Decay of the compound nucleus 74Kr formed in the reaction 16O + 58Ni

Gamma-ray and charged-particle emission has been studied after bombardment of ⁵⁸Ni with ¹⁶O at energies from 44.5–76.5 MeV. The excitation functions for various reaction channels have been measured. The locus in energy and spin of maximum cross section (the entry line) of the γ-emitting region in the 2pγ and 3pγ channels have been studied through measurement of γ-ray multiplicity, average particle energy and average γ-ray energy. The standard deviations in both energy and spin of the distributions of cross section are presented. Statistical model calculations including shell corrections in the level density show very satisfactory agreement with the experimental results. The moment of inertia at high spin and excitation energy is deduced from the measured entry line.     

Transfer reactions and sub-barrier fusion in 40Ca + 90, 96Zr

The two systems ⁴⁰Ca + ^90,96Zr have been studied by measuring nucleon transfer reactions at two energies near the Coulomb barrier, thus complementing the available sub-barrier fusion cross-sections. Angular distributions for various transfer channels have been determined. Significantly larger neutron transfer cross-sections are found for the target ⁹⁶Zr that exhibits the larger enhancement in the sub-barrier fusion cross-sections. All data have been analyzed with a new model for heavy-ion collisions that calculates simultaneously transfer cross-sections, fusion excitation functions and barrier distributions. The model gives a good account of both transfer and fusion data. Received: 2 May 2002 / Accepted: 4 June 2002 / Published online: 26 November 2002 RID="a" ID="a"e-mail: montagnoli@pd.infn.it, Fax +39049 8277102, Tel. +39049 8277117. RID="b" ID="b"On leave from the China Institute for Atomic Energy, 102413 Beijing, China. Communicated by C. Signorini     

Decay of the complete fusion nucleus,151Eu

Excitation functions have been measured for the¹³³Cs(¹³O, 3–6n)^145–148Eu reactions at incident energies from 75 to 125 MeV and compared to those previously reported for the¹³⁹La(¹²C, 3–6n) reaction system with the same complete fusion nucleus,¹⁵¹Eu. The semiclassical Diamond-Stephens approach to the treatment of angular momentum in evaporative processes is extended and a method for extraction of the moment of inertia, the critical value of angular momentum for fusion, and the yrast line for¹⁵¹Eu is presented. The model is then used to predict the excitation functions and the results are compared to experiment.     

Study of the 90Zr(p,α)87Y reaction at 22 MeV

The (p,α) reaction on ⁹⁰Zr has been studied in a high resolution experiment at an incident proton energy of 22 MeV. The cross section and asymmetry angular distributions for transitions to 36 levels of ⁸⁷Y with an excitation energy up to 3 MeV have been measured. DWBA analyses of experimental angular distributions, using either Woods-Saxon or Double Folded potentials for the exit channel, have been done, allowing either the confirmation of previous spin and parity values or the assignment of new spin and parity to a large number of states. The structure of low lying states of ⁸⁷Y has been studied in the framework of the shell model, using the OXBASH code. With the interaction PMM90 reasonable agreement is obtained for part of the negative parity spectrum. Received: 17 November 1997     

Complete and incomplete fusion in12C+93Nb and16O+89Y reactions

Excitation functions for the evaporation residues for the reactions¹²C+⁹³Nb and¹⁶O+⁸⁹Y in the projectile energy range of 4 to 6.5 MeV/amu have been measured using off-line gamma spectrometry. The excitation functions for neutron(xn), proton(pxn) and one alpha(xn) emission channels are practically similar for both the reactions. However the products formed by two alpha(2xn) emission show much higher cross sections in the¹²C+⁹³Nb than the¹⁶O+⁸⁹Y system. This has been explained in terms of the incomplete fusion process involving transfer of an alpha particle from the projectile to the target in the former case.Authors thank Shri D.C. Ephraim for making the rolled metal foils and the operation crew of PELLETRON facility for their help in carrying out the irradiations. Authors are grateful to Dr. P.R. Natarajan, Head Radiochemistry Division for his keen interest in this work.     

Core breaking and possible magnetic rotation in the semimagic nucleus 90Zr

The semimagic nucleus 90Zr,with Z =40 and N =50,is investigated in terms of large scale shell model calculations.A logical agreement is obtained between the ava...     

