3He Nuclei Research Articles

Polarization in quasielastic (p,2p) scattering on a 4He nucleus at 1 GeV

The polarization of secondary protons from the (p, 2p) reaction induced by 1-GeV protons incident to a {sup 4}He nucleus was measured in a kinematically complete experiment. By using a two-arm magnetic spectrometer, two secondary protons from this reaction were recorded in coincidence at unequal scattering angles of {theta}{sub 1} = 18{sup o}-24.21{sup o} and {theta}{sub 2} = 53.22{sup o} over a broad range of the recoil-nucleus momentum K{sub B} between 0 and 150 MeV/c. It was found that the measured polarization of either secondary proton is less than that observed in free elastic proton-proton scattering. The magnitude of this difference is determined by the mean binding energy of s-shell protons rather than by the effective nuclear density. The polarizations measured in inclusive quasielastic and elastic scattering of 1-GeV protons on {sup 4}He nuclei are presented for scattering angles in the range {theta}{sub 1} = 18{sup o} - 24.21{sup o}.

The one-body and two-body density matrices of finite nuclei with an appropriate treatment of the center-of-mass motion⋆

The one-body and two-body density matrices in coordinate space and their Fourier transforms in momentum space are studied for a nucleus (a nonrelativistic, self-bound finite system). Unlike the usual procedure, suitable for infinite or externally bound systems, they are determined as expectation values of appropriate intrinsic operators, dependent on the relative coordinates and momenta (Jacobi variables) and acting on intrinsic wavefunctions of nuclear states. Thus, translational invariance (TI) is respected. When handling such intrinsic quantities, we use an algebraic technique based upon the Cartesian representation, in which the coordinate and momentum operators are linear combinations of the creation and annihilation operators a^+ and a for oscillator quanta. Each of the relevant multiplicative operators can then be reduced to the form: one exponential of the set {a^+} times other exponential of the set {a}. In the course of such a normal-ordering procedure we offer a fresh look at the appearance of Tassie-Barker factors, and point out other model-independent results. The intrinsic wavefunction of the nucleus in its ground state is constructed from a nontranslationally-invariant (nTI) one via existing projection techniques. As an illustration, the one-body and two-body momentum distributions (MDs) for the 4He nucleus are calculated with the Slater determinant of the harmonic-oscillator model as the trial, nTI wavefunction. We find that the TI introduces important effects in the MDs.

Quark exchange and valence quark distributions in [formula omitted] mirror nuclei

The quark-exchange formalism for nuclear systems is reformulated to treat the proton, neutron and up and down quarks explicitly and it is used to calculate the structure functions of A = 3 mirror nuclei, i.e., 3He and 3H nuclei. The solution of Faddeev equations is used as the three nucleon system wave function and the initial valence quark input are taken from the GRV's (Glück, Reya and Vogt) next-to-leading order QCD calculations on F 2 p ( x , Q 2 ) which give very good fits to the available data in the ( x , Q 2 ) -plane. The momentum distributions of 3He, 3H and isoscalar A = 3 systems as well as up and down quarks are discussed. It is shown that the quark-exchange has a sizable contribution to the structure functions of A = 3 systems. So it is concluded that the nuclear structure effect is still important in 3He and 3H nuclei and the spectral functions based on the naive impulse approximation are not enough to extract the neutron structure function from these nuclei. These phenomena can be tested by the future deep inelastic electron experiments on 3He and 3H nuclei, especially those expected from the proposed 11 GeV Jefferson Laboratory.

Study of the halo nucleus 6He through the direct nuclear reactions

Differential cross sections for quasi-elastic scattering, breakup reaction as well as 1n- and 2n-transfer reactions induced by 6He at 25 MeV/nucleon from a 9Be target are reported. The experimental data were analysed in the context of the coupled-channel calculations. The internal structure of 6He is discussed and further studies are proposed.

Puzzle of the 6Li 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 T2q 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.

The effects of nuclear structure on generalized parton distributions of 3He

The effect of the nuclear medium on generalized parton distributions (GPDs) is studied for the 3He nucleus, through a realistic microscopic analysis. In Impulse Approximation, Fermi motion and binding effects, evaluated by modern potentials, are found to be larger than in the forward case and very sensitive to the details of nuclear structure at short distances.

