Emission of fast neutrons in deep-inelastic collisions of 16O on Ni

Angular correlations of neutrons with projectile-like fragments (PLF) were measured for the very asymmetric mass systems ¹⁶O+^58,64Ni at 6 MeV/amu. The emission of two fast neutron components has been observed: one is due to quasielastic and the other to deep-inelastic collisions (DIC). In both cases these neutrons seem to be ejected quasifree at the initial step of the reaction process. This is the first experimental proof for the emission of preequilibrium neutrons in DIC.     

Polarization of preequilibrium proton emission in the 93Nb + 14N reaction

Spin polarization of protons emitted at 20° and 40° in the ⁹³Nb + ¹⁴N reaction at 95 MeV has been measured by the double scattering method using a polarimeter particularly designed for protons having a continuous energy distribution.The result shows that the polarization is small for low-energy protons, but increases smoothly with the increase of proton energy, reaching around + 20 % at about 20 MeV in the center-of-mass system. Energy and angular distributions of protons measured in the same reaction indicate coexistence of equilibrium and preequilibrium components in the proton emission of interest, the fraction of the preequilibrium component increasing with the increasing proton energy. If we assume that protons emitted in equilibrium are completely unpolarized, and that the value of polarization (P) for the preequilibrium component is independent of proton energy, the present result is consistent with $\text{P ? + 22 \%}$ for the emission at 20° and $\text{P ? + 38 \%}$ for the emission at 40°.The result is discussed in terms of the hot-spot model for precompound decay.     

Continuous spectra of protons emitted in α-particle induced reactions

Proton spectra resulting from (α, p) reactions on ⁵⁴Fe, ⁵⁶Fe, ⁵⁹Co, ⁵⁸Ni, ⁶¹Ni and ⁶³Cu were measured at E_α = 23.0 MeV. Energy and angular distributions of protons present two main components due to evaporative and preequilibrium processes. The hybrid model was chosen for analysing the data: no free parameters were allowed in the computations. The analysis is consistent with the one performed for (α, n) reactions on the same target nuclei. Hybrid model calculations succeed in explaining simultaneously both neutron and proton emission in α-particle induced reactions.     

High energy proton emission in reactions induced by 315 MeV 16O ions

Inclusive proton spectra have been measured for the reaction ¹⁹⁷Au(¹⁶O, p)X at 315 MeV. The data, which are consistent with emission from a moving source, are compared with the fireball model and with models preequilibrium emission.     

Fluctuations in inelastic proton scattering from58Ni in the energy range 17.1 to 20.6 MeV

Excitation functions of proton elastic and inelastic scattering on⁵⁸Ni have been measured for proton energies in the range 17.1 to 20.6 MeV, at laboratory angles of 90°, 120° and 155°. All show cross section fluctuations. Excitation functions are presented for the elastic and for five inelastic groups in the energy range 18.1 to 18.59 MeV. These were analyzed by the Fourier analysis method, and the average level width found to be about 13 keV. The observation of strong fluctuations in the higher inelastic yields has implications for several microscopic analyses reported in the literature of⁵⁸Ni(p, p′) data obtained at 17.7 MeV incident proton energy.     

Investigation of the 60Ni(n,p) reaction mechanism at 18.5 MeV

The differential cross sections of the ⁶⁰Ni(n, p) reaction are measured at 18.5 MeV. The reaction mechanism has been discussed in the framework of the Geramb model and of the FKK statistical multistep emission theory. Special attention is paid to the excitation of the GDR as a doorway state in ⁶⁰Ni and as a parent analog in ⁶⁰Co. The total proton emission cross section is estimated.     

Microscopic calculation of 58,64Ni + 124,132Sn sub-barrier fusion

The fusion cross-sections of ^58,64Ni + ^124,132Sn are investigated through a coupled-channel approach using a density- and energy-dependent effective Brueckner G-matrix interaction. Microscopic Skyrme-Hartree-Fock proton and neutron density distributions are used in the calculations. A good agreement with the experimental data of the fusion cross-sections of these neutron-rich systems has been obtained, which favors the present microscopic approach for calculating the interaction potentials and fusion cross-sections.     

