Two-neutron Stripping Research Articles

Role of neutron transfer in the sub-barrier fusion cross section in O18 + Sn116

Background: In heavy-ion-induced fusion reactions, cross sections in the sub-barrier region are enhanced compared to predictions of the one-dimensional barrier penetration model. This enhancement is often understood by invoking deformation and coupling of the relative motion with low-lying inelastic states of the reaction partners. However, effects of nucleon transfer on fusion below the barrier, especially for the systems having positive $Q$ value neutron transfer (PQNT) channels, are yet to be disentangled completely.Purpose: We intend to study the role of the PQNT effect on the sub-barrier fusion of the $^{18}\mathrm{O} + ^{116}\mathrm{Sn}$ system having positive $Q$ value for the two-neutron stripping channel. Also we reflect on the interplay of couplings involved in the system around the Coulomb barrier.Method: The fusion excitation function was measured at energies from $11%$ below to $46%$ above the Coulomb barrier for $^{18}\mathrm{O} + ^{116}\mathrm{Sn}$ using a recoil mass spectrometer, viz., the Heavy-Ion Reaction Analyser (HIRA). Fusion barrier distributions were extracted from the data. Results from the experiment were analyzed within the framework of the coupled-channels model.Results: Fusion cross sections at energies below the Coulomb barrier showed strong enhancement compared to predictions of the one-dimensional barrier penetration model. The fusion process is influenced by couplings to the collective excitations with coupling to single- and two-phonon vibrational states of the target and the projectile respectively. Inclusion of the two-neutron transfer channel in the calculation along with these couplings could reproduce the data satisfactorily.Conclusions: The significant role of PQNT in enhancing the sub-barrier fusion cross section for the chosen system is not observed. It simply reduced the sub-barrier fusion cross section. Therefore, a consistent link between PQNT and sub-barrier fusion enhancement could not be established vividly while comparing the fusion excitation function from this work with the same from other $^{16,18}\mathrm{O}$-induced reactions. This clearly points to the need for more experimental as well as theoretical investigation in this field.

Strong neutron-transfer coupling effects in the reaction mechanism of the O18+Zn64 system at energies near the Coulomb barrier

The precise quasielastic excitation function for the $^{18}\mathrm{O}+^{64}\mathrm{Zn}$ system was measured at energies near and below the Coulomb barrier at ${\ensuremath{\theta}}_{\mathrm{lab}}={161}^{\ensuremath{\circ}}$, from which its correlated quasielastic barrier distribution was derived. The excitation functions for the two-neutron, $\ensuremath{\alpha}$, and tritium transfer reactions have also been measured at the same conditions. These data were well described by the coupled channel and coupled reaction channel calculations. The comparison of the data with theoretical calculations shows the strong influence of three inelastic channels in the coupling process: the quadrupole and octupole vibrational states of $^{64}\mathrm{Zn}$, ${2}_{1}^{+}$ and ${3}_{1}^{\ensuremath{-}}$, and the quadrupole vibrational state of $^{18}\mathrm{O}$, ${2}_{1}^{+}$. In addition, theoretical calculations indicate that the two-neutron stripping is mostly due to a one-step process, which has a striking effect on the reaction mechanism of this system.

Core longitudinal momentum distributions in the stripping reactions of two-neutron halo nuclei

The core longitudinal momentum distributions in two-neutron stripping reactions for halo nuclei of 20C and 22C are computed and compared to the experimental data obtained by detecting the core nucleus. The three-body wave function from the zero-range renormalized model is used as input in our calculations. We approximate the wave function of the projectile with a three-body structure, namely neutron-neutron-core, to an effective two-body one by integrating over neutron-neutron relative distance, such that the one has a core-dineutron wave function. The eikonal approximation is used in the description of the fragment-target interactions where the São Paulo optical potential is used for modeling the core-target and a Woods-Saxon potential is used for the dineutron-target interaction.

