Two-body photodisintegration of 3He and 4He in the Δ(1236) region

Differential cross sections for the reactions ³He(γ, p)d and ⁴He(γ, p)t were measured at proton c.m. angles of 60° and 90° for photon energies ranging from 150–450 MeV with an average resolution of 8 MeV. Bremsstrahlung was used as the photon source; deuterons/tritons were analyzed in a magnetic spectrometer whereas coincident protons were detected in a plastic scintillator telescope. The experimental method includes a calibration by means of ¹H(γ, π⁰)p differential cross section measurement at 90° c.m. in the same photon energy range. The ³He and ⁴He two-body photodisintegration differential cross sections show a monotonically decreasing variation with photon energy. In addition, partial data on the differential cross section of the reaction ⁴He(γ, n)τ at 90° and 120° neutron c.m. angle are given.     

Measurements of the (γ, n0) reaction on 12C and 16O and the investigation of the reaction mechanism

Differential (γ, n₀) cross sections on ¹²C and ¹⁶O have been measured for photon energies 60 MeV ? E_γ ? 160 MeV. These results combined with the corresponding (γ, p₀) cross sections support an absorption mechanism of the photon by neutron-proton pairs.     

Hard photons and neutral pions as probes of hot and dense nuclear matter

The dynamics of heavy-ion collisions is studied in an energy domain in the vicinity of the Fermi energy. The early history of the collision is analyzed from the theoretical and experimental point of view in which the message conveyed by bremsstrahlung photons and neutral pions is exploited. The Boltzmann-Uehling-Uhlenbeck model and the Dubna Cascade Model, both based on similar principles but each adopting different computation technics, are briefly described and their respective predictions are discussed. In particular the emission pattern of bremsstrahlung photons is discussed. The photon production has been measured in the systems ⁸⁶Kr+⁵⁸Ni at 60A MeV, ¹⁸¹Ta+¹⁹⁷Au at 40A MeV and ²⁰⁸Pb+¹⁹⁷Au at 30A MeV and energy spectra, angular distributions and two-photon correlations have been analyzed. We find that bremsstrahlung photons are emitted from two distinct sources that can be correlated with nuclear-matter density oscillations. The properties of photon emission are discussed in terms of collective properties of nuclear matter. The high energy tail of the photon spectrum is interpreted by π⁰ and Δ decay but predominantly by radiative capture of pions. The π⁰ absorption in the nuclear medium is further analyzed by examining their emission pattern.     

The 12C(γ,p) reaction at Eγ = 60 and 80 MeV

The tagged photon technique has been used to measure the ¹²C photoproton cross section over the angular range θ_p = 30°–135° for E_γ = 60 and 80 MeV. Comparison of the results with quasifree knockout and quasideuteron calculations shows that photon interaction with two-nucleon pairs is dominant at least for high excitation energies in the residual nucleus.     

The radiative capture of fast protons by medium-mass nuclei

High-energy photon spectra were obtained with moderate resolution for a number of neighboring nuclei: ⁵⁶Fe, ⁵⁹Co, ⁵⁸Ni, ⁶⁰Ni, ⁶¹Ni, ⁶³Cu and ⁶⁴Zn in bombardments with protons (8 to 22 MeV). The shapes, magnitudes and energy dependence of the observed spectra are found to be consistent in most of their essential features with the implications of the semidirect model for fast nucleon capture.     

Thermoluminescence response as a function of irradiation temperature

Thermoluminescence response at different irradiation temperatures and photon energies has been studied. Analyses of the glow curve and the peak height of TLD 200, 600, 700 and CaSO₄:Dy for irradiations using ⁶⁰Co, ¹³⁷Cs and ²²⁶Ra were made.     

Activation detection using indium foils for simultaneous monitoring neutron and photon intensities in a reactor core

In this work, the time-sequential changes in neutron and photon intensities in the core of a research reactor were monitored for 330 days following the time of shutdown. Indium foils, used as activation detectors, were irradiated during different time periods in the core. The activated nuclides ^116m1In and ^115mIn were measured and their activities were used as indices of the changes in neutron and photon intensities during the period of interest. Photon and neutron sources that may have been responsible for the activation reactions with indium foils were identified. The activities of both ^116m1In and ^115mIn rapidly decreased in the first 20 h (Phase I), and subsequently their activity levels were proportionally varied with ¹⁴⁰La activity between 20 and 3000 h (Phases II and III). After 3000 h (Phase IV), neutron fluence rate slowly decreased to levels of less than 10 cm^?2 s^?1. Then, the main neutron sources were identified as the neutron-emitting transuranic nuclides and the most important photon source that activated indium to form ^115mIn was identified as ⁶⁰Co. The activation detectors based on indium foils were found to be effective for simultaneously monitoring the variations of neutron and photon levels in reactor cores under sub-critical conditions.     

