Neutron halo in 8He

The structure of ⁸He is investigated within a three-cluster microscopic model. The threecluster configuration α+² n+₂ n is used to describe the properties of the ground state of this nucleus. The results obtained evidently indicate the existence of a neutron halo in ⁸He.     

Soft dipole mode in 8He

The low-lying spectrum of ⁸He was studied in the ³H(⁶He, p)⁸He transfer reaction for small center of mass angles. The 0⁺ ground state (g.s.) of ⁸He and excited states, 2⁺ at 3.6–3.9 MeV and (1⁺) at 5.3–5.5 MeV, were populated with cross sections of 200, 100–250, and 90–125 μb/sr. Some evidence for the excited state at about 7.5 MeV can be found in the data. The possible nature of the near-threshold anomaly above 2.14 MeV in ⁸He is related to the population of a 1⁻ continuum (soft dipole excitation) with a peak energy value at about 3 MeV. This assumption can probably resolve the problem of a large uncertainty existing in the experimental data on the ⁸He 2⁺ state energy. The article is published in the original.     

The beta-decay of 8He

A many-level, many-channel R-matrix formula is used to fit the measured ⁸He β-decay data, including the half-life, branching ratios and delayed neutron and triton energy spectra. Satisfactory fits are found with contributions from four 1⁺ levels of ⁸Li, at 0.98, 3.08, 5.15 and 9.06 MeV. The 32 reduced width amplitudes are taken from a shell-model calculation, with only two requiring adjustment. Gamow-Teller matrix elements obtained from the fits are in reasonable agreement with shell-model values.     

Nuclear charge radius of 8He

The root-mean-square (rms) nuclear charge radius of 8He, the most neutron-rich of all particle-stable nuclei, has been determined for the first time to be 1.93(3) fm. In addition, the rms charge radius of 6He was measured to be 2.068(11) fm, in excellent agreement with a previous result. The significant reduction in charge radius from 6He to 8He is an indication of the change in the correlations of the excess neutrons and is consistent with the 8He neutron halo structure. The experiment was based on laser spectroscopy of individual helium atoms cooled and confined in a magneto-optical trap. Charge radii were extracted from the measured isotope shifts with the help of precision atomic theory calculations.     

8He Nuclei Stopped in Nuclear Track Emulsion

A nuclear track emulsion (NTE) is exposed to 60 MeV ⁸He nuclei. Measurements of decays of ⁸He nuclei stopped in NTE allow one to evaluate possibilities of α-spectrometry. Thermal drift of ⁸He atoms is observed. Knowledge of the energy and emission angles of α-particles allows one to derive the energy distribution of α-decays Q_2α . The presence of a “tail” of large values Q_2α is established.     

Spectroscopy of excited states of 8He

The ¹⁰Be(¹²C,¹⁴O)⁸He-reaction has been used to study the levels of ⁸He. In addition to the known excited state of ⁸He at E_x = 3.6 MeV, with J^π = 2⁺, three additional states were found at excitation energies of 4.54(25) MeV, 6.03(10) MeV and 7.16(4) MeV.     

Beta-delayed triton emission in the decay of 8He

A β-delayed triton branch with an intensity of (0.9 ± 0.1)% has been observed in the decay of ⁸He. The triton energy spectrum was measured with a specially designed ΔE − E telescope. These data as well as an earlier measured β-delayed neutron spectrum cannot be explained from the known levels in ⁸Li, but it turns out that the main features of both the delayed triton and the delayed neutron spectra from ⁸He could be understood if one assumes a new state in ⁸Li at about 8.8 MeV excitation energy.     

Formation of dislocation loops during He clustering in bcc Fe

The clustering of helium in bcc (body centered cubic) iron and the growth of a helium bubble are simulated at the atomistic level for the helium-rich vacancy-poor condition. It is shown that a ? 111 dislocation loop is formed as a sequential collection of 111 crowdions, the latter being the most stable self-interstitial atom configuration in the presence of a He cluster.     

Direct reactions involving exotic nuclei 8He and 5H

The angular distribution of 26 MeV/n ⁸He ions elastically scattered from a gaseous helium target was measured in a wide CM angular range. Results are discussed in terms of the potential scattering and neutron transfer. The angular distributions of elastic scattering as well as 1n and 2n transfer reactions of these ions on protons are presented. Finite-range DWBA calculations made for the 2n transfer reaction leading to the ground (0⁺) and excited (2⁺) states of ⁶He underestimate the cross section for 2n transfer to ⁶He(0⁺). Analysis of data for this reaction channel suggests the importance of a direct ⁵H-cluster exchange process. A resonance state of ⁵H with an energy of 2 MeV above the decay threshold n+n+ ³H was obtained for the first time by making use of the reaction p(⁶He,²He)⁵H.     

