Quasi-free processes in 6Li(3He,pα)4He reaction at low energies

The ⁶Li(³He, pα)⁴He reaction was studied experimentally at 3.5, 4.4 and 5.5 MeV in the quasi-free reaction kinematical region. The effects of a resonance in the virtual ²H(³He, p)⁴He reaction on the three-body reaction cross section were investigated.     

Production of high energy deuterons in the reaction:3He+3He→4He+2H+e ++v

Experiments on the reaction:³He+³He→⁴He+²He using a single four detector telescope have shown evidence for the production of deuteron pulses, consistent with the²He→²H+e⁺+v fusion reaction in the final state. New measurements on the same reaction using improved techniques viz.A) a windowless³He gas target andB) α- d correlation detection with 5 ns resolving time and antipileup circuitry have been performed. Calibrations were obtained from the⁷Li+³He→²H+⁴He+⁴He reaction spectra. The correlation angles were 30° and 97° for the alphas and deuterons respectively. Deuteron pulses have been observed, correlated with alpha particles. A cross section of (1.7 ±0.5)nbsr^?1MeV^?1 was obtained consistent with previous measurements using a single telescope.     

Experimental Study of Photon Induced Reactions on 3He and 4He at Low Energies

Data are reported for the photodisintegration cross section of the reaction ³He(??, p)²H at ten energies between 7.0 and 16.0?MeV. Very preliminary data are presented for the reaction ⁴He(??, p)³H between 22.0 and 29.5?MeV in 0.5?MeV energy steps, and for the reaction ⁴He(??, n)³He at three energies around 28.0?MeV. High-pressure He/Xe gas scintillators served as target and detector. Our data are in better agreement with recent theoretical calculations than the majority of the existing data for all three reactions, but differ significantly from recent data taken with a mono-energetic photon beam and a time-projection chamber.     

Fusion-fission reactions induced by 6He-ions

The experimentally measured excitation functions for the fission and 4n evaporation channels are presented for the ⁶He + ²⁰⁹Bi reaction. The secondary ⁶He beam was produced using the special beam line (Q4DQ-spectrometer) of the U400M accelerator at FLNR, JINR. The comparison of the obtained experimental data with similar results for the ⁴He + ²⁰⁹Bi reaction shows that in the case of the ⁶He + ²⁰⁹Bi reaction a significant enhancement of the cross-section is observed for energies above the barrier. In order to get an agreement between the experimental data and the theoretical calculations it is necessary to reduce the Coulomb barrier by 15-20% , which corresponds to an increase of the parameter r₀ of the nuclear potential up to 1.5-1.6 fm. Received: 1 May 2001 / Accepted: 4 December 2001     

Microscopic study of the low-energy3He(3He, 2p)4He and3H(3H, 2n)4He fusion cross sections

We have calculated the³He(³He, 2p)⁴He and³H(³H, 2n)⁴He reaction cross sections at low energies within the microscopic multichannel resonating group method. For both reactions, we find good agreement with experiment. For the³H(³H, 2n)⁴He reaction, our calculated energy dependence reproduces that of each individual low-energy experimental data set, except for a normalization constant. Using this fact, we derive at a low-energy³H(³H, 2n)⁴He rate by taking the averaged mean of these fits.This work has been supported in part by the National Science Foundation, Grants PHY86-04197 and PHY88-17296.     

Above-barrier enhancement of fusion in the 6He + 209Bi nuclear reaction

The fission cross sections for the ⁶He + ²⁰⁹Bi reaction as functions of the ⁶He-beam energy are measured. The experimental excitation functions for the reaction ²⁰⁹Bi(⁶He, 4n)²¹¹ At are also presented. The ⁶He secondary ion beam is obtained on the basis of the extracted-beam transport system of the U400-M accelerator (the Q4DQ spectrometer). A comparison of the experimental data obtained with available results for the ⁶He + ²⁰⁹Bi reaction shows that a pronounced enhancement of the fission cross sections in the above-barrier energy region is observed in the case of the reaction with the ⁶He beam. In order to fit the results of theoretical calculations to the experimental data, it is necessary to reduce the Coulomb barrier by 15% (20%). This corresponds to an increase of 1.5 (1.6) fm in the parameter r ₀ of the nuclear potential.     

