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The desire to obtain a reconstruction image with high resolution using a short scanning duration motivates us to design a new brain positron emission tomography (PET) system. We design and simulate a wearable helmet brain PET system. This helmet PET scanner has a radius of 185 mm and comprises five parts (bottom, face, forehead, head, and top). The entire helmet PET system has 512 Lu 1.8 Y 0.2 SiO5: Ce (LYSO) PET detector blocks. Each PET detector is made of 12 × 12 LYSO crystals with 1.5 × 1.5 × 10 mm 3 elements. The proposed scanner has a global absolute sensitivity of 3.08% and a global scatter fraction of 0.56. Maximum likelihood expectation maximization (MLEM) reconstruction of the simulated data shows that full width at half maximum (FWHM) is 1.97 mm and full width at tenth maximum (FWTM) is 2.80 mm at the center of the field of view. In the radial direction, the FWHM increases from 1.97 mm to 3.82 mm, and the FWTM increases from 2.80 mm to 5.64 mm. In the axial direction, FWHM and FWTM are 1.98 mm and 2.80 mm, respectively. We use simulated line sources, a Derenzo phantom, and an MRI image to test the imaging ability of the simulated brain helmet PET. The scanner achieves higher resolution and higher sensitivity than a clinical PET scanner and will be of practical use in the diagnosis of brain illness.

Ab initio calculations of electrostatic potentials of C60 and C80 have been performed at the B3LYP/6-31G(d) level. Comparisons of the present calculations with phenomenological models and other ab initio results demonstrate that the new proposed electrostatic potentials give a better description of the confining effect than square potential well models. The present calculated electrostatic potentials are fitted with the Lorentz function. The electrostatic potentials obtained are expected to be useful for modelling atomic dynamic processes, such as photoionization and elastic scattering of endohedral fullerenes, and simulation of the interaction between energetic ions and fullerenes.

The transmission of 10-keV Cl– ions through Al2O3 insulating nanocapillaries is studied both by experiment and simulation. The double-peak structure in the transmitted angular distribution is found to be the same as our previous result. The peak around the direction of the primary beam is caused mainly by the directly transmitted Cl–, and the other peak around the tilt angle of Al2O3 nanocapillaries is mainly induced by Cl+ and Cl0. The intensity of transmitted Cl– decreases with the tilt angle increasing, which is in accord with the geometrically allowed transmission. Beyond the geometrically allowed angle, the transmitted projectiles are mainly Cl+ ions and Cl0 atoms. The ratio of transmitted Cl+ ion to Cl0 atom drops as tilt angle increases, and it turns more obvious when the tilt angle is larger than the limit of the geometrical transmission. A detailed physics process was developed within Geometry and Tracking 4 (Geant4) to perform the trajectory simulation, in which the forces from the deposited charges and the image charges, the scattering from the surfaces as well as the charge exchange are taken into consideration. The transmissions at the tilt angle of 1.6o are simulated for the cases without and with deposited charges of –100 e/capillary. For the deposition charge quantity of –100 e/capillary, the majority of the transmitted projectiles are mainly the directly transmitted Cl– ions exiting to the direction of tilt angle, and the transmitted Cl0 and Cl+ account for a very small portion. While for the case with no deposited charges, the simulation results agree well with the experimental results. The dependence of the scattering process on the tilt angle, which results in the different features in the transmitted projectiles, is studied in detail by the simulation. It is found that the transmitted Cl0 atoms exit through single to multiple scattering, and most of transmitted Cl0 atoms exit through single and double scattering, and are centered along the axis of nanocapillaries, while Cl+ ions mainly exit by single scattering, which results in the fact that the intensity of the transmitted Cl0 atoms drops slower than that of the transmitted Cl+ ions with the increase of the tilt angle, leading the ratio of the transmitted Cl+ to Cl0 to decrease as the tilt angle increases in experiment. Our results describe the physical mechanism of low-energy ions through insulating nanocapillaries in detail, i.e. how the scattering process dominates the final transmission. It is found that the transmission of the negative ions in the energy range above 10 keV is caused by the scattering and the charge exchange process.

Bentonite colloids are readily generated by eroding the compacted bentonite blocks with flowing groundwater and facilitate the transport of radionuclides due to their high mobility. The geological fate of colloids is highly relevant to the aggregation and sedimentation of colloids in porous media. In the present work, the week-scale stability of bentonite colloids concerning sedimentation was determined by photon correlation spectroscopy (PCS) as a function of pH, temperature, and electrolyte concentration. The related mechanisms governing colloidal stability were elucidated by employing the DLVO model. The results showed that the alkaline condition was favorable for bentonite colloidal stability, while high temperature and high salinity destabilized colloids due to the decrease in repulsive potential energy. Ca2+ contributed more significantly to colloid sedimentation than Na+ because of the favorable replacement in the Stern layer. The critical coagulation concentration (CCC) values calculated theoretically from DLVO theory were applicable to predict the stability of bentonite colloids. Knowledge from the present work provides deep insight into the stability of bentonite colloids and has potential implication for understanding the detailed physico-chemical processes governing colloid migration under various subsurface environments.

