Photofission of heavy nuclei at energies up to 4 GeV

Total photofission cross sections for 238U, 235U, 233U, 237Np, 232Th, and natPb have been measured simultaneously, using tagged photons in the energy range Egamma=0.17-3.84 GeV. This was the first experiment performed using the Photon Tagging Facility in Hall B at Jefferson Lab. Our results show that the photofission cross section for 238U relative to that for 237Np is about 80% over the entire energy range, implying the presence of important processes which compete with fission. If we assume that for 237Np the photofission probability is equal to unity, we observe a significant shadowing effect, starting below 1.5 GeV.     

Diffusion of long molecules through solid polyethylene. II. Measurements and results

The diffusion coefficient for two diffusants of the type n-(CH₂)NX is measured, where N ? 25, 45 and X is a suitable label; the matrix is linear polyethylene cooled at two widely differing rates from the melt. The measuring technique used is a recently developed one based on infrared microdensitometry. The results indicate that these diffusants diffuse faster in the slowly cooled matrix, in marked contrast to the behavior of gaseous diffusants. They are in agreement with the predictions of the model developed in a previous paper (part I), in which the constraints imposed by interlamellar tie molecules on long diffusants were shown to be important.     

The Kondo effect in low temperature deposited Cu(Fe) films

Highly disordered Cu(Fe) films were prepared by condensing a dilute Cu(Fe) alloy onto a lHe temperature substrate. The Kondo temperature T′_K was computed by fitting the data to the Applebaum-Kondo resistivity formula. The computed T′_K was found to be the same as that of bulk, crystalline Cu(Fe).     

Superconducting tunneling in PdH

Tunneling measurements have been made on superconducting PdH films. The measured2Δ₀/kT_c ratio was 3.80 ± 0.15 meV. Structure was found in the tunneling density of states at an energy of 48 ± 2 meV, approximately the energy of optic phonons in PdH.     

Pion scattering on 3H and 3He nuclei in the Δ33(1232) region

Elastic scattering of π ^± mesons on ³H and ³He nuclei at energies around the Δ₃₃(1232) resonance is considered. The amplitudes for single and double scattering are calculated on the basis of the nonrelativistic diagram technique. The spin and isospin structure of pion-nucleon amplitudes is fully taken into account. The resulting theoretical predictions for the differential cross sections are compared with available experimental data.     

Model dependence of single-energy fits to pion photoproduction data

Model dependence of multipole analysis has been explored through energy-dependent and single-energy fits to pion photoproduction data. The MAID energy-dependent solution has been used as input for an event generator producing realistic pseudo data. These were fitted using the SAID parametrization approach to determine single-energy and energy-dependent solutions over a range of lab photon energies from 200 to 1200MeV. The resulting solutions were found to be consistent with the input amplitudes from MAID. Fits with a χ-squared per datum of unity or less were generally achieved. We discuss energy regions where consistent results are expected, and explore the sensitivity of fits to the number of included single- and double-polarization observables. The influence of Watson’s theorem is examined in detail.     

Measurement of the total and differential cross sections for the reaction π − p → ηn with the Crystal Ball detector

The first results obtained in 1998 by the Crystal Ball collaboration from a measurement of the total and differential cross sections for the reaction π ^? p → ηn are presented. These new experimental results for the total cross sections are compared with the predictions of the K-matrix model for pion-nucleon scattering. The angular distribution at momenta near the reaction threshold (685 MeV/c) is determined by the S-wave contribution. The P-wave contribution begins to manifest itself from a momentum of 720 MeV/c.     

Applying grazing incidence X-ray reflectometry (XRR) to characterising nanofilms on mica

Molecularly smooth mica has hitherto not been widely used as a substrate for the X-ray reflectometry (XRR) technique. That is largely due to the difficulty of achieving flatness over a sufficiently large area of mica. Here we show that this difficulty can be overcome by slightly bending the mica substrate over an underlying cylinder; the enhanced rigidity of the bent mica sheet along the axis of the cylinder provides sufficient flatness along this axis for XRR measurements. To test this method, we have employed it to characterise three types of nanofilms on mica in air: (A) Cr-Au thin films; (B) a surface-grown zwitterionic polymer brush; and (C) a Langmuir-Blodgett (LB) phospholipid monolayer, using a table-top X-ray reflectometer. Fitting the obtained reflectivity curves with the standard Parratt algorithm allows us to extract the structural information of the nanofilms (both thickness and apparent roughness). Our simple method points to how XRR may be exploited as a useful characterisation tool for nanofilms on mica.     

Search for medium modifications of the rho meson

The photoproduction of vector mesons on various nuclei has been studied using the CLAS detector at Jefferson Laboratory. The vector mesons, rho, omega, and varphi, are observed via their decay to e;{+}e;{-}, in order to reduce the effects of final-state interactions in the nucleus. Of particular interest are possible in-medium effects on the properties of the rho meson. The rho mass spectrum is extracted from the data on various nuclei, 2H, C, Fe, and Ti. We observe no significant mass shift and some broadening consistent with expected collisional broadening for the rho meson.