Absorption (logic)

About: Absorption (logic) is a research topic. Over the lifetime, 5733 publications have been published within this topic receiving 236302 citations.

Model for Nuclear Reactions with Neutrons

Abstract: A simple model is proposed for the description of the scattering and the compound nucleus formation by nucleons impinging upon complex nuclei. It is shown that, by making appropriate averages over resonances, an average problem can be defined which is referred to as the "gross-structure" problem. Solution of this problem permits the calculation of the average total cross section, the cross section for the formation of the compound nucleus, and the part of the elastic-scattering cross section which does not involve formation of the compound nucleus. Unambiguous definitions are given for the latter cross sections.The model describing these properties consists in replacing the nucleus by a one-body potential which acts upon the incident nucleon. This potential $V={V}_{0}+i{V}_{1}$ is complex; the real part represents the average potential in the nucleus; the imaginary part causes an absorption which describes the formation of the compound nucleus. As a first approximation a potential is used whose real part ${V}_{0}$ is a rectangular potential well and whose imaginary part is a constant fraction of the real part ${V}_{1}=\ensuremath{\zeta}{V}_{0}$.This model is used to reproduce the total cross sections for neutrons, the angular dependence of the elastic scattering, and the cross section for the formation of the compound nucleus. It is shown that the average properties of neutron resonances, in particular the ratio of the neutron width to the level spacing, are connected with the gross-structure problem and can be predicted by this model.The observed neutron total cross sections can be very well reproduced in the energy region between zero and 3 Mev with a well depth of 42 Mev, a factor $\ensuremath{\zeta}$ of 0.03, and a nuclear radius of $R=1.45\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}13}{A}^{\frac{1}{3}}$ cm. The angular dependence of the scattering cross section at 1 Mev is fairly well reproduced by the same model. The theoretical and experimental values for the ratios of neutron width to level distance at low energies and the reaction cross sections at 1 Mev do not agree too well but they show a qualitative similarity.

Size dependence of exciton fine structure in CdSe quantum dots

Abstract: We use photoluminescence excitation and fluorescence line narrowing spectroscopies to examine structure observed in the band-edge absorption feature of CdSe quantum dots. We study eight samples ranging from \ensuremath{\sim}15 to \ensuremath{\sim}50 \AA{} in radius to probe the size dependence of this structure. We compare our results with recent theories, which predict band-edge exciton splittings in CdSe dots due to their internal crystal structure, nonspherical shape, and the exchange interaction between the electron and hole. We find reasonable agreement between our data and theory, supporting the observation of exciton fine structure. \textcopyright{} 1996 The American Physical Society.

Interband Magneto-Absorption and Faraday Rotation in InSb

Abstract: Direct interband magneto-optical transitions have been observed in pure $n$-type InSb at liquid-helium temperature using magnetic fields up to 96.5 kG. The theory of the magnetic levels in the valence and conduction bands at the zone center is carried out using a modification of the method of Luttinger and Kohn. The interaction between conduction and valence bands has been treated exactly, and the effect of higher bands to order ${k}^{2}$. Selection rules have been evaluated for both allowed ($\ensuremath{\Delta}n=0, \ensuremath{-}2$) and warping-induced ($\ensuremath{\Delta}n=2, \ifmmode\pm\else\textpm\fi{}4, \ensuremath{-}6$) direct valence-to-conduction-band transitions. Comparison of theoretical with experimental spectra (using circular and plane-polarized light in the Faraday and Voigt configurations, respectively) has shown good agreement for the magnetic field in the [110] and [100] crystal directions. The following band parameters are obtained: conduction-band effective mass ${m}_{\mathrm{c}}=0.0145{m}_{0}$: light-hole effective mass ${m}_{l.\mathrm{h}.}=0.0160{m}_{0}$; heavy-hole effective masses ${m}_{\mathrm{h}.\mathrm{h}.}[100]=0.32{m}_{0}$, ${m}_{\mathrm{h}.\mathrm{h}.}[110]=0.42{m}_{0}$, ${m}_{\mathrm{h}.\mathrm{h}.}[111]=0.44{m}_{0}$.

Optical Constants of Silicon in the Region 1 to 10 ev

Abstract: The reflectance, ${|r(\ensuremath{\lambda})|}^{2}$, of single crystal silicon was measured in the range 1 to 11.3 ev. The phase, $\ensuremath{\theta}(\ensuremath{\lambda})$, was computed from these data using the Kramers-Kronig relation between the real and imaginary parts of the complex function $\mathrm{ln}r=\mathrm{ln}|r|+i\ensuremath{\theta}$. The optical constants, $n$ and $k$, were then determined from the Fresnel reflectivity equation. The real part of the refractive index, $n$, shows a sharp maximum of magnitude 6.9 at 3.3 ev. The extinction coefficient, $k$, shows maxima of magnitude 3.1 at 3.5 ev and 5.1 at 4.3 ev; optical absorption above 3 ev is associated with the onset of strong direct transitions. The results indicate that much useful information, applicable to band structure calculations for both silicon and germanium, could be obtained from limited reflectance studies (2 to 5 ev) on Ge-Si alloys.

The Baryon Census in a Multiphase Intergalactic Medium: 30% of the Baryons May Still Be Missing

Abstract: Although galaxies, groups, and clusters contain {approx}10% of the baryons, many more reside in the photoionized and shocked-heated intergalactic medium (IGM) and in the circumgalactic medium (CGM). We update the baryon census in the (H I) Ly{alpha} forest and warm-hot IGM (WHIM) at 10{sup 5-6} K traced by O VI {lambda}1032, 1038 absorption. From Enzo cosmological simulations of heating, cooling, and metal transport, we improve the H I and O VI baryon surveys using spatially averaged corrections for metallicity (Z/Z {sub Sun }) and ionization fractions (f {sub HI}, f {sub OVI}). Statistically, the O VI correction product correlates with column density, (Z/Z {sub Sun })f {sub OVI} Almost-Equal-To (0.015)(N {sub OVI}/10{sup 14} cm{sup -2}){sup 0.70}, with an N {sub OVI}-weighted mean of 0.01, which doubles previous estimates of WHIM baryon content. We also update the Ly{alpha} forest contribution to baryon density out to z = 0.4, correcting for the (1 + z){sup 3} increase in absorber density, the (1 + z){sup 4.4} rise in photoionizing background, and cosmological proper length dl/dz. We find substantial baryon fractions in the photoionized Ly{alpha} forest (28% {+-} 11%) and WHIM traced by O VI and broad-Ly{alpha} absorbers (25% {+-} 8%). The collapsed phasemore » (galaxies, groups, clusters, CGM) contains 18% {+-} 4%, leaving an apparent baryon shortfall of 29% {+-} 13%. Our simulations suggest that {approx}15% reside in hotter WHIM (T {>=} 10{sup 6} K). Additional baryons could be detected in weaker Ly{alpha} and O VI absorbers. Further progress requires higher-precision baryon surveys of weak absorbers, down to minimum column densities N {sub HI} {>=} 10{sup 12.0} cm{sup -2}, N {sub OVI} {>=} 10{sup 12.5} cm{sup -2}, N {sub OVII} {>=} 10{sup 14.5} cm{sup -2}, using high signal-to-noise data from high-resolution UV and X-ray spectrographs.« less