A simulation is performed to study the transmission of low-energy $\mathrm{C}{\mathrm{l}}^{\ensuremath{-}}$ ions through $\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}$ nanocapillaries. For the trajectory simulations, there are several processes involved: the image forces induced by the projectile; the electrostatic force from the deposited charges; the scattering from the inner surface and charge exchange. The simulation reproduces the main features of the experiments; i.e., the double peak structure in the transmitted angular distribution and the transmitted fractions of $\mathrm{C}{\mathrm{l}}^{\ensuremath{-}}$, $\mathrm{C}{\mathrm{l}}^{+}$, and $\mathrm{C}{\mathrm{l}}^{0}$ were found in the charge state distribution. The transmitted $\mathrm{C}{\mathrm{l}}^{\ensuremath{-}}$ ions are centered around the beam direction while the transmitted fractions of $\mathrm{C}{\mathrm{l}}^{0}$ and $\mathrm{C}{\mathrm{l}}^{+}$ are centered around the tilt angles. The role of the deposited charge is also studied by simulations. With the deposited charge, it is found that $\mathrm{C}{\mathrm{l}}^{\ensuremath{-}}$ is dominant in the transmission and the majority of the ions, centered around the tilt angle, are mainly from the single deflection by the negative charge patches on the inner surfaces of the capillaries, and only a few directly transmitted $\mathrm{C}{\mathrm{l}}^{\ensuremath{-}}$ ions are centered around the incident direction. There are also a few transmitted fractions of $\mathrm{C}{\mathrm{l}}^{0}$ and $\mathrm{C}{\mathrm{l}}^{+}$ from close surface scatterings. In the case that there are no negative charge patches, the simulation agrees with the experiment in detail: The majority of the directly transmitted $\mathrm{C}{\mathrm{l}}^{\ensuremath{-}}$ ions are centered around the incident direction while only a few scattered $\mathrm{C}{\mathrm{l}}^{\ensuremath{-}}$ ions are centered around the tilt angle from the single close collisions with the inner surfaces of the capillaries. There is a portion, comparable to the transmitted fraction of $\mathrm{C}{\mathrm{l}}^{\ensuremath{-}}$, of the transmitted fractions of $\mathrm{C}{\mathrm{l}}^{0}$ and $\mathrm{C}{\mathrm{l}}^{+}$, centered around the tilt angle, from the single scatterings with the inner surfaces of the capillaries. This confirms that at the present experimental conditions there are most probably no negative charge patches formed to guide the negative ions through insulating $\mathrm{A}{\mathrm{l}}_{2}{\mathrm{O}}_{3}$ nanocapillaries.

The collisions of noble gas atoms with graphene and a graphene nanodome were investigated by employing first principles molecular dynamics calculations. By analyzing the electron-related properties of the collision process, the atom dynamics and the deformation of the graphene/nanodome, our results show a difference between the elastic properties of the nanodome and graphene. Generally, the nanodome can more easily revert to its initial conformation. The final kinetic energy, Ef, of the atom that collides with the nanodome is larger than the Ef of the atom that collides with graphene. In addition, the relationship between the initial kinetic energy of the atom, Ek0, and its corresponding proportion of energy loss, χ, is linear (except for the Kr atom). Our research will probably contribute to the investigation of the 2D materials' mechanical properties and their surface morphology. Moreover, due to its novel mechanical properties, the graphene nanodome is an extraordinary nano-architecture which can be employed to protect nano-devices from damage and injury.

Detailed calculations arc performed for 134 fine-structure levels of the 3p(6)3d(2), 3p(6)3d4s, 3p(5)3d(3) and 3p(6)3d4p configurations in Fe ViT using the multiconfiguration Dirac-Hartree-Fock (MCDHF) and relativistic configuration interaction (RCI) methods. Important electron correlation effects are systematically accounted for through active space (AS) expansions. Our results compare well with experimental measurements, emphasizing the importance of a careful treatment of electron correlation, and provide some missing data in the NIST atomic database. The data obtained are expected to be useful in astrophysical applications, particularly for the research of the solar coronal plasma.

The nonmonotonic energy dependence of the $\mathrm{L}{\mathrm{i}}^{0}$ fraction on high-work-function surfaces has not been understood so far. To further study this phenomenon, in this work, efficient neutralization of $\mathrm{N}{\mathrm{a}}^{+}$ ions has been reported on various surfaces, instead of $\mathrm{L}{\mathrm{i}}^{+}$ ions. The nonmonotonic energy dependence of the $\mathrm{N}{\mathrm{a}}^{0}$ fraction becomes more and more obvious with the increase of the surface work function. For Cu(111) and Cu(110) at the scattering angle of 7\ifmmode^\circ\else\textdegree\fi{}, the energy dependence of the neutral fraction is still nonmonotonic as compared to the scattering angles of 135\ifmmode^\circ\else\textdegree\fi{} and 53\ifmmode^\circ\else\textdegree\fi{}. For the scattering angle of 53\ifmmode^\circ\else\textdegree\fi{}, the nonmonotonic angle dependence has been observed for these surfaces. The essences of the nonmonotonic angle and energy dependences are the same. The quantum-mechanical calculations reveal that the width and position of the atomic level below the Fermi level at short ion-surface distances are responsible for high neutral fractions at large exit energies, and that the neutral fraction at much smaller exit energies is probably related to the neutralization at large distances. For the exit angle dependence, the competition between neutralization at short and large distances strongly depends on the surface work function. In particular, the neutralization is enhanced by the relatively large parallel velocity.

