Showing papers in "Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms in 2007"

Absolute calibration of 10Be AMS standards

Abstract: The increased detection sensitivity offered by AMS has dramatically expanded the utility of 10 Be. As these applications become more sophisticated attention has focused on the accuracy of the 10 Be standards used to calibrate the AMS measurements. In recent years it has become apparent that there is a discrepancy between two of the most widely used 10 Be AMS standards, the ICN 10 Be standard and the NIST 10 Be standard. The ICN (ICN Chemical & Radioisotope Division) 10 Be AMS standard was calibrated by radioactive decay counting. Dilutions, ranging from 5 × 10 −13 to 3 × 10 −11 10 Be/Be, have been prepared and are extensively used in many AMS laboratories. The NIST 10 Be standard, prepared at the National Institute of Standards and Technology (NIST), is calibrated by mass spectrometric isotope ratio measurements. To provide an independent calibration of the 10 Be standards we implanted a known number of 10 Be atoms in both Si detectors and Be foil targets. The 10 Be concentrations in these targets were measured by AMS. The results were compared with both the ICN and NIST AMS standards. Our 10 Be measurements indicate that the 10 Be/ 9 Be isotopic ratio of the ICN AMS standard, which is based on a 10 Be half-life of 1.5 × 10 6 yr, is 1.106 ± 0.012 times lower than the nominal value. Since the decay rate of the ICN standard is well determined, the decrease in 10 Be/ 9 Be ratio requires that the 10 Be half-life be reduced to (1.36 ± 0.07) × 10 6 yr. The quoted uncertainty includes a ±5% uncertainty in the activity measurement carried out by ICN. In a similar fashion, we determined that the value of the NIST 10 Be standard (SRM4325) is (2.79 ± 0.03) × 10 −11 10 Be/ 9 Be, within error of the certified value of (2.68 ± 0.14) × 10 −11 . The Lawrence Livermore National Laboratory (LLNL) internal standards were also included in this study. We conclude that the 9 Be(n, γ) neutron cross section is 7.8 ± 0.23 mb, without taking into account the uncertainty in the neutron irradiation.

MICADAS: A new compact radiocarbon AMS system

Abstract: A novel tabletop AMS system with overall dimensions of only 2.5 × 3 m 2 has been built and tested. The mini radiocarbon dating System (MICADAS) is based on a vacuum insulated acceleration unit that uses a commercially available 200 kV power supply to generate acceleration fields in a tandem configuration. At the high-energy end, ions in charge state 1 + are selected and interfering molecules of mass 14 amu are destroyed in multiple collisions. The new system is now fully operational. It is the prototype of a new generation of radiocarbon spectrometers which fulfill the requirements for radiocarbon dating applications as well as for the less demanding 14 C/ 12 C isotopic ratio measurements as needed, e.g. in biomedical applications. A detailed description of the system is given and results of performance tests are discussed.

Modifying a sealed tube zinc reduction method for preparation of AMS graphite targets: Reducing background and attaining high precision

Abstract: The sealed tube zinc reduction method for converting CO2 to graphite for AMS 14C measurements was originally developed for rapid production of graphite in biomedical tracer experiments. The method was usually thought to have low precision and a high background. We have modified the zinc reduction method originally outlined in Vogel [J.S. Vogel, Radiocarbon 34 (3) (1992) 344] by carefully controlling the amounts of reagents (zinc, titanium hydride and Co or Fe catalyst) and now routinely obtain a precision of 2–3‰ and a relatively low background of ∼50,000 14C years when analyzing for 14C at the Keck Carbon Cycle AMS facility at UC Irvine. Fractionation of carbon isotopes does occur during graphitization and depends on the graphitization yield, which can be affected by the amounts of reagents used and other conditions. The δ13C of our zinc-reduced graphite is usually lighter by 2–3‰ than the CO2 from which it is made, but this is corrected for in our system by simultaneous measurement of 13C/12C along with 14C/12C by the spectrometer. This method is suitable for 14C enriched samples, as well as natural abundance 14C samples, especially those with modern 14C contents. With improved precision and background, we believe that many disciplines can benefit from this technique because of its low cost and rapid production of graphite.

