Time-of-flight measurement of the evaporation residues from fusion of 5.9 mev/u 84kr and 27al

Mass and Z-distributions of the evaporation residues from compound-nucleus formation in the reaction 5.9 MeV/u ⁸⁴Kr on ²⁷Al were measured using a time-of-flight ΔE ? E telescope, which is described in detail. The high recoil velocity attained by choosing the heavy reaction partner as projectile made it possible to resolve the reaction products by mass and atomic number. Data were taken in the angular range from 1.5 to 6°. The residue distributions are compared to evaporation calculations assuming the statistical decay by fission and particle evaporation of the compound nucleus ¹¹¹Mn formed at an excitation energy of 108 MeV with angular momenta up to $\text{L}{}_{\mathrm{<mtext>CN</mtext>}}\text{≈ 69}\text{h}\text{?}$. The data are consistent with the assumption of statistical equilibrium. Details of the de-excitation process, in particular the fission competition and the influence of nuclear deformations at high angular momenta, are discussed.     

The compound nucleus reaction 12C(11B, 2α) 15N

Energy spectra and differential cross sections of nitrogen products formed in the reaction 28 MeV ¹¹B + ¹²C have been measured using a ΔE?E counter telescope. The energy spectra are smooth and therefore indicate that the nitrogen products were formed by a compound nucleus mechanism, via the formation and decay of the compound nucleus ²³Na. The experimental results are compared with statistical model calculations and good agreement is obtained. This result provides further evidence for the importance of the compound nucleus mechanism in heavy ion reactions with light nuclei and also gives added validity to the statistical model for light compound systems.     

Light-particle emission in the reaction of 93Nb + 14N at 132, 159 and 208 MeV

Nonequilibrium light-particle emissions have been investigated in the reaction ⁹³Nb + ¹⁴N at 132, 159 and 208 MeV by measuring inclusive differential cross sections of p, d, t, ³He and α. The experimental data were analyzed in terms of three models: (i) an extended exciton model, (ii) a coalescence model, and (iii) a moving thermal source model. The angle-integrated energy spectra of the protons were well described by the extended exciton model in which projectile nucleons were assumed to be transferred to the target one by one, but those of composite particles were not. On the other hand, the composite particle spectra (except for α at forward angles) were successfully described by the coalescence model using spectra consistent with those for the protons. Extracted coalescence radii P₀ were about 140 MeV/c for d and t, and about 220 MeV/c for α. The light-particle spectra were also fitted by the moving-source model assuming isotropic emission from a source moving with approximately half of the beam velocity and with much higher temperature than that of the compound nucleus. Extracted temperatures followed the systematics of a recent compilation for the various reactions. A discussion of these analyses is given.     

The energy dependence of fusion in the 9Be+28Si system

The angular distributions of the energy spectra of the light charged particles (p, d and α) from the ⁹Be + ²⁸Si reaction have been measured in the energy range 12 ≦ E_lab ≦ 30 MeV. The particle evaporation spectra and the angular distributions were analyzed with a spin dependent statistical model. Angular distributions of ⁹Be ions elastically scattered on ²⁸Si have been measured at the energies 12 MeV, 17 MeV, 23 MeV and 30 MeV and were analysed, together with previously measured cross sections, with the optical model. The fusion cut-off angular momentum l_fus(E), the fusion cross section σ_fus(E) and the ratio σ_fus/σ_R^OM(E) were deduced. The excitation function for fusion was analyzed with the Glas and Mosel model. The parameters obtained from the fusion excitation function were compared with the corresponding ones from the ⁹Be + ²⁸Si optical-model interaction potential.     

Inclusive break-up reactions of 6Li at an incident energy of 26 MeV/nucleon

Inclusive charged particle spectra were measured from nuclear reactions induced by 156 MeV ⁶Li on ⁴⁰Ca. At forward angles the spectra exhibit broad break-up distributions centered around the energy corresponding to the beam velocity. The double differential cross sections together with previous results for a ²⁰⁸Pb target were analyzed in the framework of the DWBA approach to projectile break-up taking into account elastic and inelastic reactions of the break-up fragments. The high energy tails of the background due to preequilibrium emission of complex charged particles were estimated on the basis of the coalescence model.     

