Neutron Transfer Reactions Research Articles

Single neutron transfer coupling effects on sub-barrier elastic scattering and near-barrier fusion ofC15

To date, the possible effects of coupling to nucleon transfer reactions on the elastic scattering and fusion of weakly bound exotic nuclei have been largely neglected. The $^{15}\mathrm{C}$ nucleus presents a virtually unique opportunity to test these effects for an almost pure s${}_{1/2}$ single neutron halo nucleus. We present a series of coupled reaction channel calculations of the sub-barrier elastic scattering and single neutron transfer reactions, plus near-barrier excitation functions of the total fusion cross section for the $^{12}\mathrm{C}$, $^{13}\mathrm{C}$, and $^{15}\mathrm{C}+^{208}\mathrm{Pb}$ systems. The method is validated against data for $^{12}\mathrm{C}$ and $^{13}\mathrm{C}+^{208}\mathrm{Pb}$. A large effect on the sub-barrier elastic scattering due to coupling to the ($^{15}\mathrm{C}$,$^{14}\mathrm{C}$) single neutron stripping reaction is found, ascribed to the 2s${}_{1/2}$ halo nature of the $^{15}\mathrm{C}$ ground state, in contrast to the two stable carbon isotopes. We also find a significant diminution of the above barrier total fusion cross section for $^{15}\mathrm{C}$ due to this coupling, again in contrast to the stable isotopes.

Influence of the transfer reactions on p + 6He elastic scattering

We have studied an effect of neutron and triton transfer reactions on the p +^6He elasticscatteringat25 MeVbymeansofcoupled - reaction - channelcalculations.Itisfoundthatwhenthetransferreactionsareexplicitlyincludedinthecalculationstheimaginarypartoftheinput$p + $6He optical model potential has to be reduced by 52 percent while its real part enhanced by 15 percent in order to fit the elastic-scattering data. The effect of transfer channels on the real part of this potential is somewhat weaker than that of 6He breakup reported previously. However, for the imaginary part, the effect of transfer channels is dominant. It is concluded that while the breakup contribution to proton elastic scattering mainly affects the real part of the bare potential, the contribution of transfer channels affects mainly its imaginary part.

Excitation functions of fusion reactions and neutron transfer in the interaction of 6He with 197Au and 206Pb

Excitation functions for evaporation residues in the reactions 197Au(6He, xn)203-xnTl, x = 2-7, and 206Pb(6He, 2n)210Po, as well as for neutron transfer reactions for the production of 196Au and 198Au in the interaction of 6He with 197Au were measured. The 6He beam was obtained from the accelerator complex for radioactive beams DRIBs (JINR). The maximum energy of the beam was about 10AMeV and the intensity reached 2×107pps. The stacked-foil activation technique was used directly in the beam extracted from the cyclotron or in the focal plane of the magnetic spectrometer MSP-144. The identification of the reaction products was done by their radioactive γ- or α-decay. The fusion reaction with the evaporation of two neutrons was characterized by an increase in the cross-section compared to statistical model calculations. The analysis of the data in the framework of the statistical model for the decay of excited nuclei, which took into account the sequential fusion of 6He has shown good agreement between the experimental and the calculated values of the cross-sections in the case of sub-Coulomb-barrier fusion in the 206Pb + 6He reaction. An unusually large cross-section was observed below the Coulomb barrier for the production of 198Au in the interaction of 6He with 197Au. Possible mechanisms of formation and decay of transfer reaction products are discussed.

Reduced neutron spectroscopic factors when using potential geometries constrained by Hartree-Fock calculations

We carry out a systematic analysis of angular distribution measurements for selected ground-state to ground-state ($d,p$) and ($p,d$) neutron transfer reactions, including the calcium isotopes. We propose a consistent three-body model reaction methodology in which we constrain the transferred-neutron bound state and nucleon-target optical potential geometries using modern Hartree-Fock calculations. Our deduced neutron spectroscopic factors are found to be suppressed by $~30$% relative to independent-particle shell-model values, from $^{40}\mathrm{Ca}$ through $^{49}\mathrm{Ca}$. The other nuclei studied, ranging from B to Ti, show similar average suppressions with respect to large-basis shell-model expectations. Our results are consistent with deduced spectroscopic strengths for neutrons and protons from intermediate-energy nucleon knockout reactions and for protons from ($e,{e}^{'}p$) reactions on well-bound nuclei.

