Special features of nuclear reactions induced by loosely bound <Superscript>6</Superscript>He and <Superscript>6,7</Superscript>Li nuclei in the vicinity of the Coulomb barrier height

Excitation functions are measured for complete-fusion and transfer reactions induced by the interaction of ⁶He and ⁶Li with ²⁰⁶Pb, ²⁰⁹Bi, and Pt. Data obtained for fusion reactions induced by ⁶He ions deviate from the predictions of the statistical model of compound-nucleus decay at projectile energies in the vicinity of the Coulomb barrier height. A strong enhancement of cross sections for fusion reactions induced by the interaction of ⁶He with target nuclei is observed. The cross sections for reactions of cluster transfer, neutron transfer from ⁶He, and deuteron transfer from ⁶Li at deep-subbarrier energies are also found to be enhanced. These results are discussed from the point of view of the effect of the cluster structure of nuclei on the interaction probability at energies in the vicinity of the Coulomb barrier height.     

Near-barrier neutron transfer in reactions <Superscript>6</Superscript>He + <Superscript>45</Superscript>Sc, <Superscript>64</Superscript>Zn,and <Superscript>197</Superscript>Au

Experimental cross sections of formation of isotopes ⁴⁶Sc (in reaction ⁶He + ⁴⁵Sc), ^196,198Au (in reaction ⁶He + ¹⁹⁷Au), and ⁶⁵Zn (in reaction ⁶He + ⁶⁴Zn) are analyzed. The time-dependent Schrödinger equation for the outer neutrons of ⁶He and ¹⁹⁷Au nuclei is solved numerically to calculate the probability of neutron transfer and transfer cross sections. In reaction ⁶He + ¹⁹⁷Au, the contribution of fusion and subsequent evaporation to experimental data can be neglected, while the corresponding contributions to reactions ⁶He + ⁴⁵Sc and ⁶He + ⁶⁴Zn are considerable. Fusion–evaporation is taken into account using the computational code of the NRV knowledge base. The results of calculations are in satisfactory agreement with the experimental data.     

Momentum distributions of <Superscript>4</Superscript>He nuclei from the <Superscript>6</Superscript>He and <Superscript>6</Superscript>Li breakup

Experimental data on the momentum distributions of ⁴He nuclei originating from ⁶He and ⁶Li breakup on various targets are presented over a wide beam energy range. The experiment with ⁶He was performed at the DRIBs accelerator complex for radioactive beams at the Joint Institute for Nuclear Research (JINR, Dubna). The intensity of the ⁶He beam used was 5 × 10⁶ particles per second and its energy was 10 MeV per nucleon. The momentum distributions of breakup products were measured by means of the MSP-144 magnetic spectrometer. The distribution width was shown to be virtually independent of the target mass. A small value of this width, σ ～ 28 MeV/c, confirms the presence of a halo in ⁶He. The measurements performed with ⁶Li beams of energy 18 and 46 MeV per nucleon at the U-400M accelerator yielded a width value of σ ～ 50 MeV/c for the momentum distributions of ⁴He nuclei, which is intermediate between that for ⁶He and those for stable nuclei. A compilation of the widths of the momentum distributions of fragments originating from the breakup of various nuclei is presented versus the binding energy of one or two neutrons in these nuclei, the target mass and the projectile energy.     

Investigation of the <Superscript>6</Superscript>He structure in quasi free scattering reactions

Momentum correlations inherent to the ⁶He constituents in the ground state of this nucleus were studied in the quasi-free scattering (QFS) reaction ⁴He(⁶He, 2α)2n at ⁶He beam energy of 25 MeV/nucleon. A detailed study of nucleus structure was performed for the first time in QFS reactions with an unbound spectator. The Plane Wave Impulse Approximation was used in analyzing the experimental data. It was shown that the experimental data are described by model calculations in which the two neutron final state interaction is taken into account. t + t and t + d + n configurations were also studied in the ⁴He(⁶He, tα)t and ⁴He(⁶He, tα)dn QFS reactions.     

