few-nucleon transfer.     

Quasielastic channel of single-stage nuclear fission induced by 1-GeV protons

Experimental indications that there exists single-stage fission of ²³⁸U and ²³²Th nuclei that is induced by a nearly transverse flux of intranuclear nucleons are obtained from an experiment that employs a two-arm time-of-flight spectrometer exposed to a beam of 1-GeV protons. A nearly transverse flux of mean multiplicity about 10 to 15 nucleons is initiated by a recoil nucleon of mean momentum about 240 MeV/c in the quasielastic scattering of an incident proton.