Influence of isovector pairing and particle-number projection effects on spectroscopic factors for one-pair like-particle transfer reactions in proton-rich even-even nuclei

Isovector neutron-proton (np) pairing and particle-number fluctuation effects on the spectroscopic factors (SF) corresponding to one-pair like-particle transfer reactions in proton-rich even-even nuclei are studied. With this aim, expressions of the SF corresponding to two-neutron stripping and two-proton pick-up reactions, which take into account the isovector np pairing effect, are established within the generalized BCS approach, using a schematic definition proposed by Chasman. Expressions of the same SF which strictly conserve the particle number are also established within the Sharp-BCS (SBCS) discrete projection method. In both cases, it is shown that these expressions generalize those obtained when only the pairing between like particles is considered. First, the formalism is tested within the Richardson schematic model. Second, it is applied to study even-even proton-rich nuclei using the single-particle energies of a Woods-Saxon mean-field. In both cases, it is shown that the np pairing effect and the particle-number projection effect on the SF values are important, particularly in N = Z nuclei, and must then be taken into account.

TFF (v.4.1): A Mathematica Notebook for the Calculation of One- and Two-Neutron Stripping and Pick-Up Nuclear Reactions

The program TFF calculates stripping single-particle form factors for one-neutron transfer in prior representation with appropriate perturbative treatment of recoil. Coupled equations are then integrated along a semiclassical trajectory to obtain one- and two-neutron transfer amplitudes and probabilities within first- and second-order perturbation theory. Total and differential cross-sections are then calculated by folding with a transmission function (obtained from a phenomenological imaginary absorption potential). The program description, user instructions and examples are discussed.

Experimental elastic and quasi-elastic angular distributions provide transfer probabilities

Following a similar approach suggested recently to derive breakup probabilities [Phys. Rev. C 92, 054620 (2015)], we present a simple method to derive transfer probabilities by measuring only elastic or quasi-elastic scattering for the system under investigation with the positive transfer $Q$ values and a similar system with closed transfer channels. Our estimations and transfer data for the two-neutron stripping in the $^{18}\mathrm{O}+^{206}\mathrm{Pb}$ reaction are in a reasonable agreement.

BREAKUP AND TRANSFER EFFECTS ON NEAR-BARRIER 6He + 208Pb ELASTIC SCATTERING

Breakup couplings are known to have important effects on the near-barrier elastic scattering of 6 He . However, the (6 He ,5 He ) single neutron stripping has also been found to have a significant coupling effect, although in calculations that did not include breakup. We firstly present calculations for the 6 He + 208 Pb system that include both breakup and single neutron stripping at 22 and 27 MeV to confirm that the significant stripping coupling effect persists in the presence of breakup couplings. We then compare separate calculations including transfer couplings only with recent elastic scattering data at incident 6 He energies of 14, 16, 18, 22 and 27 MeV. We also include in an approximate way the two-neutron stripping reaction in an attempt to gain some qualitative insight into the possible coupling effects of this other important near-barrier reaction channel.

Enhanced excitation of giant pairing vibrations in heavy-ion reactions induced by weakly bound projectiles

The use of radioactive ion beams is shown to offer the possibility to study collective pairing states at high excitation energy, which are not usually accessible with stable projectiles because of large energy mismatch. In the case of two-neutron stripping reactions induced by 6He, we predict a population of the giant pairing vibration in 208Pb or 116Sn with cross-sections of the order of a millibarn, dominating over the mismatched transition to the ground state.

Evidence for correlated pair transfer of valence nucleons in multiparticle stripping channels in the18O+174Ybreaction

The single and multinucleon transfer cross sections in ${}^{16}\mathrm{O},$ ${}^{18}\mathrm{O}{+}^{174}\mathrm{Yb}$ reactions were measured at ${E}_{\mathrm{lab}}=83 \mathrm{MeV}$ bombarding energy. The data were analyzed to obtain the variation of transfer cross sections with the number of nucleons transferred from the projectile to the target nucleus, and with respect to the ground state Q value ${(Q}_{\mathrm{gg}})$ of the transfer reaction. In ${}^{18}\mathrm{O}{+}^{174}\mathrm{Yb}$ reaction, the two-neutron stripping and the two-neutron correlated cluster transfer cross sections are strongly enhanced as compared to the ${}^{16}\mathrm{O}$ ${+}^{174}\mathrm{Yb}$ reaction. The results have been discussed on the basis of the possible influence of the valence neutrons in the ${}^{18}\mathrm{O}$ nucleus on multiparticle transfer cross sections. The transfer probabilities of multinucleon transfers have been analyzed as a function of distance of closest approach and the results for two-neutron transfer is consistent with the supposition of correlated pair transfer of valence nucleons in the ${}^{18}\mathrm{O}{+}^{174}\mathrm{Yb}$ reaction.

