Neutron Transfer Research Articles

Searching for resonance in the unbound 6Be and 5Li nuclei

Resonances in 6Be and 5Li have been investigated by deuteron transfer reactions with radioactive ion beams, 2H( 8B,α) 6Be, 2H( 7Be,α) 5Li, respectively. There is an indication of a possible resonance at excitation energy of 9.6 MeV in 6Be, just below the 3He+ 3He threshold. The structure of the 6Li nucleus has been also investigated by the neutron transfer reaction 3He( 7Li,α) 6Li. In 6Li, a new resonance is observed at about 12.4 MeV in excitation energy and the presence of a resonance at 15.2 MeV is confirmed. The data are preliminary and a detailed analysis is in progress. The instalation of a Radioactive Ion Beam facility in Brazil (RIBRAS) is also reported.

Sub-barrier fusion enhancement due to neutron transfer

From the analysis of appropriate experimental data within a simple theoretical model, it is shown that the intermediate neutron transfer channels with positive Q values really enhance the fusion cross section at sub-barrier energies. The effect is found to be very large, especially for fusion of weakly bound nuclei. New experiments are proposed, which may shed additional light on the effect of neutron transfer in fusion processes.

Asymptotic normalization coefficients for8B→7Be+pfrom a study of8Li→7Li+n

Asymptotic normalization coefficients (ANCs) for 8Li->7Li+n have been extracted from the neutron transfer reaction 13C(7Li,8Li)12C at 63 MeV. These are related to the ANCs in 8B->7Be+p using charge symmetry. We extract ANCs for 8B that are in very good agreement with those inferred from proton transfer and breakup experiments. We have also separated the contributions from the p_1/2 and p_3/2 components in the transfer. We find the astrophysical factor for the 7Be(p,gamma)8B reaction to be S_17(0)=17.6+/-1.7 eVb. This is the first time that the rate of a direct capture reaction of astrophysical interest has been determined through a measurement of the ANCs in the mirror system.

Gamma coincidence study of 208Pb+350 MeV 64Ni collisions

Products of 208Pb+ 64Ni collisions at an energy 12% above the Coulomb barrier have been studied in a gamma spectroscopy thick target experiment. The product yield distribution has been established from the γ– γ coincidence analysis supplemented by target radioactivity measurements. Neutron evaporation from excited primary products has been estimated to determine the pre-emission map of fragments. We discuss the transfer of protons and neutrons between the colliding ions in terms of the N/ Z ratio equilibration.

Probing fusion dynamics with scattering experiments

The fusion of heavy nuclei at energies spanning the Coulomb barrier involves the coupling of their relative motion to the internal degrees of freedom of the binary system formed by them. The fusion process is thus a realisation of the fundamental problem of surmounting a potential barrier in an environment of many other degrees of freedom. In the nuclear binary system the number, strength, and nature of the coupling interactions can be widely varied by selecting specific projectile-target combinations. Strong coupling interactions result in a distribution of fusion barriers. These strong couplings can often be clearly identified by extracting representations of this fusion barrier distribution from precision measurements of the excitation function. Alternative representations of the fusion barrier distribution can be obtained from scattering experiments, which generally involve less experimental effort. Since they are not sensitive to the fusion barrier distribution at energies above the average fusion barrier, these alternative representations can only reveal coupling interactions with signatures in the low energy part of the distribution, such as coupling to positive Q-value neutron transfer channels.

Enhanced neutron pair transfer and collective excitations in the system 206Pb + 118Sn at barrier energies

