Neutral pion production in subthreshold heavy ion collision with16O (38 MeV/u) and20Ne (200 MeV/u) beams

Inclusive production of neutral pions has been studied in¹⁶O +²⁷Al,¹⁹⁷Au reactions at 38 MeV/u and²⁰Ne +²⁷Al reaction at 200 MeV/u. A setup of 10 Pb glass Cherenkov detector telescopes was used to detect the twoγ rays from theπ ⁰ decay.     

Multineutron photodisintegration of the 197Au nucleus behind the giant dipole resonance

An experiment in which the gold isotope ¹⁹⁷Au was irradiated with a beam of bremsstrahlung photons having an endpoint energy of E _γ ^m = 67.7 MeV and originating from the RTM-70 racetrack microtron of the Institute of Nuclear Physics at Moscow State University was performed. The gammaray spectra of the residual beta activity of an irradiated sample were measured. Multinucleon photonuclear reactions on ¹⁹⁷Au nuclei were observed in the experiment. Photonuclear reactions on a ¹⁹⁷Au nucleus that involve the emission of up to seven neutrons were recorded for the first time. The experimental results in question suggest the important role of the quasideuteron photodisintegration mechanism in the energy region behind the giant dipole resonance.     

Production of π 0 and η mesons in carbon-induced relativistic heavy-ion collisions

The production of π ⁰ and η mesons has been investigated in the system ¹²C+¹²C at 0.8A GeV, 1.0A GeV, and 2.0A GeV using the TAPS photon detector. The production cross sections and transverse-momentum distributions measured around midrapidity extend the existing systematics of neutral-meson production to light systems and the highest energy available at the GSI accelerator facility. At all beam energies scaling of the differential production cross section with the transverse mass of the emitted meson is observed. In addition π⁰ production has been studied in the mass-asymmetric systems ¹²C+¹⁹⁷Au and ¹⁹⁷Au+¹²C at 0.8A GeV.     

Experimental investigation of the reaction12C(γ, π0)X in the photon energy range between threshold and 450 MeV

The photoproduction of neutral pions on¹²C has been investigated between threshold and 450 MeV photon energy using the tagged photon beam at the Bonn 500 MeV synchrotron and a large acceptance detector. Theπ ⁰ detector consists of cylindrical wire chambers covering a solid angle of nearly 4π. Using a scintillation counter as an active¹²C target an attempt was made to separate coherent and incoherentπ ⁰ photoproduction. The measured total and differential cross sections are compared to corresponding theoretical calculations.     

Thermal hard-photons probing multifragmentation in nuclear collisions around the fermi energy

Hard-photon (E _γ>30 MeV) emission originating from photon-neutron bremsstrahlung collisions is investigated in four different heavy-ion reactions at intermediate bombarding energies (³⁶Ar+¹⁹⁷Au, ¹⁰⁷Ag, ⁵⁸Ni, ¹²C at 60 A MeV) coupling the TAPS photon spectrometer with two charged-particle multidetectors covering more than 80% of the solid angle. The hard-photon spectra of the three heavier targets result from the combination of two distinct exponential distributions with different slope parameters, a results which deviates from the behaviour expected for hard-photon production just in first-chance proton-neutron collisions. The thermal origin of the steeper bremsstrahlung component is confirmed by the characteristics of its slope and angular distribution. Such thermal hard-photons convey undisturbed information of the thermodynamical state of hot and excited nuclear systems undergoing multifragmentation.     

Neutral transverse momentum spectra in 60 and 200A·GeV16O+nucleus and proton+nucleus reactions

Transverse momentum (p _T) distributions fo inclusive photons and neutral pions at midrapidity are measured with a lead glass calorimeter in 60 and are measured with a lead glass calorimeter in 60 and 200A·Gev¹⁶O+nucleus and and proton+nucleus reactions. Inclusive photon distributions are compared for central and peripheral reactions. The degree of centrality is determined either from the charged particle multiplicity or from the remaining projectile energy in the forward direction. Deviations from a nucleus+nucleus interaction model based upon linear extrapolation from p+p reactions are observed in central¹⁶O+Au data. The variation of theaverage transverse momentum is investigated as function of centrality. The target-mass and energy dependence of π⁰ p _T distributions are presented. For¹⁶O+Au a change of slope in these distributions is observed atp _t ≈0.8 GeV/c compatible with hydrodynamic expansion models.     

