High precision measurements of the differential cross sections for π0 photoproduction at forward angles for two nuclei, 12C and 208Pb, have been performed for incident photon energies of 4.9-5.5 GeV to extract the π0→γγ decay width. The experiment was done at Jefferson Lab using the Hall B photon tagger and a high-resolution multichannel calorimeter. The π0→γγ decay width was extracted by fitting the measured cross sections using recently updated theoretical models for the process. The resulting value for the decay width is Γ(π0→γγ)=7.82±0.14(stat)±0.17(syst) eV. With the 2.8% total uncertainty, this result is a factor of 2.5 more precise than the current Particle Data Group average of this fundamental quantity, and it is consistent with current theoretical predictions.
High-resolution computed tomography (HRCT) is widely used to assess pulmonary parenchymal disease in adults. The authors used ultrafast CT with 3-mm collimation and a 100-msec scan time to obtain HRCT scans in 36 children (mean age, 49.4 months). Clinical diagnoses included normal lungs (n = 6), cystic fibrosis (n = 12), obliterative bronchiolitis (n = 6), idiopathic pulmonary hemosiderosis (n = 2), and other lung diseases (n = 10). The HRCT scans and chest radiographs were reviewed separately in blinded fashion. Pulmonary parenchymal abnormalities were categorized into interstitial, airspace, and airway processes. Nine patients with normal chest radiographs had abnormal HRCT scans. In five other patients, the extent of abnormal lung parenchyma was considerably greater on HRCT scans than on chest radiographs. HRCT scans allowed accurate characterization of the type of lung process in 24 of 30 patients when compared with clinical or biopsy findings. Early HRCT findings in cystic fibrosis included lobular air trapping, bronchial wall thickening, and centrilobular nodules. The use of cine CT with high-resolution techniques is feasible in children too young or too sick to hold their breath. HRCT may enable early detection and characterization of pulmonary disease and depiction of the extent of lung abnormality.
A theoretical model of the positive column of a rare gas glow discharge is used to calculate metastable atom densities in the pressure x radius range from 0.01 to 1.0 Torr cm. Two body quenching is shown to be an important loss process for metastables in neon and argon discharges above 0.1 Torr cm. Measurements of metastable atom flux from an anode slit are reported for the title gases and shown to represent the metastable density in the anode fall region of the discharges.
From time-of-Aight spectra of metastable Ar* atoms emerging from sustained discharges in Ar, we observe discrete nonthermal peaks due to excited products of dissociative recombination (DR) of Arz+ formed by reactions in the discharge. These peaks are in addition to the expected Boltzmann distributions of metastable atoms from direct excitation by fast primary electrons. From the peak velocities, and thus the kinetic energies released in the dissociation, the DR product states are uniquely determined, regardless of subsequent radiation. As we have previously found from Krz [A. Barrios et al. , Phys. Rev. Lett. 69, 1348(1992], most of the excited DR products are (n+ 1)s states, but Arz+ also yields smaller amounts of 4p and 3d states. We have also recently shown [G. B. Ramos et a/. , Phys. Rev. A 51, 2945 (1995)] that, surprisingly, in studies of Ne2+ and Arz+ DR, a substantial but undetermined fraction of reactions go to a purely ground-state channel. In addition, we have found that under certain conditions in Ar discharges, the relative amount of the higher excited-state products, including 4p, 3d, 5s, and 5p can be dramatically influenced. The velocity peaks also become narrower than expected from the thermal distribution of the parent Ar2+ as the energy level in the product atom increases. These studies in argon discharges have included variations in source pressure, discharge voltage, and strength of the axial magnetic field.PACS number(s): 34.50.Lf
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