Effect of cluster polarization on the spectrum of the <Superscript>7</Superscript>Li nucleus and on the reaction <Superscript>6</Superscript>Li(<Emphasis Type="Italic">n</Emphasis>, <Superscript>3</Superscript>H)<Superscript>4</Superscript>He

The effect of polarization of the ⁶Li and ³H clusters on the parameters of states of the ⁷Li nucleus and on the cross section for the reaction ⁶Li(n,³H)⁴He was studied. The ⁴He + d + n three-cluster configuration was invoked for this purpose, and cluster dynamics was determined within a microscopic model where the relative motion of the clusters was described in terms of Faddeev amplitudes. The ⁴He + d + n three-cluster configuration made it possible to consider the ⁴He + ³H and ⁶Li + n binary channels, which are dominant in ⁷Li, and to take simultaneously into account the cluster polarization of ⁶Li as a two-cluster subsystemin the ⁴He + d representation and the cluster polarization of ³Has a two-cluster subsystem in the d + n representation.     

Possibility of<Superscript>5</Superscript>He emission in neutron induced reactions

The unstable nucleus⁵He emission has never been included in the widely used statistical model codes as the evaluation tool and interpretation experimental data. The calculated threshold energies of⁵He emission from light nuclei to heavy nuclei indicate that in most cases the compound nucleus induced by incident neutron would emit⁵He rather than³He. Since⁵He is unstable and can be separated into n+α spontaneously, so neutron is produced in⁵He emission process. The formulation of the double-differential cross section of the neutron from the two-body breakup process of emitted⁵He is established. Because of the strong recoil effect, the energy balance is strictly taken into account to meet the needs in nuclear engineering. Further improvement of the statistical model calculation codes on this respect is proposed. It is expected that the correlative measurement will be available to account the outgoing neutron and alpha particle simultaneously and to test and verify the existence of⁵He emission.     

Study of fusion and nucleon transfer channels in the <Superscript>197</Superscript>Au + <Superscript>6</Superscript>He reaction in an energy range of <Superscript>6</Superscript>He to 20 Mev/A

New data on the cross sections of reactions occurring during the interaction of ⁶He nuclei with ¹⁹⁷Au at energies of ⁶He from 40 to 120 MeV are presented. The experiments were performed in the ACCULINNA secondary beam separator of the Laboratory of Nuclear Reactions, Joint Institute for Nuclear Research. To identify the reaction products, the activation method for measuring the gamma activity of the target assembly of thin foils was used. The excitation functions for fusion reactions involving the evaporation of up to ten neutrons from the compound nucleus as well as reactions with the emission of charged particles and nucleon transfer in the investigated energy region are obtained. Data analysis was carried out using two codes: ALICE-MP and NRV. The cross sections for the (⁶He, xn) reactions occurring through the compound nucleus are mostly in agreement with the results of model calculations based on the statistical approach. It is shown that, up to the energies of 114 MeV, the cross-section drop in the complete fusion reactions is negligible. The experimental excitation functions of reactions leading to the formation of isotopes of mercury and gold (transfer reaction) indicate that the main contribution to their formation is made by direct processes and that evaporation reactions (⁶He, pxn) and (⁶He, αxn) play a minor role, as is evidenced by a comparison of the measured cross sections with the calculation results.     

Diffractive dissociation of deuterons and exotic nuclei <Superscript>6</Superscript>He and <Superscript>19</Superscript>C

A theory is developed for describing the diffractive dissociation of loosely bound two-cluster nuclei in the nuclear and Coulomb fields of nuclei having a diffuse boundary. The energy spectra of charged products of the breakup of ²H, ⁶He, and ¹⁹C nuclei are calculated on the basis of the proposed approach, and the results are found to be rather sensitive to nuclear structure. For some angles of neutron and proton emission from the reaction ¹²C(d, np)¹²C, the calculated cross sections are in satisfactory agreement with the results of kinematically complete experiments performed recently to study the breakup of intermediate-energy deuterons.     

