Search Results (46 total)

The dynamics of a MeV laser-produced proton beam affected by a radio frequency (rf) electric field has been studied. The proton beam was emitted normal to the rear surface of a thin polyimide target irradiated with an ultrashort pulsed laser with a power density of 4×10¹⁸  W/cm². The energy spread was compressed to less than 11% at the full width at half maximum (FWHM) by an rf field. Focusing and defocusing effects of the transverse direction were also observed. These effects were analyzed and reproduced by Monte Carlo simulations. The simulation results show that the transversely focused protons had a broad continuous spectrum, while the peaks in the proton spectrum were defocused. Based on this new information, we propose that elimination of the continuous energy component of laser-produced protons is possible by utilizing a focal length difference between the continuous spectral protons and the protons included in the spectral peak.

The classic “Ewald-sphere construction” for determining the structure of crystalline objects from x-ray and neutron diffraction experiments is generalized to determine the correlation functions of scattering potentials of stationary random media from scattering experiments.

We carried out a demonstrative electron scattering experiment using a novel ion-trap target exclusively developed for short-lived highly unstable nuclei. Using stable ¹³³Cs ion as a target, this experiment completely mimicked electron scattering off short-lived nuclei. Achieving a luminosity higher than 10²⁶  cm^-2 s^-1 with around only 10⁶ trapped ions on the electron beam, the angular distribution of elastic scattering was successfully measured. This experiment clearly demonstrates that electron scattering off rarely produced short-lived nuclei is practical with this target technique.

A novel internal target has been developed, which will make electron scattering off short-lived radioactive nuclei possible in an electron storage ring. An “ion trapping” phenomenon in the electron storage ring was successfully utilized for the first time to form the target for electron scattering. Approximately 7×10⁶ stable ¹³³Cs ions were trapped along the electron beam axis for 85 ms at an electron beam current of 80 mA. The collision luminosity between the stored electrons and trapped Cs ions was determined to be 2.4(8)×10²⁵  cm^-2 s^-1 by measuring elastically scattered electrons.

The ordering of protons has been observed at a new storage ring, S-LSR, at Kyoto University. Abrupt jumps in the momentum spread and the Schottky noise power were observed for protons for the first time at a particle number of ∼2000, upon applying electron cooling with electron currents of 25, 50, and 100 mA. The transition temperature was 0.17 and 1 meV in the longitudinal and transverse directions, respectively. The transverse temperature of the proton beam was much below that of electrons at the transition, which played an essential role in the ordering of protons.

A demonstration of coherent quantum control for ultrafast precise selection of closely-lying states is reported. A phase-locked pair of femtosecond laser pulses is generated through a pulse shaper to excite the ground-state cesium atom to the Cs(7D_3∕2) and Cs(7D_5∕2) states by two-photon absorption. The excited state population is measured by detecting fluorescence from each spin-orbit state. By controlling the phase-difference of the pulse pair, an ultrafast precise selection is accomplished. The contrast ratio of the maximal to minimal selection ratio exceeds 10³ with the delay less than 400  fs.

We report on the solar diurnal variation of the galactic cosmic-ray intensity observed by the Tibet III air shower array during the period from 1999 to 2003. In the higher-energy event samples (12 and 6.2 TeV), the variations are fairly consistent with the Compton-Getting anisotropy due to the terrestrial orbital motion around the Sun, while the variation in the lower-energy event sample (4.0 TeV) is inconsistent with this anisotropy. This suggests an additional anisotropy superposed at the multi-TeV energies, e.g., the solar modulation effect. This is the highest-precision measurement of the Compton-Getting anisotropy ever made.

With detailed experimental studies and hydrodynamics and particle-in-cell simulations we investigate the role of the prepulse in laser proton acceleration. The prepulse or pedestal (amplified spontaneous emission) can completely evaporate the irradiated region of a sufficiently thin foil; therefore, the main part of the laser pulse interacts with an underdense plasma. The multiparametric particle-in-cell simulations demonstrate that the main pulse generates the quasistatic magnetic field, which in its turn produces the long-lived charge separation electrostatic field, accelerating the ions.

