Radiation effects in atomic cryogenic solids

Radiation effects and relaxation processes in atomic solids are discussed with the example of solid Xe preliminarily irradiated by an electron beam. The study was performed employing concurrently the combination of current and optical “activation spectroscopy” methods. Three relaxation processes were monitored simultaneously upon controlled warming-up of pre-irradiated solids: thermally stimulated exoelectron emission (TSEE), thermally stimulated luminescence (TSL) in the VUV range and the total desorption yield by pressure measuring above the sample. Anomalous strong low-temperature “post-desorption” (ALTpD) of own atoms from pre-irradiated Xe solids was observed for the first time. The data obtained demonstrated a clear correlation between the yields of exoelectrons, photons of recombination luminescence and neutral particles pointing to the common origin of the phenomena. It was shown that the key primary process of the relaxation cascade including the ALTpD is a thermally stimulated electron detrapping, promoting electrons into the conduction band. Subsequent branching of the relaxation paths results in the relaxation emissions observed. An accumulation of charges of both signs as well as excess electrons under exposure to an electron beam was found.     

Development of function-graded proton exchange membrane for PEFC using heavy ion beam irradiation

The graded energy deposition of heavy ion beam irradiation to polymeric materials was utilized to synthesize a novel proton exchange membrane (PEM) with the graded density of sulfonic acid groups toward the thickness direction. Stacked Poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) films were irradiated by Xe⁵⁴⁺ ion beam with the energy of 6 MeV/u under a vacuum condition. The induced trapped radicals by the irradiation were measured by electron spin resonance (ESR) spectroscopy. Irradiated films were grafted with styrene monomer and then sulfonated. X-ray photo-electron spectroscopy (XPS) spectra showed that the densities of sulfonic acid groups were controlled for injection “Surface” and transmit “Back” sides of the fabricated PEM. The membrane electrode assembly (MEA) fabricated by the function-graded PEM showed improved fuel cell performance in terms of voltage stability. It was expected that the function-graded PEM could control the graded concentration of sulfonic acid groups in PEM.     

Fast neutron inspection of sea containers for the presence of “dirty bomb”

The possibility of the detection of “dirty bomb” presence inside sea containers is evaluated. The method proposed for explosive and fissile material detection makes use of two sensors (X-rays and neutrons). A commercial imaging device based on the X-ray radiography performs a fast scan of the container, identifies a “suspect” region and provides its coordinates to the neutron based device for the final “confirmatory” inspection. In this two sensor system a 14 MeV neutron beam defined by the detection of associated alpha particles is used for interrogation of only volume elements marked by X-ray sensor. The object’s nature is determined from passive and neutron induced, gamma energy spectra measurements. Experimental results (time-of-flight and gamma energy spectra) obtained for the irradiation 30 kg of TNT, depleted uranium and other materials hidden inside the container are presented.     

Improved wear resistance of Al-15Si alloy with a high current pulsed electron beam treatment

A hypereutectic Al-15Si alloy (Si 15 wt.%, Al balance) was irradiated by high current pulsed electron beam (HCPEB). The HCPEB treatment causes ultra-rapid heating, melting and cooling at the top surface layer. As a result, the special “halo” microstructure centering on the primary Si phase is formed on the surface due to interdiffusion of Al and Si elements. The composition of the “halo” microstructure is distributed continuously from the center to the edge of the “halo”. Compared to an untreated matrix, the remelted layer underneath the surface presents single contrast because of the compositional homogeneity after HCPEB treatment. The thickness of the remelted layer increases slightly from 4.4 μm (5 pulses) to 5.6 μm (25 pulses). HCPEB treatment broadens and shifts the diffraction peaks of Al and Si. The lattice parameters of Al decreases due to the formation of a supersaturated solid solution of Al in the melted layer. Through analysis of Raman spectra and transmission electron microscopy (TEM), the amorphous Si (a-Si) and nanocrystalline Si are formed in the near-surface region under multiple bombardments of HCPEB. The relative wear resistance of a 15-pulse sample is effectively improved by a factor of 9, which can be attributed to the formation of metastable structures.     

