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The accurately measured ages of 89 large impact craters and layers were compared with the boundary dates for Periods, Epochs and Ages of the Geological Time Scale by a weighted least squares fit. They are highly correlated with a χ2/f=0.63. A Monte Carlo simulation of randomly chosen crater ages gives a >99.8% probability that this result is not random. No craters are found in the oceans or, until recently, in ice which collectively cover 80% of Earth’s surface indicating that far more impacts have occurred than are known. Multiple impacts cluster near the times of boundary dates so, based on the observed cluster sizes assuming a binomial distribution, it was determined that the average cluster multiplicity is five. Comparison of the impact ages with the dates of the great extinctions revealed a strong correlation with χ2/f=0.36 and a multiplicity of 8-9 impacts. It is shown that volcanism, although correlated with boundary dates, is a continuous process unrelated to sudden extinctions. During the past 125 Ma the rate of global change and the impact rate have increased dramatically as the Earth passes near the OB star association. Multiple impacts 12.9 ka ago ended the Pleistocene epoch at the onset of the Younger Dryas (YD) causing worldwide extinctions. The date and extent of the YD impact may be consistent with a 62 Ma cycle of major impact events. During the Holocene 20 crater, airburst, and impact tsunami chevron ages correspond to dates of global cooling with a χ2/f=0.75 and >99% probability. Future impacts could reverse global warming or even induce next ice age.

An evaluation of thermal neutron capture on the stable tungsten isotopes is presented, with preliminary results for the compound systems 183;184;185;187W. The evaluation procedure compares the g-ray cross-section data collected at the Budapest reactor, with Monte Carlo simulations of g-ray emission following the thermal neutron-capture process. The statistical-decay code DICEBOX was used for the Monte Carlo simulations. The evaluation yields new gamma rays in 185W and the confirmation of spins in 187W, raising the number of levels below which the level schemes are considered complete, thus increasing the number of levels that can be used in neutron data libraries.

A carbon-rich black layer, dating to ≈12.9 ka, has been previously identified at ≈50 Clovis-age sites across North America and appears contemporaneous with the abrupt onset of Younger Dryas (YD) cooling. The in situ bones of extinct Pleistocene megafauna, along with Clovis tool assemblages, occur below this black layer but not within or above it. Causes for the extinctions, YD cooling, and termination of Clovis culture have long been controversial. In this paper, we provide evidence for an extraterrestrial (ET) impact event at ≅12.9 ka, which we hypothesize caused abrupt environmental changes that contributed to YD cooling, major ecological reorganization, broad-scale extinctions, and rapid human behavioral shifts at the end of the Clovis Period. Clovis-age sites in North American are overlain by a thin, discrete layer with varying peak abundances of ( i ) magnetic grains with iridium, ( ii ) magnetic microspherules, ( iii ) charcoal, ( iv ) soot, ( v ) carbon spherules, ( vi ) glass-like carbon containing nanodiamonds, and ( vii ) fullerenes with ET helium, all of which are evidence for an ET impact and associated biomass burning at ≈12.9 ka. This layer also extends throughout at least 15 Carolina Bays, which are unique, elliptical depressions, oriented to the northwest across the Atlantic Coastal Plain. We propose that one or more large, low-density ET objects exploded over northern North America, partially destabilizing the Laurentide Ice Sheet and triggering YD cooling. The shock wave, thermal pulse, and event-related environmental effects (e.g., extensive biomass burning and food limitations) contributed to end-Pleistocene megafaunal extinctions and adaptive shifts among PaleoAmericans in North America.

