[en] The ratio of the total x-ray scattering from Ne to that from He has been determined for photon energies in the range 4--15 keV at scattering angles of 45 and 90 degrees (corresponding to momentum transfers ranging from 0.90 to 5.69 a.u.). An arrangement of two gas cells in series was employed, allowing simultaneous measurements on both gases at the same scattering angle, which eliminates possible errors due to fluctuating beam intensity. Pairs of measurements corresponding to the same momentum transfer (at momentum transfers of 1.67 and 3.08 a.u.) but to different energies, provide a direct test of the corrections to the (momentum-transfer-dependent) form-factor incoherent-scattering-factor theory. These corrections include the anomalous dot p A contributions, which are found to be important. We also consider corrections to the usual approximations made within the inelastic A² theory (the incoherent-scattering factor, using closure approximation, and the impulse approximation, assuming free-particle kinematics with a given momentum distribution). In these cases an incoherent-scattering-factor treatment is generally adequate, while anomalous scattering factor corrections to form factors are needed for elastic scattering

[en] The spatial and temporal evolution of aluminum and aluminum monoxide produced during laser ablation of an aluminum sheet were tracked by way of emission and absorption spectroscopy. Atomic Al and molecular AlO temperatures were deduced from the emission spectroscopy data at several distances from the Al sheet surface. The atomic Al temperature was found to increase as a function of distance from the Al sheet surface and decrease over time. The molecular temperature of AlO remained approximately constant near the burning temperature of Al as a function of distance and time. The concentration of atomic Al was measured using absorption spectroscopy with peak concentrations ranging from 2.0x10¹⁸-1.2x10¹⁹ atoms/m³ as a function of distance from the Al sheet

[en] We present an application of absorption spectroscopy to directly measure temperature and concentration histories of water vapor within the expansion of a high explosive detonation. While the approach of absorption spectroscopy is well established, the combination of a fast, near-infrared array, broadband light source, and rigid gauge allow the first application of time-resolved absorption measurements in an explosive environment. The instrument is demonstrated using pentaerythritol tetranitrate with a sampling rate of 20 kHz for 20 ms following detonation. Absorption by water vapor is measured between 1335 and 1380 nm. Water temperatures are determined by fitting experimental transmission spectra to a simulated database. Water mole fractions are deduced following the temperature assignment. The sources of uncertainty and their impact on the results are discussed. These measurements will aid the development of chemical-specific reaction models and the predictive capability in technical fields including combustion and detonation science

[en] Elastic photon scattering from the ground state and various excited states of carbon atoms and ions has been investigated, using the S-matrix formalism, for incident photon energies ranging from 100 eV to 10 keV, contrasting the results obtained for different configurations. The excited states considered include hollow-atom states, where one or more inner shells are completely vacated. Ionic cases are considered as a limit of excitation. Results demonstrate how cross sections for different excited states group together according to shared properties of the configurations, such as the number of K electrons. Cross sections may exhibit deep dips below the K edge, depending on the occupation of the subshells corresponding to the strongest transitions. Scattering from excited states can have significantly larger cross sections than scattering from the ground state, particularly just below the K resonance region, and therefore it needs to be considered in situations where there is a large population of these excited states. Results are interpreted in terms of form-factor arguments and the qualitative behavior of individual subshell amplitudes. The angular dependence of cross sections can be understood in terms of angle-dependent form factors and anomalous scattering factors, taken to be angle independent. Cases are identified for which excited-state total integrated cross sections are much larger than the corresponding cross sections for scattering from the ground state. Our main results use an averaging over magnetic substates at the level of the amplitude, exact only for fully filled subshells, but generally appropriate for the carbon case considered, which simplifies the discussion and explains most of the general features. We also present results for a hollow lithium atom with and without this approximation to illustrate the differences that can arise in certain circumstances. (c) 2000 The American Physical Society

[en] ⁸³Nb has been studied in order to locate the upper extent of large deformation in the N∼Z∼40 region and ascertain its sign and magnitude. The decay scheme has been considerably extended by studying gamma decay following the ⁵⁸Ni(²⁸Si, p2n)⁸³Nb reaction at 204 and 215 MeV and using Gammasphere. Both signature partners of two rotational bands have been observed. The band properties support a I^π=(5/2)⁺ ground state spin assignment. The in-band branching ratios and multipole mixing ratios were found to be consistent with significant prolate deformation, as found in other N=42 isotones. The bands are particularly similar to the isotone ⁸¹Y. A projected shell model analysis has been performed which provides good reproduction of the features of both bands. The calculations indicate the nucleus to be nearly axially symmetric. ^82,83Nb may be the heaviest nuclei in the A∼80 region with large deformation. The deformation is predicted to fall in heavier systems approaching ¹⁰⁰Sn. The conditions appear to be right for the occurrence of K isomers

[en] Input functions required for positron emission tomography (PET) tracer kinetic modeling are often obtained from arterial blood. In some situations, using short-lived radiotracers, e.g. [¹⁵O]water, rapid sample handling is required. A method used at several facilities is to pump blood through a detector system at a constant rate. We investigate the suitability of a commercial radiochromatography module (IN/US Posi-RAM) for this new use. The Posi-RAM consists of two 2.5 cm (length) x 2.5 cm (diameter) cylindrical bismuth germanate (BGO) detectors that can operate in coincidence mode. Arterial blood is transported through the system via a length of tubing with flow rate controlled by a peristalsis pump. A custom-counting loop and support frame were designed for the Posi-RAM for PET studies. System sensitivity was determined to be 1.1 x 10⁴ cps/(MBq ml^-1). Dead time as a function of count-rate was found to be less than 1% for concentrations below 3.5 MBq ml^-1, a range encompassing all human-study values. In a human study, the performance of the device was found to be similar to that of the facility's current blood monitor (Siemens Fluid Monitor). We conclude that the Posi-RAM has the necessary sensitivity and count-rate capabilities to be used as a real-time blood activity monitor