arXiv subjects

Moshe Gai

Publications and source records attributed to Moshe Gai.

33 records · Page 2Linked to original sources

arXiv2004

How Accurately Do We Know the Formation of Solar 8B?

The large value of \S17 = 22.1 \xpm 0.6 eV-b, reported by the Seattle group, suggests a larger total \b8 solar neutrino flux. Together with the two high precision values quoted for \s34 it is either 20% or 9% larger than measured by SNO. While the accuracy of the Standard Solar Model has recently been revisited, precise nuclear inputs are still relevant, but a detailed examination of current data on \xs17 (as opposed to an examination of \S17 only) excludes quoting \S17 with sufficiently small uncertainty. In contrast to suggestions that \S17 is now known with the (impressive) accuracy of \xpm 3%, the exact value of \S17 is dependent on the choice of the data and the choice of theory used for extrapolation. In addition recent high precision results (including the Seattle data) on \xs17 which are in good agreement, still differ on the measured slopes, as does the theory, precluding an accurate extrapolation to zero energy of the consistent data. Using a common extrapolation of only the consistent high precision data, suggests a value of \S17 = 21.2 \xpm 0.5 eV-b, but a value equal to or smaller than 19.0 eV-b can not be excluded due to the uncertainty in the extrapolation, leading to an additional error of $^{+0.0}_{-3.0}$ eV-b. A proposal to remedy this situation is discussed.

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arXiv2003

Dipole Symmetry Near Threshold

In celebrating Iachello's 60th birthday we underline many seminal contributions for the study of the degrees of freddom relevant for the structure of nuclei and other hadrons. A dipole degree of freedom, well described by the spectrum generating algebra U(4) and the Vibron Model, is a most natural concept in molecular physics. It has been suggested by Iachello with much debate, to be most important for understanding the low lying structure of nuclei and other hadrons. After its first observation in $^{18}O$ it was also shown to be relevant for the structure of heavy nuclei (e.g. $^{218}Ra$). Much like the Ar-benzene molecule, it is shown that molecular configurations are important near threshold as exhibited by states with a large halo and strong electric dipole transitions. The cluster-molecular Sum Rule derived by Alhassid, Gai and Bertsch (AGB) is shown to be a very useful model independent tool for examining such dipole molecular structure near thereshold. Accordingly, the dipole strength observed in the halo nuclei such as $^6He, ^{11}Li, ^{11}Be, ^{17}O$, as well as the N=82 isotones is concentrated around threshold and it exhausts a large fraction (close to 100%) of the AGB sum rule, but a small fraction (a few percent) of the TRK sum rule. This is suggested as an evidence for a new soft dipole Vibron like oscillations in nuclei.

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arXiv2003

Solar Fusion and The Coulomb Dissociation of 8B; What Have We Learned and Where Do We Go From Here?

The much needed nuclear input to the Standard Solar Model, $S_{17}(0)$, has now been measured with high precision ($\pm$5% or better) by different groups and good agreement is found, even when very different methods are employed. We review the decade long research program to measure the cross section of the $^7Be(p,γ)^8B$ reaction using the Coulomb dissociation method, including the pioneering RIKEN1 experiment carried out during March 1992, followed by RIKEN2, GSI1, GSI2 and an MSU experiment. Our RIKEN and GSI data allow us to rule out the much tooted large E2 contribution to the Coulomb dissociation of $^8B$. Specifically recent results of the MSU experiment are not confirmed. The GSI1 and GSI2 high precision measurements are in good (to perfect) agreement with the newly published high precision measurements of direct capture with $^7Be$ targets. From these GSI-Seattle-Weizmann high precision data we conclude that the astrophysical cross section factor, $S_{17}(0)$, is most likely in the range of 20 - 22 eV-b. We point out to an additional large uncertainty (-10% +3%) that still exists due to uncertainty in the measured slope of the S-factor and the theoretical extrapolation procedure which may still lower $S_{17}(0)$ down to approximately 18.5 eV-b. For quoting $S_{17}(0)$ with an uncertainty of $\pm$5% or better, yet another measurement needs to be performed at very low energies, as recently discussed by the UConn-Weizmann-LLN collaboration for the CERN/ISOLDE facility.

