Single-neutron Transfer Reactions Research Articles

Neutron transfer reactions on the ground state and isomeric state of a Sn130 beam

The structure of nuclei around the neutron-rich nucleus 132Sn is of particular interest due to the vicinity of the Z = 50 and N = 82 shell closures and the r-process nucleosynthetic path. Four states in 131Sn with a strong single-particle-like component have previously been studied via the (d,p) reaction, with limited excitation energy resolution. The 130Sn(9Be,8Be)131Sn and 130Sn(13C,12C)131Sn single-neutron transfer reactions were performed in inverse kinematics at the Holifield Radioactive Ion Beam Facility using particle-gamma coincidence spectroscopy. The uncertainties in the energies of the single-particle-like states have been reduced by more than an order of magnitude using the energies of gamma rays. The previous tentative Jpi values have been confirmed. Decays from high-spin states in 131Sn have been observed following transfer on the isomeric component of the 130Sn beam. The improved energies and confirmed spin-parities of the p-wave states important to the r-process lead to direct-semidirect cross-sections for neutron capture on the ground state of 130Sn at 30 keV that are in agreement with previous analyses. A similar assessment of the impact of neutron-transfer on the isomer would require significant nuclear structure and reaction theory input. There are few measurements of transfer reaction on isomers, and this is the first on an isomer in the 132Sn region.

Single-particle structure of neutron-rich Sr isotopes via H2(Sr94,95,96, p) reactions

Background: The region around neutron number N=60 in the neutron-rich Sr and Zr nuclei is one of the most dramatic examples of a ground-state shape transition from (near) spherical below N=60 to strongly deformed shapes in the heavier isotopes. Purpose: The single-particle structure of Sr95-97 approaching the ground-state shape transition at Sr98 has been investigated via single-neutron transfer reactions using the (d,p) reaction in inverse kinematics. These reactions selectively populate states with a large overlap of the projectile ground state coupled to a neutron in a single-particle orbital. Method: Radioactive Sr94,95,96 nuclei with energies of 5.5 AMeV were used to bombard a CD2, where D denotes H2, target. Recoiling light charged particles and γ rays were detected using a quasi-4π silicon strip detector array and a 12-element Ge array. The excitation energy of states populated was reconstructed employing the missing mass method combined with γ-ray tagging and differential cross sections for final states were extracted. Results: A reaction model analysis of the angular distributions allowed for firm spin assignments to be made for the low-lying 352, 556, and 681 keV excited states in Sr95 and a constraint has been placed on the spin of the higher-lying 1666 keV state. Angular distributions have been extracted for ten states populated in the H2(Sr95,p)Sr96 reaction, and constraints have been provided for the spins and parities of several final states. Additionally, the 0, 167, and 522 keV states in Sr97 were populated through the H2(Sr96,p) reaction. Spectroscopic factors for all three reactions were extracted. Conclusions: Results are compared to shell-model calculations in several model spaces and the structure of low-lying states in Sr94 and Sr95 is well described. The spectroscopic strength of the 0+ and 2+ states in Sr96 is significantly more fragmented than predicted. The spectroscopic factors for the H2(Sr96,p)Sr97 reaction suggest that the two lowest-lying excited states have significant overlap with the weakly deformed ground state of Sr96, but the ground state of Sr97 has a different structure.

Nonlocal interactions in the (d,p) surrogate method for (n,γ) reactions

Single-neutron transfer reactions populating states in the continuum are interesting both for structure and astrophysics. In their description often global optical potentials are used for the nucleon-target interactions, and these interactions are typically local. In our work, we study the effects of nonlocality in $(d,p)$ reactions populating continuum states. This work is similar to that of [1] but now for transfer to the continuum. A theory for computing cross sections for inclusive processes $A (d,p) X$ was explored in [2]. Therein, local optical potentials were used to describe the nucleon-target effective interaction. The goal of the present work is to extend the theory developed in [2] to investigate the effects of including nonlocality in the effective interaction on the relevant reaction observables. We implement the R--matrix method to solve the non--local equations both for the nucleon wavefunctions and the propagator. We then apply the method to systematically study the inclusive process of $(d,p)$ on $^{16}$O, $^{40}$Ca, $^{48}$Ca and $^{208}$Pb at 10, 20 and 50 MeV. We compare the results obtained when non--local interactions are used with those obtained when local equivalent interactions are included. We find that nonlocality affects different pieces of the model in complex ways. Depending on the beam energy and the target, the non-elastic breakup can either increase or decrease. While the non-elastic transfer cross section for each final spin state can change considerably, the main prediction of the model [2], namely the shape of the spin distributions, remains largely unaltered by nonlocality. [1] L. J. Titus and F. M. Nunes. Testing the Perey effect. Phys. Rev. C, 89:034609, Mar 2014. [2] G. Potel, F. M. Nunes, and I. J. Thompson. Establishing a theory for deuteron-induced surrogate reactions. Phys. Rev. C, 92:034611, Sep 2015.

Sensitivity of (d, p) Reactions to High n-p Momenta and the Consequences for Nuclear Spectroscopy Studies.