Monte Carlo study of molecular weight distribution changes induced by degradation of ion beam irradiated polymers

In this work we study a polymeric material that degrades upon irradiation due to the energy inhomogeneously deposited by heavy ion beams. Ion beam irradiation of polymers generates rather different effects than those induced by “classical” low ionizing particles such as electrons or gamma rays. This is due to the high electronic stopping power and the inhomogeneous distribution of deposited energy. This energy is transferred to the material within a small volume along the ion path forming the so called “nuclear track” or “latent track”. The track size primarily depends on the ion velocity, and it is determined by the secondary electrons (delta rays) generated along the ion trajectory. By means of Monte Carlo simulations we first obtained equilibrated polymer configurations using a coarse-grained model, and then investigated the spatially inhomogeneous chain scission process due to the passage of the ions. The number average molecular weight, weight average molecular weight and the polydispersity were calculated as a function of track radius, scission probability within the ion track and irradiation fluence. Finally we compared our results with a numerical implementation of a model for random homogeneous degradation.     

Operation of the ITER Electrostatic Residual Ion Dump with a Perturbed Field

ITER will use a novel electrostatic method to remove the unwanted residual charged component from the neutral beam injectors in place of the usual magnet separation system. This technique has not been tested experimentally and is subject to the additional complication of plasma formation perturbing the electric field. Previous calculations have shown that whilst this is not significant for the 1 MeV heating beam systems, the lower energy diagnostic beam system will be susceptible. An analytical model of the electrostatic dump has been developed that includes the perturbation of the vacuum electrostatic field by both plasma and the separating positive and negatively charged residual beams. An approximate solution of Poisson’s equation is formulated that allows analysis of the space charge field when plasma density is insufficient to ensure zero electric field at the anode. The resulting modified electric field is then incorporated into a particle trajectory code to determine the deposition of the residual ions on the ERID panels. It is shown that the effect of plasma formation is to introduce an asymmetry into the deflecting field and the effect of the separating charges is to weaken the deflection of the residual beams. As a consequence the reference design for the ITER diagnostic beam will not collect all of the residual ions and it is recommended that the deflection voltage be increased by at least 50%.     

Cross-section transmission electron microscopy of the ion implantation damage in annealed diamond

It has formerly been shown that low-damage levels, produced during the implantation doping of diamond as a semiconductor, anneal easily while high levels “graphitize” (above about 5.2 × 10¹⁵ ions/cm²). The difference in the defect types and their profiles, in the two cases, has never been directly observed. We have succeeded in using cross-section transmission electron microscopy to do so. The experiments were difficult because the specimens must be polished to ∼40 μm thickness, then implanted on edge and annealed, before final ion beam thinning to electron transparency. The low-damage micrographs reveal some deeply penetrating dislocations, whose existence had been predicted in earlier work.     

Collision cascade temperature

Interaction of a projectile with a solid has been considered in detail. It has been found that any collision cascade generated by a projectile can be characterized by the average kinetic energy of cascade atoms that represents an “instantaneous temperature” of the cascade during its very short lifetime (10⁻¹² s). We refer to this value as the “dynamic temperature” in order to emphasize the fact that cascade atoms are in a dynamic equilibrium and have a definite energy distribution. The dynamic temperature defines the electron distribution in the cascade area and, hence, the ionization probability of sputtered atoms. The energy distribution of cascade atoms and, as a consequence, the dynamic temperature can be found experimentally by measuring the energy distribution of sputtered atoms. The calculated dynamic temperature has been found to be in good agreement with the experimental data on ion formation in the case of cesium and oxygen ion sputtering of silicon. Based on the developed model we suggest an experimental technique for a radical improvement of the existing cascade sputtering models.     

Neutronic characterization of the MEGAPIE target

The MEGAPIE project aimed to design, build and operate a liquid metal spallation neutron target of about 1 MW beam power in the SINQ facility at the Paul Scherrer Institut (Villigen, Switzerland). This project is an important step in the roadmap towards the demonstration of the accelerator driven system (ADS) concept and high power liquid metal targets in general. Following the design phase, an experimental program was defined to provide a complete characterization of the facility by performing a “mapping” of the neutron flux at different points, from the center of the target to the beam lines. The neutronic performance of the target was studied using different experimental techniques with the goals of validating the Monte Carlo codes used in the design of the target; additionally, the performance was compared with the solid lead targets used before and after the MEGAPIE experiment.     