The RIKEN RI Beam Factory Project: A status report

Abstract: In order to greatly upgrade the capability of its existing heavy-ion accelerator facility, RIKEN is constructing a new radioactive isotope (RI) beam facility, called “RI Beam Factory”. This facility is of an in-flight RI beam separation scheme. Three new ring cyclotrons with K-values of 570 MeV, 980 MeV and 2500 MeV, respectively, boost energies of the output beams from the existing K540-MeV ring cyclotron up to 440 MeV/nucleon for light ions and 350 MeV/nucleon for very heavy ions. The K2500-MeV ring cyclotron, SRC is the world’s first superconducting ring cyclotron. These energetic heavy-ion beams are converted into intense RI beams via projectile fragmentation of stable isotopes or in-flight fission of uranium isotopes by a superconducting fragment separator, BigRIPS. The combination of the SRC and the BigRIPS will greatly expand our knowledge of nuclear world into presently inaccessible region on the nuclear chart. The new ring cyclotrons and the Big RIPS will be commissioned in late 2006. Construction of a part of major experimental installations planned is expected to start in 2007.

Ultra small-mass AMS 14C sample preparation and analyses at KCCAMS/UCI Facility

Abstract: We have developed techniques for accurately and precisely measuring samples containing less than a few hundred micrograms of carbon, using a compact AMS system (NEC 0.5 MV 1.5SDH-1). Detailed discussions of the sample preparation, measurement setup, data analysis and background corrections for a variety of standard samples ranging from 0.002 to 1 mgC are reported. Multiple aliquots of small amounts of CO2 were reduced to graphite with H2 over pre-baked iron powder catalyst. A reduction reaction temperature of 450 � C was adopted for graphite samples below 0.05 mgC, rather than the usual 550 � C used on samples of 0.1–1 mgC. In our regular reactors (� 3.1 cm 3 ), this reduction in temperature improved the graphite yield from � 60 to 90–100% for samples ranging from 0.006– 0.02 mgC. The combination of lower reaction temperature with a reduced reactor volume (� 1.6 cm 3 ) gave yields as high as 100% on graphite samples <0.006 mgC. High performance measurements on ultra-small samples are possible also due to a modified NEC

Ab initio calculations about intrinsic point defects and He in W

Abstract: We have used ab initio calculations in the framework of the density functional theory to determine the properties of the self-interstitial atom, the vacancy, vacancy clusters and He in tungsten. The most stable configuration for the self-interstitial atom is the 〈1 1 1〉 dumbbell. The divacancy is not stable and a strong repulsion is observed especially in second nearest neighbour position. The most stable configuration for interstitial He is in tetrahedral position. The interactions of He atoms with a vacancy are found to be in very good agreement with the experimental data.

Radiocarbon marine reservoir ages in the western Pacific estimated by pre-bomb molluscan shells

Abstract: Apparent 14C ages of the pre-bomb molluscan shells show new estimations of ΔR values for the western Pacific including the East Sakhalin Current, Oyashio Current, Kuroshio Current, Kuroshio Countercurrent, and Pacific North Equatorial Current. Clear differences in ΔR between the subarctic and subtropical gyres water masses around the Japanese archipelago are observed.

Studies on effective atomic numbers and electron densities of essential amino acids in the energy range 1 keV–100 GeV

Abstract: The effective atomic numbers and electron densities of essential amino acids histidine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine have been calculated for total and partial photon interactions by the direct method in the wide energy range of 1 keV–100 GeV using WinXCOM. The values of these parameters have been found to change with energy and composition of the amino acids. The variations of effective atomic number and electron density with energy are shown graphically for all photon interactions. The variations of photon mass attenuation coefficients with energy are shown graphically only for total photon interaction. Dosimetric implication of mass attenuation coefficient in amino acids has also been discussed.

