Angular distributions of fission and deep inelastic products of the system Ar+Au at 285 MeV

The angular distribution of the heavy products from the interaction of 285 MeV ions⁴⁰Ar on an Au target has been analysed through a large angular range (4°–165° lab angles). The results exhibit two components: the fission component corresponding to relatively symmetric masses, and a deep inelastic one which peaks in the angular range 100°–140° (c.m angles). The fission component exhibit the usual 1/sinθ pattern, proving that, in spite of a zero fission barrier, the life time of the system exceeds the rotation period. The relative cross sections for the fission and deep inelastic processes are discussed.     

X-ray study of the deep inelastic reactions 1,160-MeV136Xe+Pb and136Xe+232Th

The spectra ofK x-rays emitted by the target-like reaction products in the deep inelastic collisions 1,160-MeV¹³⁶Xe+Pb and¹³⁶Xe+²³²Th were investigated. Target-like x-ray emission probabilities per projectile-like and per target-like product were determined and compared with theoretical predictions of atomic vacancy production in the higher-Z particle (1s }-vacancy production). It is shown that a large fraction of the x-rays is produced by internal conversion ofγ-rays in the higher-Z reaction product. Measurement of target-like x-rays in coincidence with projectile-like and with target-like reaction products allowed the determination of the average survival probability against fission and of theZ-distribution of the highly excited target-like nuclei as a function of theQ value (total kinetic energy loss). The observed average survival probabilities agree qualitatively with theoretical predictions of Wilcke et al. Nuclearreaction-time effects on the 1s σ-vacancy production in these collisions could not be demonstrated with any certainty.     

Search for delayed fission products from superheavy nuclei in the reaction136Xe+208Pb

The production of superheavy elements in binary reactions of the type²⁰⁸Pb (¹³⁶Xe, X) Y was investigated atE _c.m.=470 MeV. The experiment was designed to search for delayed fission products from elements withZ between 108 and 116 and fission lifetimes ofΤ?10^?12 s. No fission events were observed the upper limit for the formation cross section being 1.2 Μbarn.     

Target-fragment angular distributions for the interaction of 86 MeV/A12C with 197Au

Target-fragment angular distributions were measured using radiochemical techniques for 69 different fragments (44 ? A ? 196) from the interaction of 86 MeV/A¹²C with ¹⁹⁷Au. The angular distributions in the laboratory system are forward-peaked with some distributions also showing a backward peaking. The shapes of the laboratory system distributions were compared with the predictions of the nuclear firestreak model. The measured angular distributions differed markedly from the predictions of the firestreak model in most cases. This discrepancy could be due, in part, to overestimation of the transferred longitudinal momentum by the firestreak model, the assumption of isotropic angular distributions for fission and particle emission in the moving frame and incorrect assumptions about how the lightest (A < 60) fragments are produced. No evidence was found for any significant number of target fragments moving sidewise to the beam direction in apparent contradiction to the expectation of hydrodynamical-model calculations. The laboratory-frame angular distributions were transformed into the moving frame using various assumptions about the moving frame velocity. The resulting light-fragment distributions showed an asymmetry in the moving frame indicative of their production in a fast process without the establishment of statistical equilibrium. No evidence was found for any production of the light fragments by a very asymmetric fission mechanism. The fission-fragment distributions were compared to standard formulas and an average fissioning-system angular momentum of $\text{J = 40?50}\text{h}\text{?}$ was deduced. It was not possible to find a moving frame in which the heavy (A > 145) fragment distributions were symmetric about 90°.     

Classical and statistical analysis of40Ar(300 MeV)+197Au deep inelastic interaction

We examine the way in which local field effects in the neutron refractive index affect the values of coherent scattering lengths determined by various kinds of neutron optical measurements. We find that under typical experimental conditions these effects are negligible for interferometry measurements but that they are significant for gravity refractometry measurements, producing changes in the effective scattering length of as much as two or three standard deviations in some cases. Refined values of the scattering lengths are obtained for the thirteen elements for which data are presently available. The special role of local field effects in neutron transmission is also discussed.     

Momentum distributions of projectile fragments produced in the cold and hot fragmentation of relativistic136Xe and197Au projectiles

The longitudinal-momentum distributions of projectile fragments from 0.8 A GeV¹³⁶Xe and 1 A GeV¹⁹⁷Au projectiles impinging on targets of beryllium and aluminium, respectively, have been measured using the projectile-fragment separator FRS at GSI. Different momentum distributions have been found for two different classes of fragmentation processes: the abundant hot fragmentation with several nucleons evaporated from the prefragments, and the rare cold fragmentation with only protons removed from the projectile, but no nucleons evaporated. The data are compared to model calculations.This article comprises part of the Ph.D. thesis of B. Voss     

Correlations between fission fragments and light charged-particles in the reaction20Ne+197Au at 13 MeV/nucleon

rgy of light charged-particles has been measured in coincidence with one or two fission fragments in the reaction²⁰Ne+¹⁹⁷Au at a bombarding energy of 13 MeV/u. The fission cross section was found equal to 1,340±260 mb. Assuming that it represents the totality of the fusion cross section, the critical valuel _c is deduced equal to 93±9?, higher thanl _Bf=0=74 in the rotating liquid drop model. The main emission source for protons and alpha particles seems to be a thermally equilibrated composite system. The competition between fission and charged particle emission is unexpected in the frame of the statistical treatment. A high energy component is observed in the forward direction. These results correspond to the first step of a study programme on the evolution of nucleus-nucleus collisions between 10 and 100 MeV per nucleon.     