At energies below the Coulomb barrier, neutron transfer and Coulomb excitation have been measured in a very heavy asymmetric nuclear system, in 206Pb + 118Sn. These are semi-magic nuclei showing super-fluid properties. Particle-γ coincidence techniques using 5 Euroball Cluster detectors (EB), combined in a set-up with the Heidelberg-Darmstadt NaI Crystal Ball (CB), have been used. Position-sensitive detectors allowed the observation of scattering processes covering angles from 110 up to 150 degrees. The fragments are identified via the known γ-decays of the lowest excited states using the high resolution of EB. Using the unique feature of the set-up with the CB, transfer to well-defined final channels with known quantum numbers is selected using the high-efficiency multiplicity filter of the CB with no second γ-ray, i.e. without feeding. The data are analysed using the semi-classical approach and transfer probabilities are obtained. Coulomb excitation has been analysed using known transition probabilities. The enhancement is deduced for the two-neutron transfer populating the low-lying super-fluid 2+ states in 120Sn and 116Sn, while the 2n transition remains in the ground state for the 20NPb nuclei. Large enhancements up to EF ≃ 103 are observed. This is the first observation of neutron pair transfer enhancement for a heavy nuclear binary system with super-fluid properties with experimentally separated levels. The calculations with microscopic 2-neutron wave functions, with configuration mixing over six shell model configurations and using the coupled reaction channels approach, reproduce well the observed probabilities and the enhancement.

Selection of cold transfer and enhanced neutron-pair transfer in the 206 Pb + 118 Sn reaction

The neutron transfer in a very heavy asymmetric nuclear system, in 206 Pb + 118 Sn has been measured using particle- $\gamma$ coincidence techniques with position-sensitive detectors, 5 EUROBALL-Cluster detectors (EB) and the Heidelberg-Darmstadt NaI-CRYSTAL BALL (CB). The fragments are identified via the known $\gamma$ -decays of the lowest excited states using the high resolution of EB. Using the unique feature of the set-up with the CB, transfer to well-defined final channels with known quantum numbers is selected using the high-efficiency multiplicity filter of the CB with no second $\gamma$ -ray, i.e. without feeding. The enhancement in the two-neutron transfer is deduced, for population of the low-lying super-fluid 2 + states in 120 Sn and 116 Sn, while the 2n-transition remains in the ground state for the recoiling 20X Pb-nuclei. Large enhancements up to EF $\simeq 10^3$ are observed. This is the first observation of neutron pair-transfer enhancement for a heavy nuclear binary system with superfluid properties with experimentally separated levels.

Accelerated radioactive beams from REX-ISOLDE

In 2001 the linear accelerator of the Radioactive beam EXperiment (REX-ISOLDE) delivered for the first time accelerated radioactive ion beams, at a beam energy of 2 MeV/u. REX-ISOLDE uses the method of charge-state breeding, in order to enhance the charge state of the ions before injection into the LINAC. Radioactive singly-charged ions from the on-line mass separator ISOLDE are first accumulated in a Penning trap, then charge bred to an A/ q<4.5 in an electron beam ion source (EBIS) and finally accelerated in a LINAC from 5 keV/u to energies between 0.8 and 2.2 MeV/u. Dedicated measurements with REXTRAP, the transfer line and the EBIS have been carried out in conjunction with the first commissioning of the accelerator. Thus the properties of the different elements could be determined for further optimization of the system. In two test beam times in 2001 stable and radioactive Na isotopes ( 23Na– 26Na) have been accelerated and transmitted to a preliminary target station. There 58Ni- and 9Be- and 2H-targets have been used to study exited states via Coulomb excitation and neutron transfer reactions. One MINIBALL triple cluster detector was used together with a double sided silicon strip detector to detect scattered particles in coincidence with γ-rays. The aim was to study the operation of the detector under realistic conditions with γ-background from the β-decay of the radioactive ions and from the cavities. Recently for efficient detection eight tripple Ge-detectors of MINIBALL and a double sided silicon strip detector have been installed. We will present the first results obtained in the commissioning experiments and will give an overview of realistic beam parameters for future experiments to be started in the spring 2002.

Kyy′(0°)for3He(d,p)4Henear theJπ=32+resonance

The polarization transfer coefficient ${K}_{y}^{{y}^{\ensuremath{'}}}(0\ifmmode^\circ\else\textdegree\fi{})$ has been measured for the ${}^{3}\mathrm{He}{(d,p)}^{4}\mathrm{He}$ reaction at energies of 520, 890, and 1490 keV. The measured values are $\ensuremath{\approx}\ensuremath{-}2/3,$ consistent with expectations based on the presence of a broad ${J}^{\ensuremath{\pi}}={\frac{3}{2}}^{+}$ resonance at ${E}_{d}=430\mathrm{keV}.$ A comparison with data taken at somewhat higher energies reveals a large change in this observable as the reaction becomes dominated by direct neutron transfer.