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arXiv2003

Open Questions in Stellar Helium Burning Addressed With Real Photons

The outcome of helium burning is the formation of the two elements, carbon and oxygen. The ratio of carbon to oxygen at the end of helium burning is crucial for understanding the final fate of a progenitor star and the nucleosynthesis of heavy elements in Type II supernova, with oxygen rich star predicted to collapse to a black hole, and a carbon rich star to a neutron star. Type Ia supernovae (SNeIa) are used as standard candles for measuring cosmological distances with the use of an empirical light curve-luminosity stretching factor. It is essential to understand helium burning that yields the carbon/oxygen white dwarf and thus the initial stage of SNeIa. Since the triple alpha-particle capture reaction, $^{8}Be(α,γ)^{12}C$, the first burning stage in helium burning, is well understood, one must extract the cross section of the $^{12}C(α,γ)^{16}O$ reaction at the Gamow window (300 keV) with high accuracy of approximately 10% or better. This goal has not been achieved despite repeated strong statements that appeared in the literature. In particular constraint from the beta-delayed alpha-particle emission of $^{16}N$ were shown to not sufficiently restrict the p-wave cross section factor; e.g. a low value of $S_{E1}(300)$ can not be ruled out. Measurements at low energies, are thus mandatory for determining the elusive cross section factor for the $^{12}C(α,γ)^{16}O$ reaction. We are constructing a Time Projection Chamber (TPC) for use with high intensity photon beams extracted from the HI$γ$S/TUNL facility at Duke University to study the $^{16}O(γ,α)^{12}C$ reaction, and thus the direct reaction at energies as low as 0.7 MeV. This work is in progress.

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arXiv2001

KeV Astrophysics With GeV Beams; Blazing a New Trail on the Summitt of Nuclear Astrophysics

GeV beams of light ions and electrons are used for creating a high flux of real and virtual photons, with which some problems in Nuclear Astrophysics are studied. GeV 8B beams are used to study the Coulomb dissociation of 8B and thus the 7Be(p,gamma)8B reaction. This reaction is one of the major source of uncertainties in estimating the 8B solar neutrino flux and a critical input for calculating the 8B Solar neutrino flux. The Coulomb dissociation of 8B appears to provide a viable method for measuring the 7Be(p,gamma)8B reaction rate, with a weighted average of the RIKEN1, RIKEN2, GSI1 and MSU published results of S17(0) = 18.9 +/- 1.0 eV-b. This result however does not include a theoretical error estimated to be +/- 10 %. GeV electron beams on the other hand, are used to create a high flux of real and virtual photons at TUNL-HIGS and MIT-Bates, respectively, and we discuss two new proposals to study the 12C(alpha,gamma)16O reaction with real and virtual photons. The 12C(alpha,gamma)16O reaction is essential for understanding Type II and Type Ia supernova. It is concluded that virtual and real photons produced by GeV light ions and electron beams are useful for studying some problems in Nuclear Astrophysics.

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arXiv2000

Nuclear Molecular Halo: Threshold Effect or Soft Dipole in 11Li?

The observation of large E1 strength near threshold in the electromagnetic dissociation of 11Li poses a fundamental question: Is the large E1 strength due to the threshold or is it due to a low lying E1 state? Such molecular cluster states were observed in 18O and in several nuclei near the drip line. We discuss the nature of the "threshold effect" as well as review the situation in Molecular (and Particle Physics) where such Molecular States are observed near the dissociation limit. We suggest that the situation in 11Li is reminiscent of the argon-benzene molecule where the argon atom is loosely bound by a polarization (van der Waals) mechanism and thus leads to a very extended object lying near the dissociation limit. Such states are also suggested to dominate the structure of mesons [a_0(980), f_0(975)] and baryons [Lambda(1405)] with proposed Kaon molecular structure (e.g. by Dalitz) near threshold. The inspection of such states throughout Physics allows us to gain insight into this phenomenon and suggest that a new collective Molecular Dipole Degree of Freedom plays a major role in the structure of hadrons (halo nuclei, mesons and baryons), and that quantitative tools such as the E1 Molecular Sum Rule are useful for elucidating the nature of the observed low lying E1 strength in halo nuclei.