Isotope shifts and nuclear radius measurements for helium and lithium

It is now well established that accurate relative values of the rms nuclear charge radius for light atoms can be extracted from a comparison between high-precision theory and experiment for the isotope shift in atomic transition frequencies. Results are available for isotopes of helium and lithium. This paper reviews and updates the interpretation of earlier measurements for 3He relative to 4He, and 6Li relative to 7Li. New results are presented for the quantum electrodynamic recoil corrections. Recent measurements for the halo nucleus 6He, and the short-lived nuclei 8Li, and 9Li are discussed. The derived nuclear charge radii of 2.054(14), 2.30(4), and 2.24(4)~fm, respectively, are compared with the predictions of various nuclear structure models.PACS Nos.: 31.15.Pf, 31.30.Jv, and 32.10.Hq

Q2 evolution of 3He spin structure functions moments

Sum rules offer the opportunity to test our understanding of the structure of a system on general grounds. In this paper we investigate the 3He nucleus using the Gerasimov-Drell-Hearn and Burkhardt-Cottingham sum rules. We have previously used the polarized 3He target measurements to extract the neutron spin structure, our goal here is to investigate rather the 3He nucleus. We have determined for the first time the Q2 evolution of $\Gamma_1(Q^2)=\int_0^{1} g_1(x,Q^2) dx$ and $\Gamma_2(Q^2)=\int_0^1 g_2(x,Q^2) dx$ for 3He in the range 0.1 GeV2 ≤ Q2 ≤ 0.9 GeV2 with good precision. Γ1(Q2) displays a smooth variation from high to low Q2. However, we do not see yet its expected turnover toward the real photon limit value. On the other hand the Burkhardt-Cottingham sum rule holds at all measured Q2s within uncertainties due mainly to cancellations between the elastic, quasielastic and the Δ(1232) resonance contributions.

Development of 3He polarized neutron spin filters at KEK

Development of neutron spin filters based on polarized gaseous 3He is underway at KEK. Spin-exchange optical pumping is employed to polarized 3He nuclei. Quartz glass cells and sapphire cells are studied as 3He containers. Spin relaxation times of more than 200 h have been observed for quartz glass cells with 3He pressures of 3 atm. Using a pulsed neutron beam, polarizations of 3He were measured for a quartz glass cell and a sapphire cell with 3He pressures of 3 atm to be 54±2% and 63±1%, respectively.

Quasielastic Scattering of the Halo Nucleus 6He at25 MeV/u from a 9Be Target

The differential cross sections of quasielastic scattering of a25 MeV/u 6He from 9Be target have been measured. Thedouble-folding model approach is applied to generate the real partof the optical potential. The imaginary potential parameters as wellas some of the real potential parameters are studied in comparisonwith the experimental data. The effect of the unstable nucleus isdiscussed.

No enhancement of fusion probability by the neutron halo of 6He.

Quantum tunnelling through a potential barrier (such as occurs in nuclear fusion) is very sensitive to the detailed structure of the system and its intrinsic degrees of freedom. A strong increase of the fusion probability has been observed for heavy deformed nuclei. In light exotic nuclei such as 6He, 11Li and 11Be (termed 'halo' nuclei), the neutron matter extends much further than the usual nuclear interaction scale. However, understanding the effect of the neutron halo on fusion has been controversial--it could induce a large enhancement of fusion, but alternatively the weak binding energy of the nuclei could inhibit the process. Other reaction channels known as direct processes (usually negligible for ordinary nuclei) are also important: for example, a fragment of the halo nucleus could transfer to the target nucleus through a diminished potential barrier. Here we study the reactions of the halo nucleus 6He with a 238U target, at energies near the fusion barrier. Most of these reactions lead to fission of the system, which we use as an experimental signature to identify the contribution of the fusion and transfer channels to the total cross-section. At energies below the fusion barrier, we find no evidence for a substantial enhancement of fusion. Rather, the (large) fission yield is due to a two-neutron transfer reaction, with other direct processes possibly also involved.

Fusion, reaction and break-up cross sections of weakly bound projectiles on 64Zn

We present new measurements and a general discussion of the behavior of the fusion, break-up and reaction cross sections of different projectiles on the same target 64Zn, at near and above barrier energies. The projectiles are the tightly bound 16O, the stable weakly bound 6Li, 7Li and 9Be and the radioactive very weakly bound 6He nuclei. We also compare the results with the ones for heavier targets.

Polarized 3He 2+ ion source based on spin-exchange collisions between 3He + ion and polarized Rb atoms

A proposal is presented on the polarized 3He ion source based on the spin-exchange collisions of the low-energy 3He + ion with polarized alkali-metal atoms (Rb) which are optically pumped under the strong magnetic field. To transfer the electron polarization of 3He + ion to the 3He nucleus, we offer to use the Sona transition based on a non-adiabatic transition method. For efficiently ionizing 3He + ion to form 3He 2+ ion, 3He + ion is stripped with a gas/foil stripper after accelerating 3He +. As a compact ionizer for polarized 3He + ions, an EBIS (Electron Beam Ion Source) ionizer operated with a high magnetic field is also proposed.

