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A carbon-rich black layer, dating to Ϸ12.9 ka, has been previously identified at Ϸ50 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 Х12.9 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 Ϸ12.9 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.comet ͉ iridium ͉ micrometeorites ͉ nanodiamonds ͉ spherules

Radiative strength functions (RSFs) for the 56,57 Fe nuclei below the separation energy are obtained from the 57 Fe( 3 He, αγ) 56 Fe and 57 Fe( 3 He, 3 He ′ γ) 57 Fe reactions, respectively. An enhancement of more than a factor of ten over common theoretical models of the soft (Eγ < ∼ 2 MeV) RSF for transitions in the quasicontinuum (several MeV above the yrast line) is observed. Two-step cascade intensities with soft primary transitions from the 56 Fe(n, 2γ) 57 Fe reaction confirm the enhancement.PACS numbers: 25.40. Lw, 25.55.Hp, 25.20.Lj, 27.40.+z Unresolved transitions in the nuclear γ-ray cascade produced in the decay of excited nuclei are best described by statistical concepts: a radiative strength function (RSF) f XL (E γ ) for a transition with multipolarity XL and energy E γ , and a level density ρ(E i , J π i ) for initial states i at energy E i with equal spin and parity J π i yield the mean value of the partial decay width to a given final state f [1]Most information about the RSF has been obtained from photon-absorption experiments in the energy interval 8-20 MeV, i.e., for excitations above the neutron separation energy S n . There, the giant electric dipole resonance (GEDR) is dominant. Data on the soft (E γ < 3-4 MeV) RSF for transitions in the quasicontinuum (several MeV above the yrast line) remain elusive. Corresponding data from discrete transitions show large fluctuations and are biased toward high transition strengths due to experimental thresholds. First data in the statistical regime have been obtained from the 147 Sm(n, γα) 144 Nd reaction [2]. They indicate a moderate enhancement of the soft E1 RSF compared to a Lorentzian extrapolation of the GEDR. For spherical nuclei, in the framework of Fermiliquid theory, this enhancement is explained by a temperature dependence of the GEDR width [3], the Kadmenskiȋ-Markushev-Furman (KMF) model. However, the experimental technique requires the presence of sufficiently large α widths and depends on estimates of both α and total radiative widths in the quasicontinuum below S n . The sequential extraction method developed at the Oslo Cyclotron Laboratory (OCL) [4] has enabled further investigations of the soft RSF by providing unique data for transitions in the quasicontinuum with sufficient averaging. For deformed rare-earth nuclei, it has been shown that the RSF can be described in terms of a KMF GEDR model, a spin-flip giant magnetic dipole resonance (GMDR), and a soft M 1 resonance [5,6]. In this work, we report on the first observation of a strong enhancement of the soft RSF in 56,57 Fe over the sum of the GEDR and GMDR models. This enhancement has been found in Oslo-type experiments and is confirmed independently by two-step cascade (TSC) measurements. To our knowledge, there exists at present no theoretical model which can explain an enhancement of this magnitude.The first experiment, the 57 Fe( 3 He, 3 He ′ γ) 57 Fe and 57 Fe( 3 He,αγ) 56 Fe reactions, was carried out with 45-MeV 3 He ions at the OCL. Particle-γ coincidences were measured ...

Photon strength functions describing the average response of the nucleus to an electromagnetic probe are key input information in the theoretical modelling of nuclear reactions. Consequently they are important for a wide range of fields such as nuclear structure, nuclear astrophysics, medical isotope production, fission and fusion reactor technologies. They are also sources of information for widely used reaction libraries such as the IAEA Reference Input Parameter Library and evaluated data files such as EGAF.arXiv:1910.06966v1 [nucl-ex] 15 Oct 2019 Fig. 1 (Color online) Schematic representation on how NLDs and PSFs are extracted from the primary γ-ray spectrum. The firstgeneration γ-ray distribution (yellow triangle) is given by the product of the level density ρ(E i − E γ ) and the γ-ray transmission coefficient T γ (E γ ). All values of the elements of the ρ(E i − E γ ) and T γ (E γ ) vectors are allowed to vary in order to give the best fit to the P(E γ , E i ) landscape.