Showing papers in "Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms in 2001"

High-energy radiation and polymers: A review of commercial processes and emerging applications

Abstract: Ionizing radiation has been found to be widely applicable in modifying the structure and properties of polymers, and can be used to tailor the performance of either bulk materials or surfaces. Fifty years of research in polymer radiation chemistry has led to numerous applications of commercial and economic importance, and work remains active in the application of radiation to practical uses involving polymeric materials. This paper provides a survey of radiation-processing methods of industrial interest, ranging from technologies already commercially well established, through innovations in the active R&D stage which show exceptional promise for future commercial use. Radiation-processing technologies are discussed under the following categories: cross-linking of plastics and rubbers, curing of coatings and inks, heat-shrink products, fiber–matrix composites, chain-scission for processing control, surface modification, grafting, hydrogels, sterilization, natural product enhancement, plastics recycling, ceramic precursors, electronic property materials, ion-track membranes and lithography for microdevice production. In addition to new technological innovations utilizing conventional gamma and e-beam sources, a number of promising new applications make use of novel radiation types which include ion beams (heavy ions, light ions, highly focused microscopic beams and high-intensity pulses), soft X-rays which are focused, coherent X-rays (from a synchrotron) and e-beams which undergo scattering to generate patterns.

Diode-like single-ion track membrane prepared by electro-stopping

Abstract: The prepn. of an asym. membrane in poly(ethylene terephthalate) (PET) is described, using a combination of chem. and electro-stopping. For this purpose, a single-ion-irradiated PET film is inserted ...

Space radiation dosimetry in low-Earth orbit and beyond

Abstract: Space radiation dosimetry presents one of the greatest challenges in the discipline of radiation protection. This is a result of both the highly complex nature of the radiation fields encountered in low-Earth orbit (LEO) and interplanetary space and of the constraints imposed by spaceflight on instrument design. This paper reviews the sources and composition of the space radiation environment in LEO as well as beyond the Earth's magnetosphere. A review of much of the dosimetric data that have been gathered over the last four decades of human space flight is presented. The different factors affecting the radiation exposures of astronauts and cosmonauts aboard the International Space Station (ISS) are emphasized. Measurements made aboard the Mir Orbital Station have highlighted the importance of both secondary particle production within the structure of spacecraft and the effect of shielding on both crew dose and dose equivalent. Roughly half the dose on ISS is expected to come from trapped protons and half from galactic cosmic rays (GCRs). The dearth of neutron measurements aboard LEO spacecraft and the difficulty inherent in making such measurements have led to large uncertainties in estimates of the neutron contribution to total dose equivalent. Except for a limited number of measurements made aboard the Apollo lunar missions, no crew dosimetry has been conducted beyond the Earth's magnetosphere. At the present time we are forced to rely on model-based estimates of crew dose and dose equivalent when planning for interplanetary missions, such as a mission to Mars. While space crews in LEO are unlikely to exceed the exposure limits recommended by such groups as the NCRP, dose equivalents of the same order as the recommended limits are likely over the course of a human mission to Mars.

High sensitivity thermoluminescence dosimetry

Abstract: This paper reviews the physics of the phenomenon of thermoluminescence (TL) related to dosimetric applications. Basic concepts are given using the simple model of one trap–one recombination centre. General characteristics of thermoluminescence dosimetry (TLD) materials are reviewed. Two high sensitivity TL materials are discussed in detail namely LiF:Mg, Cu, P and α-Al 2 O 3 :C. What is understood and what knowledge is still lacking of the TL mechanism in both materials is indicated. Field measurements show that in spite of incomplete understanding of the TL mechanism, both materials can be used to measure very low doses in a reliable way.

Nuclear instruments and methods in physics research section B : beam interactions with materials and atoms

Abstract: The impact-induced deposition of Al13 clusters with icosahedral structure on Ni(0 0 1) surface was studied by molecular dynamics (MD) simulation using Finnis–Sinclair potentials. The incident kinetic energy (Ein) ranged from 0.01 to 30 eV per atom. The structural and dynamical properties of Al clusters on Ni surfaces were found to be strongly dependent on the impact energy. At much lower energy, the Al cluster deposited on the surface as a bulk molecule. However, the original icosahedral structure was transformed to the fcc-like one due to the interaction and the structure mismatch between the Al cluster and Ni surface. With increasing the impinging energy, the cluster was deformed severely when it contacted the substrate, and then broken up due to dense collision cascade. The cluster atoms spread on the surface at last. When the impact energy was higher than 11 eV, the defects, such as Al substitutions and Ni ejections, were observed. The simulation indicated that there exists an optimum energy range, which is suitable for Al epitaxial growth in layer by layer. In addition, at higher impinging energy, the atomic exchange between Al and Ni atoms will be favourable to surface alloying.

