Measurement of the Charge-Averaged Elastic Lepton-Proton Scattering Cross Section by the OLYMPUS Experiment

We report the first measurement of the average of the electron-proton and positron-proton elastic scattering cross sections. This lepton charge-averaged cross section is insensitive to the leading effects of hard two-photon exchange, giving more robust access to the proton's electromagnetic form factors. The cross section was extracted from data taken by the OLYMPUS experiment at DESY, in which alternating stored electron and positron beams were scattered from a windowless gaseous hydrogen target. Elastic scattering events were identified from the coincident detection of the scattered lepton and recoil proton in a large-acceptance toroidal spectrometer. The luminosity was determined from the rates of Møller, Bhabha, and elastic scattering in forward electromagnetic calorimeters. The data provide some selectivity between existing form factor global fits and will provide valuable constraints to future fits.

Hard Two-Photon Contribution to Elastic Lepton-Proton Scattering Determined by the OLYMPUS Experiment

The OLYMPUS Collaboration reports on a precision measurement of the positron-proton to electron-proton elastic cross section ratio, R_{2γ}, a direct measure of the contribution of hard two-photon exchange to the elastic cross section. In the OLYMPUS measurement, 2.01 GeV electron and positron beams were directed through a hydrogen gas target internal to the DORIS storage ring at DESY. A toroidal magnetic spectrometer instrumented with drift chambers and time-of-flight scintillators detected elastically scattered leptons in coincidence with recoiling protons over a scattering angle range of ≈20° to 80°. The relative luminosity between the two beam species was monitored using tracking telescopes of interleaved gas electron multiplier and multiwire proportional chamber detectors at 12°, as well as symmetric Møller or Bhabha calorimeters at 1.29°. A total integrated luminosity of 4.5  fb^{-1} was collected. In the extraction of R_{2γ}, radiative effects were taken into account using a Monte Carlo generator to simulate the convolutions of internal bremsstrahlung with experiment-specific conditions such as detector acceptance and reconstruction efficiency. The resulting values of R_{2γ}, presented here for a wide range of virtual photon polarization 0.456<ε<0.978, are smaller than some hadronic two-photon exchange calculations predict, but are in reasonable agreement with a subtracted dispersion model and a phenomenological fit to the form factor data.

Note: Rigid holder to host and bend a crystal for multiple volume reflection of a particle beam

A holder to lodge and bend a silicon crystal to excite multivolume reflection of a high-energy particle beam has been designed and fabricated. A mechanically robust and stable structure fastens a crystal at best condition for experiments. The holder has allowed the observation of 12-time repeated volume reflection with very high efficiency. We detail the most important features behind the construction of the holder together with the characterization of the crystal being bent by the holder.

Photochemically driven models of oxygenases based on the use of iron porphyrins

Photoexcited iron porphyrins can be used to mimic the catalytic activity of cytochrome P-450 oxygenases both in the reduction of halogenated alkanes and in the oxidation of hydrocarbons by O2 itself at room temperature and atmospheric pressure. The results reported indicate that there is a similarity and a complementarity of photonic activation with other more conventional methods of activation of model systems of oxygenases. In fact, by irradiating at suitable wavelengths, it is possible to induce redox reactions which mimic those of natural oxygenases, avoiding the difficulties caused by parallel processes which could be expected when chemical reagents are used. These processes occur with good turnover values of the photocatalyst and in mild temperature and pressure conditions. By controlling the reaction environment, it is possible to address the selectivity of the process. In this regard, the very recent results obtained after heterogenization of the photocatalysts inside membranes of Nafion are particularly promising for the development of new biomimetic photocatalysts in heterogeneous or organized systems.

Atmospheric degradation of HFCs and HCFCs: chemistry and noxiousness

Hydrohaloalkanes are potential alternatives to the fully halogenated compounds (CFCs) that are believed to be responsible for depletion of the stratospheric ozone. In order to establish whether hydrofluorocarbons (HFCs) and hydrochlorofluorocarbons (HCFCs) are actually more environmentally acceptable, it is extremely important to know, in addition to ODP and HGWP, their atmospheric degradation mechanism, tropospheric lifetime, and toxicity and noxiousness of their atmospheric degradation products. The primary atmospheric sink for hydrohaloalkanes is the reaction with hydroxyl radicals (OH), so that laboratory measurements or a reliable estimation of the rate constants (kOH) of these reactions is essential in order to assess their atmospheric lifetimes. The previsional models for kOH developed in our laboratory allow the estimation of the atmospheric lifetimes of as many as 449 halocarbons (i.e., all the halocarbons containing F and/or Cl with one, two, and three carbon atoms). The harmful character of some degradation products halogenated and oxygenated is also discussed.

[Reduction of cytochrome c by NADH induced by light]

We have studied the behaviour of Fe(III) cytochrome c upon irradiation in the 290-360 nm wavelength range either in the presence or in the absence of NADH; in both cases the photoexcitation caused the reduction of the heme iron. When the irradiation was performed in the absence of NADH, the iron reduction was coupled to a non reversible modification in the protein structure; the photoreduction quantum yield was decreasing with the increase of the irradiation wavelength. Irradiation in the presence of NADH gave heme iron reduction coupled to NADH oxidation and the protein resulted finally unmodified; the quantum yield depended on the irradiation wavelength in a way similar to the observed in the absence of NADH, but it was tenfold higher. We propose that in both cases the active species is an electronic excited state of the heme iron.