In this work, we evaluated the performance of three pitches positron emission tomography (PET) detectors for building a high-resolution small-animal PET imaging experimental platform. The evaluation of PET detectors includes two-dimensional crystal identification, energy resolution, and crystal look-up table. The nuclear instrumentation module and PXI data acquisition (DAQ) system were both used to obtain flood images. The energy spectrum of the Na-22 source was measured. PET detectors of 2.0 and 1.6 mm pitches used in our present experiment were composed of lutetium yttrium oxyorthosilicate (LYSO) crystals directly coupled to position-sensitive photomultiplier tubes (PSPMTs). The 1.3-mm pitch detector comprises a LYSO crystal, a tapered light-guide, and a PSPMT. The crystal sizes of the detectors were a $$10 \times 10$$ array of $$2.0 \times 2.0 \times 10\,\hbox {mm}^{3}$$ crystals, a $$10 \times 10$$ array of $$1.6 \times 1.6 \times 10\,\hbox {mm}^3$$ crystals, and an $$18 \times 18$$ array of $$1.3 \times 1.3 \times 10\,\hbox {mm}^3$$ crystals, respectively. Due to the lower sampling frequency of the DAQ system, a spline interpolation algorithm was introduced in the signal waveform to reconstruct the peak values. The experimental results indicated that all three PET detector crystals could be clearly identified. A detailed analysis demonstrated that the key characteristic full width at half maximum (FWHM) values of the crystals profile could reach $$0.46\pm 0.03$$ , $$0.32\pm 0.03$$ , and $$0.21\pm 0.02\,\hbox {mm}$$ , while the crystal average energy resolutions were $$16.8\%\pm 1.76\%$$ , $$20.1\%\pm 2.24\%$$ , and $$31.5\%\pm 3.76\%$$ , which correspond to pitches of 2.0-, 1.6-, and 1.3-mm, respectively. The PET detector modules of LYSO crystal arrays could be potentially used in the small-animal PET imaging applications and are promising to achieve sub-millimeter PET images.

The classical over-barrier ionization model (COBI) method and trajectory calculations were utilized to simulate the ionization of He+ impacted by a positron. The calculated ionization cross sections of He+ agree with other theoretical data. Additionally, we found that the double ionization of He has a definite association with the positron-He+ impact. This result can explain why doubly ionized He seemed to be positron-scattered by the rest of the He+ in our previous study. The COBI model was also extended to study the double ionization caused by positron-Ne impacts. Our theoretical results agree with the experimental data.

We are currently investigating the feasibility of using highly pixelated Cadmium Zinc Telluride (CdZnTe) detectors for sub-500 μm resolution PET imaging applications. A 20 mm × 20 mm × 5 mm CdZnTe substrate was fabricated with 350 μm pitch pixels (250 μm anode pixels with 100 μm gap) and coplanar cathode. Charge sharing among the pixels of a 350 μm pitch detector was studied using collimated 122 keV and 511 keV gamma ray sources. For a 350 μm pitch CdZnTe detector, scatter plots of the charge signal of two neighboring pixels clearly show more charge sharing when the collimated beam hits the gap between adjacent pixels. Using collimated Co-57 and Ge-68 sources, we measured the count profiles and estimated the intrinsic spatial resolution of 350 μm pitch detector biased at −1000 V. Depth of interaction was analyzed based on two methods, i.e., cathode/anode ratio and electron drift time, in both 122 keV and 511 keV measurements. For single-pixel photopeak events, a linear correlation between cathode/anode ratio and electron drift time was shown, which would be useful for estimating the DOI information and preserving image resolution in CdZnTe PET imaging applications.

X-ray spectra of K-shell hollow krypton atoms produced in single collisions with 52 - 197 MeV/u Xe54+ ions are measured in a heavy-ion storage ring equipped with an internal gas-jet target. Energy shifts of the K{\alpha}_1,2^s, K{\alpha}_1,2^(h,s), and K\b{eta}_1,3^s transitions are obtained. Thus, the average number of the spectator L-vacancies presented during the x-ray emission is deduced. From the relative intensities of the K{\alpha}_1,2^s and K{\alpha}_1,2^(h,s) transitions, the ratio of K-shell hollow krypton to singly K-shell ionized atoms is determined to be 14 - 24%. In the considered collisions, the K-vacancies are mainly created by the direct ionization which cannot be calculated within the perturbation descriptions. The experimental results are compared with a relativistic coupled channel calculation performed within the independent particle approximation., Comment: 5 figures, 9 pages. Accepted by Physical Review A

