Analysis of the sequential fission observed in collisions of100Mo +100Mo and120Sn +120Sn around 20 A·MeV

Events with 2, 3 and 4 heavy fragments (A≥20) detected in the reactions¹⁰⁰Mo +¹⁰⁰Mo at 18.7, 23.7 A·MeV and¹²⁰Sn +¹²⁰Sn at 18.4 A·MeV were analyzed by means of an improved version of the kinematic coincidence method. The phase-space distributions prove that 3- (and possibly 4-) body events predominantly originate from a two-step mechanism and are compatible with the hypothesis of a binary deep-inelastic interaction followed by the further fissionlike decay of one (or both) of the primary fragments. The characteristics of the fission step — mass asymmetry, relative velocity, in-plane and out-of-plane angles — have been reconstructed for the 3-body events and indications are found that nonequilibrium effects at the end of the deep-inelastic phase may influence the fissionlike decay.     

A Study of the120Sn(p,nγ)120Sb reaction

Levels of¹²⁰Sb have been observed using the¹²⁰Sn(p, nγ) reaction at proton energies between 3.90 and 6.25 MeV. Sixteen previously unobserved levels below 1.03 MeV were populated. The resulting level scheme is compared with that obtained using the¹²¹Sb(p, d) reaction.     

Excited states of100Tc from100Mo(p,nγ)100Tc reaction and the parabolic rule

γ-spectra and excitation functions of the¹⁰⁰Mo(p, nγ)¹⁰⁰Tc reaction were measured in the 1.2–3.6 MeV proton energy range by using thick, enriched targets, Ge(Li) and low energy photon (hyperpure Ge) spectrometers. These detectors were used inγγ-coincidence experiments, too. Conversion electron spectrum measurements were performed by means of a superconducting magnet transporter Si(Li) spectrometer (SMS) atE _p =4 MeV and multipolarities of some transitions have been determined. Based on the experimental results a level scheme of¹⁰⁰Tc has been constructed. Level energies of¹⁰⁰Tc were calculated on the basis of the parabolic rule, derived from the cluster-vibration model.     

Near-barrier transfer in16O +144, 154Sm

The production of nitrogen and carbon fragments in the reactions induced by¹⁶O on¹⁴⁴Sm and¹⁵⁴Sm targets was studied at two bombarding energies (65 MeV and 72 MeV) around the interaction barrier. The measured angular distributions exhibit the characteristic behaviour of direct transfer reactions, and theQ-value spectra are typically centered at the values predicted by kinematic selectivity. The comparison between fusion and charged-particle transfer cross sections shows that whereas fusion is the most important process at the highest bombarding energy, transfer reactions become comparable or even dominant (as in the¹⁴⁴Sm case) at near-barrier energies. The different transfer probabilities observed for the two systems are analyzed in terms of nuclear deformation.     

Analysis of the evaporation-corrected mass and charge distributions in the Ar+Mo reactions

Partly and strongly damped fragments from the reactions³⁶Ar+⁹²Mo and⁴⁰Ar+¹⁰⁰Mo are measured atE _Lab=270 MeV. The extracted mass and charge distributions are carefully corrected forγ, n, p andα particle evaporation. The resulting primary distributions are analysed with theoretical models which assume statistical neutron and proton exchange on the potential energy surface of the projectile-target system. Dynamical-deformation effects in the framework of the surface friction model are included.     

High spin bands in120Xe

High spin states in¹²⁰Xe were populated in the reaction¹⁰⁶Cd(¹⁸O, 4n) at a bombarding. energy of 90 MeV and the subsequent de- excitation was studied using γ- ray spectroscopic methods. New levels and several. spin and parity. assignments were established. The yrast band was observed up to theK ^π=22⁺state with two band crossins athw _c=0.39MeV and 0.45Mev Negative parity levels and a new positive parity band were also identified.     

Winkelverteilung der Polarisation der Neutronen aus der Reaktion14C(d,n 0)15N bei Deuteronenenergien zwischen 1,3 und 1,9 MeV

Polarization distributions of ground state neutrons emitted from the¹⁴C(d,n ₀)¹⁵N reaction were investigated over the angular range from 15 ° (lab) to 150 ° (lab) at bombarding energies of 1.28, 1.55 and 1.88 MeV. Scattering of neutrons from helium served as polarization analyzer. The experimental results show a large variation with energy of the polarization ranging betweenP(Θ_lab=130 °)=?21% andP(Θ_lab=130 °)=+50% at 1.28 and 1.88 MeV respectively.     

Investigation of two resonances in the18O(p,n)18F reaction

Excitation functions for the¹⁸O(p,n)¹⁸F reaction were measured at bombarding energiesE _p=4.6 to 6.6 MeV. In and near two resonances of the yield curves atE _p=5.622 and 6.061 MeV, angular distributions were measured with neutron time-of-flight techniques. The strong neutron decay to theT=1 state in¹⁸F and the similarity of the¹⁸O(p,n) and¹⁸O(n,n) yield curves give good evidence that the structures in the¹⁸O(p,n) yield curve arise from the formation ofT=3/2 states in¹⁹F. A two-level-analysis does not give satisfactory fits to the strongly asymmetric angular distributions.     