Phase shift analysis of the <Superscript>6</Superscript>He+<Superscript>208</Superscript>Pb and the <Superscript>6</Superscript>He+<Superscript>197</Superscript>Au systems

We present the elastic scattering of the ⁶He+²⁰⁸Pb and the ⁶He+¹⁹⁷Au systems at the laboratory energy of E _lab=27 MeV within the framework of the McIntyre parametrization, and systematically investigate χ ²/N analysis of both systems to obtain an excellent agreement between the theoretical results and the experimental data. We find large diffusivity parameters indicating long range absorption mechanisms. We also show that both systems lack both the nuclear and the Coulomb rainbow scattering for obtained S-matrix parameters.     

Structure of the <Superscript>6</Superscript>He nucleus in the three-particle model

The properties of the ⁶He nucleus were investigated in a three-particle approximation (a alpha-particle cluster plus two neutrons) on the basis of the variational approach by using a Gaussian basis. For nn and nα interactions, potentials that make it possible to describe S-wave phase shifts for elastic scattering and, simultaneously, to reproduce faithfully the energy and the size of the ⁶He nucleus were proposed. Characteristic structural features of this nucleus that are manifested in the density distributions, elastic form factor, pair correlation functions, and momentum distributions of particles constituting the ⁶He nucleus in the three-particle model were revealed.     

Dissociation of deuteron, <Superscript>6</Superscript>He and <Superscript>11</Superscript>Be from Coulomb dissociation reaction cross-section

The fragmentation of deuteron, ⁶He and ¹¹Be have been studied during interaction with the ²⁰⁸Pb nucleus at various projectile energies. The Coulomb dissociation cross-sections and the momentum distribution of the break-up fragments have been analysed within the framework of the direct fragmentation model. The post-acceleration effect of deuteron during break-up and the halo structures of both the ⁶He and ¹¹Be have been investigated.      

Cluster radioactivity leading to doubly magic <Superscript>100</Superscript>Sn and <Superscript>132</Superscript>Sn daughters

Decay of neutron-deficient ^{128 − 137}Gd parents emitting ⁴He to ³²S clusters are studied within the Coulomb and proximity potential model. The predicted half-lives are compared with other models and most of the values are well within the present experimental limit for measurements (T _1/2 < 10³⁰ s). The lowest T _1/2 value for ²⁸Si emission from ¹²⁸Gd indicates the role of doubly magic ¹⁰⁰Sn daughter in cluster decay process. It is also found that neutron excess in the parent nuclei slows down the cluster decay process. Geiger–Nuttal plots for all clusters are found to be linear with different slopes and intercepts. The α-decay half-lives of ^{148 − 152}Gd parents are computed and are in agreement with experimental data. The role of doubly magic ¹³²Sn daughter in cluster decay process is also examined for various neutron-rich Ba, Ce, Nd, Sm and Gd parents emitting clusters ranging from ⁴He to ³²Si. Alpha-like structures are most probable in the decays leading to ¹⁰⁰Sn, while non-α-like structures are probable in the decays leading to ¹³²Sn. The neutron–proton asymmetry in parent and daughter nuclei is responsible for the reduced decay rate in the decay leading to ¹³²Sn.     

Influence of the transfer reactions on p + <Superscript>6</Superscript>He 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.     

Q-value for the Fermi beta-decay of <Superscript>46</Superscript>V

By comparing the Q -values for the ⁴⁶Ti ( ³He , t ⁴⁶V and ⁴⁷Ti ( ³He , t ⁴⁷V reactions to the isobaric analogue states the Q -value for the superallowed Fermi decay of ⁴⁶V has been determined as Q _EC( ^46V ) = (7052.11±0.27) keV. The result is compatible with the values from two recent direct mass measurements but is at variance with the previously most precise reaction Q -value. As additional input quantity we have determined the neutron separation energy S _n( ^47Ti ) = (8880.51±0.25) keV.     

High-lying excited states in <Superscript>10</Superscript>Be from the <Superscript>9</Superscript>Be(<Superscript>9</Superscript>Be,<Superscript>10</Superscript>Be)<Superscript>8</Superscript>Be reaction

A transfer-reaction experiment of ⁹Be(⁹Be, ¹⁰Be)⁸Be was performed at a beam energy of 45 MeV. Excited states in ¹⁰Be up to 18.80 MeV are produced using missing mass and invariant mass methods. Most of the observed high-lying resonant states, reconstructed from the α + ⁶He and t + ⁷Li decay channels, agree with the previously reported results. In addition, two new resonances at 15.6 and 18.8 MeV are identified from the present measurement. The 18.55 MeV state is found to decay into both the t + ⁷Li_g.s. and t + ⁷Li* (0.478 MeV) channels, with a relative branching ratio of 0.93 ± 0.33. Further theoretical investigations are encouraged to interpret this new information on cluster structure in neutron-rich light nuclei.     