A cadmium-zinc-telluride (CZT) detector with 350 μm pitch pixels was studied in high-resolution positron emission tomography (PET) imaging applications. The PET imaging system was based on coincidence detection between a CZT detector and a lutetium oxyorthosilicate (LSO)-based Inveon PET detector in virtual-pinhole PET geometry. The LSO detector is a 20 ×20 array, with 1.6 mm pitches, and 10 mm thickness. The CZT detector uses ac 20 ×20 ×5 mm substrate, with 350 μm pitch pixelated anodes and a coplanar cathode. A NEMA NU4 Na-22 point source of 250 μm in diameter was imaged by this system. Experiments show that the image resolution of single-pixel photopeak events was 590 μm FWHM while the image resolution of double-pixel photopeak events was 640 μm FWHM. The inclusion of double-pixel full-energy events increased the sensitivity of the imaging system. To validate the imaging experiment, we conducted a Monte Carlo (MC) simulation for the same PET system in Geant4 Application for Emission Tomography. We defined LSO detectors as a scanner ring and 350 μm pixelated CZT detectors as an insert ring. GATE simulated coincidence data were sorted into an insert-scanner sinogram and reconstructed. The image resolution of MC-simulated data (which did not factor in positron range and acolinearity effect) was 460 μm at FWHM for single-pixel events. The image resolutions of experimental data, MC simulated data, and theoretical calculation are all close to 500 μm FWHM when the proposed 350 μm pixelated CZT detector is used as a PET insert. The interpolation algorithm for the charge sharing events was also investigated. The PET image that was reconstructed using the interpolation algorithm shows improved image resolution compared with the image resolution without interpolation algorithm.

The effects of the lattice constant and electron affinity on the negative-ion conversion of a neutral atom undergoing grazing scattering on an ionic-crystal surface over the complete velocity range were investigated. Here, a comparison of negative-ion conversion of neutral ${\mathrm{O}}^{0}\text{\ensuremath{-}}\mathrm{KCl}(100),{\mathrm{F}}^{0}\text{\ensuremath{-}}\mathrm{KCl}(100)$, and ${\mathrm{O}}^{0}\text{\ensuremath{-}}\mathrm{KI}(100)$ surface systems shows that the pronounced difference in the efficiency of negative-ion formation between ${\mathrm{F}}^{0}\text{\ensuremath{-}}\mathrm{KCl}(100)$ and ${\mathrm{O}}^{0}\text{\ensuremath{-}}\mathrm{KCl}(100)$ is caused by the large difference in their projectile electron affinities, whereas the difference between ${\mathrm{O}}^{0}\text{\ensuremath{-}}\mathrm{KI}(100)$ and ${\mathrm{O}}^{0}\text{\ensuremath{-}}\mathrm{KCl}(100)$ is caused by the difference in their lattice constants.

Surface potential is one of the key properties of charge retain ability and energy storage performance of cellular electrets. Based on charge simulation method (CSM), a high precision simulation algorithm of surface potential of functional dielectrics is proposed in this paper, which can be applied to the simulations of cellular polypropylene after high-voltage corona discharge. In order to validate the modeling, the surface potential of insulating polymer after corona discharge is measured by an electrostatic measurement system and compared with simulation results based on CSM and finite element method (FEM). The result shows that the numerical simulation based on CSM is well consistent with the experimental data, which indicates that the modeling based on CSM can be used in the calculation and analysis of charge storage ability of capacitor and electrostatic generator based on functional electrets. Moreover, as simulation based on CSM has higher precision than FEM, investigation of surface charge storage ability and prediction of electrostatic hazards of insulating dielectrics could be realized by using CSM.

In this paper, we extend our previous work of classical over barrier ionization (COBI) model to study the multiple-ionization and mean charge state of noble gases colliding with heavy ions at energies close to the Bragg peak region ranging up to some hundreds of keV/amu. The method we report is in good agreement with experimental data and offers the advantage of very small computation time. Therefore, this model will be extremely helpful to be included in numerical codes to calculate the charge state distribution in plasma.

Muscovite mica sheets were bombarded by lithium, carbon and oxygen ions in the energy range from several hundred keV to several MeV. The induced surface structures were measured in the air with atomic force microscopy (AFM) in the tapping mode. The hillock-like structure on the mica surface was observed. The height of the hillock increases linearly when the energy loss is above 1.2 keV/nm. The induced structures are similar with the similar electronic stopping powers but different projectiles for muscovite mica.

