Jason Pruet

Nucleosynthesis in Early Supernova Winds II: The Role of Neutrinos

One of the outstanding unsolved riddles of nuclear astrophysics is the origin of the so-called p-process nuclei from A = 92 to 126. Both the lighter and heavier p-process nuclei are adequately produced in the neon and oxygen shells of ordinary Type II supernovae, but the origin of these intermediate isotopes, especially 92,94Mo and 96,98Ru, has long been mysterious. Here we explore the production of these nuclei in the neutrino-driven wind from a young neutron star. We consider such early times that the wind still contains a proton excess because the rates for νe and positron captures on neutrons are faster than those for the inverse captures on protons. Following a suggestion by Frohlich and coworkers, we also include the possibility that—in addition to the protons, α-particles, and heavy seed—a small flux of neutrons is maintained by the reaction p(e, e+)n. This flux of neutrons is critical in bridging the long waiting points along the path of the rp-process by (n, p) reactions. Using the unmodified ejecta histories from a recent two-dimensional supernova model by Janka and coworkers, we find synthesis of p-rich nuclei up to 102Pd, although our calculations do not show efficient production of 92Mo. If the entropy of these ejecta is increased by a factor of 2, the synthesis extends to 120Te. Still larger increases in entropy, which might reflect the role of magnetic fields or vibrational energy input neglected in the hydrodynamical model, result in the production of nuclei up to A ≈ 170. Elements synthesized in these more extreme outflows include numerous s- and p-process nuclei, and even some r-process nuclei can be synthesized in these proton-rich conditions.

Detecting clandestine material with nuclear resonance fluorescence

We study the performance of a class of interrogation systems that exploit nuclear resonance fluorescence (NRF) to detect specific isotopes. In these systems the presence of a particular nuclide is inferred by observing the preferential attenuation of photons that strongly excite an electromagnetic transition in that nuclide. Estimates for the false positive/negative error rates, radiological dose, and detection sensitivity associated with discovering clandestine material embedded in cargo are presented. The relation between performance of the detection system and properties of the beam of interrogating photons is also considered. Bright gamma-ray sources with fine energy and angular resolution, such as those based on Thomson upscattering of laser light, are found to be associated with uniquely low radiological dose, scan times, and error rates. For this reason a consideration of NRF-based interrogation systems may provide impetus for efforts in light source development for applications related to national security and industry.

NUCLEOSYNTHESIS IN GAMMA-RAY BURST ACCRETION DISKS

We follow the nuclear reactions that occur in the accretion disks of stellar-mass black holes that are accret- ing at a very high rate, 0.01-1 Ms � 1 , as is realized in many current models for gamma-ray bursts (GRBs). The degree of neutronization in the disk is a sensitive function of the accretion rate, black hole mass, Kerr parameter, and disk viscosity. For high accretion rates and low viscosity, material arriving at the black hole will consist predominantly of neutrons. This degree of neutronization will have important implications for the dynamics of the GRB-producing jet and perhaps for the synthesis of the r-process. For lower accretion rates and high viscosity, as might be appropriate for the outer disk in the collapsar model, neutron-proton equality persists, allowing the possible synthesis of 56 Ni in the disk wind. 56 Ni must be present to make any optically bright Type I supernova and, in particular, those associated with GRBs. Subject headings: accretion, accretion disks — gamma rays: bursts — nuclear reactions, nucleosynthesis, abundances

Nucleosynthesis in Outflows from the Inner Regions of Collapsars

We consider nucleosynthesis in outflows originating from the inner regions of viscous accretion disks formed after the collapse of a rotating massive star. We show that windlike outflows driven by viscous and neutrino heating can efficiently synthesize Fe group elements moving at near-relativistic velocities. The mass of 56Ni synthesized and the asymptotic velocities attained in our calculations are in accord with those inferred from observations of SN 1998bw and SN 2003dh. These steady windlike outflows are generally proton-rich, characterized by only modest entropies, and consequently synthesize essentially nothing heavier than the Fe group elements. We also discuss bubble-like outflows resulting from rapid energy deposition in localized regions near or in the accretion disk. These intermittent ejecta emerge with low electron fraction and are a promising site for the synthesis of the A = 130 r-process peak elements.