Improved charge-state formulas

Abstract: A large set of experimental charge-state distributions is analyzed in this work. Two fit formulas are presented for mean equilibrium charge states of projectiles ranging from protons to uranium. One formula is given for all ions in gas targets and another one for solid targets. The deviation from the experimental data is reduced by roughly a factor of two in comparison with the widely used formula by Nikolaev and Dmitriev as well as with the Bohr stripping criterion as revised by Northcliffe. Finally, the influence of the projectile charge state on the prediction of stopping powers for fast projectiles in carbon is shown and comparison is made with experimental energy-loss data.

Nanocomposites formed by ion implantation: Recent developments and future opportunities

Abstract: Ion implantation is a versatile and powerful technique for forming many types of nanocrystalline precipitates embedded in the near-surface region of a wide variety of crystalline and amorphous host materials The unique optical, electronic and magnetic properties of these nanocomposites has stimulated considerable recent research interest In this review, we discuss recent developments in the field as well as some of the problems that currently hinder the potential applications of nanocomposites formed by ion implantation

Optically stimulated luminescence dosimetry

Abstract: Models and the conceptual framework necessary for an understanding of optically stimulated luminescence (OSL) are described. Examples of various OSL readout schemes are described, along with examples of the use of OSL in radiation dosimetry.

Laser generation of proton beams for the production of short-lived positron emitting radioisotopes

Abstract: Protons of energies up to 37 MeV have been generated when ultra-intense lasers (up to 10 20 W cm −2 ) interact with hydrogen containing solid targets. These protons can be used to induce nuclear reactions in secondary targets to produce β+-emitting nuclei of relevance to the nuclear medicine community, namely 11C and 13N via (p, n) and (p,α) reactions. Activities of the order of 200 kBq have been measured from a single laser pulse interacting with a thin solid target. The possibility of using ultra-intense lasers to produce commercial amounts of short-lived positron emitting sources for positron emission tomography (PET) is discussed.

Theory of heavy charged particle response (efficiency and supralinearity) in TL materials

Abstract: We describe the theory of heavy charged particle (HCP) response (efficiency and supralinearity) in thermoluminescent (TL) materials. The HCP TL relative efficiency is treated in the framework of modified track structure theory (MTST) using Monte Carlo (MC) calculations of radial dose distributions in condensed phase LiF. HCP TL fluence response is modelled in the framework of the extended track interaction model (ETIM) which treats both HCP fluence response supralinearity and saturation using trapping centre (TC) and luminescent centre (LC) radial occupation density profiles. These are based on the MC radial dose distributions and experimental measurements of optical absorption and sensitisation dose response. Analytical ETIM calculations (up to fourth-order nearest-neighbour track interactions) are used to model the TL fluence response of the components of composite peak 5 in LiF:Mg,Ti (TLD-100). Monte Carlo track interaction calculations (MCTIM) are also described which can model even higher-order nearest-neighbour track interactions appropriate to the high temperature peaks in LiF:Mg,Ti (TLD-100), and as well, model the HCP TL fluence response in both parallel and non-parallel HCP beam geometries.

Remote optical fiber dosimetry

Abstract: Optical fibers offer a unique capability for remote monitoring of radiation in difficult-to-access and/or hazardous locations. Optical fiber sensors can be located in radiation hazardous areas and optically interrogated from a safe distance. A variety of remote optical fiber radiation dosimetry methods have been developed. All of the methods take advantage of some form of radiation-induced change in the optical properties of materials such as: radiation-induced darkening due to defect formation in glasses, luminescence from native defects or radiation-induced defects, or population of metastable charge trapping centers. Optical attenuation techniques are used to measure radiation-induced darkening in fibers. Luminescence techniques include the direct measurement of scintillation or optical excitation of radiation-induced luminescent defects. Optical fiber radiation dosimeters have also been constructed using charge trapping materials that exhibit thermoluminescence or optically stimulated luminescence (OSL).