The 12C(9Be,n)20Ne reaction

A study of the ¹²C(⁹Be, n)²⁰Ne reaction has been carried out at bombarding energies of 16 and 24 MeV. The spectra at both incident energies are dominated by a consistent set of levels between an excitation energy of 7.3 ± 0.4 and 15.7 ± 0.3 MeV. The rotational band based on the K^π = 0₃⁺ state appears to be strongly populated. Based on this selectivity, additional evidence is provided in favor of identification of the 8⁺ state at 15.9 MeV with this 0₃⁺ band. Angular distributions measured at the higher bombarding energy are compared with statistical compound-nuclear calculations. It appears that a non-statistical mechanism is responsible for the reaction's selective population of states with 8p-4h configuration. Such a mechanism, involving the preferential breakup of the ⁹Be into ⁸Be plus a neutron, is discussed.     

Two-body photodisintegration of 3He and 4He in the Δ(1236) region

Differential cross sections for the reactions ³He(γ, p)d and ⁴He(γ, p)t were measured at proton c.m. angles of 60° and 90° for photon energies ranging from 150–450 MeV with an average resolution of 8 MeV. Bremsstrahlung was used as the photon source; deuterons/tritons were analyzed in a magnetic spectrometer whereas coincident protons were detected in a plastic scintillator telescope. The experimental method includes a calibration by means of ¹H(γ, π⁰)p differential cross section measurement at 90° c.m. in the same photon energy range. The ³He and ⁴He two-body photodisintegration differential cross sections show a monotonically decreasing variation with photon energy. In addition, partial data on the differential cross section of the reaction ⁴He(γ, n)τ at 90° and 120° neutron c.m. angle are given.     

The 26Mg(14N, 10B)30Si and 26Mg(14N, 11B)29Si reactions

Reactions induced by ¹⁴N on ²⁶Mg at bombarding energies of 60–95 MeV have been studied. Angular distributions for states populated in ²⁹Si by the (¹⁴N, ¹¹B) reaction and in ³⁰Si by the (¹⁴N, ¹⁰B) reaction have been compared with Hauser-Feshbach and DWBA calculations to determine the reaction mechanism and to deduce spectroscopic information. The cross sections for the states populated in ²⁹Si and ³⁰Si are in poor agreement with statistical model calculations, indicating a non-compound nucleus mechanism.     

Emission of high-energy alpha particles in nuclear reactions of 48Ca and 56Fe ions on 181Ta and 238U targets

The results of experiments on measuring the energy spectra of alpha particles in reactions with heavy ions are presented. The measurements were performed using the high-resolution magnetic analyzer MAVR with beams of ⁴⁸Ca (280 MeV) and ⁵⁶Fe (320 and 400 MeV) on ¹⁸¹Ta and ²³⁸U targets at an angle of 0°. A strong dependence of the double differential cross sections for production of alpha particles on the atomic number of the target nucleus was observed, which indicates that fast alpha particles are mainly emitted from the target nucleus; this conclusion was also confirmed by calculations within the time-dependent Schr?dinger equation approach. An analysis of the obtained experimental data was carried out within the model of moving sources modified to consider the kinematic limits for two-body and three-body exit channels.     

Effects of exchange and final-state interactions in the reactions 2H(3He, 3H p)p and 2H(3He, 3He p)n

Absolute coincidence cross sections for the ²H(³He, ³He p)n and ²H(³He,³H p)p reactions were measured at E_He = 35.9 MeV. Spectra dominated by the nucleon-nucleon final-state interaction (FSI) are fitted by a fully antisymmetrized PWBA theory which includes the effects of FSI in all its matrix elements. Previously reported 26.8 MeV data showing both FSI and quasi-elastic scattering (both with and without charge exchange) are also fitted by the theory, which qualitatively describes the shapes of all these spectra and the ratios of the cross sections for the various processes. Predictions of Watson-Migdal theory are fitted to the FSI spectra and differences between the two theories are analyzed.     