Single nucleon transfer reactions near Coulomb barrier on 197Au

Single neutron transfer reactions on 197Au induced by 12C and 16O at near Coulomb barrier energies have been studied using radiochemical and off-line gamma-ray spectrometric techniques. High spin yield fraction (HSF = σm/(σm+σg)) for 196m,gAu and formation cross section ratio (σ198/σ196) of 198Au and 196Au have been determined for both 197Au+12C and 197Au+16O reactions at different excitation energies near the Coulomb barrier. The HSF of 196m,gAu and σ198/σ196 values in both reactions are seen to sharply increase with increasing excitation energy near the Coulomb barrier and then slowly increase reaching a constant value at high projectile energy. These observations have been analyzed in terms of nuclear structural and kinematic parameters. A comparison the effects of Coulomb excitation and nucleon transfer process during heavy-ion interactions indicates a predominant effect of the latter process in the background of the Coulomb excitation. Observed reaction systematics seem to follow the quasi-elastic transfer process.

Spectroscopy with radioactive ion beams at the HRIBF for nuclear astrophysics

Proton elastic scattering, proton inelastic scattering, proton transfer reactions and the (d, p) neutron transfer reaction have been measured in inverse kinematics with radioactive ion beams at the HRIBF to determine the properties of nuclear excited states that are important in stellar explosions. The experimental techniques and some initial results are briefly summarized.

Neutron transfer reactions with neutron-rich radioactive ion beams

Initial measurements are presented of the (d,p) reactions on neutron-rich N=50 isotones along the r-process path of nucleosynthesis with radioactive ion beams of 82Ge and 84Se. Prospects for measurements with unstable 130,132Sn beams are discussed.

New Limits for the 18F( p, α) 15O Rate in Novae

The degree to which the (p, γ) and (p,α) reactions destroy 18F at temperatures ∼ 1 − 4 × 10 8 K is important for understanding the synthesis of nuclei in nova explosions and for using the long-lived radionuclide 18F, a target of gamma ray astronomy, as a diagnostic of nova mechanisms. The reactions are dominated by low-lying proton resonances near the 18F+p threshold ( E x = 6.411 MeV in 19Ne). To gain further information about these resonances, we have used the d(18F,p)19F neutron transfer reaction to selectively populate corresponding mirror states in 19F. The results would suggest 18F(p,γ)19Ne and 18F(p,α)15O reaction rates that are 2-3 times lower than reported previously.

New constraints on theF18(p,α)O15rate in novae from the(d,p)reaction

The degree to which the (p,gamma) and (p,alpha) reactions destroy 18F at temperatures 1-4x10^8 K is important for understanding the synthesis of nuclei in nova explosions and for using the long-lived radionuclide 18F, a target of gamma-ray astronomy, as a diagnostic of nova mechanisms. The reactions are dominated by low-lying proton resonances near the 18F+p threshold (E_x=6.411 MeV in 19Ne). To gain further information about these resonances, we have used a radioactive 18F beam from the Holifield Radioactive Ion Beam Facility to selectively populate corresponding mirror states in 19F via the inverse d(18F,p)19F neutron transfer reaction. Neutron spectroscopic factors were measured for states in 19F in the excitation energy range 0-9 MeV. Widths for corresponding proton resonances in 19Ne were calculated using a Woods-Saxon potential. The results imply significantly lower 18F(p,gamma)19Ne and 18F(p,alpha)15O reaction rates than reported previously, thereby increasing the prospect of observing the 511-keV annihilation radiation associated with the decay of 18F in the ashes ejected from novae.

First study of the level structure of ther-process nucleusGe83

The first ($d,p$) neutron transfer reaction on a neutron-rich r-process nucleus has been measured at the Holifield Radioactive Ion Beam Facility. The $^{2}\mathrm{H}(^{82}\mathrm{Ge},p)^{83}\mathrm{Ge}$ reaction was studied by bombarding a 430-$\ensuremath{\mu}g/{\mathrm{cm}}^{2}$ (${\mathrm{CD}}_{2}$)${}_{n}$ target with a 330-MeV beam of radioactive $^{82}\mathrm{Ge}$. The reaction Q value ($Q=1.47\ifmmode\pm\else\textpm\fi{}0.02$ stat. $\ifmmode\pm\else\textpm\fi{}0.07$ sys. MeV) has been measured leading to the first determination of the mass of the $N=51$ nucleus $^{83}\mathrm{Ge}$. Excitation energies, angular distributions, and spectroscopic factors for the first two states of $^{83}\mathrm{Ge}$ have also been determined.

