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The photoabsorption cross section and ground-state branching ratio of l42Nd were measured using quasimonoenergetic y-ray beams at several beam energies. Two peaks corresponding to the pygmy dipole resonance (PDR) were identified. The branching ratios were compared to statistical-model calculations. We found that the Brink hypothesis is violated, and that the branching ratios are only reproduced by introducing a possible new decay mode of the observed PDR. [ABSTRACT FROM AUTHOR]

The 48Ca(γ,n) cross section was measured by using γ-ray beams of energies between 9.5 and 15.3 MeV generated at the Triangle Universities Nuclear Laboratory high-intensity γ-ray source. Prior to this experiment, no direct measurements had been made with γ-ray beams of sufficiently low energy spread to observe a structure in this energy range. The cross sections were measured at 34 different γ-ray energies with an enriched 48Ca target. Neutron emission is the dominant decay mechanism in the measured energy range that spans from the threshold, across the previously identified M1 strength, and up the low-energy edge of the E1 giant dipole resonance. This paper found B(M1) = 6.8 ± Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. for the 10.23-MeV resonance, a value greater than previously measured. Structures in the cross section have been observed whose magnitudes are in agreement with existing data and that are commensurate with extended random-phase approximation calculations. [ABSTRACT FROM AUTHOR]

Measurements of the 9Be(γ,no) reaction were performed using nearly 100% linearly polarized, high-intensity, and nearly monoenergetic γ-ray beams having energies between 5.5 and 15.5 MeV at the High Intensity γ-ray Source located at Duke University and Triangle Universities Nuclear Laboratory. Eighteen liquid scintillator detectors were used to measure neutron yields parallel and perpendicular to the plane of beam polarization. Polarization asymmetries, which are the differences between yields observed in detectors located in-plane and out-of-plane divided by their sums, were measured for the neutrons which left the residual nucleus (8Be) in its ground state, termed the n0 group. Asymmetries between 0.4 to 0.7 were discovered over this energy region in addition to a clear trend of increasing asymmetries with increasing beam energy. A prediction of the polarization asymmetry based on a pure E1 direct capture model shows good agreement with the experimental measurements. These data and the prediction could be of interest for methods that rely on neutron measurements following photofission to identify the presence of special nuclear material. [ABSTRACT FROM AUTHOR]

Measurement of a resonance's integrated cross section using nuclear resonance fluorescence can be a valuable tool for verifying the properties of the resonance because of the clear and unambiguous physical connection to the spin, lifetime, and ground state branching ratio of the level. We demonstrate this idea by measuring the integrated cross section of the 3.004-MeV level in 27Al to 4% using the monoenergetic γ-ray beam at the High Intensity γ-ray Source. That level was the subject of much debate experimentally in the 1960s, especially its spin, and even now only has a current tentative spin assignment of J = (9/2). The consistency check between this integrated cross section and the known properties of the level indicate that one (or more) of the literature properties is incorrect. Based on the range of extent of each property, a reassignment of spin to a tentative J = (7/2) may be warranted, but this would need to be confirmed with other measurements. This result demonstrates the utility of NRF as a way to verify the properties of states in the literature before undertaking more extensive measurements. [ABSTRACT FROM AUTHOR]

Photofission experiments were performed on targets of 232Th, 233,235,238U, 237Np, and 239,240pu using nearly 100% linearly polarized, high intensity (~5 x 106γ/s), and nearly-monoenergetic γ-ray beams having energies between 5.3 and 7.6 MeV at the High Intensity γ-ray Source (HlγS) located at Duke University and Triangle Universities Nuclear Laboratory. An array of 12-18 liquid scintillator detectors was used to measure prompt fission neutron yields parallel and perpendicular to the plane of beam polarization. Polarization asymmetries, the differences between the in-plane and out-of-plane yields divided by their sums, were measured. Asymmetries close to zero were found for 233,235U, 237Np, and 239Pu while significant asymmetries (~0.2-0.5) were found for 232Th, 238U, and 240Pu. Predictions based on previously measured photofission fragment angular distributions combined with a model of prompt neutron emission agree well with the experimental results. [ABSTRACT FROM AUTHOR]

The first measurement of the three-body photodisintegration of polarized ³He using a circularly polarized photon beam has been performed at incident energies of 12.8 and 14.7 MeV. This measurement was carried out at the high-intensity γ-ray source located at Triangle Universities Nuclear Laboratory. A high-pressure ³He target, polarized via spin exchange optical pumping with alkali metals, was used in the experiment. The spin-dependent double- and single-differential cross sections from ³He(γ,n)pp for laboratory angles varying from 30° to 165° are presented and compared with state-of-the-art three-body calculations. The data reveal the importance of including the Coulomb interaction between protons in the three-body calculations. [ABSTRACT FROM AUTHOR]