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arXiv2000

Precision Measurements of the 7Be(p,gamma)8B Reaction with Radioactive Beams and the 8B Solar Neutrino Flux

The 7Be(p,gamma)8B reaction is one of the major source of uncertainties in estimating the 8B solar neutrino flux and is critical for undertsanding the Solar Neutrino Problem and neutrinos. The main source of uncertainty is the existence of conflicting data with different absolute normalization. Attempts to measure this reaction rate with 7Be beams are under way by the UConn-LLN collaboration, and we discuss a newly emerging method to extract this cross section from the Coulomb dissociation of the radioactive beam of 8B. We discuss some of the issues relevant for this study including the question of the E2 contribution to the Coulomb dissociation process which was measured to be small. The Coulomb dissociation appears to provide a viable alternative method for measuring the 7Be(p,gamma)8B reaction rate, with a weighted average of the RIKEN1, RIKEN2 and GSI1 published results of S17(0) = 19.4 +/- 1.3 eV-b.

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arXiv1998

How Well Do We Know the Beta-Decay of N16 and Oxygen Formation in Helium Burning?

Contrary to claims that the problem has been solved, the astrophysical E1 S-factor of \c12ag is not yet well known. R-Matrix analyses of elastic scattering data,\c12ag data, and data on the beta-delayed alpha-particle emission of N16 are not consistent and a small S-factor solution SE1(300) cannot be ruled out. In particular, data on the beta- delayed alpha-particle emission of N16 do not agree. The unaltered Mainz data do not agree with TRIUMF, but agree with both Seattle and Yale-UConn. The TRIUMF col- laboration has recalibratedthe Mainz('71) data; however, we dispute both the alteration of the Mainz data performed by the TRIUMF col- laboration and the very justification for the recalibration.

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arXiv1998

How Well Do We Know the Beta-Decay of 16N and Oxygen Formation in Helium Burning

We review the status of the 12C(a,g)16O reaction rate, of importance for stellar processes in a progenitor star prior to a super-nova collapse. Several attempts to constrain the p-wave S-factor of the 12C(a,g)16O reaction at Helium burning temperatures (200 MK) using the beta-delayed alpha-particle emission of 16N have been made, and it is claimed that this S-factor is known, as quoted by the TRIUMF collaboration. In contrast reanalyses (by G.M. hale) of all thus far available data (including the 16N data) does not rule out a small S-factor solution. Furthermore, we improved our previous Yale-UConn study of the beta- delayed alpha-particle emission of \n16 by improving our statistical sample (by more than a factor of 5), improving the energy resolution of the experiment (by 20%), and in understanding our line shape, deduced from measured quantities. Our newly measured spectrum of the beta-delayed alpha-particle emission of 16N is not consistent with the TRIUMF('94) data, but is consistent with the Seattle('95) data, as well as the earlier (unaltered !) data of Mainz('71). The implication of this discrepancies for the extracted astrophysical p-wave s-factor is briefly discussed.

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arXiv1997

Progress In Nuclear Astrophysics Using Secondary Radioactive Beams

We review progress in studying two central problems in Nuclear Astrophysics: The \be7pg reaction is one of the major source of uncertainties in estimating the \b8 solar neutrino flux and is critical for the Solar Neutrino Problem. We discuss a newly emerging method to extract this cross section from the Coulomb dissociation of the radioactive beam of \b8. The Coulomb dissociation appears to provide a viable alternative method for measuring the \be7pg reaction rate. Several attempts to constrain the p-wave S-factor of the \c12ag reaction at Helium burning temperatures (200 MK) using the beta-delayed alpha-particle emission of \n16 have been made, and it is claimed that this S-factor is known, as quoted by the TRIUMF collaboration. In contrast reanalyses (by G.M. hale) of all thus far available data (including the \n16 data) does not rule out a small S-factor solution. Furthermore, we improved our previous Yale-UConn study of the beta-delayed alpha-particle emission of \n16. Our newly measured spectrum of the beta-delayed alpha-particle emission of \n16 is not consistent with the TRIUMF('94) data, but is consistent with the Seattle('95) data, as well as the earlier (unaltered !) data of Mainz('71). The implication of this discrepancies for the extracted astrophysical p-wave s-factor is briefly discussed.