Structure en bande du noyau 22Na: Le premier niveau Jπ = 6+

We investigate the two-body photodisintegration of ³He and its inverse, radiative p-d capture using bound-state functions corresponding to the N-N interaction being given by the Reid soft-core potential. For the two-body photodisintegration of ³He Coulomb effects and the final-state interactions between the proton and deuteron are not included. At low energy the shape of the angular distribution agrees well with experiment, but the 90° cross section exhibits an anomalous peak at 15 MeV due to electric dipole transitions connecting the deuteron and ³He D-states. The low-energy cross section is 25–40 % too small. The intermediate-energy angular distribution peaks too near the forward direction, and, contrary to experiment, has a minimum at 100°. At higher energy the 90° cross section in the center-of-momentum frame is at least an order of magnitude too small, but does display the correct energy dependence. This energy dependence is related to the properties of the bound-state wave functions and it is plausible that it will persist in an improved treatment (e.g. one which includes exchange currents) which properly accounts for the magnitude of the cross section. Contributions from the ³He S-states are negligible for photon energies between 100 and 150 MeV, but are dominant outside this energy region.     

The breakup of 16O into He: An event-by-event study of nucleus-nucleus collisions at 75, 175 and 2000 MeV/A

We present an event-by-event study of the breakup of the ¹⁶O in ¹⁶O + emulsion nucleus interactions at 75, 175 and 2000 MeV/A. The events are categorized according to their multiplicity of projectile He nuclei. The multiplicity depends on the degree of target destruction. Although the fragmentation model describes the gross features of inclusive He spectra, an event-by-event study reveals deviations from the model. The momenta of the He nuclei, emitted from the projectile, depend on helium multiplicity and the breakup properties of the target nucleus. The probability that the ¹⁶O projectile breaks up into multiple He fragments is larger at 75 MeV/A than at 2000 MeV/A. At 75 MeV/A the mean velocity of projectile He is on the average 0.06c below the projectile velocity. This recoil velocity depends on the target nucleus destruction also for the most peripheral collisions.     

Excited states of 10B from the 6Li(α, γ) and 9Be(p, γ) reactions

Excitation functions for the (α, γ) and (p, γ) reactions leading to ¹⁰B have been measured at θ = 0° in the energy range from E_x = 6.7 to 7.6 MeV. Two resonances corresponding to levels at 6.88 and 7.44 MeV are observed. Branching ratios extracted from γ-ray spectra are the same in both reactions for the 6.88 MeV (1^?, T = 0 + 1) level, but different at 7.44 MeV. The T = 0 + 1 level at 7.44 MeV (Γ = 90±10keV) is assigned 2^? or 2⁺ from its strong branch to the 3⁺ ground state. We find no evidence for a second isospin mixed 1^? state.     

Inelastic scattering of 108.5 MeV 3He particles at very small momentum transfer to the giant monopole resonance of 90Zr and 208Pb

Inelastic scattering spectra from the (³He, ³He′) reaction at 108.5 MeV have been measured down to 1.85° scattering angle on ⁹⁰Zr and ²⁰⁸Pb, allowing a giant monopole resonance study in these nuclei. DWBA analysis is reported.     

A study of the 90Zr(6Li, 6He)90Nb reaction at E6Li = 93 MeV

The energy spectra of ⁶He nuclei from the ⁹⁰Zr(⁶Li, ⁶He) reaction at E_{⁶Li = 93 MeV} have been measured over the angular range of 7 ° ? θ_lab ? 20 °. The theoretical DWBA analysis of these spectra and angular distributions was performed employing transition densities of bound and resonance states obtained on the basis of the theory of finite Fermi systems. It is shown that up to excitation energies of E_x ≈ 10 MeV in ⁹⁰Nb a direct charge-exchange mechanism dominates in this reaction, while at higher E_x the contribution made by multistep processes increases significantly. It is also shown that the quasi-elastic cross section is determined by a sum of partial contributions of spin-isospin-flip transitions with multipolarities L=0 through L=6.     