The photofission cross section of 238U was measured at sub-barrier energies as a function of the γ-ray energy using a monochromatic, high-brilliance, Compton-backscattered γ-ray beam. The experiment was performed at the High Intensity γ-ray Source (HIγS) facility at beam energies between Eγ = 4.7 MeV and 6.0 MeV and with ~3% energy resolution. Indications of transmission resonances have been observed at γ-ray beam energies of Eγ = 5.1 MeV and 5.6 MeV with moderate amplitudes. The triple-humped fission barrier parameters of 238U have been determined by fitting EMPIRE-3.1 nuclear reaction code calculations to the experimental photofission cross section. [ABSTRACT FROM AUTHOR]

Models of the r process are sensitive to the production rate of 9Be because, in explosive environments rich in neutrons, α(αn, γ)9Be is the primary mechanism for bridging the stability gaps at A = 5 and A = 8. The α(αn, γ)9Be reaction represents a two-step process, consisting of α +α → 8Be followed by 8Be(n, γ)9Be. We report here on a new absolute cross-section measurement for the 9Be(γ, rc)8Be reaction conducted using a highly efficient, 3He-based neutron detector and nearly monoenergetic photon beams, covering energies from Eγ = 1.5 MeV to Eγ = 5.2 MeV, produced by the High Intensity γ-ray Source of Triangle Universities Nuclear Laboratory. In the astrophysically important threshold energy region, the present cross sections are 40% larger than those found in most previous measurements and are accurate to ±10% (95% confidence). The revised thermonuclear α(αn, γ)9Be reaction rate could have implications for the r process in explosive environments such as type II supernovae. [ABSTRACT FROM AUTHOR]

Nuclear resonance fluorescence experiments were carried out at the High-Intensity γ-ray Source facility at Triangle Universities Nuclear Laboratory to characterize the low-energy dipole structure of 238U using 100% linearly polarized photon beams from 2.0 to 6.2 MeV. 113 transitions corresponding to de-excitations to the ground state in 238U were observed and the energy, spin, parity, integrated cross section, reduced width, and branching ratio were determined for each of these identified levels. The total E1 γ-ray interaction cross section was calculated and it was deduced that the observed concentration of low-lying El transitions were excited from the low-energy tail of the giant dipole resonance and were not a pygmy dipole resonance. Comparisons were made between quasiparticle random-phase approximation calculations and the experimentally observed strength. The observed and predicted M1 strength agreed well with each other. However, there was no similar agreement for the £1 strength. [ABSTRACT FROM AUTHOR]

A photofission experiment was performed on targets of 235U, 238U, 239Pu, and 232Th using nearly 100% linearly polarized, high-intensity (∼107 &ggr;/s), and nearly monoenergetic &ggr;-ray beams having energies between 5.6 and 7.3 MeV at the High Intensity &ggr;-ray Source (Hl&ggr;S). An array of 18 liquid scintillator detectors was used to measure prompt fission neutron polarization asymmetries. An asymmetry close to zero was found for 235U and 239Pu while a significant asymmetry (∼0.5) was found for 238U and 232Th. A simplified model of near-threshold photofission has been developed in an attempt to explain the systematic difference between the even-even and even-odd targets. The results of a simulation, based on this model and using previous measurements of fission fragment angular distributions, are shown to accurately reproduce the essential features of the data. [ABSTRACT FROM AUTHOR]

Gamma-ray transitions in 235 U were measured using the (γ, γ′ reaction below 3 MeV. The nuclear-resonance- fluorescence experiment was carried out at the High-Intensity γ-ray Source facility using nearly monoenergetic and circularly polarized photon beams. More than 20 transitions corresponding to deexcitations to the ground state and low-lying levels in 235U were observed. The integrated cross sections to the excited levels and intensities of branching transitions were deduced. The experimental results are compared with predictions from a quasiparticle random-phase approximation in a deformed basis. [ABSTRACT FROM AUTHOR]

Properties of low-energy dipole states in 232Th have been investigated with the nuclear resonance fluorescence technique. The present work used monoenergetic γ-ray beams at energies of 2-4 MeV from the high-intensity γ-ray source at Triangle Universities Nuclear Laboratory. Over 40 transitions corresponding to deexcitation to the ground state and first excited state were observed for the first time. Excitation energies, integrated cross sections, decay widths, branching ratios, and transition strengths for those states in 232Th were determined and compared with quasiparticle random-phase-approximation calculations. A large number of E1 transitions were observed for the first time in actinide nuclei with summed strength of 3.28(69)×10-3 e2 fm2. The observed summed M1 strength of 4.26(63)μN2 is in good agreement with the other actinides and with the systematics of the scissors mode in deformed rare-earth nuclei. [ABSTRACT FROM AUTHOR]