Form Factors and Two-Photon Exchange in High-Energy Elastic Electron-Proton Scattering.

We present new precision measurements of the elastic electron-proton scattering cross section for momentum transfer (Q^{2}) up to 15.75 (GeV/c)^{2}. Combined with existing data, these provide an improved extraction of the proton magnetic form factor at high Q^{2} and double the range over which a longitudinal or transverse separation of the cross section can be performed. The difference between our results and polarization data agrees with that observed at lower Q^{2} and attributed to hard two-photon exchange (TPE) effects, extending to 8 (GeV/c)^{2} the range of Q^{2} for which a discrepancy is established at >95% confidence. We use the discrepancy to quantify the size of TPE contributions needed to explain the cross section at high Q^{2}.

Transmission of megawatt relativistic electron beams through millimeter apertures.

High-power, relativistic electron beams from energy-recovering linacs have great potential to realize new experimental paradigms for pioneering innovation in fundamental and applied research. A major design consideration for this new generation of experimental capabilities is the understanding of the halo associated with these bright, intense beams. In this Letter, we report on measurements performed using the 100 MeV, 430 kW cw electron beam from the energy-recovering linac at the Jefferson Laboratory's Free Electron Laser facility as it traversed a set of small apertures in a 127 mm long aluminum block. Thermal measurements of the block together with neutron measurements near the beam-target interaction point yielded a consistent understanding of the beam losses. These were determined to be 3 ppm through a 2 mm diameter aperture and were maintained during a 7 h continuous run.

The effect of cryosurgery and polymethylmethacrylate in dogs with experimental bone defects comparable to tumor defects.

The effects of liquid nitrogen (LN) and polymethylmethacrylate (PMMA) on normal bone, bone graft incorporation, and reossification were evaluated by simulating a tumor in dogs with experimental bone cavity. Ten skeletally mature mongrel dogs (20 femora) were divided into three groups: Group I, controls; Group II, LN (with and without bone graft); and Group III, PMMA (with and without LN). Roentgenograms, whole-mount histology, and tetracycline fluorescence studies were performed on the distal femur. Correlation of these studies showed that (1) marked trabecular and bone necrosis, extending 7-12 mm around the circumference of the cavity, developed by three and seven weeks after LN but no bony necrosis occurred after PMMA; (2) the pattern of reossification following cryosurgery was delayed and abnormal, demonstrating increased calcification and metaplastic bone formation; (3) cryosurgery decreased the rate of bone graft incorporation; (4) the cryonecrotic rim following cryosurgery correlated with an abortive attempt at peripheral reossification; and (5) cryosurgery had no effect on the articular cartilage. Cryosurgery is effective in causing bone necrosis, whereas PMMA is not, and the pattern of reossification is delayed and altered by freezing. This study suggests that microvascular thrombosis with subsequent ischemic infarction of bone is a major cause of bone necrosis following cryosurgery.