Out-of-plane emission of nuclear matter in Au+Au collisions between 100 and 800 AMeV

We present new experimental results concerning the azimuthal distributions of proton-likes, light and intermediate mass fragments at midrapidity for Au(100–800 A MeV) +Au collisions measured with the FOPI phase-I detector at GSI in Darmstadt. The azimuthal distributions are investigated as a function of the collision centrality, the incident energy, the fragment charge and transverse momentum. The azimuthal anisotropy is maximum for impact parameters around 7 fm. Intermediate mass fragments present a stronger out-of-plane emission signal than light fragments and a saturation is reached for Z ⩾ 4. The azimuthal anisotropy increases with the fragment transverse momentum and decreases as the incident energy increases. The azimuthal anisotropy of Z = 2 particles investigated as a function of the scaled fragment transverse momentum follows an universal curve for bombarding energies between 250–800 A MeV. A signature for a transition from in-plane to out-of-plane emission is evidenced at the lowest beam energies.     

Comparison of the yields of high-energy gamma-rays withE γ≧10 MeV measured in the reactionsnatSn+40Ar and68Zn+64Zn

The spectra of theKX-rays of reaction products were observed in coincidence with high energy gamma-rays (E _γ≧10 MeV) in the^natSn+⁴⁰Ar (E _Ar=300 MeV) and⁶⁸Zn +⁶⁴Zn (E _Zn=290 MeV) reactions. Two groups ofKX-rays can be clearly identified in the spectra. The first one corresponds to the evaporation residue of the compound nucleus and the second one to the target-like products of the reaction. The spectra of gamma transitions gated by theKX-rays of the evaporation residues were measured in both reactions. The relative yields and the positions of the broad shoulders observed at an energyE _γ≧10 MeV in these spectra, differ drastically in the asymmetric and the symmetric systems. The ratio between the yields of gamma-rays emitted at angles of 0° and 90° to the beam direction was determined in the^natSn+⁴⁰Ar reaction. It was found to be greater than unity only atE _γ=(10–14) MeV. This experimental evidence may indicate the emission of high energy gamma-rays from the dinuclear system prior to the formation of the compound nucleus.     

Moments of charge distributions in the reaction92Mo+92Mo at 14.7 MeV/u

First and second moments of primary charge distributions were determined as a function of the total kinetic energy loss in the reaction⁹²Mo+⁹²Mo at an incident energy of 14.7 MeV/u. Up to energy losses of 200 MeV no significant drift of the first moments is observed. The second moments are in agreement with predictions of a stochastic model derived at lower bombarding energies.     

Untersuchung von57Co mit der Reaktion57Fe(p,n)57Co

A neutron time-of-flight investigation of the⁵⁷Fe(p, n)⁵⁷Co reaction performed at bombarding energies of 4.9, 5.6 and 6.2 MeV results in new energy levels of⁵⁷Co. In addition, gamma spectra andn-γ-coincidence spectra were taken to complete the decay scheme of⁵⁷Co up to an excitation energy of 4 MeV.     

‘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.     

Theβ + decay of the new isotope86Mo and the first observation of86m Nb

Theβ ⁺ decay of⁸⁶Mo has been firstly investigated by means ofβγ spectroscopy. The⁸⁶Mo nuclei were produced by fusion-evaporation reactions of⁵⁴ Fe (³⁵Cl, 1 p2n) and⁵⁸Ni (³²S,2p 2n) at beam energies of 103 and 120 MeV, respectively. Threeγ rays of 47.3, 49.8 and 187.0 keV were unambiguously identified to follow theβ ⁺ decay of⁸⁶Mo by results ofXγ andβγ coincidence and cross-bombardment. A half life and a maximumβ ⁺-ray energy of⁸⁶Mo were determined to be 19.6±1.1 s and 3.9±0.4 MeV, respectively. A decay scheme of⁸⁶Mo is proposed in this article. Furthermore, a decay of⁸⁶Nb has been studied using the same combinations of projectiles and targets, and a newβ-decaying isomer^86m Nb was observed with a half life of 56.3±8.3 s.     

Mass measurements far from stability around theN=Z line (100Sn,100In,100Cd)

Secondary ions of¹⁰⁰Ag,¹⁰⁰Cd,¹⁰⁰In and¹⁰⁰Sn were produced via the fusion-evaporation reaction⁵⁰Cr+⁵⁸Ni at an energy of 5.1 MeV/nucleon. TheA=100 secondary ions were accelerated in the second cyclotron of GANIL, which was used as a high-resolution mass spectrometer. The masses of the three latter isobars were measured with respect to the abundantly produced¹⁰⁰Ag. The known mass of¹⁰⁰Cd was very well reproduced and the masses of¹⁰⁰In and¹⁰⁰Sn were determined for the first time with precisions of 3×10^?6 and 10^?5, respectively.     