The 7Li(γ, N) and 7Li(e,N) reactions at intermediate photon energies

Cross sections for single-photonucleon emissions from ⁷Li have been measured for photon energies in the range 60–120 MeV by detecting the recoiling residual nuclei following excitation with bremsstrahlung radiation of end-point energies 140 and 155 MeV. Measurements of the ⁷Li(e, ⁶Li)e'p and ⁷Li(e, ⁶He)e'n cross sections were also made at the same electron energies. A significant difference between the ratio of electron- and bremsstrahlung-induced yields for proton and neutron emission is observed. The results are compared to a modified quasi-deuteron model and a simple direct-knockout model in which recoil terms are included.     

Analysis of triplet production by a circularly polarized photon at high energies

The possibility in principle of determining the circular polarization of a high-energy photon by measuring the created electron polarization in the process of triplet photoproduction γ + e ^? → e ⁺ e ^? + e ^? is investigated. The respective event number, which depends on polarization states of the photon and the created electron, does not decrease as the photon energy increases, and this circumstance can ensure the high efficiency in such experiments. We study different double and single distributions of the created electron (or positron), which allow probing the photon circular polarization and measuring its magnitude (the Stokes parameter ξ₂) using the technique of Sudakov variables. Some experimental setups with different rules for event selection are studied and the corresponding numerical estimations are presented.     

Gluon distributions in real and virtual photon and the photon structure function

Gluon distributions in real and virtual photons are calculated using evolution equations in the NLO approximation. The quark distributions in the photon determined on the basis of the QCD sum rule approach in [1] are taken as an input. It is shown that gluon distribution in the photon can be reliably determined up tox=0.03÷0.05, much lower than the corresponding values in the case of quark distributions. Two variants of the calculations are considered: (1) it is assumed that there are no intrinsic gluons in the photon at some low normalization pointQ ²=Q ₀ ² ∼1GeV²; (2) it is assumed that gluonic content of the photon at lowQ ₀ ² is described by gluonic content of vector mesonsρ, ω, ϕ. The gluon distributions in these two variants appear to be different. This fact permits one to clarify the origin of nonperturbative gluonic content of the photon by comparing the results with experiment. Structure functionsF ₂(x) for real and virtual photon are calculated and it is shown that in the regionx≥0.2 where QCD approach is valid, there is a good agreement with experiment.     

Three photon resonant ionization in atomic potassium via S,P, D and F series Rydberg states

Single colour three photon resonant ionization (2 + 1) is observed in atomic potassium vapour in a heat pipe oven using an excimer laser pumped dye laser. Using wavelengths between 570 nm and 603 nm various²S and²D Rydberg states are populated by two photon excitation. Third photon of the same wavelength ionizes the atoms. Rydberg states up ton ⋍ 50 are observed. Electric field as low as 1 V/cm causes extensive Stark mixing of the states. This results in progressively higher three photon ionization signals via the perturbed²P and²F Rydberg states. The three photon ionization process is studied using both linearly and circularly polarized incident light. The experiment shows qualitatively that the²P Rydberg states are perturbed primarily by the²D states in the prescence of an external electric field and to a much smaller extent by²S states. This is also explained theoretically by calculating the Stark mixing coefficients under the Bates and Daamgard (1949) approximation. Implication for a similar effect in other alkali elements is discussed.     

Photon dispersion in a strong magnetic field

In an external magnetic field analytical properties are studied of the photon polarization tensor calculated as the electron-positron loop in the Furry picture. The polarization tensor is reexpressed as a sum over singular contributions coming from e⁺e^?-pair photocreation in semidiscrete Landau states. The solutions of the photon dispersion equation (i.e., the one for the poles of the photon propagator) are described. The shape of the photon dispersion curve obtained is responsible for the effect of photon deflection by a strong magnetic field. No physically reliable solutions, apart from spurions, are found for the longitudinal photon mode. An infinite number of solutions with complex space-momentum are found, with no apparnet ghosts among them. An attempt is made to interprete the former in terms of possible bound and quasibound states of electron and positron pairs.     