90,91Zr photoproton spectra and the core-excitation model

Photoproton energy spectra have been measured for the ^90,91Zr(γ,P)^89,90Y reactions at an E_γ endpoint energy of 30 MeV, and for the ^90,91Zr,(e, e'p)^89,90Y reactions at a number of different electron beam energies around E_e = 21 MeV. Isotopically enriched target foils of metallic ⁹⁰Zr(97.65%) and ⁹¹Zr(89.2%) were used, and the proton spectra measured at 90°. Prominent proton groups are observed in the ⁹¹Zr spectra, particularly around E_p ≈ 11 MeV, which closely resemble groups produced in the ⁹⁰Zr photoreactions. Moreover, the correlating non-ground state proton groups are being produced in transitions leaving the corresponding ⁸⁹Y, ⁹⁰Y residual nuclei in excited levels which also correlate in energy. These photoproton groups from ⁹⁰Zr have previously been identified as representing T_> strength. A qualitative explanation is proposed in terms of the core-excitation model, in which the structure in the ⁹¹Zr proton spectrum is described as representing dipole $\text{T}{}_{\mathrm{>}}\text{(T =}\text{13}\text{2}\text{)}$ strength formed by coupling the 2d$\text{5}\text{2}$ neutron to dipole T_> (T = 6) excitations of a ⁹⁰Zr core.     

The excitation function for the 89Y(n, 2n)88Y reaction

Activation cross sections for the ⁸⁹Y(n, 2n)⁸⁸Y reaction at neutron energies between 12.6 and 17.8 MeV have been measured by using the mixed-powder method and γ-ray detection by a Ge(Li) spectrometer. Using the ²⁷Al(n, α)²⁴Na reaction for monitoring the neutron flux, the measured cross-section values for the ⁸⁹Y(n, 2n)⁸⁸Y reaction were found to be 331±32 mb, 603±58 mb, 820±79 mb, 1040±100 mb, 1072±103 mb, 1172±112 mb, 1221±117 mb and 1218±117 mb at the respective incident neutron energies of 12.6±0.1 MeV, 13.3±0.1 MeV, 14.0±0.4 MeV, 14.9±0.3 MeV, 15.1±0.3 MeV, 16.0±0.4 MeV, 16.7±0.5 MeV and 17.8±0.7 MeV. The measured values are compared with the experimental values of others and with the theoretical values obtained from calculations based on the statistical model for the formation of a compound nucleus and its subsequent emission of neutrons.     

Excited states of 96Nb from the 96Zr(p,nγ)96Nb reaction

The γ-ray spectra of the ⁹⁶Zi(p, nγ)⁹⁶Nb reaction have been measured with Ge(Li) detectors at different bombarding proton energies between 1.3 and 5.1 MeV. γγ coincidences were observed at E_p = 4.7 and 5.0 MeV. On the basis of experimental results a level scheme of ⁹⁶Nb was deduced, and γ-threshold energies and γ-branching ratios were determined. Computed Hauser-Feshbach (p, n′) cross sections have been compared with experimental data obtained from the γ-ray measurements, and level spins and parities have been determined. The energies of ⁹⁶Nb levels were calculated on the basis of the parabolic rule, derived from the cluster-vibration model. The Racah multipole decomposition method was used also for the theoretical interpretation of several ⁹⁶Nb multiplet states.     

Differential cross sections for radiative capture of energetic protons by 110Cd and 115In

Spectra of high-energy photons following the radiative capture of 8–22 MeV protons in ¹¹⁰Cd and ¹¹¹In are measured. The (p, γ) differential cross sections at 90° with respect to the beam axis is deduced from the integration of measured spectra. The photon angular distribution is measured for the ${}^{110}\text{Cd}\text{(}\text{p}\text{, γ}{}_0\text{)}$ reaction, too, at 13 MeV incident energy. Satisfactory agreement between theory and experiment is obtained by using the direct-semidirect model for dipole and quadrupole fast nucleon radiative capture.     

High resolution investigation of 91Mo and 92Mo via (p,d) and (p,t) reactions on 90Mo

The (p, d) and (p, t) reactions on ⁹²Mo have been studied at a proton energy of 28.0 MeV. Using an Enge split-pole spectrograph, resolutions of 11–15 keV for the detected deuterons and tritons were obtained. A total of 87 levels up to 4.9 MeV in ⁹¹Mo and 25 levels up to 4.0 MeV in ⁹⁰Mo were found, several of which were previously unknown. By comparing the measured angular distributions with DWBA calculations l-values and spectroscopic factors were determined. The results are compared with data from previous experiments on ⁹¹Mo and ⁹⁰Mo, with experiments on other N = 49 and 48 nuclei, and with theoretical calculations.