T invariance and polarization effects in the reactions p+3He → π++4He and π++4He → p+3He

On the basis of T invariance, it is established that the dependence of the effective cross section for a binary reaction of the a+b → c+d type on the polarization vectors of primary particles a and b determines completely the polarization vectors and spin correlations for the same particles in the inverse reaction c+d → a+b induced by collisions between unpolarized primary particles c and d. By using the formalism of helicity amplitudes, polarization effects are studied in the process p+³He → π⁺+⁴He and in the inverse process π⁺+⁴He → p+³He. It is shown that, in the reaction π⁺+⁴He → p+³He, the spins of the final-state particles (proton and ³He nucleus) are strongly correlated. An expression for the correlation tensor is obtained for arbitrary values of the (p, ³He) emission angle.     

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.     

Effects of electron screening on the 3He(d,p)4He low-energy cross sections

The reaction ³He(d, p)⁴He has been investigated for E_CM = 5.9−41.6 keV with the use of D projectiles and ³He atomic gas target nuclides as well as with ³He projectiles and D₂ molecular gas target nuclides. These studies show for the first time the effects of electron screening on low-energy fusion cross sections, i.e. a nearly exponential enhancement of the cross sections compared to the case of bare nuclei. The enhancement is about a factor 2 smaller for the case d(³He, p)⁴He due to the molecular nature of the D₂ target nuclides.     

Nuclear vertex constants for the virtual decay <Superscript>7</Superscript>Be → <Superscript>3</Superscript>He + <Superscript>4</Superscript>He of the 7Be bound states from a phase-shift analysis in the effective-range theory

The phase-shift-analysis data for the ³He⁴He scattering are analyzed using the effective-range theory, with the Coulomb interaction taken into account. We find both the renormalized nuclear vertex constants for the vertex ⁷Be → ³He + ⁴He in the ground (3/2⁻) and in the first excited (1/2⁻) bound states of ⁷Be, and the corresponding asymptotic normalization constants of the radial wave functions in these states. The results obtained can be used in an astrophysical S-factor calculation for radiative capture reaction ⁴He(³He, γ)⁷Be     

Structure effects on reaction mechanisms in collisions induced by radioactive ion beams

The present paper concerns the study of reactions induced by radioactive beams of halo and weakly bound nuclei at energies around and above the Coulomb barrier. The results obtained for the reaction induced by the halo nucleus ⁶He on ⁶⁴Zn have been compared with the results for the reaction induced by ⁴He on the same target. The results of the reaction induced by the weakly bound unstable ¹³N on the weakly bound ⁹Be have been compared with those for the reaction ¹⁰B + ¹²C. The text was submitted by the authors in English.     

Angular distribution measurements of Li(p,α)He reaction at 140 keV proton energy using nuclear track detectors

Angular distributions of a ⁶Li(p,α) ³He reaction were measured at six angles for 140 keV proton energy using nuclear track detectors (NTDs). The measurements were carried out over 60°–160° lab. angles in 20° increments using a scattering chamber of 80° beam line of the 350 kV accelerator. A semiconductor silicon surface barrier (SSB) detector was placed at +160° and was used as a monitor. The results have shown that the CR-39 detector has excellent capabilities to distinguish 1.4–2.7 MeV α+ ³He particles from the ⁶Li(p,α) ³He reaction and 8–9.4 MeV α-particles from the ⁷Li(p,α) ⁴He reaction through their track diameters. However, it was not possible to distinguish between the 2.3 MeV ³He ions and the 1.7 MeV ⁴He ions from the ⁶Li(p,α) ³He reaction from their track diameter measurements, but it was possible to differentiate between the two, from the darker contrast of the ³He particles caused by its deeper tracks as compared to those of ⁴He.     

Fission reactions of 469-MeV56Fe+238U: Detection of4He/1H emission from pre- and post-fission sources

The emission of⁴He and¹H has been measured in coincidence with fission for reactions of 469-MeV⁵⁶Fe+²³⁸U. By using a gas-ionization telescope in kinematic coincidence with a position-sensitive avalanche detector, the folding angle between two fission fragments was determined in order to distinguish fusion reactions from fission following smaller-momentum-transfer collisions. In both fusion fission and sequential fission reactions, the⁴He/¹H energy spectra are relatively narrow with relatively flat angular distributions at backward angles and become broader in energy with enhanced cross-sections at forward angles. The extent of forward peaking is significantly greater for peripheral collisions than for central collisions. The light-charged-particle multiplicities are quite similar for⁴He and¹H, being much larger for fusion fission than for sequential fission. Detailed comparisons of the spectral shapes with Monte Carlo simulations of reaction kinematics impose strong constraints on the participation of different emission sources. We find important contributions to the observed⁴He/¹H emission both from accelerated fragments (FE) and from the composite system prior to fission (CE). For⁴He emission, the multiplicity of CE is much larger for fusion fission than for sequential fission, possibly as a consequence of the higher spins and shorter reaction times associated with deeply inelastic and quasi-elastic processes. For¹H emission, a corresponding but somewhat smaller difference is observed for the CE multiplicities. An excess of⁴He/¹H particles, found at forward angles in both fusion and sequential fission processes, cannot be attributed to evaporative emission from any fragments and therefore must originate in pre-thermalization emission.     

