Calculation of resonance reduced neutron widths by semiclassical description

Peripheral interactions of¹²C nuclei with protons and carbon nuclei have been investigated in a 2m propane bubble chamber at 4.2 GeV/c per nucleon. Energetic and angular characteristics ofγ-quanta accompanying the production of multicharged fragments withZ _f =5 and 6 have been analyzed. It is shown that in the fragment rest system there is an excess of γ-quanta with ε_γ<40 MeV over the background from known sources (π ⁰→2γ decays, bremsstrahlung and nuclear excitation).     

Shadowing of α-emitting fragments observed in 313 MeV32S+93Nb collisions

Energy- and angular correlations ofα particles in coincidence with deeply inelastic fragments from³²S+⁹³Nb reactions at 313 MeV were measured and the assumption of sequentialα emission from the fragments tested. In the direction of the detected light fragment too fewα particles from the heavy fragment are observed. This pronounced anomaly is ascribed to nuclear shadowing of theα particles by the proximate light fragment.     

Fragmentation of gold projectiles: From evaporation to total disassembly

We have studied the fragmentation of Au projectiles interacting with targets of C, Al and Cu at an incident energy ofE/A=600 MeV. The employed inverse kinematics allowed a nearly complete detection of projectile fragments with chargeZ≧2. The recorded fragmentation events were sorted according to three observables, the multiplicityM _lp of light charged particles, the largest atomic numberZ _max within an event, and a new observable,Z _bound, representing the sum of the atomic numbersZ of all fragments withZ≧2. Using these observables, the impact parameter dependence of the fragmentation process was investigated. For all three targets, a maximum mean multiplicity of 3 to 4 intermediate mass fragments (IMFs) is observed. The corresponding impact parameters range from central collisions for theC target to increasingly peripheral collisions for the heavier targets. It is found that the correlation between the IMF multiplicity andZ _bound, extending from evaporation type processes (largeZ _bound) to the total disassembly of the projectile (smallZ _bound), is independent of the target nucleus. This universal behaviour may suggest an — at least partial — equilibration of the projectile fragment prior to its decay.     

Incomplete deep-inelastic scattering in20Ne+197Au collisions at 20 MeV/nucleon

Correlations between emission angles and energies of coincidentα-particle projectile-fragment pairs have been measured for the²⁰Ne+¹⁹⁷Au system at 390 MeV beam energy. Theα-particles observed close to the direction of the projectile-like fragment were found to result essentially from sequential projectile decay. Starting from the close-geometry data, the contribution of sequentially emittedα-particles was calculated by Monte Carlo simulations for the entire angular range. For deep inelastic events, the measured angular correlations exhibit a spectacular excess over the calculated correlation. This excess is centred close to the beam direction but on the opposite side of it with respect to the detected projectile-like fragment. The correspondingα-particles have velocities around 85% of the projectile velocity indicating emission in an early reaction phase.     

Possible mechanisms of fragmentation of 16O nuclei with a momentum of 4.5 GeV/c per nucleon in track emulsions

The angular distributions of doubly charged fragments of ¹⁶O nuclei having a momentum of 4.5 GeV/c per nucleon and interacting with track-emulsion nuclei were studied. The experimental angular distributions of doubly charged fragments of a ¹⁶O nucleus are not described by the statistical model of the fragmentation of nuclei. The possible channels of fragmentation of ¹⁶O nuclei may include ¹⁶O → 2⁸Be → 4α, ¹⁶O → ⁸Be +⁸ Be* → 4α, ¹⁶O → 2⁸Be* → 4α, ¹⁶O → α+¹²C, ¹⁶O → α +¹²C* → α + 3α, ¹⁶O → α +¹²C* → α + pα ⁷Li, ¹⁶O → α +¹²C* → α + 2⁶Li, ¹⁶O → α +¹²C* → α + pt2α, ¹⁶O → Li + B, and ¹⁶O → Li* + B*.     