Refractive effects in the scattering of loosely bound nuclei

A study of the interaction of the loosely bound nuclei $^{6,7}\mathrm{Li}$ at 9 and $19\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}∕\text{nucleon}$ with light targets has been undertaken. With the determination of unambiguous optical potentials in mind, elastic data for four projectile-target combinations and one neutron transfer reaction $^{13}\mathrm{C}(^{7}\mathrm{Li},^{8}\mathrm{Li})^{12}\mathrm{C}$ have been measured over a large angular range. The kinematical regime encompasses a region where the mean field (optical potential) has a marked variation with mass and energy, but turns out to be sufficiently surface transparent to allow strong refractive effects to be manifested in elastic scattering data at intermediate angles. The identified exotic feature, a ``plateau'' in the angular distributions at intermediate angles, is fully confirmed in four reaction channels and is interpreted as a prerainbow oscillation resulting from the interference of the barrier and internal barrier far-side scattering subamplitudes.

Nuclear structure ofGd159

The well deformed atomic nucleus $^{159}\mathrm{Gd}$ was investigated by means of radiative neutron capture and single neutron transfer reactions. Nearly 70 secondary $\ensuremath{\gamma}$ rays from the $(n,\ensuremath{\gamma})$ reaction studied with high-resultion bent-crystal spectrometers at Grenoble are assigned to $^{159}\mathrm{Gd}$. About 200 levels with spin up to $\frac{11}{2}$ are observed in this nucleus below $2.3\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ in $(d,p)$ and $(\mathbf{d},t)$ reactions investigated at the Tandem Van de Graaff accelerator in Garching using unpolarized $(18\phantom{\rule{0.3em}{0ex}}\mathrm{MeV})$ and polarized $(22\phantom{\rule{0.3em}{0ex}}\mathrm{MeV})$ deuteron beams, respectively. The proposed level scheme for this nucleus is arranged into 21 rotational bands. Experimentally observed levels are interpreted using predictions obtained within the quasiparticle-phonon model and the quasiparticle-rotor approach for a well deformed nucleus.

Analysis of the nuclear fusion process based on potential barriers modified by coupling effects

Partial wave analysis and the decoupling of the wave equations based on potential barriers modifi ed by coupling effects are applied in the study of the fusion process in medium heavy nuclei. Several relevant physical quantities are calculated in order to provide evidence of underline reaction mechanisms that determine characteristics of fusion excitation functions and barrier distributions. The influence of Q- values is especially considered. The method is applied for the neutron transfer reaction in the 17O +144Sm system.

Systematic extraction of spectroscopic factors fromC12(d,p)C13andC13(p,d)C12reactions

Existing measurements of the angular distributions of the ground-state to ground-state transitions of the 12C(d,p)13C and 13C(p,d)12C neutron-transfer reactions have been analyzed systematically using the Johnson-Soper adiabatic and distorted-wave theories. When using a consistent set of physical inputs the deduced spectroscopic factors are consistent to within 20% for incident deuteron energies from 6 to 60 MeV. By contrast, original analyses of many of these data quoted spectroscopic factors that differed by up to a factor of five. The present analysis provides an important reference point from which to assess the requirements of future spectroscopic analyses of transfer reactions measured in inverse kinematics using rare nuclei.

Structure of neutron-rich Be and C isotopes

The structure of neutron-rich beryllium isotopes has been investigated using different heavy-ion-induced transfer reactions. In neutron transfer reactions, the population of final states shows a strong sensitivity to the chosen core nucleus, i.e., the target nuclei 9Be or 10Be, respectively. Molecular rotational bands up to high excitation energies are observed with 9Be as the core due to its pronounced 2α-cluster structure, whereas only a few states at low excitation energies are populated with 10Be as the core. For 11Be, a detailed investigation has been performed for the three states at 3.41, 3.89, and 3.96 MeV, which resulted in the most probable spin-parity assignments 3/2+, 5/2−, and 3/2−, respectively. Furthermore, we have studied particle-hole states of 16C using the 13C(12C, 9C)16C reaction and found 14 previously unknown states. Using the 12C(12C, 9C)15C reaction, five new states were observed for 15C.

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.

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.

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.

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%).

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.