The neutron capture reaction is useful in identifying and analyzing the gamma‐ray spectrum from an unknown assembly as it gives unambiguous information on its composition. This can be done passively or actively where an external neutron source is used to probe an unknown assembly. There are known capture gamma‐ray data gaps in the ENDF libraries used by transport codes for various nuclear applications. The Evaluated Gamma‐ray Activation file (EGAF) is a new thermal neutron capture database of discrete line spectra and cross sections for over 260 isotopes that was developed as part of an IAEA Coordinated Research Project. EGAF is being used to improve the capture gamma production in ENDF libraries. For medium to heavy nuclei the quasi continuum contribution to the gamma cascades is not experimentally resolved. The continuum contains up to 90% of all the decay energy and is modeled here with the statistical nuclear structure code DICEBOX. This code also provides a consistency check of the level scheme nuclear structure evaluation. The calculated continuum is of sufficient accuracy to include in the ENDF libraries. This analysis also determines new total thermal capture cross sections and provides an improved RIPL database. For higher energy neutron capture there is less experimental data available making benchmarking of the modeling codes more difficult. We are investigating the capture spectra from higher energy neutrons experimentally using surrogate reactions and modeling this with Hauser‐Feshbach codes. This can then be used to benchmark CASINO, a version of DICEBOX modified for neutron capture at higher energy. This can be used to simulate spectra from neutron capture at incident neutron energies up to 20 MeV to improve the gamma‐ray spectrum in neutron data libraries used for transport modeling of unknown assemblies.

Improved neutron‐capture γ‐ray spectra, not only of interest to the nuclear structure and reactions communities, are needed in a variety of applied and non‐proliferation programs. This requires an evaluation of the existing experimental capture‐γ data. Elemental neutron‐capture data taken from direct measurements at the Budapest Reactor have been used to collate the Evaluated Gamma‐ray Activation File, a database of capture γ‐ray cross sections. These cross sections are then compared to Monte Carlo simulations of γ‐ray emission following the thermal neutron‐capture process using the statistical‐decay code DICEBOX. The aim of this procedure is to obtain the total radiative neutron‐capture cross section and confidently increase the number of levels and γ rays that can be assigned to a given isotope in the neutron data libraries. To achieve these goals and provide as complete information as possible in the neutron data libraries, it is also necessary to remain current with recent advances in nuclear structur...

Four supernovae (SNe), exploding ≤300 pc from Earth, were recorded 44, 37, 32, and 22 kyr ago in the radiocarbon (14C) record during the past 50 kyr. Each SN left a nearly identical signature in the record, beginning with an initial sudden increase in atmospheric radiocarbon, when the SN exploded, followed by a hiatus of 1500 yr, and concluding with a sustained 2000 yr increase in global radiocarbon due to γ-rays produced by diffusive shock in the SN remnant (SNR). For the past 18 kyr excess radiocarbon has decayed with the 14C half-life. SN22kyrBP, is identified as the Vela SN that exploded 250 ± 30 pc from Earth. These SN are confirmed in the 10Be, 26Al, 36Cl, and NO geologic records. The rate of near-Earth SNe is consistent with the observed rate of historical SNe giving a galactic rate of 14 ± 3 kyr–1 assuming the Chandra Galactic Catalog SNR distribution. The Earth has been used as a calorimeter to determine that ≈2 × 1049 erg were released as γ-rays at the time of each SN explosion and ≈1050 erg in γ-rays following each SN. The background rate of 14C production by cosmic rays has been determined as 1.61 atoms cm–2 s–1. Approximately 1/3 of the cosmic ray energy produced by diffusive shock in the SNR was observed to be emitted as high-energy γ-rays. Analysis of the 10Be/9Be ratio in marine sediment identified 19 additional near-Earth SNe that exploded 50-300 kyr ago. Comparison of the radiocarbon record with global temperature variations indicated that each SN explosion is correlated with a concurrent global warming of ≈3°C-4°C.

The properties of even-even nuclei with 30 {le} Z {le} 82, A {ge} 60 have been examined to find examples displaying the characteristics of E(5) critical-point behavior for the shape transition from a spherical vibrator to a triaxially soft rotor. On the basis of the known experimental state energies and E2 transition strengths, the best candidates that were identified are {sup 102}Pd, {sup 106,108}Cd, {sup 124}Te, {sup 128}Xe, and {sup 134}Ba. The closest agreement between experimental data and the predictions of E(5) is for {sup 128}Xe and for the previously suggested example of {sup 134}Ba. It is proposed that {sup 128}Xe may be a new example of a nucleus at the E(5) critical point.