Strong Evidence for

The boundaries of the chart of nuclides contain exotic isotopes that possess extreme proton-to-neutron asymmetries. Here we report on strong evidence of ^{9}N, one of the most exotic proton-rich isotopes where more than one half of its constitute nucleons are unbound. With seven protons and two neutrons, this extremely proton-rich system would represent the first-known example of a ground-state five-proton emitter. The invariant-mass spectrum of its decay products can be fit with two peaks whose energies are consistent with the theoretical predictions of an open-quantum-system approach; however, we cannot rule out the possibility that only a single resonancelike peak is present in the spectrum.

Observation of the Exotic Isotope

A ^{13}F resonance was observed following a charge-exchange reaction between a fast ^{13}O beam and a ^{9}Be target. The resonance was found in the invariant-mass distribution of 3p+^{10}C events and probably corresponds to a 5/2^{+} excited state. The ground state was also expected to be populated, but was not resolved from the background. The observed level decays via initial proton emissions to both the ground and first 2^{+} state of ^{12}O, which subsequently undergo 2p decay. In addition, there may also be a significant proton decay branch to the second 2^{+} level in ^{12}O. The wave function associated with the observed level may be collectivized due to coupling to the continuum as is it located just above the threshold for proton decay to the 2_{2}^{+} state of ^{12}O.

First Observation of the Four-Proton Unbound Nucleus

^{18}Mg was observed, for the first time, by the invariant-mass reconstruction of ^{14}O+4p events. The ground-state decay energy and width are E_{T}=4.865(34) MeV and Γ=115(100) keV, respectively. The observed momentum correlations between the five particles are consistent with two sequential steps of prompt 2p decay passing through the ground state of ^{16}Ne. The invariant-mass spectrum also provides evidence for an excited state at an excitation energy of 1.84(14) MeV, which is likely the first excited 2^{+} state. As this energy exceeds that for the 2^{+} state in ^{20}Mg, this observation provides an argument for the demise of the N=8 shell closure in nuclei far from stability. However, in open systems this classical argument for shell strength is compromised by Thomas-Ehrman shifts.

First Observation of Unbound

The structure of the extremely proton-rich nucleus _{8}^{11}O_{3}, the mirror of the two-neutron halo nucleus _{3}^{11}Li_{8}, has been studied experimentally for the first time. Following two-neutron knockout reactions with a ^{13}O beam, the ^{11}O decay products were detected after two-proton emission and used to construct an invariant-mass spectrum. A broad peak of width ∼3.4 MeV was observed. Within the Gamow coupled-channel approach, it was concluded that this peak is a multiplet with contributions from the four lowest ^{11}O resonant states: J^{π}=3/2_{1}^{-}, 3/2_{2}^{-}, 5/2_{1}^{+}, and 5/2_{2}^{+}. The widths and configurations of these states show strong, nonmonotonic dependencies on the depth of the p-^{9}C potential. This unusual behavior is due to the presence of a broad threshold resonant state in ^{10}N, which is an analog of the virtual state in ^{10}Li in the presence of the Coulomb potential. After optimizing the model to the data, only a moderate isospin asymmetry between ground states of ^{11}O and ^{11}Li was found.

Observation of long-range three-body coulomb effects in the decay of (16)Ne.

The interaction of an E/A=57.6-MeV ^{17}Ne beam with a Be target is used to populate levels in ^{16}Ne following neutron knockout reactions. The decay of ^{16}Ne states into the three-body ^{14}O+p+p continuum is observed in the High Resolution Array (HiRA). For the first time for a 2p emitter, correlations between the momenta of the three decay products are measured with sufficient resolution and statistics to allow for an unambiguous demonstration of their dependence on the long-range nature of the Coulomb interaction. Contrary to previous measurements, our measured limit Γ<80 keV for the intrinsic decay width of the ground state is not in contradiction to the small values (of the order of keV) predicted theoretically.

Democratic decay of 6Be exposed by correlations.

The interaction of an E/A=70-MeV (7)Be beam with a Be target was used to populate levels in (6)Be following neutron knockout reactions. The three-body decay of the ground and first excited states into the α+p+p exit channel were detected in the High Resolution Array. Precise three-body correlations extracted from the experimental data allowed us to obtain insight into the mechanism of the three-body democratic decay. The correlation data are in good agreement with a three-cluster-model calculation and thus validate this theoretical approach over a broad energy range.

Propagation in planar waveguides and the effects of wall roughness.