Neutron-deficient and neutron-rich Fe and Ni isotopes near the drip line

Positions of the proton and neutron drip lines and characteristics of the neutron-deficient and neutron-rich Fe and Ni isotopes have been studied by the Hartree-Fock method with the Skyrme forces (Ska, SkM*, Sly4) with allowance for deformation. Pairing was taken into account on the basis of the BCS approximation with the pairing constant G = (19.5/2)[1 ± 0.51(N ? Z)/A]. Manifestation of magic numbers for the nickel isotopes ⁴⁸Ni, ⁵⁶Ni, and ⁷⁸Ni and for the nucleon-stable isotope ¹¹⁰Ni which is beyond the drip line is discussed.     

Incident energy dependence of preequilibrium (p,p′) spectra

Proton energy spectra from (p, xp) reactions on⁹⁸Mo and¹⁰⁶Pd have been measured at incident energies of 12, 14, and 16 MeV to investigate the incident energy dependence of preequilibrium (p, p′) process. The (p, xp) energy spectrum for⁶⁰Ni has also been measured at an incident energy of 18 MeV to confirm the mass number dependence. These spectra were compared with a calculation based on the exciton model in which the square of the average effective matrix element ¦M¦² was assumed to beKA ^?3 E ^?1 and isospin conservation was taken into account. The calculated spectra using a constantK-value (430 MeV³) were in good agreement with all the measured ones. The role of isospin conservation in preequilibrium process was discussed through the present analyses.     

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.     

Clustering states in the62Ni(58Ni,58Ni)62Ni reaction

Excitation function and angular distributions of the⁶²Ni(⁵⁸Ni,⁵⁸Ni)⁶²Ni elastic scattering have been measured at incident⁵⁸Ni energies from 220.0 to 230.0 MeV in steps of 0.5 MeV. Evidence of two structures was found in the excitation function; a statistical analysis suggests a possible nuclear cluster quasi-molecular nature for these structures.     

Evidence of non-statistical structures in the elastic and inelastic scattering of58Ni+58Ni and58Ni+62Ni and intermediate dinuclear states

Excitation functions and angular distributions of⁵⁸Ni+⁵⁸Ni and⁵⁸Ni+⁶²Ni scattering at energies just above the Coulomb barrier have been measured aroundθ _cm=90° in energy stepsΔE _cm=0.25 MeV fromE _cm ? 110 MeV toE _cm ? 120 MeV for⁵⁸Ni+⁵⁸Ni and fromE _cm ? 110 MeV toE _cm ? 118 MeV for⁵⁸Ni+⁶²Ni. Evidence for structure of non-statistical character has been found in the angle-summed excitation functions; this evidence is corroborated by the analysis of the angular distributions. This is the first time that non-statistical structure in elastic and inelastic scattering is reported with high confidence level for this mass and excitation energy ranges. Attempts are presented to understand the nature of this structure, including the presence of intermediate dinuclear states and virtual states in a potential well.     

Sub-barrier fusion of35, 37Cl+58, 64Ni evidence of effects of proton transfer channels

Sub-barrier fusion excitation functions of^35,37Cl+^58,64Ni have been measured. They are discussed in comparison to similar data of^34,36S+^58,64Ni with the aim of revealing the influence of coupled proton transfer channels. Using the simplified coupled channel code CCFUS, differences in the effects of inelastic channels are eliminated in an approximative manner to stress the differences in the effects of transfer channels. Signatures of positiveQ-value transfer channels coupled to fusion are clearly identified.     