Solid state microdosimetry.

Abstract: A review of solid state microdosimetry is presented with an emphasis on silicon-based devices. The historical foundations and basics of microdosimetry are briefly provided. Various methods of experimental regional microdosimetry are discussed to facilitate a comparison with the more recent development of silicon microdosimetry. In particular, the performance characteristics of a proportional gas counter and a silicon microdosimeter are compared. Recent improvements in silicon microdosimetry address the issues of requirement specification, non-spherical shape, tissue equivalence, sensitive volume definition (charge collection complexity) and low noise requirements which have previously impeded the implementation of silicon-based microdosimetry. A prototype based on silicon-on-insulator technology is described along with some example results from clinical high LET radiotherapy facilities. A brief summary of the applications of microdosimetry is included.

Radiation dosimetry and spectrometry with superheated emulsions

Abstract: Detectors based on emulsions of overexpanded halocarbon droplets in tissue equivalent aqueous gels or soft polymers, known as “superheated drop detectors” or “bubble (damage) detectors”, have been used in radiation detection, dosimetry and spectrometry for over two decades. Recent technological advances have led to the introduction of several instruments for individual and area monitoring: passive integrating meters based on the optical or volumetric registration of the bubbles, and active counters detecting bubble nucleations acoustically. These advances in the instrumentation have been matched by the progress made in the production of stable and well-specified emulsions of superheated droplets. A variety of halocarbons are employed in the formulation of the detectors, and this permits a wide range of applications. In particular, halocarbons with a moderate degree of superheat, i.e. a relatively small difference between their operating temperature and boiling point, can be used in neutron dosimetry and spectrometry since they are only nucleated by energetic heavy ions such as those produced by fast neutrons. More recently, halocarbons with an elevated degree of superheat have been utilised to produce emulsions that nucleate with much smaller energy deposition and detect low linear energy transfer radiations, such as photons and electrons. This paper reviews the detector physics of superheated emulsions and their applications in radiation measurements, particularly in neutron dosimetry and spectrometry.

High-energy particle transport code NMTC/JAM

Abstract: We have developed a high-energy particle transport code NMTC/JAM, which is an upgraded version of NMTC/JAERI97. The applicable energy range of NMTC/JAM is extended in principle up to 200 GeV for nucleons and mesons by introducing the high-energy nuclear reaction code JAM for the intra-nuclear cascade part. We compare the calculations by NMTC/JAM code with the experimental data of thin and thick targets for proton-induced reactions up to several tens of GeV. The results of the NMTC/JAM code show excellent agreement with the experimental data.

Developments in Dynamic Analysis for quantitative PIXE true elemental imaging

Abstract: Dynamic Analysis (DA) is a method for projecting quantitative major and trace element images from PIXE event data-streams (off-line or on-line) obtained using the Nuclear Microprobe. The method separates full elemental spectral signatures to produce images that strongly reject artifacts due to overlapping elements, detector effects (such as escape peaks and tailing) and background. The images are also quantitative, stored in ppm-charge units, enabling images to be directly interrogated for the concentrations of all elements in areas of the images. Recent advances in the method include the correction for changing X-ray yields due to varying sample compositions across the image area and the construction of statistical variance images. The resulting accuracy of major element concentrations extracted directly from these images is better than 3% relative as determined from comparisons with electron microprobe point analysis. These results are complemented by error estimates derived from the variance images together with detection limits. This paper provides an update of research on these issues, introduces new software designed to make DA more accessible, and illustrates the application of the method to selected geological problems.

Etched heavy ion tracks in polycarbonate as template for copper nanowires

Abstract: 30 and 40 μm thick polycarbonate foils were irradiated with Au197 and Pb208 ions of kinetic energy 1–2 GeV with fluences between 106 and 109 ions/cm2. The latent tracks generated by the heavy ions were chemically etched providing membranes with cylindrical pores of diameters between 30 and 200 nm. These membranes have been used as templates for the creation of metallic nanowires of very high aspect ratio. A thin metal film deposited on one side of the membrane acted as cathode in the two-electrode electrochemical cell, while a copper cone served as anode. The wires were grown potentiostatically. The electrochemical process was monitored by registering chronoamperometric curves for different cathode overvoltages, temperatures and concentrations of the electrolyte. Under suitable conditions, single-crystalline needles were produced. The morphology and crystallinity of the copper nanowires were studied by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction.