Radiation synthesis of PVP/CMC hydrogels as wound dressing

Abstract: Carboxymethyl cellulose (CMC) and poly( N -vinyl pyrrolidone) (PVP) were blended to synthesize hydrogels by 60 Co γ-ray irradiation. A series of hydrogels were prepared with three compositions, i.e. PVP:CMC = 5:5, 6:4 and 7:3. The properties of the hydrogels such as gel fraction, gel strength, swelling behavior and moisture retention were investigated. Compared with pure PVP and CMC hydrogels, PVP/CMC blend hydrogels possessed improved gel strength, flexibility and transparency. As the content of PVP in PVP/CMC blend hydrogels increased, the gel fraction increased while the swelling rate decreased. The hydrogels with optimal properties were obtained in a composition of PVP/CMC = 6/4 with sterilization dose (25 kGy) and further characterized by FT-IR and SEM. Properties concerning the application of wound dressing were performed by comparison with a commercial hydrogel wound dressing to find a similar moisture retention capability and improved swelling rate. The improved properties as well as the cheapness of the materials suggest that PVP/CMC blend hydrogel can be good candidate as wound dressing.

Measurements of secondary ions emitted from organic compounds bombarded with large gas cluster ions

Abstract: We propose to use cluster ions that are much larger than the molecular ions as primary ions for organic secondary ion mass spectrometry. Incident Ar cluster ion beams with energies from 10 to 20 keV and a mean size of about 1000 atoms/cluster were used. Secondary ions were measured for a thin arginine film target (200 nm) bombarded with large Ar cluster ions using a time-of-flight technique. Molecular ions of arginine and characteristic fragment ions were detected with high sensitivity. When large Ar cluster ions such as Ar1500 were incident on the arginine target, molecular ions of arginine were detected with little fragment ions. This indicates that large cluster ions can ionize arginine molecules without damaging them.

Nanostructured Cu/Nb multilayers subjected to helium ion-irradiation

Abstract: Helium ion-irradiation experiments have been performed in single layer Cu films, Nb films and Cu/Nb multilayer films with layer thickness varying from 2.5 nm to 100 nm each layer. Peak helium concentration approaches a few atomic percent with 6-9 displacement-per-atom in Cu and Nb. He bubbles were observed in single layer Cu and Nb films, as well as in Cu 100 nm/Nb 100 nm multilayers with helium bubbles aligned along layer interfaces. Helium bubbles are not resolved via transmission electron microscopy in Cu 2.5 nm/Nb 2.5 nm multilayers. These studies indicate that layer interface may play an important role in annihilating ion-irradiation induced defects such as vacancies and interstitials and have implications in improving the radiation tolerance of metallic materials using nanostructured multilayers.

A Review of Ion Beam Induced Charge Microscopy.

Abstract: Since its development in the early 1990’s, ion beam induced charge (IBIC) microscopy has found widespread applications in many microprobe laboratories for the analysis of microelectronic devices, dislocations, semiconductor radiation detectors, semi-insulating materials, high power transistors, charge-coupled arrays, solar cells, light emitting diodes, and in conjunction with Single Event Upset imaging. Several modalities of the techniques have been developed, such as lateral IBIC and time-resolved IBIC. The theoretical model of IBIC generation and collection has developed from a one-dimensional model of charge drift and diffusion to a detailed model of the motion of ion charge carriers in semiconductors and insulators. This paper reviews the current state-of-the-art of IBIC theory and applications.