We are currently investigating the feasibility of using highly pixelated Cadmium Zinc Telluride (CdZnTe) detectors for sub-500 μm resolution PET imaging applications. A 20 mm × 20 mm × 5 mm CdZnTe substrate was fabricated with 350 μm pitch pixels (250 μm anode pixels with 100 μm gap) and coplanar cathode. Charge sharing among the pixels of a 350 μm pitch detector was studied using collimated 122 keV and 511 keV gamma ray sources. For a 350 μm pitch CdZnTe detector, scatter plots of the charge signal of two neighboring pixels clearly show more charge sharing when the collimated beam hits the gap between adjacent pixels. Using collimated Co-57 and Ge-68 sources, we measured the count profiles and estimated the intrinsic spatial resolution of 350 μm pitch detector biased at -1000 V. Depth of interaction was analyzed based on two methods, i.e., cathode/anode ratio and electron drift time, in both 122 keV and 511 keV measurements. For single-pixel photopeak events, a linear correlation between cathode/anode ratio and electron drift time was shown, which would be useful for estimating the DOI information and preserving image resolution in CdZnTe PET imaging applications.

Water-Cu(110) interaction is of particular importance during the routine use of graphene-based devices. In this work, water adsorption, dissociation, and desorption at elevated temperatures have been well studied using the time-of-flight ion scattering technique. It is found that water adsorption meets the first-order Langmuir adsorption model at room temperature. The variation of the ratio between residual O and H on the surface with temperature has been well determined, which profoundly reveals the dynamical process of surface composition. Furthermore, the change in the surface electronic properties has been probed by measuring negative-ion fractions as a function of the annealing temperature for fast ion scattering. It suggests that charge transfer is a very sensitive method for studying specific electronic processes in real time.

Previous designs of conventional aerodynamic lens have been widely used to focus large nanoparticles. It is a big challenge to focus small sub-10 nm nanoparticles. The focusing of small nanoparticle is controlled by two factors: high diffusivity and low inertia. Here, we made some modification for the aerodynamic lens system designed by Wang et al. (Aerosol Sci Technol 39:624, 1). The capability of focusing sub-10 nm nanoparticles was evaluated by the computational fluid dynamics software FLUENT. From numerical simulations of size-resolved particle trajectories, it was found that most of the small nanoparticles lost to the wall by diffusion. The effect of Brownian diffusion on the particle extraction from lens system is investigated. Simulations have shown that the Brownian diffusion perturbs the trajectories of nanoparticles that are dictated by the carrier gas and increase the probability of nanoparticle deposition on the wall. The effect of lens system geometry on the nanoparticles is also investigated. The results of simulation showed that the geometry of lens system affected the beam diameter and the transmission efficiency of nanoparticle.

We propose a simple theoretical approach to consider negative-ion conversion of neutral atoms grazing on alkali-metal-halide crystal surfaces over the complete velocity range. The conversion process is viewed as a series of successive binary collisions between the projectile and the negatively charged sites on the surface along their trajectories due to localization of valence-band electrons at the anionic sites of the crystal. Conversion from F-0 to F- via grazing scattering in LiF(100) and KI(100) is demonstrated with this model, which incorporates the key factors of image interaction and Mott-Littleton polarization interaction for electron capture. It also incorporates the decrease in the electron affinity due to Coulomb barrier tunneling of large-velocity negative ions to the vacuum level near surface anion sites. The pronounced differences in the efficiency of F- formation at LiF(100) and KI(100) surfaces are well explained by the proposed model. The relative efficiency and related saturation of the negative-ion formation for LiF and KI crystals compare well with experimental results.

The interaction of 16 keV Cu − ions with Al 2 O 3 nano-capillaries of 50 nm diameter and 10 μm length was studied. The angular and charge state distributions of the transmitted particles were measured by a position-sensitive microchannel plate detector. It shows that transmitted particles appear as two peaks and consist of three charge states, Cu − , Cu 0 and Cu + . As the tilt angle increases, more Cu − ions lose electrons resulting in the formation of Cu 0 and Cu + during multiple collisions with the capillary walls. Unlike previously reported results on highly charged ions, keV-energy Cu − ions undergo significant charge exchange processes with the inner walls of the capillaries. This indicates that the guiding effect is absent in the negative ion transmission under the very low ion current conditions.

We are investigating the feasibility of a high-resolution PET insert device based on a Cadmium Zinc Telluride (CdZnTe) detector with 350μm anode pixel pitch to be integrated into a conventional animal PET scanner to improve its image resolution to sub-500 micrometer range. In this work, we have used a simplified version of the future 2048-pixel CdZnTe planar detector with 250μm anode pixel size and 100μm gap. This simplified 9 anode pixel structure makes it possible to conduct experiments without a complete ASIC readout system (with 2048 channels) that is still under development. We characterized this CdZnTe detector by investigating it charge sharing, spatial resolution, and energy resolution. We imaged a Na-22 point source using the coincidence events between this 350μm pixelated CdZnTe detector and a lutetium oxyorthosilicate (LSO) based Siemens Inveon PET detector. The reconstructed PET image shows a resolution of 590μm full width at half maximum (FWHM) by using single-pixel events. When we included double-pixel charge sharing events in the image reconstruction, the image resolution was degraded to 655μm, but the sensitivity of the coincidence system increased 2.5 to 3 times.