An empirical approach to the stopping power of solids and gases for ions from 3Li to 18Ar – Part II

Abstract: A large collection of stopping power data for projectiles from 3Li to 18Ar is investigated as a possible basis for producing a table of stopping powers. We divide the experimental stopping powers for a particular projectile (nuclear charge Z1) by those for alpha particles in the same element, as given in ICRU Report 49. With proper normalization, we then obtain experimental stopping power ratios Srel that lie approximately on a single curve, provided we treat solid and gaseous targets separately, and provided we exclude H2 and He targets. For every projectile, this curve is then fitted by a 3-parameter sigmoid function Srel=Srel(a,b,c). We find that the three parameters a, b and c depend smoothly on Z1 and can themselves be fitted by suitable functions af,bf and cf of Z1, separately for solid and gaseous targets. The low energy limit (coefficient a) for solids agrees approximately with the prediction by Lindhard and Scharff. We find that agas

The Pierre Süe Laboratory nuclear microprobe as a multi-disciplinary analysis tool

Abstract: The nuclear microprobe at the Pierre Sue Laboratory is a facility exclusively devoted to microanalysis. The microprobe consisting of a single stage Van de Graaff accelerator and two microbeam lines will be described. Simultaneous detection (X-rays, γ-rays, charged particles, etc.) and imaging are routinely performed by a PC-based multi-parameter data acquisition system. Telescope mounting is frequently used for particle identification particularly when performing nuclear reaction analysis (NRA). A unique feature of the Pierre Sue Laboratory nuclear microprobe is the ability to analyze radioactive samples. One of the two beamlines has been specifically designed for that purpose. Radioactive environment requires suited target handling and detection setups. Main application fields of the microprobe are material, earth, planetary, environmental sciences and electronuclear related topics.

Production of nanodiamonds by high-energy ion irradiation of graphite at room temperature

Abstract: It has previously been shown that graphite can be transformed into diamond by MeV electron and ion irradiation at temperatures above approximately 600°C. However, there exists geological evidence suggesting that carbonaceous materials can be transformed to diamond by irradiation at substantially lower temperatures. For example, submicron-size diamond aggregates have been found in uranium-rich, Precambrian carbonaceous deposits that never experienced high temperature or pressure. To test if diamonds can be formed at lower irradiation temperatures, sheets of fine-grain polycrystalline graphite were bombarded at 20°C with 350 ± 50 MeV Kr ions to fluences of 6×10 12 cm −2 using the Argonne tandem linear accelerator system (ATLAS). Ion-irradiated (and unirradiated control) graphite specimens were then subjected to acid dissolution treatments to remove untransformed graphite and isolate diamonds that were produced; these acid residues were subsequently characterized by high-resolution and analytical electron microscopy. The acid residue of the ion-irradiated graphite was found to contain nanodiamonds, demonstrating that ion irradiation of graphite at ambient temperature can produce diamond. The diamond yield under our irradiation conditions is low, ∼0.01 diamonds/ion. An important observation that emerges from comparing the present result with previous observations of diamond formation during irradiation is that nanodiamonds form under a surprisingly wide range of irradiation conditions. This propensity may be related to the very small difference in the graphite and diamond free-energies coupled with surface-energy considerations that may alter the relative stability of diamond and graphite at nanometer sizes.

Theory of thermoluminescence gamma dose response: The unified interaction model

Abstract: We describe the development of a comprehensive theory of thermoluminescence (TL) dose response, the unified interaction model (UNIM). The UNIM is based on both radiation absorption stage and recombination stage mechanisms and can describe dose response for heavy charged particles (in the framework of the extended track interaction model – ETIM) as well as for isotropically ionising gamma rays and electrons (in the framework of the TC/LC geminate recombination model) in a unified and self-consistent conceptual and mathematical formalism. A theory of optical absorption dose response is also incorporated in the UNIM to describe the radiation absorption stage. The UNIM is applied to the dose response supralinearity characteristics of LiF:Mg,Ti and is especially and uniquely successful in explaining the ionisation density dependence of the supralinearity of composite peak 5 in TLD-100. The UNIM is demonstrated to be capable of explaining either qualitatively or quantitatively all of the major features of TL dose response with many of the variable parameters of the model strongly constrained by ancilliary optical absorption and sensitisation measurements.