Effect of viscosity on the fission dynamics of the excited compound nucleus 251Es produced in fusion reaction

A stochastic approach based on two-dimensional dynamical model has been used to simulate the fission dynamics of the excited compound nucleus ²⁵¹Es produced in ¹⁹F+²³²Th reaction. In the dynamical calculations the elongation parameter has been used as the first dimension and the projection of the total spin of the compound nucleus onto the symmetry axis, K, considered as the second dimension. The value of post-saddle friction was inferred by reproducing experimental data on the mean pre-scission neutron multiplicity for ²⁵¹Es. It was shown that the results of calculations are in good agreement with the experimental data by applying values of the post-saddle friction equal to 13−15×10²¹s⁻¹.     

Decay of 0 2 + states in88,92,94Zr andE0 systematics of Zr isotopes

Branching ratios ofE0 andE2 transitions depopulating the 0 ₂ ⁺ states of^{88, 92, 94}Zr have been determined using conversion-electron and γ-ray spectroscopy. Two different lifetime-measurement methods were applied in the remeasurement of the half-lives of the 0 ₂ ⁺ states in^{92, 94}Zr, yielding consistent results ofT _1/2(0 ₂ ⁺ ,⁹²Zr)=85(15) ps andT _1/2(0 ₂ ⁺ ,⁹⁴Zr)=280(40) ps. The monopole strengths extracted areρ ²(⁹²Zr)=8.4(17) ×10^?3 andρ ²(⁹⁴Zr)=11.9(20)×10^?3. The proton configurations of the 0 ₂ ⁺ states are discussed in view of these values and proton transfer data.     

Formation and decay of the compound nucleus studied in the reaction20Ne+27Al

Energy and velocity spectra, angular and mass distributions have been measured for evaporation residue products of the^{20, 22}Ne+²⁷Al system in the energy range ofE _L (²⁰Ne)=51 to 395 MeV and in the angular rangeθ=2° and 30°. Calculations with simple assumptions of the velocity and angular distributions of the evaporation residues are presented and compared to the data. The structure seen in the mass distributions, a competition between α-particle and nucleon evaporation in the deexcitation of the compound nucleus, is described well by calculations with the computer code CASCADE. The evaporation residues exhibit mass distributions varying systematically as a function of the excitation energy. The excitation function of the evaporation residue cross section is compared with theoretical models. At higher incident energies contributions of incomplete momentum transfer (incomplete fusion) are observed. A limitation for complete compound nucleus formation with following light particle evaporation is found.     

Fission dynamics of the compound nucleus 213Fr formed in heavy-ion-induced reactions

A stochastic approach based on one-dimensional Langevin equations was used to calculate the average pre-fission multiplicities of neutrons, light charged particles and the fission probabilities for the compound nucleus ²¹³Fr and the results are compared with the experimental data. In these calculations, a modified wall and window dissipation with a reduction coefficient, k _s , has been used in the Langevin equations. It was shown that the results of the calculations are in good agreement with the experimental data by using values of k _s in the range 0.3?≤?k _s ?≤?0.5.     

Mean life of the 21/2+ isomer of89Zr

The mean life of the lowest 21/2(⁺ level of⁸⁹Zr has been measured with a pulsed⁷Li beam and the⁸⁵Rb(⁷Li, 3n)⁸⁹Zr reaction. The mean life is τ=7.39 ± 0.23 ns and the reduced transition probability to the 17/2⁺ state isB(E2)=73±2e²fm⁴. The comparison with shell-model predictions requires a large value of the effective chargee _p ≈ 2) to account for the experimental result. The mean life of the 13/2^? state has also been measured and results to be τ=3.22±0.17ns.     

Mössbauer study of cold nuclear fusion in Fe−Zr alloy

... Mössbauer spectroscopy as well as neutron and gamma spectroscopy were used to study the possibility of cold nuclear fusion in Fe?Zr amorphous alloys deuterized electrolytically both in air and in nitrogen atmosphere. The change in the localization and concentration of deuterium can be sensitively followed by Mössbauer spectroscopy. Significant differences were observed between the spectra of samples deuterized in air and in nitrogen atmosphere. In the case of deuterization performed in air these spectral changes can be explained by decrease of deuterium uptake and small heat effect. These changes can be attributed to reaction of deuterium with oxygen dissolved into water if the isotope effect could be taken into consideration.     