Scattering of radioactive nuclei 6He and 3H by protons: Effects of neutron skin and halo in 6He, 8He,and 11Li

In an experimental study of ⁶He+p and ³H+p collisions at ∼70 A MeV, the elastic scattering angular distributions were measured and the known excited state ⁶He_1.8¹ was observed. Comparative analysis of existing experimental data on proton elastic scattering by ⁴He, ⁶He, ⁶Li, ⁸He, ⁹Li, and ¹¹Li was performed. Effects of valence neutrons were investigated using the eikonal approach. A difference between ¹¹Li and ^6,8He was found.     

Dineutron configurations in neutron-excess nuclei from the viewpoint of the diffraction model

Cross sections measured for the interactions of incident neutron-excess nuclei ⁶He and ¹¹Li with other nuclei are analyzed in terms of expressions obtained for relevant integrated cross sections within the diffraction approximation. The results of this analysis suggest that there exists a dineutron configuration in the ⁶He nucleus and that there is no such configuration in the ¹¹Li nucleus. On the basis of calculations performed for various observables, some new specific experiments are proposed for discovering dineutrons more reliably in nuclei having a neutron halo.     

核子与镜象核~3H和~3He弹性散射中电荷对称破环效应

采用微观的动量空间光学势及R.Crespo和J．A．Tostevin建议的处理动量空间库仑作用的方法,计算了500MeV的核子与镜象核3H和3He弹性散射微分截面,截面比r1,r2和超比R．理论计算结果r1,r2和R均不等于1,显示出由于p-核库仑作用直接引起的电荷对称破坏效应．     

Formation of a direct Kolmogorov-like cascade of second-sound waves in He II

Based on measurements of nonlinear second-sound resonances in a high-quality resonator, we have observed a steady-state wave energy cascade in He II involving a flux of energy through the spectral range towards high frequencies. We show that the energy balance in the wave system is nonlocal in K space and that the frequency scales of energy pumping and dissipation are widely separated. The wave amplitude distribution follows a power law over a wide range of frequencies. Numerical computations yield results in agreement with the experimental observations. We suggest that second-sound cascades of this kind may be useful for model studies of acoustic turbulence.     

Formation Cross Sections of He+ Ions in Specific Electronic States in Collisions of He2+ Ions with Hydrogen Atoms

Technical Physics - The method of collision spectroscopy, which is based on precision measurement of the kinetic energy of projectile ions after the electron capture process, is used to measure the...     

Direct mass measurement of the four-neutron halo nuclide 8He

A high-precision Penning trap mass measurement of the exotic 8He nuclide (T(1/2)=119 ms) has been carried out resulting in a reduction of the uncertainty of the halo binding energy by over an order of magnitude. The new mass, determined with a relative uncertainty of 9.2 x 10(-8) (deltam=690 eV) is 13 keV less bound than the previously accepted value. The mass measurement is of great relevance for the recent charge-radius measurement of 8He [P. Mueller, Phys. Rev. Lett. 99, 252501 (2007).10.1103/PhysRevLett.99.252501]. The 8He mass is the first result from the newly-commissioned Penning trap: TITAN (TRIUMF's Ion Trap for Atomic and Nuclear science) at the ISAC (Isotope Separator and Accelerator) radioactive beam facility at TRIUMF.     

First direct mass measurement of the two-neutron halo nucleus 6He and improved mass for the four-neutron halo 8He

The first direct mass measurement of {6}He has been performed with the TITAN Penning trap mass spectrometer at the ISAC facility. In addition, the mass of {8}He was determined with improved precision over our previous measurement. The obtained masses are m({6}He)=6.018?885?883(57) u and m({8}He)=8.033?934?44(11) u. The {6}He value shows a deviation from the literature of 4σ. With these new mass values and the previously measured atomic isotope shifts we obtain charge radii of 2.060(8) and 1.959(16) fm for {6}He and {8}He, respectively. We present a detailed comparison to nuclear theory for {6}He, including new hyperspherical harmonics results. A correlation plot of the point-proton radius with the two-neutron separation energy demonstrates clearly the importance of three-nucleon forces.