Peculiarities in total cross sections of reactions with weakly bound nuclei <Superscript>6</Superscript>He, <Superscript>9</Superscript>Li

The energy dependence of the total cross sections for the ⁶He + Si and ⁹Li + Si reactions was measured at beam energies between 5 and 20 MeV per nucleon. The results agree with experimental data published for the ⁶He + Si reaction. New data are obtained for the ⁹Li + Si reaction in the vicinity of a local enhancement of the total cross section. A theoretical analysis of the possible reasons behind the appearance of this peculiarity in the case of collisions of ⁶He and ⁹Li nuclei with silicon target nuclei is performed. In particular, the enhancement may owe its origin to the effect of loosely bound projectile nucleons.     

Unexpected transparency in the scattering of fragile <Superscript>6</Superscript>Li and <Superscript>6</Superscript>He Nuclei

It is found that the scattering of the fragile nucleus ⁶Li from ¹²C and ¹⁶O is unexpectedly transparent. It is shown that the internal-wave contribution is significantly large in the scattering, which suggests that some transparency could persist in the scattering involving the fragile nucleus ⁶He.     

Semiclassical description of the <Superscript>6</Superscript>He+<Superscript>12</Superscript>C elastic scattering

The results of the elastic scattering of ⁶He+¹²C systemat E _Lab = 18 MeV by using the barrier and internal wave decomposition of the S-matrix element within the framework of the WKB method are presented. This is the first detailed study for the interaction of the exotic ⁶He nucleus on different stable nuclei by using a semiclassicalmethod. In this paper, we show that in order to obtain the elastic scattering cross section of the ⁶He+¹²C systemat energies close to the Coulomb barrier, it is vitally important to take into account the inner complex turning points in the calculations and the tunneling effects play a crucial role to explain the experimental data. The semiclassical results are compared with the experimental data as well as the quantum-mechanical one.     

Evidence for a K<Superscript>π</Superscript> = 1/2<Superscript>+</Superscript> isomer in neutron-rich <Superscript>185</Superscript>Ta

In this paper we present for the first time the measured delayed neutron (DN) yields and time spectra from high-energy protons interacting with thick ^natPb targets. The 1GeV protons from the accelerator impinged on targets of different thicknesses producing a huge number of spallation-fission products, some of which can be DN precursors. After the beam is switched off, the DNs were detected with optimized ³He counter. The production yields of light DN precursors as ¹⁷N and “usual” fission products as ⁸⁷Br and ⁸⁸Br, which dominate the total DN activity, are obtained both for thin and thick targets. These new data are of great interest for the new generation high-power spallation targets based on liquid-metal technologies. Our findings also should help to constrain the physics models within the simulations codes.     

Puzzle of the <Superscript>6</Superscript>Li quadrupole moment: Steps toward solving it

The problem of the origin of the quadrupole deformation in the ⁶Li ground state is investigated with allowance for the three-deuteron component of the ⁶Li wave function. Two long-standing puzzles related to the tensor interaction in the ⁶Li nucleus are known: that of an anomalous smallness of the ⁶Li quadrupole moment (being negative, it is smaller in magnitude than the ⁷Li quadrupole moment by a factor of 5) and that of an anomalous behavior of the tensor analyzing power T_2q in the scattering of polarized ⁶Li nuclei on various targets. It is shown that a large (in magnitude) negative exchange contribution to the ⁶Li quadrupole moment from the three-deuteron configuration cancels almost completely the “direct” positive contribution due to the αd folding potential. As a result, the total quadrupole moment proves to be close to zero and highly sensitive to fine details of the tensor nucleon-nucleon interaction in the ⁴He nucleus and of its wave function.