arXiv subjects

Attila Csoto

Publications and source records attributed to Attila Csoto.

35 records · Page 2Linked to original sources

arXiv1997

Parity conserving gamma asymmetry in n-p radiative capture

The importance of n-p radiative capture, utilizing polarized cold neutrons, as a means of measuring the weak pion coupling constant is reviewed. Parity conserving processes of the form k_gamma*[s_n,k_n] can contribute to the s_n*k_gamma photon asymmetry in any such experiment, if the apparatus is not perfectly symmetric. For an incident laboratory neutron energy of 0.003 eV a value of A^{PC}_{gamma}=0.67*10^{-8} is obtained for two different potential models (Argonne AV14 and Nijmegen Reid93). Serving as an extreme test case, the Reid soft core potential yields 0.61*10^{-8}, close to the result of the contemporary forces. Implications for extracting the weak pion coupling constant and for monitoring the beam polarization are discussed.

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arXiv1997

Be7(p,gamma)B8 and the high-energy solar neutrino flux

The importance of the Be7(p,gamma)B8 reaction in predicting the high-energy solar neutrino flux is discussed. I present a microscopic eight-body model and a potential model for the calculation of the Be7(p,gamma)B8 cross section.

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arXiv1997

Importance of core polarization in halo nuclei

A comment on the importance of core polarization in halo nuclei. I point out that although core polarization is suppressed in neutron halos, it plays an essential role in their binding mechanism.

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arXiv1997

Three-alpha structures in C-12

We search for three-alpha resonances in C-12 by using the complex scaling method in a microscopic cluster model. All experimentally known low-lying natural-parity states are localized. For the first time we unambiguously show that the 0+_2 state in C-12, which plays an important role in stellar nucleosynthesis, is a genuine three-alpha resonance.

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arXiv1997

Nature of the first excited state of He-4

We study the first excited state of He4 in a microscopic {H3+p,He3+n} cluster model, including H3 and He3 distortions. The phenomenological 1S0 H3+p scattering phase shift is well reproduced. We localize a complex pole of the S-matrix between the H3+p and He3+n thresholds. The corresponding resonance parameters are E_r=93 keV position relative to H3+p, and Gamma=390 keV width. A pole search is also performed in an extended R-matrix method, and a resonance is found with parameters E_r=114 keV and Gamma=392 keV. The R-matrix approach gives several additional poles, some of which may be connected with an enhanced threshold effect.

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arXiv1996

Off-shell effects in the energy dependence of the Be7(p,gamma)B8 astrophysical S factor

I show that off-shell effects, like antisymmetrization and Be-7 distortions, can significantly influence the energy dependence of the nonresonant Be7(p,gamma)B8 astrophysical S factor at higher energies. The proper treatment of these effects results in a vitrually flat E1 component of the S factor at E_cm = 0.3-1.5 MeV energies in the present eight-body model. The energy dependence of the nonresonant S factor, predicted by the present model, is in agreement with the low-energy direct capture data and the existing high-energy Coulomb dissociation data. Irrespective of whether or not the present energy dependence is correct, off-shell effects can cause 15--20% changes in the value of S(0) extrapolated from high-energy (E_cm > 0.7 MeV) data.

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arXiv1996

S-matrix and R-matrix determination of the low-energy He-5 and Li-5 resonance parameters

We study the low-energy 3/2- and 1/2- states of He-5 and Li-5 in a microscopic cluster model. The scattering phase shifts of Bond (alpha+n) and of Schwandt (alpha+p), respectively, are reproduced well. We determine the resonance parameters by localizing the poles of the analytically continued S-matrix at complex energies. Our results differ from conventional R-matrix resonance parameters, which were extracted from experimental data using the definition of a resonance based on the positions and widths of reaction cross section peaks. However, they nicely agree with the results of an extended R-matrix method which works at complex energies.

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arXiv1996

Low-energy M1 strength in the Li-7(p,gamma_0)Be-8 reaction

The Li-7(p,gamma_0)Be-8 reaction is studied in a microscopic cluster model. All relevant subsystem properties are well reproduced. The calculated astrophysical S factor is in good agreement with the experimental data, although some M1 strength is missing in the 1+;1 resonance region. There is no contribution from the 1+;0 state. We estimate an M1 strength of 3.5% at 80 keV. By assuming the experimental resonance parameters and channel spin ratio for the first 1+ state that M1 strength is 6.3%, in good agreement with an R matrix analysis. The large (20%) M1 strength determined from a transition matrix element analysis may be caused by assuming a vanishing 0p3/2 spectroscopic factor, which is not supported by shell model calculations.