Role of natural polysaccharides in radiation formation of PVA–hydrogel wound dressing

Abstract: Radiation processed PVA–polysaccharides hydrogels have been observed to be suitable for producing transparent, flexible, mechanically strong, biocompatible, effective and economical hydrogel dressings. The dressings were formed in single stage irradiation process achieving gel formation and sterilization at 25–30 kGy gamma radiation dose. No synthetic plasticizers and additives were used. Different formulations containing poly-vinylalcohol (PVA) and polysaccharides selected from combinations of agar and carrageenan were used to make the dressings. The selected polysaccharides themselves form thermo-reversible gels and degrade on irradiation. Using concentration of polysaccharides as low as 0.5–2% resulted in increase of tensile strength from 45 g/cm 2 to 411 g/cm 2 , elongation from 30% to 410% and water uptake from 25% to 157% with respect to PVA gel without polysaccharides. Besides improving mechanical strength, agar contributes more to elongation and carrageenan to mechanical strength of the gel dressing. PVA formulations containing the polysaccharides show significantly different pre-gel viscosities behaviour. Increasing the concentration of agar in the formulation to about 2% converts the sheet gel to paste gel useful for filling wound cavities. The results indicate that pre irradiation network structure of the formulation plays an important role in determining mechanical properties of the irradiated gel dressing. Formulations containing 7–9% PVA, 0.5–1.5% carrageenan and 0.5–1% agar gave highly effective usable hydrogel dressings. Scanning electron micrographs show highly porous structure of the gel. Clinical trials of wound dressing on human patients established safety and efficacy of the dressing. The dressing has been observed to be useful in treating burns, non-healing ulcers of diabetes, leprosy and other external wounds. The dressings are now being marketed in India under different brand names.

XPCS at the European X-ray free electron laser facility

Abstract: The European X-ray free electron laser source (XFEL) will provide highly brilliant ( B > 10 33 ph/s/mm 2 /mrad 2 /0.1% bw) and coherent X-ray beams. The pulse structure and the unprecedented brightness will allow one for the first time to study fast dynamics in the time domain, thus giving direct access to the dynamic response function S ( Q , t ), instead of S ( Q , ω ), which is of central importance for a variety of phenomena such as fast non-equilibrium dynamics initiated, e.g. by a short pump pulse. X-ray photon correlation spectroscopy (XPCS) measures the temporal changes in speckle patterns produced when coherent light is scattered by a disordered system and therefore allows to measure S ( Q , t ). This paper summarizes important aspects of the scientific case for an XPCS instrument at the planned XFEL. Novel XPCS set-ups are illustrated.

An inter-comparison of 10Be and 26Al AMS reference standards and the 10Be half-life

Abstract: We have completed a survey and inter-comparison of several 10 Be and 26 Al standard reference materials (SRMs) that are in routine use at various AMS laboratories to assess their relative values and the accuracy of their quoted nominal ratios. The accelerator measurement cycle, analysis procedure and setup used at the ANTARES AMS facility for this survey are described. We focused on a new set of 10 Be and 26 Al serial dilutions of standard reference materials (SRMs) prepared by Kuni Nishiizumii at the University of California, Berkeley, and found excellent systematic reproducibility and internal consistency. For other standard materials, minor deviations are evident even when the results have been recalibrated to a common half-life. In particular, we confirm that the NIST 10 Be SRM-4325 has a 14% greater 10 Be/Be ratio than that certified by NIST when it is calibrated against other SRMs whose ratios have been normalized to a common 1.5 Ma 10 Be half-life. In order to investigate this apparent discrepancy, we report on the results of an absolute, normalization independent, measure of the NIST-4325 10 Be/Be ratio. Within the constraints of this type of measurement and its systematic errors, we determine an absolute value for the 10 Be/Be SRM-4325 ratio in the range 26,050 to 24,800 × 10 −15 in support of the certified value of 26,800 × 10 −15 given by NIST. We hesitate to directly infer as a consequence that the 10 Be half-life is 1.34 Ma because such an inference is contingent on a direct and accurate specific activity in the parent solution, which at present is not available.