Differential cross sections for photoemission of deuterons from the nitrogen nucleus

Momentum distributions for inclusive photoemission of deuterons from nitrogen have been measured at laboratory angles of 43° and 50°, and for maximum energies of the bremsstrahlung photon beam of 725, 775 and 825 MeV. The double differential cross sections of deuterons emitted from nitrogen are obtained for photon energies of 750 and 800 MeV and for laboratory angles of 43° and 50°. The photon energy is determined by the photon subtraction method. Results are examined by a π⁰ photoproduction process off a quasi-deuteron in nitrogen (γ″d″→π⁰d) and by a two-step process. The two-step process means the following: first, a nucleon is produced by a photo-pion production process from a quasi-free nucleon in the nitrogen nucleus (γ″N″→πN); second, the nucleon produced at the first step picks up another nucleon in the nucleus and then a deuteron is emitted (N+A→d+anything).     

Hard gamma as probe of nuclear dynamics

Experimental results concerning the ⁵⁸Ni + ⁵⁸Ni at 30 MeV/u and ⁵⁸Ni + ¹⁹⁷Au at 30 and 45 MeV/u reactions are presented. The emission mechanism of hard photons has been exploited to get information on the time evolution of the reactions. From the measured hard gamma multiplicity associated to different classes of heavy residues a quantitative estimate of the reaction centrality has been obtained. Moreover, evidence of a prompt Intermediate Mass Fragment (IMF) emission is shown for the Ni + Au reaction at 45 MeV/u for central collisions exploring the correlation function between thermal photons and IMF’s.     

Photon emission from bare quark stars

We investigate the photon emission from the electrosphere of a quark star. We show that at temperatures T ≈ 0.1–1 MeV, the dominating mechanism is the bremsstrahlung due to bending of electron trajectories in the mean Coulomb field of the electrosphere. The radiated energy for this mechanism is much larger than that for the Bethe-Heitler bremsstrahlung. The energy flux from the mean field bremsstrahlung also exceeds the one from the tunnel e ⁺ e ^? pair creation. We demonstrate that the LPM suppression of the photon emission is negligible.     

Multineutron photonuclear reactions on the <Superscript>197</Superscript>Au isotope

An experiment on irradiation of the ¹⁹⁷Au isotope by a beam of bremsstrahlung γ-rays of maximum energy E _γ ^m = 70 MeV has been carried out at the RTM-70 race-track microtron of the INP MSU. Residual activity spectra of the irradiated sample were measured. Multiparticle photonuclear reactions on the ¹⁹⁷Au nucleus are observed. Photonuclear reactions with emission of as many as six neutrons are observed for the first time.     

Possible dynamical limitations to excitation energy storage in nuclei

The dependence of the relative populations of particle unbound states on the associated charged particle multiplicity and on the total kinetic energy of the two decay products was investigated for ⁴⁰Ar induced reactions on ¹⁹⁷Au at E/A = 60 MeV. The measurements indicate that the relative populations exhibit little sensitivity to the violence of the collision and to the time of emission. Implications of these results on the dynamics of the reaction are discussed.     

Bremsstrahlung in intermediate-energy nucleon reactions within an effective one-boson exchange model

Within a covariant effective one-boson exchange model for the T matrix of NN interactions we present detailed calculations of bremsstrahlung cross sections for proton-proton and proton-neutron reactions at beam energies in the 1 GeV region. Besides pure bremsstrahlung processes we consider photons from Λ decays and contributions from the η → γγ process. At beam energies above 700 MeV the Λ decay channel dominates the spectra at large photon energies, where the interference between non-resonance processes and the Λ decay channel becomes also important. Low energy photons stem from pure bremsstrahlung processes. The available experimental data at 730 MeV beam energy is well described. We extrapolate the model down to 280 MeV, where more detailed experimental data exist, and find agreement with angular distributions.     