Stability of a partially compensated electron beam

The author discusses the conditions for stability of a partially compensated electron beam in relation to deflection (snaking). It is shown that, with a continuous spectrum of perturbation wave vectors, there is always a region of strong instability (with relatively large increments). With a discrete spectrum (e.g., with a beam of finite length in an accelerator), instability occurs only at beam currents greater than a certain critical value. Landau damping and radiation friction do not eliminate the instability. A weak dissipative instability is discovered, caused by radiation friction. In some cases Landau damping stabilizes this instability, but can also increase it.The investigation is based on a model beam in the form of two pinches, electron and ion, with constant dimensions and uniform densities.Translated from Atomnaya Énergiya, Vol. 19, No. 3, pp. 239–244, September, 1965     

Negative differential conductivity of positrons in gases

This paper reports on a new series of calculations of positron transport properties based on current experimental cross section data. It is found that negative differential conductivity (NDC) occurs in the bulk drift velocity W but not the flux drift velocity w. The origin of the phenomenon lies in the “reactive” nature of positron collisions associated with positronium Ps formation, and is quite different in origin to the better known NDC effect in w arising from certain combinations of inelastic-elastic cross sections. Moreover, while the Ps formation process is qualitatively similar (at least from a kinetic theory perspective) to electron attachment, it is characterized by a cross section several orders of magnitude larger and hence the “reactive” NDC effect is correspondingly more pronounced. In this paper we test both established conditions for NDC, and develop new criteria, using simple mathematics and physical arguments where possible.     

Formation of Electron Internal Transport Barrier in Edge Plasma of Small Size Divertor Tokamak

The formation of electron internal transport barrier (EITB) during using counter-neutral beam injection (NBI) heating in the edge plasma of small size divertor tokamak can be simulated by using fluid transport code B2SOLPS0.5.2D. The results of simulations give us the following: (1) Plasma heating with counter-neutral beam injection leads to, strong, parabola type electron internal transport barrier (EITB) was formed in the edge plasma of small size divertor tokamak. (2) In case of plasma heating by counter-neutral beam injection, the radial electric field shear (E _r –gradient) was increased, while electron transport coefficients were reduced in conjunction with the formation of electron internal transport barrier (EITB). (3) The plasma heating by counter-neutral beam injection play significantly role in redistribution of parallel (toroidal) velocity in edge plasma of small size divertor tokamak.     

Prospects and challenges in application of gamma, electron and ion beams in processing of nanomaterials

Application of radiation techniques for nanotechnology has been known for years. X-ray, electron beam and ion beam lithography are good examples of applications. By using electron beams, ion beams and X-rays structures as small as 10 nm can be produced. Ion track membranes with track diameters from 10 nm to 100 nm are used as such or as templates for electroplating of nanowires of metal, semiconductor and magnetic materials. In the near future X-rays, focused ion beams and electron beams will be used for nanolithography and 3D fabrication; heavy ion beams on the other hand can be useful for fabrication of nanopores and nanowires. The use of radiation has proved to be an essential technique in the fabrication of nanostructures with high resolution as the radiation beams can be focused into few nanometer scales or less. Three groups of products could be considered to be fabricated by radiation techniques: nanoparticles, nanogels and nanocomposites. These and other possible fields of radiation processing applications in nanotechnology are discussed in the paper.     

Using a current method for measuring ion-induced electron emission from LiF

The interaction of ions with an insulating substrate leads to surface charging. For intense ion beams these charges are not sufficiently fast removed between successive ion impact events. As a result the trajectories of the incident beam and the electron emission yield may be altered in a hardly predictable way. We demonstrate, that by heating an alkali-halide sample to a sufficiently high temperature, this macroscopic charging can be avoided and a simple current method can be used to study electron emission from such a surface. Measured electron emission yields from a LiF(1 0 0) single crystal due to impact of singly and multiply charged heavy noble gas ions (argon and xenon) are presented. For argon projectiles our results compare well with previous data, which were obtained using a more sophisticated electron statistics method and six orders of magnitude less intense ion beams.     

Coherent Smith-Purcell radiation: Theories and simulations

Smith-Purcell (SP) radiation has been observed many times over the past fifty years, and several theories have been proposed to explain it. However, it is only quite recently that Andrews, Brau and collaborators made a considerable advance in understanding how coherent SP radiation may be produced from an initially continuous beam. Their work received support from 2-D simulations which were performed using the Particle-in-Cell (PIC) code “MAGIC”. Here we present a review of our 2-D simulations of coherent SP and discuss how they relate to the model of Andrews and Brau. We also describe briefly a SP experiment in the microwave domain using a sheet beam that is planned for 2008.     