Fragmentation cross sections of16O at 60 and 200 GeV/nucleon

We used CR39 plastic nuclear track detectors (C₁₂H₁₈O₇) in combination with automatic track measurement to determine total charge changing and partial cross sections for the production of fragments of chargeZ=6 andZ=7 in collisions of¹⁶O beam nuclei at energies of 60 GeV/nucleon and 200 GeV/nucleon in targets H, C, CR39, CH₂, Al, Cu, Ag and Pb. Total charge changing cross sections due to the process of electromagnetic dissociation are calculated based on a theoretical model and found to be consistent with total and partial electromagnetic dissociation cross sections derived from this experiment. The energy dependence of pure nuclear fragmentation is investigated.     

Breakup effects of 6,7li on elastic and inelastic scattering from 12C at 18–28 MeV/nucleon

The differential cross sections for elastic scattering of ^6,7Li from ¹²C and inelastic one from the lowest three excited states of ¹²C have been measured at bombarding energies of 18–28 MeV/nucleon. Theoretical analyses of the data have been performed in which consistent treatments of density distributions for the ground and excited states of both projectile and target nuclei are made in the framework of microscopic cluster models for ^6,7Li and ¹²C and projectile-target interactions are generated by the double folding of the M3Y effective nucleon-nucleon interaction. About 25% reduction of the real part of folded potentials is required both in the analyses of elastic scattering with the single-channel calculation and in those of inelastic scattering with the coupled-channel calculation including the excited states of ¹²C. This reduction can be explained as a projectile breakup effect on elastic and inelastic scattering in comparison with a coupled-discretized-continuum-channels (CDCC) calculation and an extended CDCC one which allows mutual excitations of both projectile and target nuclei for ⁶Li case, respectively. It is also seen that an effect due to the target excitation on elastic scattering is of less importance than that of the ^6,7Li projectile breakup processes even fora deformed nucleus like ¹²C. Discrepancy between the extended CDCC calculation and inelastic data for the 0⁺₂ state of ¹²C suggests a strong influence from the ¹²C → 3α breakup channels in the ⁶Li case.     

Dissociation of 10C nuclei in a track nuclear emulsion at an energy of 1.2 GeV per nucleon

The charge topology in the fragmentation of ¹⁰C nuclei in a track nuclear emulsion at an energy of 1.2 GeV per nucleon is studied. In the coherent dissociation of ¹⁰C nuclei, about 82% of events are associated with the channel ¹⁰C → 2α+ 2p. The angular distributions and correlations of product fragments are presented for this channel. It is found that among ¹⁰C → 2α+ 2p events, about 30% are associated with the process in which dissociation through the ground state of the unstable ⁹Be_g.s. nucleus is followed by ⁸Be_g.s. + p decays.     

Structures and angular distributions of slow and relativistic charged particles produced in 7Li-emulsion interactions at 3 GeV/c per projectile nucleon

Experimental results on the multiplicity distributions of various particles produced in the interactions of ⁷Li with emulsion nuclei at a momentum of 3 GeV/c per projectile nucleon are reported. A comparison with data on collisions induced by other nuclei at a nearly identical momentum per nucleon is presented in order to reveal the dependence on the projectile mass. The internal structure of ⁷Li is explored by studying the projectile fragment. The mean multiplicity of shower particles, 〈n _s〉, induced by ⁷Li is found to be less than that in the case of ⁶Li projectiles. The angular distributions of target fragments and relativistic charged secondaries are investigated. No shock-wave phenomena are observed. Forward-to-backward ratios are calculated for each case. The probability distributions for relativistic secondaries produced per unit rapidity are studied in detail, along with the rapidity densities and their dependence on the projectile and the target mass. A comparison of the angular spectra of shower particles produced in central and peripheral collisions supports the limiting-fragmentation hypothesis. The collisions in question seem to become more central with increasing shower-particle multiplicity.     

Sequential emission of neutrons in the reaction16O+64Ni at 7.5–12 MeV per nucleon

Neutron energy and angular distributions have been studied in coincidence with projectilelike fragments (PF) for quasielastic and deeply inelastic collisions of¹⁶O on⁶⁴Ni at 7.5–12 MeV/u bombarding energy. The neutron yield can be explained assuming only two isotropically emitting sources:i) sequential emission from fully accelerated PF andii) evaporation from fully accelerated targetlike fragments (TF). ForZ=6 andZ=8 ejectiles known excited states in¹³C and¹⁷O could be identified. For oxygen, this suggests a direct excitation process,n pickup from the target. No significant evidence for preequilibrium neutron emission was found even at the highest bombarding energy. The TF temperatures and neutron multiplicities are consistent with the assumption that thermal equilibrium was reached during the binary reaction.     

