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.     

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.     

Precise mass measurement of 68Se, a waiting-point nuclide along the rp process

Mass measurements of 68Ge, 68As, and 68Se have been obtained with the Canadian Penning Trap mass spectrometer. The results determine the mass excess of 68Se as -54 232(19) keV, the first measurement with a precision and reliability sufficient to address the light-curve and energy output of x-ray bursts as well as the abundances of the elements synthesized. Under typical conditions used for modeling x-ray bursts, 68Se is found to cause a significant delay in the rp process nucleosynthesis.     

Ultraprecise atomic mass measurement of the alpha particle and 4He

The atomic masses of the alpha particle and 4He have been measured by means of a Penning trap mass spectrometer which utilizes a frequency-shift detector to observe single-ion cyclotron resonances in an extremely stable 6.0 T magnetic field. The present resolution of this instrument approaches 0.01 ppb [10 ppt (parts per trillion)] and is limited primarily by the effective stability (<5 ppt/h) of the magnet over hundreds of hours of observation. The leading systematic shift [at -202(9) ppt] is due to the image charge located in the trap electrodes. The new value for the atomic mass of the alpha particle is 4 001 506 179.147(64) nu and the corresponding value for the mass of 4He is 4 002 603 254.153(64) nu (nu=10(-9) u). The 16 ppt uncertainty is at least 20 times smaller than any previous determination.     

Direct measurement of quantum phase gradients in superfluid 4He flow

We report a new kind of experiment in which we generate a known superfluid velocity in a straight tube and directly determine the phase difference across the tube's ends using a superfluid matter wave interferometer. By so doing, we quantitatively verify the relation between the superfluid velocity and the phase gradient of the condensate macroscopic wave function. Within the systematic error of the measurement (approximately 10%) we find nu(s)=Planck's/m4InvertedDeltaphi.     

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.     

Toward a universal formulation of the halo mass function

We compute the dark matter halo mass function using the excursion set formalism for a diffusive barrier with linearly drifting average which captures the main features of the ellipsoidal collapse model. We evaluate the non-Markovian corrections due to the sharp filtering of the linear density field in real space with a path-integral method. We find an unprecedented agreement with N-body simulation data with deviations ?5% over the range of masses probed by the simulations. This indicates that the excursion set in combination with a realistic modeling of the collapse threshold can provide a robust estimation of the halo mass function.     

Pionic fusion study of the halo nucleus 6He, the reaction d+4He→6He+π + at CELSIUS

The halo nucleus ⁶He has been studied in a pionic fusion experiment at the CELSIUS storage ring facility in Uppsala. The ⁶He nuclei were produced in reactions with a deuteron beam incident on a ⁴He jet target 0.9–5.4 MeV above threshold in the center-of-mass frame. The ⁶He ions were detected in a ΔE-E solid-state detector telescope inserted into the CELSIUS ring. The aim of the experiment was to investigate, in particular, the high-momentum part of the halo wave function by measuring the total and differential cross sections of the reaction d+⁴He→⁶He+π ⁺.     

Investigation of halo structure of 6He by hyperspherical three-body method

Hyperspherical harmonics expansion method is applied to a three-body model of two neutron halo nuclei. Convergence of the expansion has been ensured. A repulsive part is introduced in the interaction between the core and the extra-core neutron, to simulate Pauli principle. Two neutron separation energy, r.m.s. radii, correlation factor and probability density distributions have been calculated for ⁶He. It is found that the convergence of the two neutron separation energy is relatively slow, while other quantities reach convergence quickly.     

Electron scattering on two-neutron halo nuclei: The case of He

The formalism to describe electron scattering reactions on two-neutron halo nuclei is developed. The halo nucleus is described as a three-body system (core +n + n), and the wave function is obtained by solving the Faddeev equations in coordinate space. We discuss elastic and quasielastic scattering using the impulse approximation to describe the reaction mechanism. We apply the method to investigate the case of electron scattering on ⁶He. Spectral functions, response functions, and differential cross sections are calculated for both neutron knockout and knockout by the electron.     

Observation of a proton halo in8B

The longitudinal momentum distribution of⁷Be was measured after the break-up reaction of⁸B in C, Al, and Pb targets at 1471 A·MeV. We observed a narrow distribution with a FWHM of (81±6) MeV/c in all targets. The experimental results indicate an extended spatial distribution of the loosely bound proton in⁸B, and agree with QRPA calculations.     

Electron scattering on two-neutron halo nuclei: The case of 6He

The formalism to describe electron scattering reactions on two-neutron halo nuclei is developed. The halo nucleus is described as a three-body system (core +n + n), and the wave function is obtained by solving the Faddeev equations in coordinate space. We discuss elastic and quasielastic scattering using the impulse approximation to describe the reaction mechanism. We apply the method to investigate the case of electron scattering on ⁶He. Spectral functions, response functions, and differential cross sections are calculated for both neutron knockout and α knockout by the electron.     

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.