Double pi(0) Photoproduction on the Proton at GRAAL

The double pi(0) photoproduction off the proton has been measured in the beam energy range of 0.65-1.5 GeV. The total and differential cross sections and the Sigma beam asymmetry were extracted. The total cross section measured for the first time in the third resonance region of the nucleon shows a prominent peak. The interpretation of these results by two independent theoretical models infers mostly the selective excitation of P11- and D13-nucleon resonances.     

Investigation of the reaction D(γ, <Emphasis Type="Italic">n</Emphasis>)H near the threshold by means of powerful femtosecond laser radiation

The possibility of studying photonuclear reactions near the threshold by means of powerful femtosecond lasers is explored by considering the example of deuteron photodisintegration. The respective experiment was performed by employing the terawatt femtosecond laser facility of the International Laser Center at Moscow State University. The radiation from this facility is characterized by a pulse energy of up to 50 mJ, a duration of 50 fs, a repetition rate of 10 Hz, and a wavelength of 805 nm. This provides a power above 10¹⁸ W/cm². Intense relativistic-electron and photon beams of energy up to 10 MeV were obtained after the optimization of relevant experimental parameters, including the focus of the laser beam, its time structure, and the choice of target. The use of these beams made it possible to study neutron generation in heavy water, to measure the time of neutron moderation, and to determine the detection efficiency. The experimental data obtained in this way are in qualitative agreement with the results of simulations based on the GEANT-4 and LOENТ code packages and indicate that it is possible to create a neutron source on the basis of the aforementioned laser. The cross section measured for deuteron photodisintegration complies with theoretical estimates available in the literature.     

On Eta-Meson Interaction with Nuclear Matter

Physics of Atomic Nuclei - The transparency of the carbon nucleus with respect to eta mesons of energy 25 to 95 MeV was measured. Mesons of this energy originated from the photoproduction on...     

Measurement of \Sigma beam asymmetry in \pi^{0}_{} photoproduction off the neutron in the second and third resonances region⋆

The $ \Sigma$ beam asymmetry in the photoproduction of neutral pions from quasi-free nucleons in a deuteron target was measured for the first time between 0.60 and 1.50GeV, with the GRAAL polarized and tagged photon beam. The asymmetry values from the quasi-free proton were found equal to the ones extracted from a pure proton target. The asymmetries from quasi-free proton and quasi-free neutron were found equal up to 0.82GeV and substantially different at higher energies. The results are compared with recent partial-wave analyses.     

Measurement of beam-recoil observables Ox , Oz and target asymmetry T for the reaction \gammap → K+ \Lambda

The double polarization (beam-recoil) observables $\ensuremath O_x$ and $\ensuremath O_z$ have been measured for the reaction $ \gamma$ p → K ⁺ $ \Lambda$ from threshold production to $\ensuremath E_{\gamma} \sim 1500$ MeV. The data were obtained with the linearly polarized beam of the GRAAL facility. Values for the target asymmetry T could also be extracted despite the use of an unpolarized target. Analyses of our results by two isobar models tend to confirm the necessity to include new or poorly known resonances in the 1900MeV mass region.     

Novel photonuclear techniques based on femtosecond lasers

The results of X-ray diagnostics of plasma created at a material surface by a laser pulse with an intensity of up to 2.5 × 10¹⁸ W/cm² are presented. The experimental data are compared with the results of numerical modeling of the laser-plasma interaction while accounting for the detector-response function. Examples of possible nuclear problems that can be solved with the use of laser plasma as a source of charged particles and X-ray quanta are given.     

Photofission of 238U in the giant-resonance region

Data on cross sections for the reaction ²³⁸U(γ,F) in the giant-resonance region were analyzed in connection with the preparation of new experiments aimed at studying ²³⁸U photofission—in particular, in beams of photons from in-flight positron annihilation on internal targets of positron storage rings. These data were taken from measurements also performed with annihilation photons but from positron beams external to the accelerators used. The procedures applied in such measurements and based both on processing the multiplicity of detected neutrons and on detecting fission fragments were also analyzed.     

The quantum magnetic size effect in cylindrical conductors

Quantum magnetic size oscillations of the electron characteristics of cylindrical conductors are associated with the Fermi surface anisotropy. The oscillations predicted may be a source of important information of the electron energy spectrum of conductors.     

Photofission of actinide nuclei in the nucleon-resonance region: First photonuclear experiment at the Siberia-2 storage ring

Results of an experiment performed to study ²³⁸U photofission with the aid of the initial section of the GAMMA channel of the Siberia-2 storage ring at the Kurchatov Institute are presented. These results are predominantly of a methodological value, because a photonuclear experiment was conducted for the first time in this channel. However, the data obtained in this way allowed us to evaluate an upper limit on the probability of the fast fission (fragmentation) of ²³⁸U nuclei that was induced by photons of energy up to 2.5 GeV. This is pertinent to the problem of the deviations of the total photoabsorption cross sections for actinide nuclei from a “universal curve.”     

The graal experiment: Beam polarization asymmetries in pseudoscalar meson photoproduction

The GRAAL tagged and polarized γ beam has been operating with a maximum energy of 1100 MeV. For the first time asymmetry results in ν photoproduction were obtained and will be shown. Interesting results have also been obtained for π⁰ and π⁺ photo-production, that largely confirm the already existing ones, but also extend in not yet explored angular and energetic regions, especially for backward scattering and energies greater than 900 MeV.