Dressed bosons theory for nuclear structure

The structure of nuclei of even number of nucleons is investigated in the boson-dynamic correlation model where valence-particle-excitation as well as core-excitation modes coexist. The model is nonperturbative and does not employ effective operators. The eigenstates of the model are the self-consistent solution of a set of nonlinear dynamical equations, which represents valence-particle pairs dressed by their interactions with the nuclear medium. The model is applied to medium mass as well as light nuclei. Our study of 18O reveals that the inclusion and the nonperturbative treatment of core excitations are the principal reasons for obtaining a correct ordering of the energy levels, a more complete spectrum and a much better agreement with the experimental low-lying as well as high-lying states than other published calculations. Our study of light nuclei 6He and 6Li shows that the halo formation in these nuclei is also a consequence of the excitation of the core protons and that halo can also be associated with valence proton–neutron pairs. The calculated matter distributions of these two nuclei reproduce well the differential cross sections obtained in proton elastic scattering experiments. The cluster-factorization method that expedites nonperturbative calculations is elucidated.

Nuclear spin-lattice relaxation in finely dispersed carbonizate powders

Finely dispersed carbonizate powders were studied with the aim of revealing their suitability for producing hyperpolarized noble gases. In the temperature and frequency dependences obtained over a wide range of temperatures and magnetic fields for the spin-lattice relaxation times of the magnetic moments of 3He, 1H, and 13C nuclei, anomalous features caused by the suppression of the exchange between surface paramagnetic centers in a magnetic field were observed. It is shown that the interaction with magnetic moments of the 1H nuclei situated near the paramagnetic centers is the main polarization-leakage channel for the noble-gas nuclear spins.

Studies on optical pumping cells (OPC) to polarize 3He

The technique applied at HMI to obtain nuclear-spin-polarized 3He, used in neutron spin filters (NSFs), is metastability-exchange optical pumping. To prepare efficient NSF, one must highly polarize 3He nuclei in the optical pumping volume (OPV) and reduce the polarization losses during the compression phase. Great progress has been achieved in reducing of depolarization due to the recent development of both, large polarization preserving piston compressors and long relaxation time filter cells. It is even more important to significantly enhance the 3He polarization rate during optical pumping in order to increase NSF efficiency. Different cells materials were tested, such as Duran and quartz glass. In order to use the laser light more efficiently and to decrease the risk of 3He depolarization due to unfavorable reflections, antireflection (AR) coatings were used on cell windows made of quartz glass. They were compared with the ones without coating, made of quartz, Duran and BK7 glass. The comparison of various techniques to mount the windows such as blowing, gluing or molecular diffusion was also conducted. It indicated that the molecular diffusion is the most suitable technique because of a better purity of the gas in the cell and the preservation of the optical flatness of the windows. Cells, for practical reasons each entirely made from the same material (Duran, Quartz glass) with windows mounted using this method, showed the best polarization performance.

Photodisintegration of Light Nuclei

The first microscopic calculation of inelastic reactions on nuclei with mass number A = 6 is presented. In particular, we consider the total photoabsorption cross section of 6He and 6Li. It is shown that the halo nucleus 6He exhibits two types of dipole resonances, the normal giant dipole resonance and a low-energy soft dipole resonance associated with the excitation of the outer neutrons. Comparing our results with available experimental data we note the role of P-wave interaction. Calculations are performed using the Lorentz integral transform (LIT) method and the effective interaction in the hyperspherical harmonics formalism (EIHH).

Unbound exotic nuclei studied by transfer to the continuum reactions

In this paper we show that the theory of transfer reactions from bound to continuum states is well suited to extract structure information from data obtained by performing “spectroscopy in the continuum”. The low energy unbound states of nuclei such as 10Li and 5He can be analyzed and the neutron–core interaction, necessary to describe the corresponding borromean nuclei 11Li and 6He can be determined in a semi-phenomenological way. An application to the study of 10Li is then discussed and it is shown that the scattering length for s-states at threshold can be obtained from the ratio of experimental and theoretical cross sections. The scattering single particle states of the system n+ 9Li are obtained in a potential model. The corresponding S-matrix is used to calculate the transfer cross section as a function of the neutron continuum energy with respect to 9Li. Three different reactions are calculated 9Li(d,p) 10Li, 9Li( 9Be, 8Be) 10Li, 9Li( 13C, 12C) 10Li, to check the sensitivity of the results to the target used and in particular to the transfer matching conditions. Thus the sensitivity of the structure information extracted from experimental data on the reaction mechanism is assessed.

Loosely bound three-body nuclear systems in the J-matrix approach

We discuss the extension of the oscillator-basis J-matrix formalism on the case of true A-body scattering. The formalism is applied to loosely bound 11Li and 6He nuclei within three-body cluster models 9 Li+n+n and α+ n+ n. The J-matrix formalism is used not only for the calculation of the three-body continuum spectrum wave functions but also for the calculation of the S-matrix poles associated with the 11Li and 6He ground states to improve the description of the binding energies and ground state properties. The effect of the phase equivalent transformation of the n– α interaction on the properties of the 6He nucleus is examined.

Radioactive ion beam research made in Dubna

Operational parameters are given for ACCULINNA set-up used in experiments with 20–40 MeV secondary beams of 6He and 8He nuclei. The lay-out of DRIBs and the results of the 6He beam acceleration made at this new ISOL type facility are presented. The main directions of recent experimental studies made at ACCULINNA are briefly discussed.