We consider the propagation of guided waves in a planar waveguide that has smooth walls except for a finite length segment that has random roughness. Maxwell's equations are solved in the frequency domain for both TE and TM polarization in 2-D by using modal expansion methods. Obtaining numerical solutions is facilitated by using perfectly matched boundary layers and the {\bf R}-matrix propagator. Varying lengths of roughness segments are considered and numerical results are obtained for guided wave propagation losses due to roughness induced scattering. The roughness on each waveguide boundary is numerically generated from an assumed Gaussian power spectrum. The guided waves are excited by a Gaussian beam incident on the waveguide aperture. Considerable numerical effort is given to determine the stability of the algorithm.

Infrared light scattering from surfaces covered with multiple dielectric overlayers.

Formulas are derived, using first-order perturbation theory, which predict light scattering from substrates coated with multiple dielectric overlayers. The scattering is assumed to result from surface irregularity. Numerical analysis is given for three types of dielectric stack configurations designed to typify experimental situations. The three configurations are substrate profile replicated at each dielectric interface, profile present at outermost dielectric layer only, and random roughness present at each interface. The formulas and configurations are used to model low efficiency rectangular groove gratings as beam sampling optical components and multilayer-overcoated metallic mirror substrates. Consideration is given to thermal expansion and fabrication error of thicknesses of the dielectric layers.

Calculation of the power spectral density from surface profile data.

The power spectral density (PSD), in its two-dimensional form, has been designated as the preferred quantity for specifying surface roughness on a draft international drawing standard for surface texture. The correct calculation of the one-dimensional PSD from discrete surface profile data is given, and problems in using fast Fourier-transform routines that are given in some of the standard reference books are flagged. The method given here contains the correct normalizing factors. Two ways to reduce the variance of the PSD estimate are suggested. Examples are shown of the variance reduction possible in the PSD's.

Intrinsic and roughness-induced absorption of electromagnetic radiation incident on optical surfaces.

The intrinsic and the surface-induced optical absorption of typical metals are calculated for various wavelengths and roughness parameters as a function of incident angle. In the infrared region the surface-induced absorption is <10% of the total absorption and is insensitive to the wavelength for high-quality mirrors. However, for mirrors of certain periodic structure the presence of a surface plasma resonance can generate substantial absorption. Absorption caused by Gaussian-shaped surface defects is also considered. The results are discussed with some experiments in laser damage research.

Wavelength and angular dependence of light scattering from beryllium: comparison of theory and experiment.

The purpose of this study was (1) to measure angle-resolved scattering (ARS) and the wavelength dependence of scattering from polished and sputtered beryllium, (2) to measure surface-roughness parameters by profilometry, (3) to calculate the power spectral density (PSD) of the surface roughness and use it with ARS theory to compare with ARS measurements, and (4) to investigate a dual-source scattering theory and compare calculations from the theory with measured ARS. The wavelengths considered here were 0.633, 1.06, and 10.6 µm, the angle of incidence was 60°, and s and p polarization of the incident and scattered radiation were taken into account both in the theory and in the experiments.Two types of theoretical comparison were attempted. First, two-dimensional PSD functions were calculated from one-dimensional, mechanical-stylus-profile data, and these data were used in first-order roughness ARS theory to compare with ARS measurements. The calculated ARS curves were much too low at some wavelengths and showe incorrect wavelength scaling. Second, a dual-source (surfaceroughness and dielectric inhomogeneity) ARS theory was used. In this model the surface-roughness PSD functions calculated from surface-profile data were used, and reasonable values for parameters characterizing the dielectric inhomogeneities were used in an analytic PSD function. This dual-source ARS theory can predict a significantly different angle and wavelength dependence of ARS than that predicted by roughness-only ARS theory. In contrast to ARS predicted by surface roughness alone, it was found that the dual-source theory was able to provide good agreement with measured ARS in angular and wavelength dependence. It is concluded that, for the two beryllium samples studied here, scattering caused by spatial variations in the near-surface dielectric response (variations in reflectance) is a significant and sometimes dominant source of scattering, especially at a 10.6-µim wavelength.

Light scattering from multilayer optics: comparison of theory and experiment.

Measurements of angular scattering due to surface roughness were taken from a 24-layer dielectric mirror and compared to theory. In addition, the top surface roughness of the multilayer stack is analyzed from Talystep profilometer measurements. These roughness data are used to obtain a roughness spectral density function to be used in a vector multilayer scattering theory. The theory uses three multilayer stack models to incorporate possible effects of different degrees of correlation between interfaces of the stack. It was found that the angular scattering calculated using the experimentally obtained roughness spectral density function agreed remarkably well with the measured angular scattering data. This is especially true if care is taken to differentiate between particulate and roughness scattering. For the sake of comparison, the angular scattering from an aluminum film is also given, and differences from scattering from the multilayer mirror are noted.