A novel superconducting (sc) structure is proposed, to accelerate a high intensity proton beam within the energy range of 5–20 MeV. The comparison with different sc structures from the point of view of both rf efficiency and technological aspects of manufacture looks promising. Results of M.A.F.I.A. simulations are given.

We perform an extension of the continuum-distorted-wave (CDW) approximation for single electron capture by introducing model potentials to describe the interaction of the active electron with the residual-target and projectile ions. The cross sections for electron transfer in collisions of bare and dressed projectile ions with H and He atoms are calculated and compared with experimental data and previous CDW calculations that make use of approximate analytical wave functions to represent the active electron initial and final states. For electron capture in proton-He collisions into definite final states the present calculations are in better agreement with experiments. We trace the differences in the results from both models to a different behavior at projectile scattering angles close to the Thomas peak. In the case of dressed projectiles with different charge states impinging on H and He the present version of CDW gives a better representation of the filling of the unoccupied orbitals of the dressed ion and, therefore, of the cross section as a function of the impinging ion charge state.

A compact proton synchrotron using combined function magnets is proposed to help realize the wider availability of charged particle cancer therapy facilities. This combined function magnet was designed with the help of three-dimensional magnetic field calculations to take account of a realistic fringe and the interference among the magnetic poles. An evaluation scheme for tune values based on particle tracking was developed to improve the magnet design. To verify the magnet design, a model magnet was fabricated and measured. In order to achieve a tune value evaluation from the measured magnetic field, schemes for accurate field mapping and field interpolation were developed. From the tune value evaluation of the measured magnetic field, it was thought that the performance of the model magnet was good enough to construct a synchrotron. In this paper, we report details of the design and the evaluation scheme for the combined function magnet and the results of the field measurements of the model magnet.

A computational scheme, based on the theory of the continuum-bound transitions of Bell and Seaton [J. Phys. B 18, 1589 (1985)] and the close-coupling R-matrix approach, has been developed to treat dielectronic recombination (DR) in high-lying resonance-energy regions. This scheme and our presented numerical method to compute DR in low-lying resonance-energy regions [Phys. Rev. A 62, 022706 (2000)] have been applied together to elucidate the experimental spectra of the DR of O⁶⁺ ions in the metastable 1s2s ³S and 1s2s ¹S states. For comparison, a perturbative theoretical calculation of DR for O⁶⁺ has also been accompanied. The reasonable representation of the general dielectronic spectral shape is yielded by both our close-coupling and perturbative calculations. However, both the methods do not reproduce the experimental double-peak structure at ∼6–8 eV. This shows that the further investigation on DR of this kind of ions is required both experimentally and theoretically.

Dielectronic recombination (DR) of Li⁺ ions in the metastable 1s2s ³S state has been calculated by using the close-coupling R-matrix method and perturbation theory, and compared with the high-resolution experiment. Good agreement has been shown. The occurrence of the experimental double peak structure at energies of 0.1–0.2 eV can be surprisingly attributed to relativistic effects. A very strong radiation damping effect on the resonances in the second peak position was discovered. Furthermore, in the 1s2p(¹P)nl (n=5-7) resonance energy region, our calculations have displayed that the contribution to DR from high-angular-momentum (l>3) configurations is very small. This point is markedly different from the result of Saghiri et al. [Phys. Rev. A 60, R3350 (1999)].

Since 1996, a hybrid experiment consisting of the emulsion chamber and burst detector array and the Tibet-II air-shower array has been operated at Yangbajing (4300 m above sea level, 606 g/cm²) in Tibet. This experiment can detect air-shower cores, called burst events, accompanied by air showers in excess of about 100 TeV. We observed about 4300 burst events accompanied by air showers during 690 days of operation and selected 820 proton-induced events with its primary energy above 200 TeV using a neural network method. Using this data set, we obtained the energy spectrum of primary protons in the energy range from 200 to 1000 TeV. The differential energy spectrum obtained in this energy region can be fitted by a power law with the index of -2.97±0.06, which is steeper than that obtained by direct measurements at lower energies. We also obtained the energy spectrum of helium nuclei at particle energies around 1000 TeV.