Photon mass and the fieldB (3)

The connection between finite photon mass and the fieldB ⁽³⁾ is developed with reference to special relativity in the vacuum. The existence of the physical and longitudinal fieldB ⁽³⁾ implies that there are three degrees of polarization associated with the photon, which cannot therefore be a massless boson. The fieldB ⁽³⁾ can be observed experimentally through the magnetization of a plasma with microwave pulses, and this experiment serves to demonstrate unequivocally the existence of photon mass.     

Real and virtual photon contributions to inelasticep scattering

We investigate direct and resolved photon contributions to hardep scattering at high energies. Some terms of the direct contribution are already included in the resolved photon one, described in terms of the photon structure functions. A suitable subtraction mechanism has to be found in order to avoid double counting. Inelasticep scattering requires the knowledge of photon structure functions over a wide range in photon virtuality, in particular from approximately zero up to almost 10⁵ GeV² at HERA. We analyse the real and virtual photon structure function and we comment on the structure of the photon in the intermediate region of low virtuality. We finally describe howep cross sections can be obtained as functions of the virtuality of the exchanged photon.     

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.     

Virtual photon structure

The structure functions of a real photon are calculable in QCD. The leading contribution is proportional to 1n Q², with a coefficient reflecting the gluon flux in a real photon. We investigate this leading term for non-zero target photon mass. In an appropriate limit the gluon content in a virtual photon is found to vanish. The gluon radiative corrections of QCD can thus be turned off by tuning the target photon mass.     

Preparation, structure, and properties of metal nanocomposites in lithium niobate

Crystals of lithium niobate LiNbO₃ are implanted with 60-keV Cu^? ions at different ion fluxes to a fluence of 2 × 10¹⁷ ions/cm². The structure and the linear and nonlinear optical properties of the implanted layers are investigated. The optical transmission and ion-induced photon spectra of the LiNbO₃ crystals are measured in the course of implantation. It is revealed that the implantation brings about the formation of complex nanocomposites consisting of metallic copper nanoparticles and nanodomains of the matrix. The distributions of nanoparticles and nanodomains in the implanted layers do not correlate with each other. It is shown that the variations in the linear and nonlinear optical absorption of the nanocomposites are predominantly determined by the changes in the chemical composition and the structure of the matrix.     

Photon production in Au+Au collisions at \sqrt {s_{NN} } = 130 GeV

We report raw photon, neutral pion and eta measurements at RHIC. Photons in the energy range from 100MeV ? 4GeV were detected by reconstructing e ⁺ e ^? pair production, γ+Z→e ⁺+e ^?+Z, with the STAR Time Projection Chamber (TPC). Along with the photon detection technique we discuss the purity of the photon candidates and measurements of hadronic decays via their electromagnetic decay channels. The π⁰→γ and η→γγ decay channels are addressed.     

A theoretical study of the photoelectron spectrum of atomic lithium

The 1s photoelectron spectrum of atomic lithium has been calculated for incident photon energies of 151 and 1487 eV. For this open-shell system there are four possible final-state manifolds for the residual ion, with symmetries ³S, ¹S, ³P^o and ¹P^o. The line energies for the first four members of each manifold have been calculated using relaxed Hartree—Fock or configuration interaction wavefunctions as required. The agreement with available optical data is excellent.The intensity expressions neglecting interchannel coupling have been evaluated to allow a comparison of line intensities between final-state manifolds and hence the synthesis of theoretical spectra. The variation of the spectral profile with photon energy is clearly demonstrated. Conjugate shake-up, as well as shake-up processes are predicted to be important at high photon energy.     

Neutral Dissociation of Superexcited Nitric Oxide Induced by Intense Laser Fields

利用波长800 nm、脉宽60 fs、强度0.2 PW/cm²的强激光激发NO分子,随后通过荧光光谱检测到了处于激发态的N和O的中性碎片. 说明强激光导致的多光子激发同样可以引起NO分子的超激发,并且发生中性解离. 采用离子实解离模型,建立了NO分子的超激发态势能曲线. 提出了直接解离和预解离两种解离过程,分别对应生成N^*+O或O^*+N两个通道.