Formation of highly excited states of 6He in the 9Be(π−, tt)t reaction

The hypothesis of a heavy stable quark of the fourth family can provide a nontrivial solution for cosmological dark matter if baryon asymmetry in the fourth family has a negative sign and an excess of ū antiquarks with charge (?2/3) is generated in the early Universe. Excessive ū antiquarks form (ūūū) antibaryons with electric charge ?2, which are all captured by ⁴He and trapped in a [⁴He⁺⁺(ūūū)^??] O-helium OHe “atom” as soon as ⁴He is formed in Big Bang nucleosynthesis. Interaction of O-helium with nuclei opens a new path to the creation of heavy nuclides in Big Bang nucleosynthesis. Due to the large mass of the U quark, OHe “atomic” gas decouples from baryonic matter and plays the role of dark matter in large-scale structure formation with structures on small scales being suppressed. Owing to nuclear interaction with matter, cosmic O-helium from the galactic dark matter halo is slowed in the Earth below the thresholds of underground dark matter detectors. However, an experimental test of this hypothesis is possible in the search for OHe in balloon-borne experiments and for U hadrons in cosmic rays and accelerators. OHe “atoms” might form anomalous isotopes and could cause cold nuclear transformations in matter, offering a possible way to exclude (or prove) their existence.     

Evidence for the constant-matrix-element mechanism in the breakup of the3He+7Li system and highly excited states in6Li

The inclusive alpha particle spectra from the breakup of the³He+⁷Li system atE ³He=42.9 MeV are analyzed in the frame of the nuclear reaction model whose matrix element does not depend on the relative energies of the outgoing particles and is therefore constant. The comparison with a recent analysis of thep+⁹Be interaction, leading to the same composite system, has shown the correlation of the final-state configuration on the entrance-channel cluster structure.the existence of broad excited states for⁶Li excitation energies between 19 and 28 MeV is confirmed.     

Search for highly excited states in light nuclei with three-body reactions

Various three-body break-up reactions resulting from the bombardment of ⁷Li with 120 MeV ³He have been measured simultaneously in a kinematically complete experiment. Missing-mass spectra have been deduced for ⁴H, ⁴He, ⁴Li, ⁵He, ⁵Li, ⁶He, ⁶Li and ⁷Li. The ground states of ⁴H and ⁵He are well described as n-t and n-α P-wave final-state interactions, respectively. Evidence is found for relatively narrow states at very high excitation energies in several nuclei. The results are compared with those of other authors and with various theoretical predictions.     

Low-lying states of 6He studied via the 6Li(t,3He)6He reaction

We present the recent experimental results on the ⁶He structure studied by the ⁶Li(t, ³He)⁶He reaction at 336 MeV. Above the conspicuous peaks for ground and first excited states for ⁶He, we have observed a broad structures at E _x∼ 5 MeV, and E _x∼ 15 MeV. The angular distribution of this structure exhibits the dominance of a ΔL = 1 transition, indicating the existence of intruder dipole states at low excitation energies in ⁶He. A slight admixture of positive-parity states in this structure has been indicated as well. Received: 1 May 2001 / Accepted: 4 December 2001     

A study of excited states of5He by the7Li (d, α)5He-reaction

The interactions of deuterons with⁷Li, in which two α-particles and a neutron are produced in the final state are studied atE _d =1.2 MeV. The dual parameter α-α coincidence spectrum was analyzed with a search program. The results indicate two excited states of⁵He: One at excitation energy 2.9±0.1 MeV and with a reduced width of γ²=1.1±0.4 MeV, and the other at 4.8±0.4 MeV, with γ²=0.9±0.4 MeV. No contributions of sequential decay through the 4⁺ state in⁸Be or direct three body decay was observed.     

Detailed test of distorting potentials for the transfer reactions 40Ca(7Li, 6Li/6He)

Angular distributions for the transfer reactions ⁴⁰Ca(⁷Li, ⁶Li)⁴¹Ca and ⁴⁰Ca(⁷Li, ⁶He)⁴¹Sc have been measured at 34 MeV, in small angular increments out to angles where the cross section has fallen by four orders of magnitude. Finite range DWBA calculations show a high sensitivity to different distorting potentials at the largest angle cross sections. It is found that a combination of Woods-Saxon and double-folded potentials does the best job of reproducing the data over the whole angular range.     

A microscopic DWBA analysis of (6Li, 6He) reactions

The spin-isospin flip (⁶Li, ⁶He) reaction has been studied. The experiment was performed with a 34 MeV ⁶Li beam incident upon ^{25, 26}Mg and ²⁷Al targets. Angular distributions associated with the lowest $\text{5}\text{2}{}^{\mathrm{+}}\text{,}\text{1}\text{2}{}^{\mathrm{+}}\text{,}\text{7}\text{2}{}^{\mathrm{+}}{}^{25}\text{Al}\text{, 5}{}^{\mathrm{+}}\text{, 3}{}^{\mathrm{+}}\text{, 1}{}^{\mathrm{+}}{}^{26}\text{Al and}\text{5}\text{2}{}^{\mathrm{+}}{}^{27}\text{Si}$ states were measured. Microscopic DWBA calculations were performed using Yukawa central and r². Yukawa (r² · Y) tensor (μ = 1.0 fm^?1) potentials averaged over the ⁶Li(⁶He) cluster model wave functions. Knock-on (KO) exchange corrections corresponding to the central interaction are included using the local-energy approximation (LEA) method. The conclusion is that the quasi-elastic process is of major importance in the (⁶Li, ⁶He) transition studied in this work.     