Comparative study of cascade damage in Fe simulated with recent potentials

Abstract: Considerable quantitative uncertainty has remained regarding the amount and structure of defects produced in molecular dynamics simulations of collision cascades in Fe. The problem is most likely related to the description of interstitial energetics in the interatomic potentials. Three potentials have recently been developed for Fe, which, even though they have different physical motivations and functional forms, describe the interstitial energetics well. Using these potentials, we simulate recoil collision cascades in Fe in the recoil energy range 0.5–20 keV. Prior to the cascade simulations a realistic high-energy repulsive part was added to two of the potentials, adjusting the fit to reproduce the experimentally obtained threshold displacement energies. The results show that the total Frenkel pair production, as predicted by the three potentials, is the same within the statistical uncertainty, but also that some differences remain in the fraction of clustered defects. However, these differences are smaller than those predicted by previous potentials.

Relation between acid dissolution time in the vacuum test tube and time required for graphitization for AMS target preparation

Abstract: Availability of an effective graphitization system is essential for the successful operation of an AMS laboratory for radiocarbon measurements. We have set up a graphitization system consisting of metal vacuum lines for cleaning CO 2 sample gas which is then converted to graphite. CO 2 gas from a carbonate sample is produced in vacuum in a test tube by injecting concentrated phosphoric acid. The tube is placed into a heated metal block to accelerate dissolution. However, we have observed systematic differences in the time required to convert the CO 2 gas to graphite under a hydrogen atmosphere, from less than 3 h to over 10 h. We have conducted a series of experiments including background measurements and yield measurements to monitor secondary carbon contamination and changes in isotopic fractionation. All of the tests show that the carbon isotope ratios remain unaffected by the duration of the process. We also used a quadrupole mass spectrometer (QMS) to identify possible contaminant gases. Contaminant peaks were identified at high mass (larger than 60) only for long duration experiments. This suggests a possible reaction between the rubber cap and acid fumes producing a contaminant gas that impeded the reduction of CO 2 .

New capabilities of the Zagreb ion microbeam system

Abstract: The installation of a new 1.0 MV Tandetron accelerator and a recent upgrade of the 6.0 MV Tandem Van de Graaff accelerator increased the application possibilities of the Zagreb ion microbeam system. Several ion sources enable now the selection of a wide variety of ions. Most of them can be focused by the existing microprobe system. Sample positioning tools and new scanner control options are implemented in the new generation of SPECTOR data acquisition system. Details of the upgrades are presented.

Study on the ablation threshold induced by pulsed lasers at different wavelengths

Abstract: A study of the effects induced by pulsed laser ablation on different materials as a function of the laser wavelength is presented. In particular the ablation at low laser fluence, of the order of 10 8 –10 10 W/cm 2 with ns pulse width, is investigated experimentally on different metals, semiconductors and polymers. Two theoretical models, explain the experimental results about the fluence threshold value measurements, as depending on the laser wavelength are discussed. The photothermal process is valid for the estimation of the threshold fluence for IR and visible radiation, both inducing thermal heating in metals and semiconductors through the photon-free electron energy transfer. This model is not valid for polymers. The photochemical process is valid for the estimation of the threshold fluence for UV radiation, which photon energy is higher with respect to the chemical binding energy. This radiation induces chemical bond breaking in insulators and scission and cross linking effects can be produced. This last model is not valid for metals and semiconductors.

Radiation cross-linking in ultra-high molecular weight polyethylene for orthopaedic applications

Abstract: The motivation for radiation cross-linking of ultra-high molecular weight polyethylene (UHMWPE) is to increase its wear resistance to be used as bearing surfaces for total joint arthroplasty. However, radiation also leaves behind long-lived residual free radicals in this polymer, the reactions of which can detrimentally affect mechanical properties. In this review, we focus on the radiation cross-linking and oxidative stability of first and second generation highly cross-linked UHMWPEs developed in our laboratory.

