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V. D. Efros

Publications and source records attributed to V. D. Efros.

31 records · Page 2Linked to original sources
arXiv1998

The first excited states of 9Be and 9B

It is found here that the 1/2+ first excited state of 9Be is a virtual state with the energy of -23.5 KeV. The line shape for the excitation of the state is approximated with a simple analytic form based on the effective range expansion. The partner in 9B of this state is found to be a resonance with a maximum in the peak at about 1.1 MeV, FWHM of 1.5 MeV, and complex energy of 0.6-i0.75 MeV. The line shape for its excitation is calculated in terms of the p-8Be phase shift. The phase shifts are obtained from N-8Be effective potentials deduced from the data on on the photodisintegration of 9Be. A possibility for direct extraction of the energy of the resonant state from experimental data is also discussed, and an expression for a residue at a virtual state pole in terms of a quadrature taken over the virtual state eigenfunction is given.

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arXiv1998

Exact 4He Spectral Function in a Semirealistic NN Potential Model

The spectral function of 4He is calculated with the Lorentz integral transform method in a large energy and momentum range. The excitation spectrum of the residual 3N-system is fully taken into account. The obtained spectral function is used to calculate the quasi elastic longitudinal (e,e') response R_l of 4He for q=300, 400, and 500 MeV/c. Comparison with the exact R_l shows a rather sizeable disagreement except in the quasi elastic peak, where the differences reduce to about 10% at q=500 MeV/c. It is shown as well that very simple momentum distribution approximations for the spectral function provide practically the same results for R_l as the exact spectral function.

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arXiv1998

Low-energy photodisintegration of 9Be and alpha + alpha + n <-> 9Be + gamma reactions at astrophysical conditions

A semi-microscopic model for the low-energy photodisintegration of the 9Be nucleus is constructed, and the experimental data are analyzed with its help. The older radioactive isotope data are supported by this analysis. The theoretical photodisintegration cross section is derived. The astrophysical rates for the reaction alpha + alpha + n -> 9Be + gamma and the reverse photodisintegration of 9Be are calculated. The new reaction rate for alpha + alpha + n -> 9Be + gamma is compared with previous estimations.

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arXiv1997

Photodisintegration of the Three-Nucleon Systems and their Polarizabilities

The total photodisintegration cross sections of three-body nuclei are calculated with semirealistic NN potentials below pion threshold. Full final state interaction with Coulomb force is taken into account via the Lorentz integral transform method. The experimental total cross sections are well described and the sum rule $σ_{-1}(^3$H) agrees with elastic electron scattering data. The calculated ^3He polarizability is 0.15 fm^3.

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arXiv1997

Is there a Pronounced Giant Dipole Resonance in ^4He?

A four-nucleon calculation of the total ^4He photodisintegration cross section is performed. The full final-state interaction is taken into account for the first time. This is achieved via the method of the Lorentz integral transform. Semi-realistic NN interactions are employed. Different from the known partial two-body ^4He(γ,n)^3He and ^4He(γ,p)^3H cross sections our total cross section exhibits a pronounced giant resonance. Thus, in contrast to older $(γ,np)$ data, we predict quite a strong contribution of the $(γ,np)$ channel at the giant resonance peak energy.

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arXiv1997

Accurate Four-Body Response Function with Full Final State Interaction: Application to Electron Scattering off 4He

The longitudinal (e,e') response function of 4He is calculated precisely with full final state interaction. The explicit calculation of the four-body continuum states is avoided by the method of integral transforms. Precision tests of the response show the high level of accuracy. Non-relativistic nuclear dynamics are used. The agreement with experimental data is very good over a large energy range for all considered momentum transfers (q=300, 400, 500 MeV/c). Only at higher q the theoretical response overestimates the experimental one beyond the quasi-elastic peak.