Since 1996, a hybrid experiment consisting of an emulsion chamber and a burst detector array and the Tibet-II air-shower array has been operated at Yangbajing (4300 m above sea level) in Tibet. This experiment can detect air shower cores, called burst events, accompanied by air showers in excess of about 100 TeV. Using the burst event data observed by this experiment, we discuss the primary cosmic ray composition around the knee in comparison with the Monte Carlo simulations. In this paper, we show that all the features of burst events are wholly compatible with the heavy enriched composition in the knee energy region.

Within the framework of the rigorous continuum-bound transition theory of Davies and Seaton [J. Phys. B 2, 757 (1969)], we have developed a numerical method to calculate low-lying resonance photorecombination by directly evaluating the Cauchy principal value of the integral in scattering matrices. The required dipole matrix was obtained using the close-coupling R-matrix code. The advantage of this method is that radiation damping can be accurately estimated and it can naturally be applied to stronger resonance-resonance interference systems. On the basis of this scheme, photorecombination cross sections of C⁴⁺ ions in the KLL, KLM; and KLN resonance energy regions were calculated, and compared with synchrotron storage ring experimental data and perturbative and close-coupling theoretical results. The comparisons showed that our damped cross sections reproduce the results of the high-resolution experimental measurements, and are in agreement with the theoretical calculations. However, our undamped cross sections are larger than the close-coupling results in the parametrized method by a factor of up to 3. The discrepancies were interpreted.

Doubly differential cross sections for inclusive single-electron emission in 3.6 MeV/amu Au⁵³⁺+Ar collisions are calculated in the continuum distorted wave with eikonal initial-state approximation. Previously observed structures in the low-energy continuum are traced back to a known deficiency of the Hartree-Fock-Slater target potential that has been used in the work [R. Moshammer et al., Phys. Rev. Lett. 83, 4721 (1999)]. The structures do not occur, if the argon atom is described in terms of the optimized potential method, in which the electronic exchange interaction is treated exactly.

In high-energy atomic collisions between bare high-Z projectiles and low-Z target atoms, an electron may be captured radiatively into the ground state or, alternatively, into an excited projectile state, which subsequently decays by x-ray emission. These processes are the inverse of a single-step or a two-step ionization, in which the first photon resonantly excites an electron from the hydrogenic 1s_1/2 ground state and a second photon ionizes the excited electron. In this paper, we present a theoretical analysis with a particular emphasis on a detailed multipole decomposition of the photon wave function. This treatment is suitable for connecting angular correlations with alignment studies of the excited state. The anisotropy factors for angular correlations beween the beam axis and the decay x rays following radiative electron capture (REC) into the 2p_3/2,  3p_3/2,  3d_3/2, and 3d_5/2 levels of bare Xe⁵⁴⁺,  Au⁷⁹⁺,  Pb⁸²⁺, and U⁹²⁺ projectiles with energies ranging from 10 MeV/u to 10 GeV/u are explicitly presented and the connection with the alignment is given. It is predicted that REC occurs predominantly into states with the minimal magnetic quantum number ±1 / 2 indicating a strong alignment perpendicular to the beam axis.

In high-energy atomic collisions between bare high- Z projectiles and low- Z target atoms, an electron may be captured radiatively into an excited projectile state which subsequently decays by x-ray emission. This process is the inverse of two-photon–one-electron ionization, in which the first photon resonantly excites an electron from the hydrogenic 1s_1/2 ground state and a second photon ionizes the excited electron. We present an experimental and theoretical study of the angular distribution of the Ly- α₁ ( 2p_3/2→1s_1/2) x rays following radiative electron capture. From the observed anisotropic emission pattern a significant alignment of the intermediate 2p_3/2 state is deduced.

