arXiv subjects

P. Vogel

Publications and source records attributed to P. Vogel.

At least 73 records · Page 4Linked to original sources

arXiv1998

Pairing and symmetry energy in N \simeq Z nuclei

It is shown that in N \simeq Z nuclei the effects of isovector pairing and the symmetry energy are intimately related. In particular, in the odd-odd N = Z nuclei these two effect are essentially equal in magnitude, causing near degeneracy of the lowest T = 1 and T = 0 states and appearance of the T = 1 ground state isospin in many such nuclei. Following the earlier work of Jaenecke, it is shown that the global symmetry energy fit can be reproduced using just the excitation energies of the lowest T = 3/2 states in N = Z+1 nuclei. Similarly, the global pairing gap fit can be related to the excitation energy of the T = |N - Z|/2 +1 states in N \simeq Z even A nuclei.

nucl-th↗

arXiv1998

Neutrino fluence after r-process freeze-out and abundances of Te isotopes in presolar diamonds

Using the data of Richter et al. (1998) on Te isotopes in diamond grains from a meteorite, we derive bounds on the neutrino fluence and the decay timescale of the neutrino flux relevant for the supernova r-process. Our new bound on the neutrino fluence F after freeze-out of the r-process peak at mass number A = 130 is more stringent than the previous bound F < 0.045 (in units of 10**37 erg/cm**2) of Qian et al. (1997) and Haxton et al. (1997) if the neutrino flux decays on a timescale tau > 0.65 s. In particular, it requires that a fluence of F = 0.031 be provided by a neutrino flux with tau < 0.84 s. Such a fluence may be responsible for the production of the solar r-process abundances at A = 124-126 (Qian et al. 1997; Haxton et al. 1997). Our results are based on the assumption that only the stable nuclei implanted into the diamonds are retained while the radioactive ones are lost from the diamonds upon decay after implantation (Ott 1996). We consider that the nanodiamonds are condensed in an environment with C/O > 1 in the expanding supernova debris or from the exterior H envelope. The implantation of nuclei would have occurred 10**4-10**6 s after r-process freeze-out. This time interval may be marginally sufficient to permit adequate cooling upon expansion for the formation of diamond grains. The mechanisms of preferential retention/loss of the implanted nuclei are not well understood.

astro-ph↗

arXiv1998

Mass signature of supernova $ν_μ$ and $ν_τ$ neutrinos in SuperKamiokande

The $ν_μ$ and $ν_τ$ neutrinos (and their antiparticles) from a Galactic core-collapse supernova can be observed in a water-Čerenkov detector by the neutral-current excitation of $^{16}$O. The number of events expected is several times greater than from neutral-current scattering on electrons. The observation of this signal would be a strong test that these neutrinos are produced in core-collapse supernovae, and with the right characteristics. In this paper, this signal is used as the basis for a technique of neutrino mass determination from a future Galactic supernova. The masses of the $ν_μ$ and $ν_τ$ neutrinos can either be measured or limited by their delay relative to the $\barν_e$ neutrinos. By comparing to the high-statistics $\barν_e$ data instead of the theoretical expectation, much of the model dependence is canceled. Numerical results are presented for a future supernova at 10 kpc as seen in the SuperKamiokande detector. Under reasonable assumptions, and in the presence of the expected counting statistics, $ν_μ$ and $ν_τ$ masses down to about 50 eV can be simply and robustly determined. The signal used here is more sensitive to small neutrino masses than the signal based on neutrino-electron scattering.

hep-ph↗

arXiv1998

Mass signature of supernova $ν_μ$ and $ν_τ$ neutrinos in the Sudbury Neutrino Observatory