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.

Investigation of the role of neutron transfer in the fusion of 32,34S with 197Au, 208Pb using quasi-elastic scattering

Excitation functions for quasi-elastic scattering have been measured at backward angles for the systems 32,34S + 197Au and 32,34S + 208Pb for energies spanning the Coulomb barrier. Representative distributions, sensitive to the low energy part of the fusion barrier distribution, have been extracted from the data. For the fusion reactions of 32,34S with 197Au couplings related to the nuclear structure of 197Au appear to be dominant in shaping the low energy part of the barrier distribution. For the system 32S + 208Pb the barrier distribution is broader and extends further to lower energies, than in the case of 34S + 208Pb. This is consistent with the interpretation that the neutron pick-up channels are energetically more favoured in the 32S induced reaction and therefore couple more strongly to the relative motion. It may also be due to the increased collectivity of 32S, when compared with 34S.

Solution to a long-standing problem:12C(14N,13N)13C1/2+

The failure of finite-range distorted-wave Born approximation (DWBA) calculations to describe neutron transfer to ${2s}_{1/2}$ states in nuclei such as ${}^{13}\mathrm{C}{,}^{13}\mathrm{N}{,}^{17}\mathrm{O},$ and ${}^{27}\mathrm{Mg}$ is a long-standing problem. Detailed coupled-channels Born approximation and coupled-reaction-channels (CRC) calculations failed to improve on the much simpler DWBA calculations. This work presents results suggesting that the vital step necessary to obtain agreement between CRC calculations and data in the case of the ${2s}_{1/2}$ state in ${}^{13}\mathrm{C}$ is an increase in the radius of the potential binding the neutron to the ${}^{12}\mathrm{C}$ core, leading to a considerable extension of the neutron radial wave function. The source of this extension is shown to arise from the deformation of the ${}^{12}\mathrm{C}$ core.

Particle spectroscopy of low-lying collective states in 100Ru

One neutron stripping and pick-up results obtained in São Paulo with good energy resolution show that levels interpreted in previous works as two-phonon and three-phonon states in 100Ru are excited with considerable intensity through predominantly direct one-step single neutron transfers. Furthermore, a comparative analysis of excitation energy values obtained in recent experiments, including the present results, with previous ones shows that there are at least 4 spurious levels in the adopted 100Ru level scheme below 2.5 MeV. Several levels seen in recent gamma work are here confirmed and three new ones indicated below 3.2 MeV of excitation energy.

Towards complete spectroscopy of 193Os

Strong evidence for the existence of supersymmetry in atomic nuclei has been found in the quartet of nuclei 194,195Pt, 195,196Au [1–3]. Our attention is now focused a second quartet of nuclei, that is 193,194Ir, 192,193Os where the model is applicable [4]. We investigate the heaviest osmium isotope which is accessible in a single neutron transfer reaction: 193Os. Excitation energies are known from (d,p)-and (n,γ)-measurements [5,6], but there is limited information about quantum numbers. To measure absolute cross-sections and asymmetries of the 192Os(\(\bar d\), p)193Os reaction we used our facilities at the Munich tandem accelerator. The 22 MeV beam of vector-polarised deuterons was produced by our new Stern-Gerlach-Source. Outgoing protons were analysed by the Q3D magnetic spectrograph and detected with a strip-detector. The target consisted of a metallic foil of 146 μg/cm2 highly enriched 192Os (99%).

Low Energy Reactions with Unstable Nuclei

Several new ISOL radioactive nuclear beam facilities with post acceleration are coming into operation. The first experiments at REX-ISOLDE at CERN and SPIRAL at GANIL were made towards the end of 2001. Both facilities canre-accelerate beams of various radioactive nuclei to energies of about 2 MeV per nucleon and there are plans to increase this energy. Low energy beams with energies up to about 15 MeV per nucleon are very effective for studying reactions involving transfer of neutrons, protons or clusters of two or three nucleons. This contribution presents some simple formulae which are useful for estimating the energy and angular momentum dependence of transfer probabilities. They could be helpful in the design of experiments. The last part of the contribution is about a different topic: the calculation of pre-equilibrium gamma emission in heavy ion fusion reactions.