Probing of complete and incomplete fusion dynamics in heavy-ion collision

Three different types of experiments have been performed to explore the complete and incomplete fusion dynamics in heavy-ion collisions. In this respect, first experiment for the measurement of excitation functions of the evaporation residues produced in the ²⁰Ne + ¹⁶⁵Ho system at projectile energy ranges ≈2–8 MeV /nucleon has been done. Measured cumulative and direct cross-sections have been compared with the theoretical model code PACE-2, which takes into account only the complete fusion process. It has been observed that, incomplete fusion fraction is sensitively dependent on projectile energy and mass asymmetry between the projectile and the target systems. Second experiment for measuring the forward recoil range distributions of the evaporation residues produced in the ²⁰Ne + ¹⁶⁵Ho system at projectile energy ≈8 MeV /nucleon has been done. It has been observed that, some evaporation residues have shown additional peaks in the measured forwardrecoil range distributions at cumulative thicknesses relatively smaller than the expected range of the residues produced via complete fusion. The results indicate the occurrence of incomplete fusion involving the breakup of ²⁰Ne into ⁴He + ¹⁶O and /or ⁸Be + ¹²C followed by one of the fragments with target nucleus ¹⁶⁵Ho. Third experiment for the measurement of spin distribution of the evaporation residues produced in the ¹⁶O + ¹²⁴Sn system at projectile energy ≈6 MeV /nucleon, showed that the residues produced as incomplete fusion products associated with fast α and 2 α-emission channels observed in the forward cone, are found to be distinctly different from those of the residues produced as complete fusion products. The spin distribution of the evaporation residues also inferred that in incomplete fusion reaction channels input angular momentum (J ₀) increases with fusion incompleteness when compared to complete fusion reaction channels. Present observation clearly shows that the production of fast forward α-particles arises from relatively larger angular momentum in the entrance channel leading to peripheral collision.     

Fragment spin distribution in 400 MeV40Ar+93Nb collisions investigated by light-particle anisotropies

In and out-of-plane angular correlations between light particles and projectile-like fragments from deeply inelastic collisions of 400 MeV⁴⁰Ar+⁹³Nb were measured. At backward angles, the equilibrium emission of a particles and protons from the target-like fragment was identified. By analysing the out-of-plane angular correlations in the framework of a semi-classical statistical model, an average spin of 30±2? and an average alignment ofP _zz =0.80±0.15 for the target-like fragment were deduced.     

Excitation of high-spin particle-hole states in A = 28 nuclei by 27Al(α, t) and (α, 3He) reactions

The ²⁷Al(α, t) and (α, ³He) reactions have been measured at E_α = 64.5 MeV. The experimental angular distributions were analyzed by the exact finite-range DWBA calculations assuming a nucleon stripping mechanism. The distributions of spectroscopic strengths for the single-particle transitions with transferred angular momenta l = 0, 1, 2, 3 and 4 have been obtained. The strengths for the transitions to the stretched 6^? states in ²⁸Si and ²⁸Al are compared to those obtained from inelastic scattering on ²⁸Si. The present results show no distinctive differences in the structures for 6^?T = 0 and 1 states in ²⁸Si such as are observed in proton and pion inelastic scattering on ²⁸Si.     

Fragmentation cross sections of32S at 0.7, 1.2 and 200 GeV/nucleon

We used CR39 plastic nuclear track detectors (C₁₂H₁₈O₇) in combination with automatic track measurement techniques to determine total charge changing and partial cross sections for the production of fragments of chargeZ _F =6 toZ _F =15 in collisions of³²S beam nuclei at energies of 0.7, 1.2 and 200 GeV/nucleon in targets H, C, CR39, CH₂, Al, Cu, Ag and Pb. By application of factorization rules measured partial cross sections are separated into pure nuclear and electromagnetic components. Total and partial cross sections for electromagnetic dissociation are compared with theoretical models. The energy dependence of pure nuclear cross sections is investigated.     