First study of the (14C, 12C) reaction

The first measurements are reported for ( 14 C, 12 C) two-neutron stripping reactions. Energy spectra up to an excitation energy of 12 MeV have been measured at 69 MeV for the 26 Mg( 14 C, 12 C) and 28 Si( 14 C, 12 C) reactions. A strong selectivity of this reaction is observed. Using this selectivity, the comparison of the spectra suggests J π assignments for several 30 Si and 28 Mg states. The results from the other two-neutron stripping reactions, (t, p) and ( 18 O, 16 O) are compared with those of the ( 14 C, 12 C) reaction.

Center-of-mass corrections to overlap factors for two-nucleon transfer

The following simple recipe is presented for making c.m. corrections to the overlap factors involved in two-nucleon transfer: Expand the shell-model overlap in products of harmonic oscillator wave functions of the relative and c.m. coordinates of the two nucleons. Change the oscillator ν-value to Aν (A + 2) in the c.m. functions, and multiply each term in the sum by [ (A + 2) A ] 1 2 N , where N is the energy quantum number associated with the c.m. state of that term. The recipe depends on the value of ν used in the expansion. The proper choice of ν is discussed, and numerical examples are presented which indicate the magnitude of the correction. The zero-range form factor for two-neutron stripping on 16O is increased by a factor of about two, or more, in the surface region, resulting in a factor of about five in the cross section. The correction is greater at larger distances, implying that heavy-ion reactions will be effected more than light ion reactions.

Observation of a shell gap at N = 152 in the 246 Cm(t, p) 248 Cm reaction

Previous reports of two-neutron stripping reactions in the actinide nuclei have not indicated any excited 0 + strength below an excitation energy of 1.5 MeV. In contrast, the 246 Cm(t,p) 248Cm reaction shows a relatively strong 0 + state at 1.084 MeV. We believe that this result, coupled with a marked change in ground-state transition strenghts between 246 Cm and 248 Cm targets, indicates a gap in the single-particle orbitals that is larger than the pairing gap.

Excitation of particle-vibration multiplets in theA=110region by two-neutron stripping reactions

The $^{103}\mathrm{Rh}$, $^{106,108}\mathrm{Pd}$, and $^{107,109}\mathrm{Ag}(t, p)$ reactions have been studied at a bombarding energy of 17 MeV. Excitation energies and enhancement factors were obtained for a number of levels up to about 2.5 MeV excitation in the residual nuclei. The $L=0$ transitions observed indicate that these nuclei are of a transitional character, though more closely resembling pairing rotational than pairing vibrational nuclei. The low-lying negative parity levels in the Ag nuclei appear to be well described as a ($2{p}_{\frac{1}{2}}$) proton weakly coupled to the appropriate core state in the even Pd cores. $^{105}\mathrm{Rh}$ appears to display similar weak coupling characteristics. However, the higher-lying levels such as the ${3}^{\ensuremath{-}}$ based states are severely mixed with single particle levels and perhaps with other elementary modes of excitation as well.NUCLEAR REACTIONS $^{103}\mathrm{Rh}$, $^{106,108}\mathrm{Pd}$, $^{107,109}\mathrm{Ag}(t, p)$, $E(t)=17.0$ MeV, magnetic spectrograph, enriched targets; measured $\ensuremath{\sigma}(\ensuremath{\theta}, {E}_{p})$, excitation energies, DWBA analysis; $^{105}\mathrm{Rh}$, $^{108,110}\mathrm{Pd}$, $^{109,111}\mathrm{Ag}$ deduced $L$, $J$, $\ensuremath{\pi}$, enhancement factors; weak coupling model.

Analyzing power in two-neutron stripping reactions

Analyzing powers have been measured for six states from the 208Pb( t, p) 210Pb reaction and seven states from the 90Zr( t, p) 92Zr reaction. These studies were performed with a beam from a new polarized triton source, and the reaction protons were detected in a Q3D spectrometer. The analyzing powers and differential cross sections are adequately described by DWBA calculations for both the medium and heavy targets up through L-transfers of 4. Above this value, some disagreement is seen.

Levels Excited inPb206with Both Two-Neutron Stripping and Pickup Reactions

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