Core-collapse supernovae emit of order $10^{58}$ neutrinos and antineutrinos of all flavors over several seconds, with average energies of 10--25 MeV. In the Sudbury Neutrino Observatory (SNO), which begins operations this year, neutrinos and antineutrinos of all flavors can be detected by reactions which break up the deuteron. For a future Galactic supernova at a distance of 10 kpc, several hundred events will be observed in SNO. The $ν_μ$ and $ν_τ$ neutrinos and antineutrinos are of particular interest, as a test of the supernova mechanism. In addition, it is possible to measure or limit their masses by their delay (determined from neutral-current events) relative to the $\barν_e$ neutrinos (determined from charged-current events). Numerical results are presented for such a future supernova as seen in SNO. Under reasonable assumptions, and in the presence of the expected counting statistics, a $ν_μ$ or $ν_τ$ mass down to about 30 eV can be simply and robustly determined. If zero delay is measured, then the mass limit is independent of the distance $D$. At present, this seems to be the best possibility for direct determination of a $ν_μ$ or $ν_τ$ mass within the cosmologically interesting range. We also show how to separately study the supernova and neutrino physics, and how changes in the assumed supernova parameters would affect the mass sensitivity.

hep-ph↗

arXiv1998

Supernovae as the Site of the r-Process: Implications for Gamma-Ray Astronomy

We discuss how detection of gamma-ray emission from the decay of r-process nuclei can improve our understanding of r-process nucleosynthesis. We find that a gamma-ray detector with a sensitivity of 10**(-7)/cm**2/s at 100-700 keV may detect the emission from the decay of Sb125, Cs137, Ce144, Eu155, and Os194 produced in a future Galactic supernova. In addition, such a detector may detect the emission from the decay of Sn126 in the Vela supernova remnant and the diffuse emission from the decay of Sn126 produced by past supernovae in our Galaxy. The required detector sensitivity is similar to what is projected for the proposed Advanced Telescope for High Energy Nuclear Astrophysics (ATHENA). Both the detection of gamma-ray emission from the decay of several r-process nuclei (e.g., Sb125 and Os194) produced in future Galactic supernovae and the detection of emission from the decay of Sn126 in the Vela supernova remnant would prove that supernovae are a site of the r-process. Furthermore, the former detection would allow us to determine whether or not the r-process nuclei are produced in relative proportions specified by the solar r-process abundance pattern in supernova r-process events. Finally, detection of diffuse emission from the decay of Sn126 in our Galaxy would eliminate neutron star/neutron star mergers as the main source for the r-process nuclei near mass number A=126.

astro-ph↗

arXiv1997

SMMC studies of N=Z pf-shell nuclei with pairing-plus-quadrupole Hamiltonian

We perform Shell Model Monte Carlo calculations of selected N=Z pf-shell nuclei with a schematic hamiltonian containing isovector pairing and quadrupole-quadrupole interactions. Compared to realistic interactions, this hamiltonian does not give rise to the SMMC ``sign problem'', while at the same time resembles essential features of the realistic interactions. We study pairing correlations in the ground states of N=Z nuclei and investigate the thermal dependence of selected observables for the odd-odd nucleus Co54 and the even-even nuclei Zn60 and Ni60. Comparison of the present results to those with the realistic KB3 interaction indicates a transition with increasing temperature from a phase of isovector pairing dominance to one where isoscalar pairing correlations dominate. In addition, our results confirm the qualitative reliability of the procedure used to cure the sign problem in the SMMC calculations with realistic forces.

nucl-th↗

arXiv1997

Gamow-Teller strength distributions in fp-shell nuclei

We use the shell model Monte Carlo method to calculate complete 0f1p-shell response functions for Gamow-Teller (GT) operators and obtain the corresponding strength distributions using a Maximum Entropy technique. The approach is validated against direct diagonalization for 48Ti. Calculated GT strength distributions agree well with data from (n,p) and (p,n) reactions for nuclei with A=48-64. We also calculate the temperature evolution of the GT+ distributions for representative nuclei and find that the GT+ distributions broaden and the centroids shift to lower energies with increasing temperature.