Light-Element Synthesis in High-Entropy Relativistic Flows Associated with Gamma-Ray Bursts

We calculate and discuss the light-element freezeout and nonthermal reaction nucleosynthesis in high-entropy winds and fireballs for broad ranges of entropy per baryon, dynamic timescales characterizing relativistic expansion, and neutron-to-proton ratios. With conditions characteristic of gamma-ray bursts (GRBs), we find that deuterium production can be prodigious, with final abundance values 2H/H 2%, depending on the fireball isospin, late-time dynamics, and the effects of neutron-decoupling-induced high-energy nonthermal nuclear reactions. This implies that there could potentially be detectable local enhancements in the deuterium abundance associated with GRB events.

On the Origin of the Lightest Molybdenum Isotopes

We discuss implications of recent precision measurements for the 93Rh proton separation energy for the production of the lightest molybdenum isotopes in proton-rich Type II supernova (SN II) ejecta. It has recently been shown that a novel neutrino-induced process makes these ejecta a promising site for the production of the light molybdenum isotopes and other p-nuclei with atomic mass near 100. The origin of these isotopes has long been uncertain. A distinguishing feature of nucleosynthesis in neutrino-irradiated outflows is that the relative production of 92Mo and 94Mo is set by a competition governed by the proton separation energy of 93Rh. We use the detailed nuclear network calculations and the recent experimental results for this proton separation energy to place constraints on the outflow characteristics that produce the lightest molybdenum isotopes in their solar proportions. It is found that for the conditions calculated in recent two-dimensional SN simulations, and also for a large range of outflow characteristics around these conditions, the solar ratio of 92Mo to 94Mo cannot be achieved. This suggests that either proton-rich winds from SNe II do not exclusively produce both isotopes, or that these winds are qualitatively different than calculated in todays SN models.

On the Contribution of Gamma Ray Bursts to the Galactic Inventory of Some Intermediate Mass Nuclei

Light curves from a growing number of gamma-ray bursts (GRBs) indicate that GRBs copiously produce radioactive Ni moving outward at fractions of the speed of light. We calculate nuclear abundances of elements accompanying the outflowing Ni under the assumption that this Ni originates from a wind blown off of a viscous accretion disk. We also show that GRBs likely contribute appreciably to the Galactic inventory of 42Ca, 45Sc, 46Ti, 49Ti, and 63Cu, and may be an important site for the production of 64Zn.

Neutrinos from the Propagation of a Relativistic Jet through a Star

We discuss the neutrino signature of a relativistic jet propagating through a stellar envelope, a scenario realized in the collapsar model for gamma-ray bursts (GRBs). It is shown that the dramatic slowing of the jet deep within the star is accompanied by inelastic neutron-nucleon collisions and the conversion of a substantial fraction of the jet kinetic energy to neutrinos. These neutrinos have observed energies in the range from 2 to tens of GeV and an estimated detection rate comparable to or larger than the detection rate of GeV neutrinos from other GRB-related processes. The time delay between the arrival of these neutrinos and the GRB photons is tens of seconds. An observation of this delay would provide an indication that the GRB jet originated in a massive star.

Effects of nonequilibrium particle distributions in deuterium-tritium burning

We investigate the effects of non-equilibrium particle distributions resulting from rapid deuterium-tritium burning in plasmas using a Fokker-Planck code that incorporates small-angle Coulomb scattering, Brehmsstrahlung, Compton scattering, and thermal-nuclear burning. We find that in inertial confinement fusion environments, deviations away from Maxwellian distributions for either deuterium or tritium ions are small and result in 1% changes in the energy production rates. The deuterium and tritium effective temperatures are not equal, but differ by only about 2.5% near the time of peak burn rate. Simulations with high Z (Xe) dopants show that the dopant temperature closely tracks that of the fuel. On the other hand, fusion product ion distributions are highly non-Maxwellian, and careful treatments of energy-exchange between these ions and other particles is important for determining burn rates.

Nucleosynthesis in Early Neutrino Driven Winds

Two recent issues realted to nucleosynthesis in early proton‐rich neutrino winds are investigated. In the first part we investigate the effect of nuclear physics uncertainties on the synthesis of 92Mo and 94Mo. Based on recent experimental results, we find that the proton rich winds of the model investigated here can not be the only source of the solar abundance of 92Mo and 94Mo. In the second part we investigate the nucleosynthesis from neutron rich bubbles and show that they do not contribute to the nucleosynthesis integrated over both neutron and proton‐rich bubbles and proton‐rich winds.