Isotopic dependence and channel coupling effects in the fusion of16O+112,116Snand32S+112,116,120Snat energies around the barrier

We report here measurements of fusion cross-sections for the ${}^{16}\mathrm{O}{+}^{112,116}\mathrm{Sn}$ and ${}^{32}\mathrm{S}{+}^{112,116,120}\mathrm{Sn}$ systems. The set of systems studied with widely varying ground state Q values for the transfer of neutrons is a good case to understand the influence of single and multineutron transfer on the fusion process. The evaporation residue cross-sections were measured in small energy steps covering the region around the Coulomb barrier $(\ensuremath{\approx}{0.8V}_{b}$ to ${1.2V}_{b}).$ The data have been analyzed in the framework of a coupled-channels formalism using the coupled-channels code CCFULL. A comparative study of the ${}^{16}\mathrm{O}{+}^{A}\mathrm{Sn}$ and ${}^{32}\mathrm{S}{+}^{A}\mathrm{Sn}$ systems suggests that the fusion process is predominantly influenced by coupling to collective excitations with coupling to multiphonon states (of target and projectile) becoming increasingly important for heavier systems. From the present analysis an unambiguous signature of the role of coupling to neutron transfer channels could not be inferred.

Spectrometer equipment for neutron spectra measurements in mixed neutron/photon fields

The paper describes spectrometer equipment in the IPPE experimental laboratory for neutron spectra measurement in mixed (n,γ) fields. The laboratory was founded in 1957 and it occupies a leading position in the field of nuclear facilities radiation spectrometry and benchmark experiments in Russia. Spectrometer equipment includes spectrometers based on the organic stilbene scintillator, hydrogen counter and Bonner balls. Basic fields of spectrometer application are mixed radiation neutron spectra measurement of radionuclide sources, of nuclear reactors and accelerators; study of neutron transfer through the material, including benchmark experiments and measurement of neutron spectra in the rooms of nuclear facilities.

On the slope anomaly in heavy-ion transfer reactions

We discuss a semiclassical model of transfer reactions in heavy-ion collisions, in which the nuclei are assumed to move along classical trajectories governed by the Coulomb and the real part of the optical potential. The model, originally proposed for the case of spherical nuclei, is here extended to deformed ones. It takes into account tunneling around the point of closest approach of the collision partners, and the effect of other channels is included as an absorption due to the imaginary part of the optical potential. The interplay between absorption and tunneling effects explains both the observed energy dependence of the transfer probabilities at large distances, and the so-called “slope anomaly” in neutron transfer reactions.

Direct Reactions Involving Exotic Nuclei 8 He and 5 H

The angular distribution of 26 MeV/n 8He ions elastically scattered from a gaseous helium target was measured in a wide CM angular range. Results are discussed in terms of the potential scattering and neutron transfer. The angular distributions of elastic scattering as well as 1n and 2n transfer reactions of these ions on protons are presented. Finite-range DWBA calculations made for the 2n transfer reaction leading to the ground (0+) and excited (2+) states of 6He underestimate the cross section for 2n transfer to 6He(0+). Analysis of data for this reaction channel suggests the importance of a direct 5H-cluster exchange process. A resonance state of 5H with an energy of 2 MeV above the decay threshold n+n+ 3H was obtained for the first time by making use of the reaction p(6He,2He)5H.

Influence of higher-order deformations in the34S+168Erfusion reaction

The shape of the measured barrier distribution for the ^{34}S + ^{168}Er reaction is analysed using the coupled-channels description. The ^{168}Er nucleus is a good candidate to test current fusion models description of deformation since it has a large quadrupole deformation with an insignificant hexadecapole deformation. Coupling to weaker channels, the 2^+_1 state in ^{34}S, the 3^-_1 state in ^{168}Er, and the pair neutron transfer channel, all were found to have little influence on the barrier distribution. A successful description of the barrier distribution was only obtained after the hexacontatetrapole deformation term in ^{168}Er (beta_6) was included in the coupling scheme. However, a positive value for beta_6 was needed where the macroscopic-microscopic model predicts a negative one.