The hyperspherical coupled-channel method is applied to the positron-helium scatterings in a low-energy region below 54.4 eV. An independent electron model is used for the helium atom under the approximation that the active electron is bound by a central model potential. All the hyperspherical adiabatic states associated with Ps(n,l; n=1-4, l=0-2) and He(1snl; n=1-5, l=0-3) in the separated-atom limit are coupled for the expansion of the scattering equation, which is solved by means of a hybrid procedure of the diabatic-by-sector and the conventional adiabatic-basis-expansion methods. Excellent agreement is obtained with experiment for the positronium formation. © 1996 The American Physical Society.

In high-energy atomic collisions between bare high-Z projectiles and low-Z target atoms, an electron may be captured radiatively into the projectile [radiative electron capture (REC)]. The photon angular distributions can be very well represented by radiative recombination (RR) of the projectile with free electrons. This process is the inverse of the photoelectric effect. In this paper, we present exact differential RR cross sections for Au⁷⁹⁺ and U⁹²⁺ at projectile energies of 0.2, 0.5, 1, 2, 5, and 10 GeV/u. We also show the differential cross sections for the photoelectric effect at x-ray energies corresponding to the former projectile energies. It is seen that because of the dramatic forward-peaking of the cross sections at high energies, measurements of REC from low-Z targets are the most practical way to study the photoelectric effect. In one particular example, 10.8 GeV/u Au⁷⁹⁺ on Au targets, we show that the RR cross section multiplied with the number of target electrons is very close to the REC cross section calculated within the impulse approximation. © 1996 The American Physical Society.

Total cross sections have been examined theoretically by using the continuum distorted wave–eikonal initial state (CDW-EIS) and the plane-wave Born approximations for the ionization of hydrogenlike ions in the excited states with the principal quantum number n≤5 in collisions with bare ions in the keV/amu to MeV/amu range. For the ionization of hydrogenlike ions up to C⁵⁺(1s) by H⁺ at low energies, the present CDW-EIS prediction is in good accord with close-coupling calculation, while the Born prediction leads to a considerable overestimate. A comparison is made between the present results and other theoretical predictions as well as experimental measurements.

A detailed discussion is given of the general behavior of differential cross sections for radiative electron capture in relativistic and nonrelativistic heavy-ion–atom collisions. From angular-momentum conservation and a simple approximate treatment, the basic qualitative features can be derived in an analytic form. Subsequently, the results of rigorous relativistic calculations are presented graphically for a sequence of projectile charges and projectile energies. It is shown that in radiative electron capture, the influence of the electron spin manifests itself in a particularly clear-cut way.

The process of radiative electron capture (REC) in relativistic collisions of high-Z ions with low-Z gaseous and solid targets is studied experimentally and theoretically. The observed x-ray spectra are analyzed with respect to photon angular distributions as well as to total K-REC cross sections. The experimental results for angle-differential cross sections are well reproduced by exact relativistic calculations which yield significant deviations from standard sin²θ distributions. Total cross sections for K-REC are shown to follow a simple scaling rule obtained from exact relativistic calculations as well as from a nonrelativistic dipole approximation. The agreement between these different theoretical approaches must be regarded as fortuitous, but it lends support to the use of the nonrelativistic approach for practical purposes.

The photon angular distributions for radiative electron capture (REC) into the j=1 / 2 and j=3 / 2 L subshell levels were measured and calculated for U⁹⁰⁺ → C collisions at 89 MeV/u. The experiment provides the first study of the photon angular distribution of REC into a projectile p state (j=3 / 2) which was found to exhibit a slight backward peaking in the laboratory frame. For radiative capture to the j=1 / 2 states, the measured angular distribution deviates considerably from symmetry around 90°. The results demonstrate that the usual sin²θ_lab distribution is not valid in the high-Z regime.