nucl-th↗

arXiv1997

Diverse Supernova Sources for the r-Process

(Abridged) It is shown that a semi-quantitative agreement with the gross solar r-process abundance pattern near and above mass number A=130 can be obtained by a superposition of two distinctive kinds of supernova r-process events. These correspond to a low frequency case L and a high frequency case H, which takes into account the low abundance of I129 and the high abundance of Hf182 in the early solar nebula. The lifetime of Hf182 associates the events in case H with the most common Type II supernovae. These events would be mainly responsible for the r-process nuclei near and above A=195. They would also make a significant amount of the nuclei between A=130 and 195, including Hf182, but very little I129. In order to match the solar r-process abundance pattern and to satisfy the I129 and Hf182 constraints, the events in case L, which would make the r-process nuclei near A=130 and the bulk of those between A=130 and 195, must occur 10 times less frequently but eject 10--20 times more r-process material in each event. We speculate that the usual neutron star remnants, and hence prolonged ejection of r-process material, are associated with the events in case L, whereas the more frequently occurring events in case H have ejection of other r-process material terminated by black hole formation during the neutrino cooling phase of the protoneutron star.

astro-ph↗

arXiv1997

Neutron-proton pairing in the BCS approach

We investigate the BCS treatment of neutron-proton pairing involving time-reversed orbits. We conclude that an isospin-symmetric hamiltonian, treated with the help of the generalized Bogolyubov transformation, fails to describe the ground state pairing properties correctly. In order for the np isovector pairs to coexist with the like-particle pairs, one has to break the isospin symmetry of the hamiltonian by artificially increasing the strength of np pairing interaction above its isospin symmetric value. We conjecture that the np isovector pairing represents part (or most) of the congruence energy (Wigner term) in nuclear masses.

nucl-th↗

arXiv1996

Neutrino-induced nucleosynthesis and the site of the r-process

If the r-process occurs deep within a Type II supernova, probably the most popular of the proposed sites, abundances of r-process elements may be altered by the intense neutrino flux. We point out that the effects would be especially pronounced for 8 isotopes that can be efficiently synthesized by the neutrino reactions following r-process freeze-out. We show that the observed abundances of these isotopes are entirely consistent with neutrino-induced nucleosynthesis, strongly arguing for a supernova r-process site. The deduced fluences place stringent constraints on the freeze-out radius and dynamic timescale of the r-process.

astro-ph↗

arXiv1996

Neutrino-induced neutron spallation and supernova r-process nucleosynthesis

In order to explore the consequences of the neutrino irradiation for the supernova r-process nucleosynthesis, we calculate the rates of charged-current and neutral-current neutrino reactions on neutron-rich heavy nuclei, and estimate the average number of neutrons emitted in the resulting spallation. Our results suggest that charged-current $ν_e$ captures can be important in breaking through the waiting-point nuclei at N=50 and 82, while still allowing the formation of abundance peaks. Furthermore, after the r-process freezes out, there appear to be distinctive neutral-current and charged-current postprocessing effects. A subtraction of the neutrino postprocessing effects from the observed solar r-process abundance distribution shows that two mass regions, A=124-126 and 183-187, are inordinately sensitive to neutrino postprocessing effects. This imposes very stringent bounds on the freeze-out radii and dynamic timescales governing the r-process. Moreover, we find that the abundance patterns within these mass windows are entirely consistent with synthesis by neutrino interactions. This provides a strong argument that the r-process must occur in the intense neutrino flux provided by a core-collapse supernova.

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arXiv1996

Neutron-Proton Correlations in an Exactly Solvable Model

We examine isovector and isoscalar neutron-proton correlations in an exactly solvable model based on the algebra SO(8). We look particularly closely at Gamow-Teller strength and double beta decay, both to isolate the effects of the two kinds of pairing and to test two approximation schemes: the renormalized neutron-proton QRPA (RQRPA) and generalized BCS theory. When isoscalar pairing correlations become strong enough a phase transition occurs and the dependence of the Gamow-Teller beta+ strength on isospin changes in a dramatic and unfamiliar way, actually increasing as neutrons are added to an N=Z core. Renormalization eliminates the well-known instabilities that plague the QRPA as the phase transition is approached, but only by unnaturally suppressing the isoscalar correlations. Generalized BCS theory, on the other hand, reproduces the Gamow-Teller strength more accurately in the isoscalar phase than in the usual isovector phase, even though its predictions for energies are equally good everywhere. It also mixes T=0 and T=1 pairing, but only on the isoscalar side of the phase transition.