States in95Mo and99Mo populated in the (3He, α) reaction

The (³He,α) reaction on⁹⁶Mo and¹⁰⁰Mo targets has been studied at a bombarding energy of 18 MeV. Thel _n transfer assignments have been made on the basis of angular distribution patterns and on an analysis of the ratios of the experimental and theoretical cross-sections of (³He,α) and (d, t) reactions data leading to the same final states. New states are observed in⁹⁵Mo at 3373 keV (9/2⁺); spin and parity assignments are made to levels in⁹⁹Mo at 1621 keV 7/2⁺ (9/2⁺), 1778 keV (5/2^?) and 2078 keV (11/2^?).     

Folding-model analysis of the inelastic scattering of 32S on 40Ca at 100, 120 and 151.5 MeV

The angular distributions for the 2⁺ level of ³²S at 2.24 MeV and 3⁻ level of ⁴⁰Ca at 3.74 MeV have been measured at 100, 120 and 151.5 MeV ³²S bombarding energies and the 3⁻ level of ³²S at 5.01 MeV at 120 and 151.5 MeV. The levels have been analysed by the coupled-channel approximation using real potentials and form factors deduced from the folding model. It has been found that the energy dependence of the folding-model renormalization coefficient decreases in comparison with that found in the elastic-channel analysis and that an overall energy-independent renormalization coefficient seems to be needed.     

Structure of excited states in100Mo and102Mo from spectroscopy using18O induced reactions

The lifetimes have been determined for the 2⁺, 0^+～ and 4⁺ states in¹⁰⁰Mo and¹⁰²Mo using the recoil-distance Doppler-shift method. The states have been excited in¹⁰⁰Mo by Coulomb excitation and in¹⁰²Mo by the two-neutron transfer reaction induced by¹⁸O ions on a¹⁰⁰Mo target. The study of the excitation function for the elastic and inelastic scattering on the ground and first excited 2⁺ state in¹⁰⁰Mo at beam energies between 20 and 61 MeV shows that 40 MeV is the highest incident energy for pure Coulomb excitation. Above this energy nuclear absorption sets in and nuclear scattering contributes to the excitation of the 2⁺ state of¹⁰⁰Mo. From the lifetimes of the 2⁺ and 4⁺ states deformation parameters of ¦β¦= 0.21 and ¦β¦=0.31 for¹⁰⁰Mo and¹⁰²Mo respectively were deduced. The 0^+′ levels are not shape isomeric states, as suggested earlier, but they decay by enhancedE2 transitions to the first 2⁺ states. From a comparison with similar states in other transitional nuclei it is suggested that they are band heads forβ vibrational bands.     

Pre-equilibrium alpha emission accompanying deep-inelastic16O+58Ni collisions

α particles were measured in coincidence with projectile-like reaction products (oxygen and carbon) produced in deep-inelastic¹⁶O+⁵⁸Ni collisions at about 6 MeV/N bombarding energy. The kinematic analysis of the HI andα energies measured as a function ofΘ _α gives strong evidence for a sequential process: the target-like fragments are excited by the deep-inelastic collision and undergo subsequentα decay. In contrast, the angular correlations show a pronounced forward peak, indicative of direct or pre-equilibriumα emission. The emission time for the latter is estimated to be of the order of 2×10^?21 s. To resolve this conflict of co-existing statistical and direct features of the pre-equilibrium emission, the concept of a hot spot is proposed. From the angular correlation and from theα multiplicities, a local temperature ofT?3.5 MeV is deduced which agrees well with the temperature derived from the shape of theα spectra. The spot size is estimated to be 1/5 of the sphere.     

The study of the (α, α′ f) reaction at 120 MeV on 232Th and 238U: (II). Fission barrier properties deduced from fission probabilities and angular distributions

The fission probabilities and angular distributions of the fission fragments for the (α, α'f) reaction on ²³²Th and ²³⁸U at a bombarding energy of 120 MeV have been measured from about 4 to 14 MeV excitation energy. Evidence for sub-barrier resonances has been found, the negativeparity ones occurring at the same excitation energy where photofission resonances have been observed. The data are analyzed with the two-humped barrier model. For ²³⁸U the data are reasonably well fitted with barriers similar to those known from the literature. For ²³²Th though, the outer barrier parameters are quite different: the height E_B = 6.6 MeV and the width (khω)_B = 1.2 MeV. Also for ²³²Th, introducing an additional mass symmetric and axially asymmetric outer barrier, as was previously found necessary for ²³⁸U, does not result in a good fit to the data at higher excitation energies.