International Atomic Energy Agency intercomparison of ion beam analysis software

Abstract: Ion beam analysis (IBA) includes a group of techniques for the determination of elemental concentration depth profiles of thin film materials. Often the final results rely on simulations, fits and calculations, made by dedicated codes written for specific techniques. Here we evaluate numerical codes dedicated to the analysis of Rutherford backscattering spectrometry, non-Rutherford elastic backscattering spectrometry, elastic recoil detection analysis and non-resonant nuclear reaction analysis data. Several software packages have been presented and made available to the community. New codes regularly appear, and old codes continue to be used and occasionally updated and expanded. However, those codes have to date not been validated, or even compared to each other. Consequently, IBA practitioners use codes whose validity, correctness and accuracy have never been validated beyond the authors’ efforts. In this work, we present the results of an IBA software intercomparison exercise, where seven different packages participated. These were DEPTH, GISA, DataFurnace (NDF), RBX, RUMP, SIMNRA (all analytical codes) and MCERD (a Monte Carlo code). In a first step, a series of simulations were defined, testing different capabilities of the codes, for fixed conditions. In a second step, a set of real experimental data were analysed. The main conclusion is that the codes perform well within the limits of their design, and that the largest differences in the results obtained are due to differences in the fundamental databases used (stopping power and scattering cross section). In particular, spectra can be calculated including Rutherford cross sections with screening, energy resolution convolutions including energy straggling, and pileup effects, with agreement between the codes available at the 0.1% level. This same agreement is also available for the non-RBS techniques. This agreement is not limited to calculation of spectra from particular structures with predetermined parameters, but also extends to extracting information from real data. In particular, we have shown data from an Sb implanted sample where the Sb fluence was certified with an uncertainty of 0.6%. For this sample, and using SRIM03 stopping powers for 1.5 MeV 4He in Si, the codes were able to extract the Sb fluence with an average 0.18% deviation from the certified value and a 0.11% agreement between the codes. Thus IBA is a suitable technique for accurate analysis where traceability is critical. These results confirm that available IBA software packages are, within their design limitations, consistent and reliable. The protocol established may be readily applied to validate future IBA software as well.

The single stage AMS machine at Lund University: Status report

Abstract: In May 2003 Lund University, Sweden, ordered a new AMS facility from National Electrostatics Corp. (NEC), Wisconsin. The system is the first low energy, open air, single stage AMS system (SSAMS). The machine is equipped with a dual ion source sequential injector, which allows measurements of carbon with the masses 12, 13 and 14. The machine operates at 250 kV with the gas stripper, high energy magnet and detector mounted on a high voltage deck and with the injector at ground potential. The new machine was installed during August-October 2004 and about 500 samples were dated during the first nine months. The dual ion source injector allows both low-activity samples for radiocarbon dating and high-activity samples for medicine, biochemistry and ecology studies to be measured, with minimum risk of cross-contamination. The precision in radiocarbon dating is 50 years or better and this is expected to improve with careful tuning of the machine. The background for processed graphite equates to about 43 000 radiocarbon years. (c) 2007 Elsevier B.V. All rights reserved. (Less)

Gamma ray interaction with bismuth silicate glasses

Abstract: UV–visible, infrared and Raman spectroscopy together with thermal properties were measured for glasses from the system Bi2O3 · SiO2 with varying Bi2O3 content. The spectroscopic properties were also measured after gamma irradiation. The experimental results indicate that UV–visible spectra reveal intense charge transfer ultraviolet bands which seem to originate from trace iron impurities from the raw materials and the possible sharing of Bi3+ ions. The studied glasses show resistant to gamma irradiation on all the measured spectra. Infrared and Raman spectra reveal characteristic bands suggesting that Bi2O3 takes part in the formation of the glass network. The thermal properties and density values indicate that the measured data are related to heavy metal cation (Bi3+) and its large polarizability in oxide glasses.

