Neutron-rich Heavy Nuclei Research Articles

The thermal runaway r-process

view Abstract Citations (30) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The thermal runaway r-process Cowan, J. J. ; Cameron, A. G. W. ; Truran, J. W. Abstract One or more processes of the type of the rapid neutron capture process (r-process) have been responsible for synthesizing the neutron-rich heavy nuclei. It has been assumed that the r-process must be associated in some way with supernova explosions. However, recent work has suggested that a nonsupernova site should also be considered as a possibility for an r-process. The results of the present investigation demonstrate the possibility that an r-process can occur as a result of a thermal runaway process in a degenerate helium gas, with C-13 providing both the energy and the neutron source. It is found that the general character of the r-process abundance distribution can be reproduced, starting with a solar system abundance distribution of the heavy elements, if a series of localized flashes takes place in the helium region, with mixing throughout the region between flashes, or in a single flash if the neutron source is sufficiently strong. Publication: The Astrophysical Journal Pub Date: January 1982 DOI: 10.1086/159562 Bibcode: 1982ApJ...252..348C Keywords: Helium; Neutron Sources; Nuclear Fusion; Particle Production; Stellar Evolution; Thermal Neutrons; Abundance; Heavy Elements; Photodissociation; Radioactive Decay; Supernovae; Astrophysics full text sources ADS |

Nucleosynthesis of neutron-rich heavy nuclei during explosive helium burning in massive stars

The production of heavy nuclei during explosive helium burning has been calculated using a hydrodynamical model of a 15-solar mass (Type II) supernova and an n-process nuclear reaction network. It is found that the resulting neutron-rich heavy nuclei are not produced in the relative abundances of solar-system r-process material, especially in the vicinity of Pt, nor are any actinides produced. These deficiencies reflect an inadequate supply of neutrons. However, some neutron-rich isotopes, normally associated with the r-process, are produced which may be significant for the production of isotopic anomalies in meteorites.

Expansion of neutron star matter and its final nuclear composition

The final nuclear composition of the matter expanding from the density of a neutron star is investigated. It is assumed that starquakes cause the cracks which penetrate the neutron star crust and that the neutron star fluid can flow out through the cracks into space. The change with time of the nuclear composition of this matter is calculated by use of the compressible nuclear mass formula, and the hydrodynamics of the system is followed by the effect of nuclear transformation with time of the second fission of heavy neutron-rich nuclei, which is followed by a rapid rise to above 109 K. If the value of the β-strength function exceeds about 10−5.5 MeV−1 s−1, the system proceeds to a state of nuclear equilibrium in the later expansion stage and the nuclear composition is reshuffled, finally to be transformed into neutron-excess, stable nuclei within the atomic mass region 80≲A≳120. It also becomes clear that if the strength function has a value smaller than the above critical value, then the neutron-rich nuclides withA∈[200, 400] are copiously produced. These results will also be applied in the cases of a neutron-star-black-hole collision and the explosion of a neutron star associated with the catastrophic phase transition within the neutron star core. The astrophysical implications are briefly discussed.

The decompression of cold neutron star matter

view Abstract Citations (208) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The decompression of cold neutron star matter. Lattimer, J. M. ; Mackie, F. ; Ravenhall, D. G. ; Schramm, D. N. Abstract The ejection of cold neutron-star matter is examined, and an attempt is made to determine whether the final composition of this matter may be similar to that normally associated with the hot high-neutron-flux r-process. A semiempirical liquid-drop model is used for the nucleus, and the equilibrium composition of the matter is determined by assuming it to be in its absolute ground state at a given density. Physical mechanisms operating during the expansion are analyzed, and the composition of the ejected matter is found as a function of its density during expansion. The results indicate that it is virtually impossible for deuterium to form, that neutrons can be captured only after beta decay increases the atomic numbers of nuclei, and that no free neutrons can escape. It is concluded that neutron-star ejecta can produce heavy neutron-rich nuclei and may produce somewhat heavier nuclei than a standard r-process. Publication: The Astrophysical Journal Pub Date: April 1977 DOI: 10.1086/155148 Bibcode: 1977ApJ...213..225L Keywords: Black Holes (Astronomy); Expansion; Neutron Stars; Nuclear Reactions; Stellar Evolution; Density (Mass/Volume); Heavy Nuclei; Matter (Physics); Stellar Mass Ejection; Stellar Models; Stellar Temperature; Supernovae; Astrophysics full text sources ADS |

Additional results concerning nuclei in the terminal parts of the r-process path

An investigation of the nuclear stability of heavy neutron-rich nuclei, on the basis of more recent model refinements and the microscopic-macroscopic method, gives additional support to an earlier suggestion regarding the unlikelihood of the synthesis of superheavy elements by the r-process. The use of microscopically calculated inertial masses gives some probability for a moderate longevity around Z = 110, N = 172.