Strain relaxation of pseudomorphic heterostructures after hydrogen or helium ion implantation for virtual substrate fabrication

Abstract: Strain relaxed Si1−xGex layers on Si(1 0 0) are used as virtual substrates for the growth of e.g. Si/Si1−xGex quantum well structures. We investigated the effects of H+ and He+ ion implantation and subsequent annealing on pseudomorphic Si 1−x Ge x / Si (1 0 0) heterostructures grown by molecular beam epitaxy (MBE). A narrow defect band is generated by ion implantation slightly underneath the interface inducing the formation of strain-relieving misfit dislocations (MDs) during subsequent thermal annealing. Using H+ ion implantation, nearly complete strain relaxation of Si1−xGex layers with Ge fractions up to 22 at.% was obtained at temperatures as low as 800°C and the samples appeared free of threading dislocations (TDs) within the SiGe layer to the limit of transmission electron microscopy (TEM) analysis. Efficient strain relaxation was demonstrated even for Si1−xGex layers with Ge fractions up to 30 at.% using He+ ion implantation. We have thus developed a method for producing high-quality, thin, relaxed Si1−xGex films on Si(1 0 0) with TD densities well below 10 7 cm −2 by standard techniques as MBE and ion implantation. The heterostructures were analyzed using X-ray diffraction (XRD), Rutherford backscattering/channeling spectrometry and TEM. We propose a model of strain relaxation in which dislocations generated in conjunction with the formation of H or He filled overpressurized cavities glide to the interface where they form strain-relieving misfit segments. On the basis of this assumption, the conditions for efficient strain relaxation are discussed.

Conetrap: A compact electrostatic ion trap

Abstract: We have developed a very simple and compact fully electrostatic ion storage device. It consists of only three electrodes: two conical and one central cylindrical. This simple setup works as two focusing electrostatic mirrors forming a resonator-like structure and thereby confining the ions. We present the design and show the first test results demonstrating an ion-storage lifetime in the 100 ms-range at modest vacuum conditions. Storage lifetimes in the order of minutes are foreseen when UHV conditions are introduced.

Pore shape control in nanoporous particle track etched membrane

Abstract: This paper shows the possibility of preparing nanoporous particle track etched membranes (nanoPTM) with perfectly smooth and cylindrical pores from polycarbonate film Interest in the template use of these nanoPTM for the production of polymeric or metallic nanoscale materials is also emphasized (C) 2001 Elsevier Science BV All rights reserved

A gas cell for thermalizing, storing and transporting radioactive ions and atoms. Part II: On-line studies with a laser ion source

Abstract: The application of a gas cell filled with noble gas (helium or argon) for thermalizing, storing and transporting trace radioactive ions and atoms has been studied in on-line conditions. Radioactive ions produced in nuclear reactions and stable energetic ions have been resonantly re-ionized by laser light via a two-step resonant process after thermalization and neutralization in high-pressure noble gas. The influence of the ion–electron density created by the projectile beam on the recombination of exotic ions has been investigated in different experimental conditions, including DC and RF electrical fields in the gas cell. Results for the laser ion source efficiency and selectivity for heavy-ion induced fusion reaction products are given. For the Rh isotopes the efficiency reaches up to 12%.

Morphology of latent and etched heavy ion tracks in radiation resistant polymers polyimide and poly(ethylene naphthalate)

Abstract: The initial phase of chemical etching of ion tracks was studied in the radiation resistant polymers polyimide (PI) and poly(ethylene naphthalate) (PEN). Stacks of thin films were irradiated with Kr, Xe and Bi ions with energies of several MeV per nucleon. During the etching process, the evolution of the pore growth was monitored by conductometric measurements. Similar to earlier observations in other polymers, the etching process revealed the existence of two different radial damage zones, namely, a track core and a track halo. Compared to the virgin polymer, the etching rate in the core region is largely increased, whereas the halo exhibits a higher resistance to the chemical attack indicating radiation-induced cross-linking effects. Typical diameter of the central part of the latent track is measured for different stopping powers of the projectiles. The track to bulk etch ratio is found to reach 500–600 in PI and ∼10 000 in PEN. Due to this property, both materials seem to be suitable for producing micro- and even nanostructures with high aspect ratios.