Theoretical models of low-energy (d, p) single-neutron transfer reactions are a crucial link between experimentation, nuclear structure, and nuclear astrophysical studies. Whereas reaction models that use local optical potentials are insensitive to short-range physics in the deuteron, we show that including the inherent nonlocality of the nucleon-target interactions and realistic deuteron wave functions generates significant sensitivity to high n-p relative momenta and to the underlying nucleon-nucleon interaction. We quantify this effect upon the deuteron channel distorting potentials within the framework of the adiabatic deuteron breakup model. The implications for calculated (d, p) cross sections and spectroscopic information deduced from experiments are discussed.

One-neutron transfer study ofXe137and systematics of13/21+and13/22+levels inN=83nuclei

Excited states in 137Xe have been studied by using the near-barrier single-neutron transfer reactions 13C(136Xe,12C )137Xe and 9Be(136Xe,8Be )137Xe in inverse kinematics.Particle- and particle- coincidence measurements have been performed with the Phoswich Wall and Digital Gammasphere detector arrays. Evidence is found for a 13/2+2 level (E = 3137 keV) and for additional high-lying 3/2– and 5/2– states. The results are discussed in the framework of realistic shell-model calculations. These calculations are also extended to the 13/2+1 and 13/2+2 levels in the N = 83 isotonic chain. Furthermore, they indicate that there is a need for a value of the neutron 0i13/2 single-particle energy (ESPE = 2366 keV) lower than the one proposed in the literature. It is also demonstrated that the population patterns of the j = l ± 1/2 single-particle states in 137Xe are different for the two targets used in these measurements and the implications of this effect are addressed.

Strong multistep interference effects inC12(d,p)to the9/21+state inC13

The population of the 9.50 MeV $9/{2}_{1}^{+}$ resonance in $^{13}\mathrm{C}$ by single-neutron transfer reactions is expected to be dominated by the two-step route through the $^{12}\mathrm{C}$ ${2}_{1}^{+}$ (4.44 MeV) state, with another possible contribution via the strongly excited ${3}_{1}^{\ensuremath{-}}$ (9.64 MeV) resonance in $^{12}\mathrm{C}$. However, we find that a good description of the angular distribution for population of this state via the $^{12}\mathrm{C}(d,p)^{13}\mathrm{C}$ reaction is only possible when both direct ${0}_{1}^{+}\ensuremath{\bigotimes}{g}_{9/2}$ and two-step (via the 4.44 MeV $^{12}\mathrm{C}$ ${2}_{1}^{+}$ state) ${2}_{1}^{+}\ensuremath{\bigotimes}{d}_{5/2}$ paths are included in a coupled reaction channel calculation. While the calculated angular distribution is almost insensitive to the presence of the two-step path via the 9.64 MeV $^{12}\mathrm{C}$ ${3}_{1}^{\ensuremath{-}}$ resonance, despite a much greater contribution to the wave function from the ${3}_{1}^{\ensuremath{-}}\ensuremath{\bigotimes}{f}_{7/2}$ configuration, its inclusion is required to fit the details of the experimental angular distribution. The very large interference between the various components of the calculations, even when these are small, arises through the ``kinematic'' effect associated with the different transfer routes.

Single-neutron energies nearN=28and the absence of theN=34subshell closure in the Ti isotopes

The surprising absence of the $N=34$ subshell closure in the exotic nucleus $^{56}\mathrm{Ti}$ is highlighted by examining the experimental single neutron energies along the $N=28$ isotopes, which are extracted from previously existing data on single-neutron transfer reactions on stable isotones and nuclear masses. The results point out the role of the monopole effect of the tensor force on the ${f}_{7/2}\text{\ensuremath{-}}{f}_{5/2}$ splitting and confirm the prediction of the reduction of this splitting in the $Z>20$ isotones. In addition, the importance of making similar measurements of the distribution of single neutron and single neutron hole strength in the exotic $Z<20 N=28$ isotones is discussed. Measurements of the ($d,p$) reaction and the neutron knockout reaction on the exotic nucleus $^{46}\mathrm{Ar}$ have provided proof of principle for the necessary reactions, and advances presently taking place in detector technology will soon provide the tools necessary for reliably mapping the distribution of neutron strength in detail.

Two-quasineutron states in98248Cfand98250Cfand the neutron-neutron residual interactions

Two-quasineutron states in {sup 248}Cf and {sup 250}Cf were investigated by single-neutron transfer reactions, {sup 249}Cf(d,t){sup 248}Cf and {sup 249}Cf(d,p){sup 250}Cf. The majority of levels observed were assigned to 12 bands in {sup 248}Cf and six bands in {sup 250}Cf, constructed from the single-particle states in neighboring Cf nuclei. The effective two-body interactions between two odd neutrons coupled outside a deformed core were deduced from the differences in the energies of the bandheads formed by the parallel and antiparallel coupling of the intrinsic spins of the two single-particle states.

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.

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.