Study of emission characteristics of the projectile fragments produced in the interaction of 84Kr36 with nuclear emulsion detector at 1 GeV

This article is focused on the characteristics of the projectile fragments of charge 1 ≤ Z ≤ 10 produced in the interaction of the ⁸⁴Kr₃₆ with nuclear emulsion detector at 1 GeV per nucleon. We have studied the average charge distribution and multiplicity distribution of the projectile fragments having charge 1 ≤ Z ≤ 10. We have also studied the emission behavior of various projectile fragments produced from the interaction with different target groups of nuclear emulsion detector. From this study we have observed that the emission of projectile fragments is strongly dependent on the interaction with different type of the target groups of nuclear emulsion detector as well as on the mass of the projectile beam. The results are compared with other experimental observations carried out at relativistic energy and found to be consistent.     

The effect of nuclear structure in the emission of reaction products in heavy-ion reactions

Study of intermediate mass fragments (IMFs) and light charged particles (LCPs) emission has been carried out for a few reactions involving α-cluster and non- α-cluster systems to see how the emission processes are affected by nuclear clustering. Li, Be, B and α-particles have been studied from α-clustered system ¹⁶O + ¹²C for 117, 125, 145 and 160 MeV bombarding energies respectively. The enhanced yields of near-entrance channel fragment B and large quadrupole deformation of the produced composite ²⁸Si^? extracted from LCP spectra indicate the survival of orbiting-like process in ¹⁶O + ¹²C system at these energies. The same IMFs emitted from the α-cluster system ¹²C (77 MeV) + ²⁸Si and nearby non- α cluster ¹¹B (64 MeV) + ²⁸Si and ¹²C (73 MeV) + ²⁷Al (all having the same excitation energy of ～67 MeV) have also been studied. 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.     

Light particle emission in deeply inelastic collisions of 280 MeV40Ar+58Ni: Thermal equilibrium and angular momentum transfer

Protons andα-particles emitted in coincidence with a deep inelastic fragment produced in the reaction⁴⁰Ar (280 MeV)+⁵⁸Ni have been measured. From a detailed study of their in-plane angular distribution, it is shown that the bulk of the light charged particle emission can be attributed to a secondary evaporation process by the two primary deep inelastic fragments. A lowest estimate of the fragment spin (assuming total alignment perpendicular to the reaction plane) is obtained from the anisotropy of the out-of-plane angular distribution of theα-particles. For very asymmetric splitting, the heavy fragment spin estimate as obtained from theα-anisotropy measurement is consistent with the observed proton toα-particle multiplicity ratio. The results are in agreement with the hypothesis of a sticking configuration between two deformed nuclei.     

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.     

‘Slow’ residues observed in the reaction20Ne+208Pb at E/A=8.6, 11.4 and 15.0 MeV/u

Velocity distributions of heavy residuesA _Res>A _tar,Z _Res>Z _tar identified by means ofα spectroscopy, have been investigated at the velocity filter SHIP in reactions²⁰Ne+²⁰⁸Pb at projectile energies E/A=8.6, 11.4 and 15.0 MeV/u. Besides products from complete or nearly complete fusion, characterized by velocity distributions peaking atν/ν _CN?0.8–1.0, heavy residues with mean velocities of about half of the compound nucleus velocity were observed. The Z-distribution of this component was found to peak atZ=87. It is interpreted as residues from fusion of target nuclei with projectile fragments produced by nearly symmetric break-up. The experimental results were compared with predictions of theoretical models: cross sections for incomplete fusion were calculated using the sum-rule model of Wilcynski et al., while residue cross sections were calculated using the evaporation code HIVAP. A fair agreement between experimental and calculated mass distributions of heavy residues and transferred projectile fragments is achieved if an energy dissipation of ?23% (at E/A=8.6 MeV/u) and ?8% (at E/A=11.4 MeV/u) of the incident projectile energy is introduced. The observed peak of theZ-distribution atZ=87 is predominantly effected due to a higher fission probability of products withZ>87 during the deexcitation process and experimental limitations in the identification of products withZ≦86 by means of a spectroscopy, which cause a decrease of the observed production rates towards lowerZ.