The first results with the new JYFL 14 GHz ECR ion source

Abstract: A new 14 GHz ECR ion source has been built for the Accelerator Laboratory in the Department of Physics (JYFL), University of Jyvaskyla. This source belongs to the family of the LBNL AECR-U-based ECR ion sources. The operation during the first four months has shown that the new ion source performs well and is able to produce intensive highly charged ion beams. For example, 145 μA of O7+ ion beam was recorded. The production of iron and boron ion beams was tested using the MIVOC method. The 56Fe11+ ion beam current reached a value of 115 μA. The intensities of 11B3+ and 11B5+ ion beams were 235 and 52 μA, respectively. This iron beam intensity is the second highest and the boron beam intensities are the highest ever produced by an ECR ion source. In all the tests an extraction voltage of 10 kV was applied.

Electronic dosimeters based on solid state detectors

Abstract: This paper attempts to give an idea on the state of the art in electronic solid state dosimetry, including devices mainly based on semiconductors and diamond. Research in this area has made great progress.

Electron paramagnetic resonance (EPR) biodosimetry

Abstract: Radiation-induced electron paramagnetic resonance (EPR) signals were first reported by Gordy et al. [Proc. Natl. Acad. Sci. USA 41 (1955) 983]. The application of EPR spectroscopy to ionizing radiation dosimetry was later proposed by Brady et al. [Health Phys. 15 (1968) 43]. Since that time EPR dosimetry has been applied to accident and epidemiologic dose reconstruction, radiation therapy, food irradiation, quality assurance programs and archaeological dating. Materials that have been studied include bone, tooth enamel, alanine and quartz. This review paper presents the fundamentals and applications of EPR biodosimetry. Detailed information regarding sample collection and preparation, EPR measurements, dose reconstruction, and data analysis and interpretation will be reviewed for tooth enamel. Examples of EPR biodosimetry application in accidental overexposures, radiopharmaceutical dose assessment and retrospective epidemiologic studies will also be presented.

Degradation of segmented poly(etherurethane) Tecoflex® induced by electron beam irradiation: Characterization and evaluation

Abstract: We have studied the influence of electron beam irradiation on a polyurethane Tecoflex ® (TFX) used in medical applications; this study has been performed in order to evaluate the capability of such materials to be sterilized by electrons in industrial conditions. With this aim, thin films have been prepared and have been irradiated under a dose-rate of 5 MGy h −1 , with absorbed doses varying from 25 to 1000 kGy under O 2 . Analytical techniques used were size exclusion chromatography (SEC) and Fourier transform infrared spectroscopy (FTIR). Evolved gas analysis has been performed using thermogravimetric analysis (TGA) coupled with FTIR spectroscopy (TG–FTIR). TFX films analyzed by SEC showed simultaneous scission and cross-linking that were both increasing with the irradiation dose. Various modifications of FTIR spectra were induced, with appearance of oxidation groups, identified as mainly formates, esters and carboxylic acids. Scission of chains were localized in soft (SS) and hard (HS) segments by decrease of both urethane and aliphatic ether absorbance. Finally, TG–FTIR analysis confirmed previous results: TG analysis of non-irradiated films showed a two-steps profile that was globally shifted to lower temperatures after irradiation. The coupling with FTIR allowed identification of degradation molecules: (i) oxidized SS fragments, (ii) long SS slightly oxidized and (iii) HS accompanied by SS.

Ion irradiation of gold inclusions in SiO2: Experimental evidence for inverse Ostwald ripening

Abstract: When sub mm size gold inclusions in SiO 2 are irradiated with MeV gold ions, a ring of nanometer-size clusters is formed in their immediate vicinity. The size, density and radial distribution of these nanoclusters were determined. Our observations are in agreement with a recent prediction that in a driven system, under appropriate ion beam and temperature conditions, the steady-state solute concentration's dependence on precipitate size may be opposite to that expected from the equilibrium Gibbs–Thomson relation. Kinetic Monte-Carlo simulations account for additional experimental results: the irradiation-induced increase in solute concentration can promote nucleation of additional precipitates around the first ring. The latter finding contradicts the usual assumption that ion beam mixing hinders nucleation.