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arXiv1996

Ground--state energies and widths of $^5$He and $^5$Li

We extract energies and widths of the ground states of $^5$He and $^5$Li from recent single--level R--matrix fits to the spectra of the $^3$H$({\rm d},γ$)$^5$He and the $^3$He$({\rm d},γ$)$^5$Li reactions. The widths obtained differ significantly from the formal R--matrix values but they are close to those measured as full widths at half maxima of the spectra in various experiments. The energies are somewhat lower than those given by usual estimates of the peak positions. The extracted values are close to the S--matrix poles calculated previously from the multi--term analyses of the N-$^4$He elastic scattering data.

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arXiv1996

Reaction rate for two--neutron capture by $^4$He

Recent investigations suggest that the neutrino--heated hot bubble between the nascent neutron star and the overlying stellar mantle of a type--II supernova may be the site of the r--process. In the preceding $α$--process building up the elements to $A \approx 100$, the $^4$He(2n,$γ$)$^6$He-- and $^6$He($α$,n)$^9$Be--reactions bridging the instability gap at $A=5$ and $A=8$ could be of relevance. We suggest a mechanism for $^4$He(2n,$γ$)$^6$He and calculate the reaction rate within the $α$+n+n approach. The value obtained is about a factor 1.6 smaller than the one obtained recently in the simpler direct--capture model, but is at least three order of magnitude enhanced compared to the previously adopted value. Our calculation confirms the result of the direct--capture calculation that under representative conditions in the $α$--process the reaction path proceeding through $^6$He is negligible compared to $^4$He($α$n,$γ$)$^9$Be.

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arXiv1995

Nuclear Reactions of Astrophysical Interest Involving Light Nuclei

An introduction to nucleosynthesis, the creation of the elements in the big bang, in interstellar matter and in stars is given. The two--step process $^4$He(2n,$γ$)$^6$He and the reverse photodisintegration $^6$He($γ$,2n)$^4$He involving the halo nucleus $^6$He could be of importance in the $α$--process in type--II supernovae. The reaction rates for the above processes are calculated using three--body methods and show an enhancement of more than three orders of magnitude compared to the previous adopted value. Direct--capture calculations give similar values for the above reaction rates. Therefore, this method was also used to calculate the reaction rates of the two--step processes $^6$He(2n,$γ$)$^8$He and $^9$Li(2n,$γ$)$^{11}$Li and the reverse photodisintegration of $^8$He and $^{11}$Li that could be also of importance in the $α$-process.

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arXiv1994

Study of 3he(e,e') Longitudinal Response Functions with the Integral-Transform Method

The method of integral transforms is first applied for studying the $^3$He longitudinal response functions. The transforms are calculated from localized bound-state-type solutions to an inhomogenous Schrödinger-type three-body equation. Several versions of local $s$-wave spin-dependent potentials supplemented with a singlet $p$-wave potential and with the proton-proton Coulomb interaction are used as a two-nucleon input. The conventional charge density operator is utilized. The three-body equations are solved with a high acuracy. It is found that the contribution of the $T=3/2$ final states to the problem is suppressed and it amounts about 15\%. This might be ascribed to symmetry requirements. The contributions of the $p$-wave $NN$ interaction and of the Coulomb interaction are found to amount several per cent. Uncertainty due to different choices of $s$-wave $NN$ forces is of a similar magnitude provided that the low-energy $NN$ data are properly described. The results are compared with the integral transforms of the experimental response functions. For $q=300$ MeV/c experimental and theoretical results coincide within their uncertainties. For $q=500$ MeV/c a noticeable difference is detected.

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arXiv1993

Study of the Three-Nucleon (e,e') Longitudinal Response Function with a New Approach

A new method for studying the many-body response functions is elaborated and first applied to the $^3$He longitudinal response. An integral transform of the response function is calculated from the bound-state-type equations for several versions of the N-N force. The equations are solved with the help of the hyperspherical expansion. The final-state interaction is completely taken into account. The results are compared with the integral transform of the experimental response function for 250 MeV/c$\leq q \leq 500$ MeV/c. The difference amounts to 20-25\% with the experimental response substantially exceeding the theoretical one in the low-energy region.

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