Pionic fusion study of the halo nucleus 6He, the reaction d+4He→6He+π + at CELSIUS

The halo nucleus ⁶He has been studied in a pionic fusion experiment at the CELSIUS storage ring facility in Uppsala. The ⁶He nuclei were produced in reactions with a deuteron beam incident on a ⁴He jet target 0.9–5.4 MeV above threshold in the center-of-mass frame. The ⁶He ions were detected in a ΔE-E solid-state detector telescope inserted into the CELSIUS ring. The aim of the experiment was to investigate, in particular, the high-momentum part of the halo wave function by measuring the total and differential cross sections of the reaction d+⁴He→⁶He+π ⁺.     

Radiative fusion from very symmetric reactions: the giant dipole resonance in the Au nucleus

The spectra of high-energy γ rays emitted by the Giant Dipole Resonance (GDR) built on moderately excited states associated with the evaporation of 0, 1 and 2 nucleons were measured in the ⁹⁰Zr + ⁸⁹Y symmetric fusion reaction. The radiative fusion data suggest statistical emission from the compound nucleus. In addition, the analysis of the high-energy γ-ray spectra associated with the different evaporation channels at the present temperature of 0.7 MeV and spin range 15–20 show a fairly narrow width of 5.0±0.35 MeV. This value is smaller than what would be expected in a nucleus where shell effects do not play a role.     

Dependence of the evaporation residue production on the excitation energy of the compound nucleus in the reaction40Ar+164Dy

With the velocity filter SHIP at the heavy ion accelerator UNILAC the energy dependence of the evaporation residues production in the reaction⁴⁰Ar+¹⁶⁴Dy was investigated. The results show a much smaller shift of the (²⁰⁴Po, xn) excitation functions with respect to theoretical predictions than has been reported by other authors.     

Near-barrier transfer and fusion of the systems33S +90,91,92Zr

Double differential cross sections of all prominent transfer channels have been measured in the systems³³S +^99,91,92Zr at two energies close to the nominal Coulomb barrier. In addition the fusion excitation functions of these systems have been measured below and around the barrier. The angular- andQ-distributions of the most important transfer reactions have been analysed in the framework of a simple semiclassical formalism. Particularly the two-nucleon transfer angular distributions exhibit strong multi step coupling effects which manifest themselves in reduced cross sections at large angles corresponding to close distances. From the angular distributions at forward angles, where a single step character of the transfer reaction can be assumed, approximate form factors have been extracted employing a first order perturbation theory. Within the uncertainties of a schematic coupled channels calculation the isotopic differences of the sub-barrier fusion enhancement can be understood on the basis of the isotopic differences of the transfer form factors andQ-values.     

Shell-model studies of the 90Y, 91Zr, 92Nb and 93Mo nuclei

A shell-model calculation of the N = 51, 39 ≦ Z ≦ 42 nuclei is presented. The ⁸⁸Sr nucleus is assumed to be an inert closed core. The extra-core protons are restricted to the ($\text{2}\text{p}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, $\text{1}\text{g}{}_{\mathrm{<mtext>9</mtext><mtext>2</mtext>}}$) configurations, and the active neutron is allowed to occupy the $\text{2}\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$, $\text{3}\text{s}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, $\text{2}\text{d}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ and $\text{1}\text{g}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ orbits. The proton-proton effective interaction is directly taken from the previous analysis on the energy levels for N = 50 isotones by Ball et al. The proton-neutron effective interaction is assumed to be of the form of the surface δ-interaction. The energy spectra are calculated from a least-squares fit to the experimental data, varying the T = 0 and T = 1 strengths of the surface δ-interaction. Spectroscopic factors, E2 transition rates and two-body matrix elements are also calculated and compared with the observed values and the previous theoretical results.     

Production of fragments in He+Au reaction at 12.7 GeV

The emission of fragments (A≥16) from reaction of ⁴He with ¹⁹⁷Au has been studied at bombarding energy of 12.7 GeV. Policarbonate Makrofol in sandwich form was used as a detector. Fragments produced in the experiment were identified and an event by event model-free analysis was performed. The decay channels ending with one, two or more (three, four) fragments were detected. A classification scheme based on the multiplicity (M_H) of heavy fragments (A≥40) was used in order to define multifragmentation (M_H = 0), spallation (M_H = 1) and fission (M_H = 2) events. The various experimental features of the fragmentation process are discussed.