Isovector giant resonances in light nuclei observed by (7Li, 7Be) reaction

Isovector giant resonances were studied with the (⁷Li, ⁷Be) reaction on ¹²C, ²⁴Mg, ²⁷Al, ²⁸Si and ⁴⁸Ti at an incident energy of 150 MeV. Differential cross sections were measured at forward scattering angles including 0°. We observed the octupole resonance at E_x ∽ 16 MeV and the giant dipole resonance at E_x ∽ 20 MeV in all residual nuclei.     

Microscopic form factor calculations for the quasielastic (6Li, 6He) reactions

We present a simple procedure for calculating microscopic form factors for quasielastic reactions which does not make us of a multipole expansion of the interaction potential. The form factor of the (⁶Li, ⁶He) reactions is studied to reveal its general properties.     

Investigating highly excited states of the 8Li isotope

Highly excited states of the ⁸Li isotope are sought in absorption reactions of π^? mesons stopped by ^10,11B isotopes. The ground and three excited states of the ⁸Li isotope are observed in inclusive measurements of reactions ¹⁰B(π^?, d)X and ¹¹B(π^?, t)X. The states with excitation energies E _x ≈ 8.7 and 10.1 MeV are observed for the first time in correlation measurements of reactions ¹⁰B(π^?, dt)X and ¹¹B(π^?, tt)X.     

Analog states of 7He observed via the 6He(p,n) reaction

Isobaric analog states of 7He have been investigated by a novel technique involving the observation of the resonant yield of neutrons from the 6He(p,n) reaction in coincidence with gamma rays from the decay of the (0(+),T=1) state in 6Li. The gamma rays provide a clean signature for the isospin-conserving neutron decay of the low-lying isobaric analog resonances. It is conclusively shown that the analog of the recently observed low-lying spin-orbit partner of the 7He ground state does not exist. Evidence is presented that this state lies at much higher energies, in agreement with microscopic calculations.     

Structure and reactions of the Li and Be halo nuclei

The phenomenon of s-wave intruder orbits is examined for a range of Li and Be isotopes. Whether these states are mixed or stand alone, they contribute distinctively to the properties of these nuclei. For weakly bound states, they accentuate those features of large sizes, narrow momentum widths and low-lying E1 strengths that are associated with halo nuclei.     

Experimental evidence of (2)He decay from (18)Ne excited states

Two-proton decay from (18)Ne excited states has been studied by complete kinematical reconstruction of the decay products. The (18)Ne nucleus has been produced as a radioactive beam by (20)Ne primary projectile fragmentation at 45 AMeV incident energy on a Be target. The (18)Ne at 33 AMeV incident energy has been excited via Coulomb excitation on a (nat)Pb target. The obtained results unambiguously show that the 6.15 MeV (18)Ne state two-proton decay proceeds through a (2)He diproton resonance (31%) and democratic or virtual sequential decay (69%). The quoted branching ratio has been deduced from relative angle and momentum correlations of the emitted proton pairs.     

Spectroscopic factors and nuclear vertex constants from the (p,d) reactions on7Li,9Be and13C nuclei

The angular distributions of the (p, d) reactions on the⁷Li,⁹Be and¹³C nuclei have been analysed within the framework of the new approach to the extraction of the structural information from the direct nuclear reactions. In this method the additional physical information on the nuclear vertex constants is introduced into the standard DWBA calculations, which makes possible to eliminate the strong dependence of the spectroscopic factors on the geometry of bound state potentials. The possibility of the model — independent determination of the spectroscopic factors — is discussed.The authors wish to express their gratitude to I. Borbely for assistance in the calculations by the method of subtraction of singularities.     

Population densities of He(I) and He(II) excited states in plasmas

Reduced population coefficients for He(I) and He(II) levels, both for optically thin and partially optically thick conditions, have been calculated from a collisional-radiative model using recent experimental and theoretical cross sections. The calculated population densities are compared with experimental results on helium plasmas. Agreement with experimental results is better at low electron temperatures.     

First measurement of the 2H(6He,7Li)n angular distribution and proton spectroscopic factor in 7Li

The angular distribution of the ²H ( ⁶He,⁷Li n reaction was measured with a secondary ⁶He beam of 36.4MeV for the first time. The proton spectroscopic factor of ⁷Li ground state was extracted to be 0.42±0.06 by normalizing the calculational differential cross-sections with the distorted-wave Born approximation to the experimental data. It was discussed that the uncertainty of extracted spectroscopic factors from the one-nucleon transfer reactions induced by deuteron might be reduced by determining the volume integrals of imaginary optical potentials precisely.     

A useful spectroscopic tool: The (7Li, 6He) reaction

Angular distribution measurements for the reactions ²⁴Mg(⁷Li, ⁶He)²⁵ Al and ⁵²Cr(⁷Li, ⁶He)⁵³ Mn at$\text{E(}{}^7\text{Li) = 34}$ MeV show that for angles less than 15°, the shapes of the measured angular distributions allow $\text{d}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ or $\text{d}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$, and $\text{f}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}$ or $\text{f}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ final-state configurations to be distinguished when compared with finite-range DWBA calculations.