Effect of metal ion location in reaction medium on formation process and structure of PtCu–CuO nanoparticles supported on carbon and γ-Fe2O3

The process of nanoparticle formation by radiochemical synthesis in a heterogeneous system has been investigated considering the effects of the metal ion location in the reaction medium. PtCu nanoparticles supported on carbon and γ-Fe₂O₃ were synthesized using a high-energy electron beam. The metal ions in the precursor were categorized as those dissolved in solution, adsorbed on support, and precipitated. The ratio of metal ions in the solution was varied prior to the electron beam irradiation and its effects on the synthesized particle structures were examined. The nanoparticles were characterized by inductively coupled plasma-atomic emission spectrometry, transmission electron microscopy, X-ray diffraction, and X-ray absorption spectroscopy. A PtCu alloy and CuO were immobilized on the support in all the samples. The PtCu alloy nanoparticle composition depended on the Cu ion content in the solution. The nanoparticle formation mechanism could be explained using the obtained results. Metal ions present in the solution resulted in formation of the alloy. The adsorbed ions also contributed to the alloy formation by desorbing from the support when irradiated. On the other hand, alloy formation with Pt from the precipitated Cu ions was found to be difficult.     

Neutral beam deposition in large,finite-beta noncircular tokamak plasmas

A parametric pencil beam model is introduced for describing the attenuation of an energetic neutral beam moving through a tokamak plasma. The nonnegligible effects of a finite beam cross-section and noncircular shifted plasma cross-sections are accounted for in a simple way by using a smoothing algorithm dependent linearly on beam radius and by including information on the plasma flux surface geometry explicitly. The model is bench-marked against more complete and more time-consuming two-dimensional Monte Carlo calculations for the case of a large D-shaped tokamak plasma with minor radiusa=120 cm and elongationb/a=1.6. Deposition profiles are compared for deuterium beam energies of 120–150 keV, central plasma densities of 8×10¹³ to 2×10¹⁴ cm^–3, and beam orientation ranging from perpendicular to tangential to the inside wall.     

Potential nuclear explosive yield of reactor-grade plutonium using the disassembly theory of early reactor safety analysis

The disassembly theory of reactor safety analysis developed for early metal-fueled criticals is applied to determining the potential nuclear explosive yield of reactor-grade plutonium. After verification of the theoretical models, materials data, and equation of state by recalculation of published data, this disassembly theory is applied to so-called hypothetical nuclear explosive devices (HNEDs) based on reactor-grade plutonium. The masses for k_eff = 0.98 and the neutron life times are calculated for such devices by applying neutron transport theory and Monte Carlo codes. Spherical shock compression models describe the density variations as a function of space and time for spherical shock compression of the reactor-grade plutonium sphere with a natural-uranium reflector. Reactivity calculations are performed to determine “Rossi alpha” as a function of time during spherical shock compression. Pre-ignition theory shows that pre-ignition by spontaneous fission neutrons occurs just after prompt criticality is achieved. The chain reaction and power excursion initiated lead to internal pressure buildup which stops the spherical shock wave after it has penetrated between 1.3 cm and 1.8 cm into the plutonium metal sphere. This limits the maximum reactivity input and the potential nuclear explosive yield to 0.12 up to 0.35 kt TNT (equivalent).However, these results do not describe the full reality. In a companion paper, thermal analysis shows such HNEDs to be technically unfeasible for all reactor-grade plutonium from spent LWR fuel with a burn-up of more than 30 GWd/t as long as low technology is used in the spherical implosion lenses of chemical high explosives. More advanced medium technology would raise this burn-up limit to approximately 55 GWd/t.     

Materials science and biophysics applications at the ISOLDE radioactive ion beam facility

The ISOLDE isotope separator facility at CERN provides a variety of radioactive ion beams, currently more than 800 different isotopes from ∼70 chemical elements. The radioisotopes are produced on-line by nuclear reactions from a 1.4 GeV proton beam with various types of targets, outdiffusion of the reaction products and, if possible, chemically selective ionisation, followed by 60 kV acceleration and mass separation. While ISOLDE is mainly used for nuclear and atomic physics studies, applications in materials science and biophysics account for a significant part (currently ∼15%) of the delivered beam time, requested by 18 different experiments. The ISOLDE materials science and biophysics community currently consists of ∼80 scientists from more than 40 participating institutes and 21 countries. In the field of materials science, investigations focus on the study of semiconductors and oxides, with the recent additions of nanoparticles and metals, while the biophysics studies address the toxicity of metal ions in biological systems. The characterisation methods used are typical radioactive probe techniques such as Mössbauer spectroscopy, perturbed angular correlation, emission channeling, and tracer diffusion studies. In addition to these “classic” methods of nuclear solid state physics, also standard semiconductor analysis techniques such as photoluminescence or deep level transient spectroscopy profit from the application of radioactive isotopes, which helps them to overcome their chemical “blindness” since the nuclear half life of radioisotopes provides a signal that changes in time with characteristic exponential decay or saturation curves. In this presentation an overview will be given on the recent research activities in materials science and biophysics at ISOLDE, presenting some of the highlights during the last five years, together with a short outlook on the new developments under way.