Mass asymmetric fission and the termination of the astrophysical r-process

We use a recent calculation of the mass-asymmetric fission barriers for heavy neutron-rich nuclei to estimate the heavy to light mass fragment ratio ( M H M L ) resulting from fission at the termination of the astrophysical r-process. The fragments due to mass-asymmetric fission at the termination of the r-process may account for the solar system abundance distribution of neutron-rich nuclei in the mass region 150 ⪅ A ⪅ 170.

Cooling of Dense Stars

Cooling rates are first calculated for neutron stars of about 1 M⊙ and 10 km radius, with magnetic fields from zero to about 1014 G, for two extreme cases of maximum and no superfluidity. The results show that most pulsars are so cold that thermal ionization of surface atoms would be negligible. Next, nucleon superfluidity and crystallization of heavy nuclei are treated more quantitatively, and more realistic hadron star models are chosen. Cooling rates are thus calculated for a stable hyperon star near the maximum mass limit, a medium weight neutron star, and a light neutron star with neutron-rich heavy nuclei near the minimum mass limit. Results show that cooling rates are a sensitive function of density. The lightest star is cooler than others in earlier stages but the trend is reversed later. The medium weight star is generally the coldest of all in lower temperature regions where the effect of superfluidity becomes significant. However, if a heavy star contains pions, its cooling will be even faster. The Crab pulsar and Vela pulsar, expected to be the two youngest, can be as hot as (2 ∼ 4) x 106 K (on the surface), comparable with the results obtained from internal frictional heating by Greenstein, if they are medium weight to heavy hadron stars. However, older pulsars are cold. In fact, at about a few million years, the age of average radio pulsars, the surface temperature becomes only several hundred to several thousand degrees. Thus, the earlier conclusions about cold pulsars are still valid. Cooling of a massive white dwarf star is also shown.

Production of Superheavy Nuclei by Multiple Capture of Neutrons

A new calculation of fission barriers and neutron separation energies for heavy neutron-rich nuclei indicates that it is unlikely that superheavy nuclei can be produced either in the astrophysical r'' process or in man-made nuclear explosions. (auth)

Possible Paths for Synthesis of Superheavy Elements in Nature

We discuss the possibility of producing superheavy elements in the astrophysical r-process. Thus, we consider a detailed calculation of fission-barrier heights and neutron separation energies for heavy neutron-rich nuclei. The dependence of the neutron-induced fission cutoff of the r-process on the uncertainty in the nuclear models is discussed in some detail.

Fission and the Synthesis of Heavy Nuclei by Rapid Neutron Capture

The role of fission is examined in the synthesis of heavy nuclei by multiple capture of neutrons in thermo-nuclear explosions. We begin by reviewing evidence from the recent Tweed and Cyclamen experiments indicating that neutron-induced fission is a serious source of depletion in neutron capture chains which start from targets of $^{242}\mathrm{Pu}$ and $^{243}\mathrm{Am}$. An analysis of Tweed abundances (Sec. 2) is made to obtain capture-to-fission ratios for the odd-$A$ plutonium isotopes through $A=253$. We next use the liquid-drop model of Myers and Swiatecki plus empirical shell corrections and pairing energies, in order to correlate and predict spontaneous fission lifetimes (Sec. 3) and fission barriers (Sec. 4). For nuclei having $Z\ensuremath{\le}101$ and $N\ensuremath{\le}157$, we extrapolate the shell correction, assuming it to be a function of $N$ plus a function of $Z$, and thus obtain neutron binding energies, fission barriers, and spontaneous fission lifetimes for neutron-rich heavy nuclei (Sec. 6). Capture-to-fission ratios are estimated for many of these nuclei in Sec. 7, and qualitative agreement is found with laboratory and Tweed results. In Sec. 8, the Sec. 8, extrapolation is continued out to $N=159$ and $Z=104$. We conclude that by using the liquid-drop model plus semiempirical shell corrections, one can obtain capture-to-fission ratios and spontaneous fission half-lives which are usefully accurate. However, for predicting properties of nuclei having $Z>104$, $N\ensuremath{\gtrsim}159$, one needs, in this formalism, an accurate way of predicting shell corrections or nuclear masses.