Mass distribution of fission fragments in the19F+197Au reaction

Mass distributions of fission fragments have been measured for the reaction of¹⁹F on¹⁹⁷Au as a function of scattering angle. The mean fragment mass is dependent on the scattering angle, the asymmetry increasing with increasing projectile energy.     

Fragment angular distributions from fission of238U,209Bi and197Au by 140-MeV4He Ions

The fission fragment angular distributions from reactions of 140-MeV⁴He ions with²³⁸U,²⁰⁹Bi and¹⁹⁷Au have been studied. From the anisotropies in the angular distributions, values for? _eff, the effective moment of inertia at the fission saddle point, have been estimated and compared with results obtained at lower bombarding energies. The derivation of? _eff values has included corrections for the effects of incomplete fusion mechanisms on the orbital angular momentum distribution of the fissioning nuclei and for neutron evaporation prior to fission. The results are also compared with heavy-ion-induced fission data for systems of similar fissility. Also, examination of the forward-backward symmetry of the²³⁸U angular distribution substantiates other results which show that the fraction of fission reactions which follow complete fusion of the target and projectile is less than 0.5 for 140-MeV⁴He-ion bombardment of²³⁸U.     

Experimental evidence of highly energy relaxed products in the 35 MeV/u84Kr+197Au reaction

The Q_β-values of⁹⁹Rb,^99,100Sr and¹⁰⁰Y have been measured for the first time at the mass separator OSTIS of the ILL; for⁹⁹Y,^99,100Zr and¹⁰⁰Nb, the results of earlier experiments were confirmed with reduced experimental errors. The two-neutron binding energies derived from the experimental Q_β-values clearly indicate that the nuclei¹⁰⁰Sr and¹⁰⁰Y are strongly deformed.     

Understanding of the dissipation mechanism in ternary fission for the system 197Au+197Au

The mass number distributions of three fragments from the ternary fission of the system 197Au+197Au are reproduced rather well by using the improved quantum molecular dynamics (ImQMD) model without any adjusting parameter. It is found that the probability of ternary fission evidently depends on the incident energy and the impact parameter, and the two-body dissipation is the main mechanism responsible for the formation of the third fragment with comparable mass.     

Gamma ray multiplicities in deep inelastic collisions for the Cu + Au 400 MeV system

For the Cu + Au 400 MeV system γ-ray multiplicities have been measured. The mean value of the transferred angular momentum, the variances, and the repartition of angular momentum between the fragments have been deduced. It is shown that, early in the reaction the building up of angular momentum is a very fast process. For longer times, additional angular momentum is induced in the fragments. The excitation of collective modes can account for this effect. The properties of such modes have been analysed.     

Mass and charge flow in the reaction 7.1 MeV/u197Au+63Cu

Mass-yield and angular-distribution data are presented for products from the reaction of 7.1 MeV/ ¹⁹⁷Au with⁶³Cu. With help from information derived from the latter, the former are classified into components corresponding to quasielastic transfer (580±80 mb), deep-inelastic transfer plus quasifission (1300±130 mb), fusionfission (195 mb), and sequential fission (195±45 mb). The fusion excitation function calculated with the Dynamic Capture model standard parameter set reproduces our deduced fusion-fission cross section well. Moreover, using this cross section as well as additional published data for the same reaction system, we extract ans-wave fusion-barrier shift (extra push) for this system of 35±7 MeV, which is in good agreement with the systematics derived from other fusion-barrier shifts which have been reported in the literature. Lastly, support is found for the Dissipative Diabatic Dynamics model prediction that dynamically-hindered fusion trajectories are reflected into quasielastic channels.Research supported by the Bundesministerium für Forschung und Technologie under contract number 06 MR 553     

Coincidence studies of heavy fragment pairs in the reaction 343 MeV40Ar+197Au

Measurements are reported for correlated energies and angles of heavy fragments from reactions of 343 MeV⁴⁰Ar with¹⁹⁷Au. The results show a continuous evolution of the mass distributions from fission-like products at angle pairs of ≈60 and 300 degrees to deeply inelastic projectile-like products at ≈45° and 300°. Certain difficulties are emphasized for the use of the “symmetric fragmentation” cross sections in the context of current theoretical models (e.g. the “extra push” theory). The fission-like reactions have been studied in some detail and the results have been compared to a Monte Carlo kinematic simulation. To account for the experimental results forward-peaked light particle emission (n, H, He) must occur in a large fraction of the fission-like reactions.     

Fission products from the197Au(d,f) reaction

Cumulative and independent fission product yields of the reaction¹⁹⁷Au(d,f) were determined at a beam energy of 80 MeV at the Jülich isochronous cyclotron. Off-beamγ-ray measurements applying a catcher foil technique and chemical separation, and in inbeam kinetic energy measurements of fission products were performed. The complete yield distribution of post neutron emission fission fragments is represented by a single function, consisting of the product of two Gaussians. The mass distribution is characterized by the widthσ _A =12.5amu. The width of theZ-distribution for eachA-chain is given byσ _Z =0.63amu.     

Charge and mass transfer in the reaction136Xe+208Pb at energies close to the coulomb barrier

Mass and charge transfer was investigated for the system¹³⁶Xe+²⁰⁸Pb at 5.9 MeV/nucleon incident energy with aΔE —E-time of flight telescope. The angle and energy integrated cross section for the strongly damped events was found to be 510 mb, very close to the total reaction cross section. The width of the mass distribution as function of the total kinetic energy loss was measured and is compared to the width of the corresponding charge distribution. An upper limit of 1 μb has been found for processes with very large mass transfer.