Theoretical calculations employing the modified over-barrier model to investigate single and double ionization cross sections of helium in collisions with highly charged ions are presented in the strong perturbation region. It shows the velocity and charge state dependences of cross sections, which coincides with the available experimental data. In order to determine the reliability of this model for partially stripped projectiles, the cross-section ratios have been measured for C Z p + , O Z p + – He ( Z p = 1 – 4 ) by the coincidence technique in the velocity range of 1–4.5 a.u. The calculated results are approximately in agreement with the experimental data.

X-ray emission from Xe30+ ions at 350–600 keV impacting on an Au target was investigated at the Heavy Ion Research Facility at Lanzhou. Characteristic X-rays of Xe ions at energy of about 1.65 keV were observed. This X-ray emission is induced by the decay of very high Rydberg states of Xe ions. It was also found that the yield of such characteristic X-rays is decreasing with increasing the projectile kinetic energy. Simultaneously, the yield of the characteristic Au X-rays from the M shell increases also. These phenomena are qualitatively analyzed with the classical Coulomb over the Barrier Mode (COBM) for highly charged ions interacting with solid state surfaces.

The InGaN-based blue light-emitting diodes (LEDs) with double AlGaN/InGaN superlattice (SL) electron-blocking layers are investigated theoretically. The simulation results indicate that the output power, the radiative recombination rates, and the spontaneous emission rates of the blue LED with double AlGaN/InGaN superlattice layers are evidently improved, meanwhile, the efficiency droop phenomenon is remarkably alleviated, compared with the original structure. The reasons are that the new structure with double AlGaN/InGaN SLs can not only suppress the electron leakage but also enhance the hole injection. Specifically, it can improve the radiative recombination rates, which leads to higher output power than original structure.

In this paper, the projectile electron loss cross sections of He+, Li+, Li2+ and C2+ colliding with atomic hydrogen are studied in the frame work of the extended over-barrier model at intermediate velocities (25–600 keV/u). The electron loss cross sections are calculated in terms of the interaction between the screened target nucleus and the active projectile electron and of the interaction between the projectile electron and the target electron. Compared with the measurements, this model satisfactorily reproduces the experimentally obtained energy dependence of the single electron loss cross sections over the energy range investigated here.

We measured the transmission of 10 keV, 14 keV and 18 keV Cu - ions through Al 2 O 3 nanocapillaries of 50 nm diameter and 12 μm length at a series of different tilt angles. The surfaces of Al 2 O 3 capillaries were coated by Au . The angular distributions and the charged states of transported particles were investigated. Our results indicated the existence of a guiding effect when ions pass through the capillary. The transmitted particles were observed to lose the electrons easier at higher projectile energy and larger tilt angle. It was also found that the transmitted ions lose the electrons easier with lower ionization energy.

Molybdenum L-shell X-rays were produced by Xeq+ (q = 25-30) bombardment at low energies from 2.65 to 4.55 keV/amu (350-600 keV). We observed a kinetic energy threshold of Mo L-shell ionization down to 2.65-3.03 keV/amu (350-400 keV). The charge state effect of the incident ions was not observed which shows that the ions were neutralized, reaching an equilibrium charge state and losing their initial charge state memory before production of L-shell vacancies resulted in X-ray production. The experimental ionization cross sections were compared with those from Binary Encounter Approximation theory. Taking into account projectile deflection in the target nuclear Coulomb field, the ionization cross section of Mo L-shell near the kinetic energy threshold was well described. (C) 2010 Published by Elsevier B.V.

In this paper, we investigated the applications of D-D and D-T neutron sources in Neutron Stimulated Emission Computed Tomography (NSECT). Four NSECT models were established, including the TiT/TiD targets to generate neutron sources and the presence/absence of iron balls in water phantom to mimic tumors. NSECT system was simulated by using MCNP software, in which the D-D and D-T neutron sources were generated, collimated, transported, and recorded. We studied flux distributions and typical spectrum of both outgoing neutrons and the characteristic gamma-rays for D-D and D-T NSECT. Results show that the optimized NSECT gamma-ray detector locations will be in the direction from 43.6° to 50.9° along the neutron beam propagation direction. The best NSECT gamma-ray detector locations for D-D and D-T NSECT are slightly different, but in both cases the detectors should be aligned in the direction from 43.6° to 50.9°. We found the characteristic peaks of 56Fe and 16O from the energy spectrum of characteristic gamma-ray. This demonstrates that NSECT has the ability to identify elements, which would be very useful in tracking the treatment process and in studying the molecular process of living body (including human-beings). This study gives a comprehensive understanding on the selection of neutron sources and the placement of gamma-ray detectors in NSECT applications.