nucl-th↗

arXiv1996

Pairing and Isospin Symmetry in Proton-Rich Nuclei

Unlike their lighter counterparts, most odd-odd N=Z nuclei with mass A > 40 40 have ground states with isospin T=1, suggesting an increased role for the isovector pairing interaction. A simple SO(5) seniority-like model of this interaction reveals a striking and heretofore unnoticed interplay between like-particle and neutron-proton isovector pairing near N=Z that is reflected in the number of each kind of pair as a function of A and T. Large scale shell-model calculations exhibit the same trends, despite the simultaneous presence of isoscalar pairs, deformation, and other correlations.

nucl-th↗

arXiv1996

Double beta decay of $^{48}$Ca

$^{48}$Ca, the lightest double beta decay candidate, is the only one simple enough to be treated exactly in the nuclear shell model. Thus, the $ββ(2ν)$ half-life measurement, reported here, provides a unique test of the nuclear physics involved in the $ββ$ matrix element calculation. Enriched $^{48}$Ca sources of two different thicknesses have been exposed in a time projection chamber, and yield T$_{1/2}^{2ν} = (4.3^{+2.4}_{-1.1} [{\rm stat.}] \pm 1.4 [{\rm syst.}]) \times 10^{19}$ years, compatible with the shell model calculations.

nucl-ex↗

arXiv1996

Neutrino induced transitions between the ground states of the A=12 triad

Neutrino induced reactions on $^{12}$C, an ingredient of liquid scintillators, have been studied in several experiments. We show that for currently available neutrino energies, $E_ν \le$ 300 MeV, calculated exclusive cross sections $^{12}$C$_{gs}(ν,l)$$^{12}$N$_{gs}$ for both muon and electron neutrinos are essentially model independent, provided the calculations simultaneously describe the rates of several other reactions involving the same states or their isobar analogs. The calculations agree well with the measured cross sections, which can be therefore used to check the normalization of the incident neutrino spectrum and the efficiency of the detector.

nucl-th↗

arXiv1995

Limit on T-Violating P-Conserving $ρN N$ Interaction from the $γ$ Decay of $^{57}$Fe

We use the experimental limit on the interference of M1 and E2 multipoles in the $γ$-decay of $^{57}$Fe to bound the time-reversal-violating parity-conserving $ρN N$ vertex. Our approach is a large-basis shell-model calculation of the interference. We find an upper limit on the parameter $\bar{g}_ρ$, the relative strength of the T-violating $ρN N$ vertex, of close to $10^{-2}$, a value similar to the best limits from other kinds of experiments.

nucl-th↗

arXiv1995

Signal for supernova $ν_μ$ and $ν_τ$ neutrinos in water Čerenkov detectors

We suggest that photons with energies between 5 and 10 MeV, generated by the ($ν,ν'pγ$) and ($ν,ν'nγ$) reactions on $^{16}$O, constitute a signal which allows a unique identification of supernova $ν_μ$ and $ν_τ$ neutrinos in water Čerenkov detectors. We calculate the yield of such $γ$ events and estimate that a few hundred of them would be detected in Superkamiokande for a supernova at 10 kpc distance.

nucl-th↗

arXiv1995

SMMC method for two-neutrino double beta decay

Shell Model Monte Carlo (SMMC) techniques are used to calculate two-neutrino double beta decay matrix elements. We validate the approach against direct diagonalization for $^{48}$Ca in the complete $pf$-shell using the KB3 interaction. The method is then applied to the decay of $^{76}$Ge in the $(0f_{5/2},1p,0g_{9/2})$ model space using a newly calculated realistic interaction. Our result for the matrix element is $0.13\pm0.05$ MeV$^{-1}$, in agreement with the experimental value.

nucl-th↗