Elastische und inelastische Streuung von 104 MeV Alpha-Teilchen an58,60,62,64Ni

Differential cross sections are measured for the elastic and inelastic scattering of 104 MeV α-particles from^{58, 60, 62, 64}Ni. The experimental results are analyzed in terms of coupled channels on the basis of an anharmonic vibrational model and prove to be quite sensitive to the values of the deformation parameters. For forward angles the 4⁺ angular distributions of^58,60Ni are dominated by aL=4 single excitation mechanism. This is in keeping with theE4-transitions observed in (e, e′) scattering. From the 2 ₁ ⁺ - and 3 ₁ ⁺ -results transition rates are derived which can be compared to results of electromagnetic methods and of inelastic proton scattering studies. The comparison indicates that the transition rates differ and are generally higher for the proton scattering. Especially in the case of the 3^? states of^58,60Ni the differences are obvious. The elastic cross sections are analyzed both on the basis of the usual phenomeno-logical model and in terms of a semimicroscopic folding model resulting in values of rms-radii for the nuclear matter distribution.     

An EPR study of Ni+ centers in SrF2:Ni and BaF2:Ni

An EPR study of RT X-irradiated SrF2:Ni and BaF2:Ni has been performed. Different kinds of Ni⁺ centers showing small deviations with respect to a basic structure have been observed. This basic structure consists of a Ni⁺ ion displaced along a <100> direction from the cation substitutional position toward the center of a face of the cube of fluorines. These results are similar to those previously reported on CaF2:Ni.

The spin Hamiltonian parameters of the different centers obtained from the analysis of the corresponding rotational diagrams are also reported.     

Enhanced collectivity in Ni

The neutron-rich nucleus ⁷⁴Ni was studied with inverse-kinematics inelastic proton scattering using a ⁷⁴Ni radioactive beam incident on a liquid hydrogen target at a center-of-mass energy of 80 MeV. From the measured de-excitation γ rays, the population of the first 2⁺ state was quantified. The angle-integrated excitation cross section was determined to be 14(4) mb. A deformation length of δ=1.04(16) fm$\delta =1.04(16)\text{fm}$ was extracted in comparison with distorted wave theory, which suggests that the enhancement of collectivity established for ⁷⁰Ni continues up to ⁷⁴Ni. A comparison with results of shell model and quasi-particle random phase approximation calculations indicates that the magic character of Z=28$Z=28$ or N=50$N=50$ is weakened in ⁷⁴Ni.     

Direct observation of new proton rich nuclei in the region 23≦Z≦29 using A 55A.MeV58Ni beam

Using magnetic separation and identification through time of flight andΔExE measurements, the nuclei⁵⁶Cu,⁵²Co (T _z =?1);⁵⁵Cu,⁵¹Co,⁴⁷Mn,⁴³V(T _z =?3/2)⁵²Ni,⁵⁰Co,⁴⁸Fe,⁴⁶Mn,⁴⁴Cr (T _z =?2); and⁵¹Ni (T _z =?5/2) are observed for the first time from interactions of 55A.MeV⁵⁸Ni projectiles with a Ni target. The predicted proton drip-line has probably been reached for the odd elements.     

Isobaric analogue states excited in the59Co(p, γ)60Ni reaction

The⁵⁹Co(p, γ)⁶⁰Ni reaction has been investigated in the proton energy regionE _p=1365–2150 keV. Decay schemes and branching ratios have been determined for ten resonances, five of which have been identified as possible analogues or fragments of analogues of the ground state (5⁺) and the 58.6 keV (2⁺), 277.1 keV (4⁺), 288.4 keV (3⁺), and 435.7 keV (5⁺) levels in⁶⁰Co. At eight of the resonances most of the decay seems to go via a group of states with an excitation energy of 5–9 MeV. The investigated analogue states give a Coulomb displacement energy of 9118±7 keV.     

Isospin dependence of the isoscalar giant quadrupole response: Relevance to pion scattering in the Ni isotopes

We study the isospin dependence of the isoscalar giant quadrupole response to pion excitation, with the aim of showing that the apparent anomaly in the π^?/π⁺ analyses is due to the highly selective nature of surface pion absorption. In that context, we present results of continuum RPA calculations which are consistent with recent inelastic pion scattering data on^58–64Ni. In general, the calculation predicts a drastic increase in the ratio of neutron to proton transition density in going from⁵⁶Ni to⁶⁶Ni, a trend well substantiated by recent LAMPF experiments.