The ratios R(k1) of k-fold to single ionization of the target atom with simultaneous one-electron capture by the projectile have been measured for 15-480 keV/u (nu(p) = 0.8-4.4 a.u.) collisions of C(q+), O(q+) (q=1-4) with Ar, using time-of-flight techniques which allowed the simultaneous identification of the final charge state of both the low-velocity recoil ion and the high-velocity projectile for each collision event. The present ratios are similar to those for He(+) and He(2+) ion impact. The energy dependence of R(k1) shows a maximum at a certain energy, E(max). which approximately conforms to the q(1/2)-dependence scaling. For a fixed projectile state, the ratios R(k1) also vary strongly with outgoing reaction channels. The general behavior of the measured data can be qualitatively analyzed by a simple impact-parameter, independent-electron model. (C) 2009 Elsevier B.V. All rights reserved.

K-shell X-ray emission was observed in the neutralization process of Ar 16+ interacting with beryllium and molybdenum at slow speed (less than Bohr velocity). The K-shell X-ray emission came from the metastable component of Ar 16+ beam with the configuration (1s2s) produced from electron cyclotron resonance ion source. The intensity ratio of K β X-ray emission to K α X-ray emission (I K β/I K α) in the neutralization process decreased when the kinetic energy of the projectile increased. This is closely related to the flight time the projectiles spend above the surface, which is dominated by both the work function of the metal and the normal velocity component of the incident ions to the surface according to the classical over-barrier model. As the relation between I K β/I K α and the above-flight time shows, the longer time above the surface the larger ratio due to the above-surface contribution. Furthermore, the smaller work function leads to the smaller contribution from the above surface.

The transmission of 60 keV O + ions through Al 2 O 3 nanocapillaries target in a series of different tilted angles was measured. The capillaries with diameters of 50 nm and 30 nm respectively, and 10 μm in length were used. Our results indicate the existence of guiding effect when ions through the capillary and a significant dependence of the ion transmission on the diameters of the capillaries.

This paper reports that the transmission of O6+ ions with energy of 150keV through capillaries in an uncoated Al2O3 membrane was measured, and agreements with previously reported results in general angular distribution of the transmitted ions and the transmission fractions as a function of the tilt angle well fitted to Gaussian-like functions were observed. Due to using an uncoated capillary membrane, our c is larger than that using a gold-coated one with a smaller value of E-p/q, which suggests a larger equilibrium charge Q(infinity) in our experiment. The observed special width variation with time and a larger width than that using a smaller E-p/q were qualitatively explained by using mean-field classical transport theory based on a classical-trajectory Monte Carlo simulation.

Coupled-channel cross-sections for electron capture, ionization and electron loss due to polarization effects are calculated. The maximum impact parameter for electron escape is analyzed within the classical framework. The probabilities of ionization and capture are analyzed simultaneously by a semi-empirical method. Differing from the n-body classical trajectory Monte Carlo method, the condition for electron escape is determined by Coulomb forces related to the two nuclei. This method can be used to calculate coupled-channel cross-sections rather than single-channel ones in other methods. Therefore the calculated results can be compared with experimental data directly. In the low energy range, neglecting the ionization effect, the single-capture cross-sections of hydrogen atoms induced by various partially-stripped ions were calculated. In the high energy range, neglecting the capture effect on ionization, the pure-ionization cross-sections of neon atoms induced by Ne q + ( q = 4, 6, 8) and Ar q + ( q = 4, 6, 8, 10) at an incident energy E = 1.05 MeV/u were calculated. Good agreement was found between our calculation and experimental data in the literature. This method had been partially applied for intermediate energy successfully.

The transmission of 60 keV O+ ions through Al2O3 nanocapillaries target in a series of different tilted angles was measured. The capillaries with diameters of 50 nm and 30 nm respectively, and 10 μm in length were used. Our results indicate the existence of guiding effect when ions through the capillary and a significant dependence of the ion transmission on the diameters of the capillaries.

Since the successful completion of the cooling storage ring (CSR) project in China at the end of 2007, high qualitative heavy ion beams with energy ranging from keV to GeV/u have been available at the Heavy Ion Research Facility at Lanzhou (HIRFL). More than 10 9 1 GeV/u C 6+ particles or 10 8 235 MeV/u Xe particles can be stored in the CSR main-ring and extracted within hundred nano-seconds during the test running, the beam parameters will be improved in the coming years so that high energy density (HED) conditions could be achieved and investigated there. Recent scientific results from the experiments relevant to plasma research on HIRFL are summarized. Dense plasma research with intense heavy ion beams of CSR is proposed here.

The X-ray spectra of Nb surface induced by Arq+ (q = 16,17) ions with the energy range from 10 to 20 keV/q were studied by the optical spectrum technology. The experimental results indicate that the multi-electron excitation occurred as a highly charged Ar16+ ion was neutralized below the metal surface. The K shell electron of Ar16+ was excited and then de-excited cascadly to emit K X-ray. The intensity of the X-ray emitted from K shell of the hollow Ar atom decreased with the increase of projectile kinetic energy. The intensity of the X-ray emitted from L shell of the target atom Nb increased with the increase of projectile kinetic energy. The X-ray yield of Ar17+ is three magnitude orders larger than that of Ar16+.

L-shell X-ray spectra of Mo surface induced by Xe25+ and Xe29+ were measured. The X-ray intensity was obtained in the kinetic energy range of the incident ions from 350 to 600 keV. The relationship of X-ray intensity with kinetic energy of the projectile and its charge state were studied, and the simple explanation was given.