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arXiv1995

Parity-violating alpha-decay of the 3.56-MeV J,T=0+,1 state of Li-6

We study the parity-violating alpha+d decay of the lowest 0+ state of Li-6 in a microscopic three-cluster model. The initial bound and the final scattering states are described consistently within the same model. The parity-violating decay width is calculated in perturbation theory using the parity-nonconserving nucleon-nucleon interaction of Desplanques, Donoghue, and Holstein (DDH). We find that the decay width is sensitive to dynamical degrees of freedom which are beyond the alpha+p+n model, for example, alpha excitation and breakup. In our full model, which contains breathing excitations of the alpha particle and H-3 + He-3 rearrangement, the parity-nonconserving decay width is Gamma=3.97x10^{-9} eV, using the DDH coupling constants.

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arXiv1995

On the balance of the solar p-p chain

I show that the solar neutrino fluxes, predicted by standard nuclear and solar physics, can get closer to the experimental observations if one takes the freedom to introduce two free parameters into the model, as it is done in the MSW solution. I point out that the plasma electron capture of Be7 and the He3(He3,2p)He4 nuclear reactions deserve further experimental and theoretical attention.

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arXiv1995

Comment on ``Large-space shell-model calculations for light nuclei''

In a recent publication Zheng, Vary, and Barrett reproduced the negative quadrupole moment of Li-6 and the low-lying positive-parity states of He-5 by using a no-core shell model. In this Comment we question the meaning of these results by pointing out that the model used is inadequate for the reproduction of these properties.

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arXiv1994

Three-body resonances in He-6, Li-6, and Be-6, and the soft dipole mode problem of neutron halo nuclei

Using the complex scaling method, the low-lying three-body resonances of $^6$He, $^6$Li, and $^6$Be are investigated in a parameter-free microscopic three-cluster model. In $^6$He a 2$^+$, in $^6$Li a 2$^+$ and a 1$^+$, and in $^6$Be the 0$^+$ ground state and a 2$^+$ excited state is found. The other experimentally known 2$^+$ state of $^6$Li cannot be localized by our present method. We have found no indication for the existence of the predicted 1$^-$ soft dipole state in $^6$He. We argue that the sequential decay mode of $^6$He through the resonant states of its two-body subsystem can lead to peaks in the excitation function. This process can explain the experimental results in the case of $^{11}$Li, too. We propose an experimental analysis, which can decide between the soft dipole mode and the sequential decay mode.

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arXiv1993

Three-body resonances by complex scaling

It is demonstrated that the complex scaling method can be used in practical calculations to localize three-body resonances. Our model example emphasizes the fact that in three-body systems several essentially different asymptotic behaviors can appear. We show that the possibility of these different asymptotic configurations can lead to an apparent, resonance like structure in the three-body continuum.

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arXiv1993

Microscopic description of the beta delayed deuteron emission from \bbox{^6}He

The beta delayed deuteron emission from $^6$He is studied in a dynamical microscopic cluster model. This model gives a reasonably good description for all the subsystems of $^6$He and $^6$Li in a coherent way, without any free parameter. The beta decay transition probability to the $^6$Li ground state is underestimated by a few percents. The theoretical beta delayed deuteron spectrum is close to experiment but it is also underestimated by about a factor 1.7. We argue that, in spite of their different magnitudes, both underestimations might have a common origin. The model confirms that the neutron halo part of the $^6$He wave function plays a crucial role in quenching the beta decay toward the $α$ + d channel.

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arXiv1993

Proton halo of $^8$B in a microscopic model

The experimentally predicted existence of a proton halo in $^8$B is confirmed by dynamical microscopic multiconfiguration cluster model calculations. The thickness of the proton halo in $^8$B is 0.5 $fm$ while in $^8$Li a 0.4 $fm$ thick neutron halo has been found. It is shown that the huge quadrupole moment of $^8$B partly comes from the very distortable $^7$Be core.

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arXiv1993

Localization of shadow poles by complex scaling

Through numerical examples we show that the complex scaling method is suited to explore the pole structure in multichannel scattering problems. All poles lying on the multisheeted Riemann energy surface, including shadow poles, can be revealed and the Riemann sheets on which they reside can be identified.

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arXiv1993

The neutron halo of $^6$He in a microscopic model

The two--neutron separation energy of $^6$He has been reproduced for the first time in a realistic parameter--free microscopic multicluster model comprising the $α+n+n$ and $t+t$ clusterizations, with $α$ cluster breathing excitations included. The contribution of the $t+t$ channel is substantial. A very thick (0.85 fm) neutron halo has been found in full agreement with the results of the latest phenomenological analysis.

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