Single-neutron excitations in neutron-rich N = 51 nuclei

Single-neutron transfer reactions have been measured on two N = 50 isotones at the Holifield Radioactive Ion Beam Facility (HRIBF). The single-particle-like states of 83Ge and 85Se have been populated using radioactive ion beams of 82Ge and 84Se and the (d, p) reaction in inverse kinematics. The properties of the lowest-lying states —including excitation energies, orbital angular momenta, and spectroscopic factors— have been determined for these N = 51 nuclei.

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.

Physics with heavy neutron-rich RIBs at the HRIBF

The Holifield Radioactive Ion Beam Facility at Oak Ridge National Laboratory has recently produced the world's first post-accelerated beams of heavy neutron-rich nuclei. The first experiments with these beam are described, and the results discussed. B(E2;0+ → 2+) values for neutron-rich 126,128Sn and 132,134,136Te isotopes have been measured by Coulomb excitation in inverse kinematics. The results for 132Te and 134Te (N = 80, 82) show excellent agreement with systematics of lighter Te isotopes, but the B(E2) value for 136Te (N = 84) is unexpectedly small. Single-neutron transfer reactions with a 134Te beam on natBe and 13C targets at energies just above the Coulomb barrier have also been studied.

Study of 190Ir via single-proton and single-neutron transfer reactions

Levels in the odd-odd nucleus 190Ir were investigated with the 191Ir(d,t), 189Os( 3He,d) and 189Os(α,t) reactions. For the 191Ir(d,t) reaction, spectra were measured at 16 angles with a typical overall resolution of 5–6 keV FWHM. The angular distributions of cross sections were compared with predicted curves from DWBA calculations, and spectroscopic strengths were extracted for the different l-values populating the states. From the ratios of ( 3He,d) and (α,t) cross sections it was possible to obtain information on the dominant l-value involved in the transfer. Relative excitation energies, parities, possible spin values and spectroscopic strengths were determined for levels up to approximately 1 MeV excitation energy. The large number of peaks observed to be populated in common to both the single-proton and single-neutron transfer reactions is strong evidence for a large amount of configuration mixing. Assignments based on the Nilsson model were made for some low-lying states.

Single neutron transfer in 238U + 238U collisions

Cross sections measured for single neutron transfer reactions in 238U + 238U collisions at sub-Coulomb energies are qualitatively reproduced using a simple semi-classical formula.

Single neutron transfer reactions in the systems:139La+16O,93Nb+12C,14N,16O and20Ne

Data from the quasi-elastic transfer reaction products140La and92mNb exhibit near zero effective ranges, flat excitation functions over large energy ranges and Q independent yields. These features are used to characterise quasi elastic reactions via the target-like product.

Search for the Second Excited 11/2-State and Weak-Coupling Schema in117Sn

The second excited 11/2 - state in 117 Sn has been searched for by four different single-neutron transfer reactions, (d, t), (p, d), (d, p) and ( 3 He, α). The state has been found at E ex =1.589 MeV, very near to the 13/2 - and 15/2 - states observed in an in-beam γ-ray study. These very closely degenerate states are possible three members of the quintet of the weak-coupling scheme [h 11/2 ×2 + ] in this nucleus.

The 48Ti(t, d) 49Ti reaction: A comparison with magnetic electron scattering

The reaction 48Ti(t, d) 49Ti leading to the ground and first excited states of 49Ti has been studied at triton energies of 2.75 and 3.0 MeV. The cross section to the ground state of 49Ti has been analysed using the DWBA with a previously determined bound-state well geometry to obtain a value for the (t, d) normalization factor of D 2 = (3.29 ± 0.40) × 10 4 MeV 2 · fm 3. This value is in agreement with that obtained from a comparison of the (d, t) reaction with heavy-ion single-neutron transfer reactions. Using this value of the normalization factor the rms radius of the 2 p 3 2 component in the 3 2 − first excited state of 49Ti is found to be 4.42 ± 0.07 fm (point neutron), corresponding to the use of a local bound-state potential well.

A study of the second-order processes in the elastic scattering of polarized deuterons

Angular distributions for the differential cross sections σ 0( θ) and the vector analyzing powers iT 11( θ) for 76, 78, 80, 82Se( d , d) scattering at 12 MeV incident deuteron energy, have been analyzed using a coupled-channel Born approximation. Two-step scattering processes involving inelastic excitations and single-neutron transfer reactions, have been included in the analysis. Detailed study of these processes has been carried out in order to determine their relative strengths and their parameter dependence. Excitation of 2 1 + states is found to dominate over all other second-order processes considered here and it has a strong influence upon the elastic scattering.

Single-neutron transfer reactions 76, 78, 80, 82Se(p,d) 75, 77, 79, 81Se at EP = 33MeV

Single-neutron transfer reactions on even-even Se nuclei have been studied using 33 MeV incident proton energy from the ANU cyclograaff facility. A total of 120 levels have been seen below 4 MeV excitation energy in 75, 77, 79, 81Se isotopes. Angular distributions for 88 states were extracted and analysed with DWBA theory yielding a number of new l n assignments. Coriolis coupling calculations have been carried out for low-level spin states in all four isotopes.