We have developed a one-dimensional position-sensitive detector (1-D PSD) for X-ray backscatter imaging which had a scintillator-fiber- position-sensitive photomultiplier (PSPMT) structure. Scintillators of the detector were covered by diffused reflection material. Teflon and the fiber of the detector was covered by mirror reflection material an aluminum foil. The detector was composed of a 1 x 25 LYSO array which was coupled with 25 plastic fibers. Another end of these fibers was twisted into a 5 x 5 array and coupled with a PSPMT R8520. The detector element's area was 4.2 x 20 mm(2) and the position resolution was 4.2 mm. We tested the positioning histogram of the detector set up an X-ray backscatter imaging system, and obtained an X-ray backscatter image from it. (C) 2008 Elsevier B.V. All rights reserved.

The ratios of the cross section of the transfer-ionization to the single-electron-capture of Argon induced by Cq+ (q = 1, 2, 3) ions are measured by means of position sensitive and time-of-flight techniques. Our experimental results are compared with the data of Heq+ (q = 1, 2) − Ar of DuBois. A qualitative interpretation is presented based on the Classical-Over-Barrier Model of Bohr.

Cross-section ratios σDDI/σDSI of direct double ionization (DDI) to direct single ionization (DSI) of Cq+– and Oq+–He (q = 1–4) collisions in the energy range of 15–480 keV/u (0.8 a.u. ≤ v p ≤ 4.4 a.u.) are experimentally determined. For a fixed projectile charge the ratios depend strongly on the projectile energy and there is a maximum at about 100q1/2 keV/u. Comparison is made with other available experimental data. The present results are in agreement with the previously published data. On the whole, the ratios appear in two groups according to the different projectile ions. Combining the Bohr–Lindhard model and statistical model, a theoretical estimate is presented. The estimate shows the trend in good agreement with experimental data.

The impact ionization of atomic hydrogen in collisions with H+, He2+, Li 2 , 3 + , C 2 , 3 , 4 , 5 , 6 + , N 2 , 3 , 4 , 5 + , O 2 , 3 , 4 , 5 , 6 + , and Ar 3 , 4 , 5 , 6 + ions has been studied by the modified over-barrier model at low-to-intermediate velocities. Compared with the calculated results of the classical trajectory Monte Carlo (CTMC) method and the unitarized-distorted-wave approximation (UDWA), this model satisfactorily reproduces the experimentally obtained velocity dependence of the absolute ionization cross sections at intermediate impact velocities.

Pure target ionization was investigated for 0.4–6.4 MeV C q + ( q = 1–4) + He and O q + ( q = 1–4) + He collisions. The double-to-single target ionization ratios R 21 were measured using coincidence techniques. We compare our results with existing experimental results and find they are in good agreement. The ratio R 21 is nearly independent of projectile charge state. The relation of R 21 ∼ V is analyzed using the over barrier model (OBM) and ionization probability, which is described in our extended over barrier model. Our calculation agrees well with the experimental results.

In this paper, an extension of the already-developed and modified over-barrier model is presented to study theoretically one and two electron transitions between helium atoms and multiply charged ion collisions at a low-to-intermediate velocity (v(Bohr) similar to 5v(Bohr)). It is found that the present model can partially reproduce the experimentally obtained velocity dependence of the absolute cross-sections which is also supported by the results of the classical trajectory Monte Carlo (CTMC) method. (c) 2007 Elsevier B.V. All rights reserved.

In this paper a projectile ions-recoil ions coincidence technique is employed to investigate the target ionization and projectile charge state changing processes in the collision of 0.22–6.35 MeV Cq+ (q = 1 – 4) ions with argon atoms. The partial cross section ratios of the double, triple, quadruplicate ionization to the single ionization (or the single capture) of argon associated with single electron loss (or single electron capture) by the projectile are measured and compared with the previous experimental results. In the present experiment, it is observed that the ratios of ionization cross sections R associated with single loss and single capture depend strongly on the projectile charge state and vary significantly with different reaction channels as impact energy increases. In addition, this paper gets empirical scaling laws for the ionization cross section ratios R corresponding to the projectile single loss and finds that the ratios of the double ionization to the single ionization associated with single electron capture remain constant in the present energy range.

L-shell x-ray yields of molybdenum bombarded by highly charged Arq+ ions (q = 11-16) are measured. The x-ray production cross-sections are extracted from the yields data. The energy of the incident Ar ions ranges from 200 to 350 keV. After the binding energy correction, experimental data are explained in the framework of binary-encounter-approximation (BEA). The direct ionization is treated in the united atom (UA) limit (Lapicki and Lichten 1985 Phys. Rev. A 31 1354), not in the separate atom (SA) limit. The calculation results of BEA (Gacia and Fortner 1973 Rev. Mod. Phys. 45 111) are much lower than the experimental results, while the results of binding energy modified BEA are basically in agreement with the experimental results.