Positive pion photoproduction from hydrogen at photon energies between 500 MeV and 1,400 MeV in forward direction

The differential cross section of the reactionγ+p→π ⁺ was measured at pion CM-angles of 20° and 30° for photon energies between 500 MeV and 1,400 MeV. The pions were detected in a magnetic spectrometer. By measuring each pion trajectory and by offline calculation of the initial pion parameters an energy resolution of about 2.5% FWHM was achieved. The results complete a set of data which were measured in recent years at the Bonn 2.5 GeV synchrotron. In comparison to photoproduction analyses two effects were revealed: The η cusp appears in the energy dependence of the cross section as a sharp drop atK _γ=710 MeV. In the region of the third resonance the data show a greater enhancement than predicted by most of the analyses.     

Directed and elliptic flow in 197Au+197Au at intermediate energies

Directed and elliptic flow for the ¹⁹⁷Au+¹⁹⁷Au system at incident energies between 40 and 150 MeV per nucleon has been measured using the INDRA 4π multi-detector. For semi-central collisions, the excitation function of elliptic flow shows a transition from in-plane to out-of-plane emission at around 100 MeV per nucleon. The directed flow changes sign at a bombarding energy between 50 and 60 MeV per nucleon and remains negative at lower energies. Molecular dynamics calculations (CHIMERA) indicate sensitivity of the global squeeze-out transition on the σ _NN and demonstrate the importance of angular momentum conservation in transport codes at low energies.     

Investigation of the reaction <Superscript>4</Superscript>Нe(γ, <Emphasis Type="Italic">pn</Emphasis>)<Emphasis Type="Italic">d</Emphasis> at energies below the meson-production threshold

Themomentum distributions of deuterons and nucleons from the reaction ⁴Не(γ, pn)d induced by bremsstrahlung photons whose spectrum extends up to the endpoint energy of 150 MeV weremeasured by means of a diffusion chamber placed in a magnetic field. These measurements were performed in four photon-energy intervals for deuterons and in the energy range between 100 and 150 MeV for nucleons. Angular and energy correlations of nucleons were measured at photon energies in the interval between 50 and 70 MeV. The results obtained in this way were analyzed on the basis of the quasideuteron model. The probability for final-state nucleon–deuteron interaction was estimated.     

New approach to imaging of two-proton source functions

The influence of protons emitted from both fast and slow emission mechanisms in 2-proton correlations measured in heavy ion collisions at intermediate energies is discussed. The size of the fast emitting source is found to be determined mainly by the width of the correlation peak at 20 MeV/c, rather than by its magnitude. To illustrate the new analysis technique, the total momentum dependence of p-p correlations measured in ¹⁴N+¹⁹⁷Au reactions at E/A=75 MeV is studied with an imaging analysis. The extracted results and the basic ideas behind the proposed analysis technique provide new tools for using particle-partcile correlation measurements as a probe of nuclear reactions.     

Preequilibrium nucleon jets in TDHF calculations of medium-energy,heavy-ion collisions

We study collisions of ¹²C and ¹⁹⁷Au projectiles at E_lab/A_proj = 30 to 85 MeV/A on ¹⁹⁷Au targets, using the 3-dimensional TDHF model. Most of the qualitative phenomena observed experimentally are reproduced by the TDHF calculations. The results are compared with recent data from the 1 GeV ¹²C beam at CERN and show some quantitative deviations from experiment. Possible reasons for these discrepancies are discussed and give valuable insight into intermediate energy collision processes.     

Photofission of 238U at the energy of the giant dipole resonance

Photofission of ²³⁸ U by bremsstrahlung photons is studied at four energies of an electron accelerator: 19.5, 29.1, 48.3, and 67.7 MeV. The yields of the fission fragments after the emission of prompt neutrons are obtained using the gamma-ray spectroscopic technique. The mass distributions of photofission are obtained at different upper energies of the bremsstrahlung spectrum. The ratio the symmetric-fission mode to the asymmetric mode is obtained from the mass distribution. The symmetric mode becomes 3–4 times greater than the asymmetric as the excitation energy of the ²³⁸U nucleus increases from 12 to 16 MeV.     

A search for πNN bound systems

The d(π⁺, π⁻)T⁺⁺⁺ and the d(π⁻, π⁺) T⁻ reactions were studied at an incident pion energy of 260 MeV. Outgoing negative pions were detected at 35° and 55° in the first reaction, and the positive pions at 45° in the second. Indications for structures corresponding to a (π⁺pp) system with a mass of 2003.0±1.0 MeV and a (π⁻nn) system with a mass of 200.1±1.5 MeV were observed with marginal statistical significance. Upper limits for the cross sections are obtained.