The low solar neutrino flux: A search for a resonance in 6Be

A study of neutron time-of-flight spectra from the ⁴He(³He, n)⁶ reaction at E_3He = 38.61 MeV, sets a one standard deviation upper limit of 7 μb/sr for the cross section to a possible narrow level near the ³He + ³He threshold.     

Consideration on direct reaction mechanisms in the unified microscopic nuclear theory

In the framework of a unified microscopic nuclear theory the starting equations for the discussion of the different phenomenological direct reaction theories are derived. Subsequently in continuation of a previous investigation the⁶Li(p,³He)⁴He reaction as a realistic example is discussed to obtain a better physical insight into the different kinds of reaction mechanisms.     

Evaluated Integral Cross Sections of the <Superscript>3</Superscript>H(<Emphasis Type="Italic">t</Emphasis>,2<Emphasis Type="Italic">n</Emphasis>)<Superscript>4</Superscript>He Reaction in the Low-Energy Region Obtained with Regard to Electron Screening

procedure is considered for analyzing ³H(t,2n)⁴He reaction proceeding in a gas environment with regard to electron screening [1–4]. Results from such an analysis are presented. An electron screening potential of 121 eV is obtained. The magnitude of this potential is three times higher than the one given in [5]. Starting with a 100 eV energy of particle interaction the cross sections of ³H(t,2n)⁴He reaction are calculated using the above potential. The reaction rates are calculated using the evaluated cross sections in the lowenergy region. Enhancement factors for cross sections and reaction rates are defined.     

Measurements of the two-neutron transfer reaction induced by a 6He radioactive beam

To estimate the probability of two-neutron configuration in halo nuclei, an experimental method for studying neutron-neutron correlations at the periphery of such nuclei by measuring the two-neutron transfer reaction is proposed. Experimental investigation of the ⁶He + A → ⁴He + B reaction was performed using a 60-MeV ⁶He radioactive beam at the Flerov Laboratory of Nuclear Reactions (Joint Institute for Nuclear Research, Dubna); the technique of nuclear photoemulsions was applied. Search for events of the two-nucleon transfer reaction was performed at the PAVIKOM setup (Lebedev Physical Institute, Russian Academy of Sciences).     

T–T Neutron Spectrum from Inertial Confinement Implosions

A new technique that uses inertial confinement implosions for measuring low-energy nuclear reactions important to nuclear astrophysics is described. Simultaneous measurements of n–D and n–T elastic scattering at 14.1 MeV using deuterium–tritium gas-filled capsules provide a proof of principle for this technique. Measurements have been made of D(d,p)T (dd) and T(t,2n)⁴He (tt) reaction yields relative to the D(t,n)⁴He (dt) reaction yield for deuterium–tritium mixtures with f _T /f _D between 0.62 and 0.75 and for a wide range of ion temperatures to test our understanding of the implosion processes. Measurements of the shape of the neutron spectrum from the T(t,2n)⁴He reaction have been made for each of these target configurations.     

Surprising properties of the nuclear stratosphere indicated by energy spectra and large-angle correlations between 4He and 1,2,3H or 4He

Angular and energy distributions are reported for ⁴He in correlation with ^1,2,3H and ⁴He in the reaction 337 MeV ⁴⁰Ar+^natAg. Comparisons are made to a statistical model that includes emitter. Energy spectra important barrier reductions compared to those for cold nuclei, ≈25%, 12%, 8% and 0 for ¹H, ⁴He, ²H and ³H respectively. Emitter deformation alone cannot give an explanation. A transitory nuclear stratosphere is suggested with proton emission favored from its fringes of lowest density.     

Possible influence of ground state baryonic resonance admixture on the yield of the 4He(γ, p)T reaction

The inclusion of a reasonable amount of N′(1470) in the ⁴He ground state is consistent with the available experimental cross section values of the ⁴He(γ, p)T reaction for intermediate energy photons (E_γ < 400 MeV).