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arXiv1997

A Study of the Coulomb Dissociation of 8B and the 7Be(p, gamma)8B Reaction

We study the breakup reactions of 8B projectiles in high energy (50 and 250 MeV/u) collisions with heavy nuclear targets (208Pb). The intrinsic nuclear wave functions are calculated using a simple model, as well as a simple optical potential. We demonstrate that nuclear effects are negligible and evaluate the contributions of various (E1, E2 and M1) multipolarities. A good agreement with measured data is obtained with insignificant M1 contribution (at 50 MeV/u) and very small E2 contribution.

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arXiv1997

Comment on Constraints on the Low-Energy E1 Cross Section of 12C(alpha,gamma)16O from the Beta-Delayed Alpha Spectrum of 16N

We dispute the alteration by Azuma et al. of the energy calibration of the Mainz('71) data on the beta-delayed alpha-particle emission of $^{16}N$ as well as the very justification of the recalibration. We use the unaltered data to observe a pronounced disagreement between the TRIUMF('94) and Mainz('71) data sets on both the high and low energy sides of the primary peak (at 2.36 MeV) of the $^{16}N$ alpha-spectrum. We cannot support the dismissal of the Mainz('71) spectrum by the TRIUMF collaboration and emphasize the need to include it in R-matrix fits. We discuss the need for new improved data to resolve this disagreement and its implication for the extracted p-wave astrophysical S-factor of the $^{12}C(α,γ)^{16}O$ reaction.

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arXiv1995

NUCLEAR ASTROPHYSICS WITH SECONDARY (RADIOACTIVE) BEAMS

Some problems in nuclear astrophysics are discussed with emphasize on the ones central to the field which were not solved over the last two decades, including Helium Burning in Massive stars (the 12C(a,g)16O reaction) and the 8B Solar Neutrino Flux Problem (the 7Be(p,g)8B reaction). We demonstrate that a great deal of progress was achieved by measuring the time reverse process(es): the beta-delayed alpha-particle emission of 16N and the Coulomb dissociation of 8B, using radioactive beams (of 16N and 8B). In this way an amplification of the sought for cross section was achieved, allowing a measurement of the small cross section(s) of relevance for stellar (solar) process(es).

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arXiv1994

Comment on E2 Contribution to the 8B ---> p + 7Be Coulomb Dissociation Cross Section [K. Langanke and T.D. Shoppa, Phys. Rev. C49(1994)R1771, ER(1994)]

The E2 cross section calculated by Langanke and Shoppa for the Riken experiment on the Coulomb Dissociation of 8B uses E2 nuclear matrix elements from one specific model. Other nuclear models predict a considerably smaller E2 cross section (by a pproximately a factor of 3 to 4), and Langanke and Shoppa appear to assume the most optimistic scenario predicting large E2 cross section. We also note that Barker has already criticised the nuclear model used by Langanke and Shoppa. A MODEL INDEPENDENT Chi-Square analysis of the Riken data, suggest the best fit for the current Riken data is obtained with E1 amplitudes only. The upper limit (90% confidence) on the E2 component derived from our Chi-Square analysis is considerably smaller than used by Langanke and Shoppa. The MODEL DEPENDENT analysis of Langanke-Shoppa should not be considered as a correction to the Riken result, as claimed, and their quoted S17(0) is not substantiated.

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arXiv1994

Laboratory Mesurements in Nuclear Astrophysics

After reviewing some of the basic concepts, nomenclatures and parametrizations of Astronomy, Astrophysics and Cosmology, we introduce a few central problems in Nuclear Astrophysics, including the hot-CNO cycle, helium burning in massive stars, and solar neutrino's. We demonstarte that SECONDARY (RADIOACTIVE) NUCLEAR BEAMS allow for considerable progress on these problems.

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