The initial 129I/I ratio and the presence of ‘old’ iodine in continental margins

Abstract: The natural radioisotope 129 I covers an important age range for applications in geological systems, particularly for fluids derived from or associated with organic material. Crucial for the application of this isotopic system is the initial ratio used for the calculation of ages, the marine input ratio of 129 I/I. Determinations of this ratio in recent marine sediments led to a value R i = (1500 ± 150) × 10 −15 , which has been used in the determination of source ages in hydrocarbon systems, in gas hydrate occurrences and the tracing of fluids in subduction related volcanoes. We investigated the validity of this ratio in view of a number of determinations which resulted in ratios considerably below this value. Investigations of fluids along continental margins demonstrate that the likely cause for these low values is the release of methane-rich fluids with high iodine concentrations derived from old organic sources. Although these fluids locally influence the ratio in some areas along continental margins, they do not disturb the isotopic balance of the open ocean, justifying the use of R i for the application of the 129 I/I system.

The Nuclear Car Wash: Neutron interrogation of cargo containers to detect hidden SNM☆

Abstract: LLNL is actively involved in the development of advanced technologies for use in detecting threats in sea-going cargo containers, particularly the presence of hidden special nuclear materials (SNM). The “Nuclear Car Wash” (NCW) project presented here uses a high-energy (En ≈ 3.5–7.0 MeV) neutron probe to scan a container and then takes high-energy (Eγ ⩾ 2.5 MeV), β-delayed γ-rays emitted during the subsequent decay of any short-lived, neutron-induced fission products as a signature of fissionable material. The components of the proposed system (e.g. neutron source, gamma detectors, etc.) will be discussed along with data processing schemes, possible threat detection metrics and potential interference signals. Results from recent laboratory experiments using a prototype system at LLNL will also be presented.

Dual-energy X-ray radiography for automatic high-Z material detection

Abstract: There is an urgent need for high-Z material detection in cargo. Materials with Z > 74 can indicate the presence of fissile materials or radiation shielding. Dual (high) energy X-ray material discrimination is based on the fact that different materials have different energy dependence in X-ray attenuation coefficients. This paper introduces the basic physics and analyzes the factors that affect dual-energy material discrimination performance. A detection algorithm is also discussed.

Life with MC-SNICS. Part II: Further ion source development at the Keck carbon cycle AMS facility

Abstract: We report on an ongoing program of improvements to the National Electrostatics (NEC) MC-SNICS ion source at the KCCAMS laboratory at UC Irvine. Recent work has focused on vacuum improvements and on increasing the source output and efficiency. We replaced the extractor and preacceleration assemblies for a threefold improvement in pumping conductance, leading to lower source pressures and quicker startup after sample wheel changes. A switch to spherical ionizers caused a marked improvement in the focusing of the Cs beam, leading to major gains in ion source efficiency and negative ion beam quality. Better confinement of Cs within the new ionizer assembly resulted in lower Cs consumption and a significant reduction in arcing problems.

Ion irradiation effects in nanocrystalline TiN coatings

Abstract: Nitride materials and coatings have attracted extensive research interests for various applications in advanced nuclear reactors due to their unique combination of physical properties, including high temperature stability, excellent corrosion resistance, superior mechanical property and good thermal conductivity. In this paper, the ion irradiation effects in nanocrystalline TiN coatings as a function of grain size are reported. TiN thin films (thickness of 100 nm) with various grain sizes (8-100 nm) were prepared on Si substrates by a pulsed laser deposition technique. All the samples were irradiated with He ions to high fluences at room temperature. Transmission electron microscopy (TEM) and high resolution TEM on the ion-irradiated samples show that damage accumulation in the TiN films reduces as the grain size reduces. Electrical resistivity of the ion-irradiated films increases slightly compared with the as-deposited ones. These observations demonstrate a good radiation-tolerance property of nanocrystalline TiN films.

Defects and threshold displacement energies in SrTiO3 perovskite using atomistic computer simulations

Abstract: The mode and energy of simple defect incorporation in SrTiO 3 (vacancies and interstitials) is quantified using computer simulation techniques with an empirical partial charge model of interatomic forces, as well as using density functional theory calculations. Oxygen and strontium interstitials form split-interstitial configurations whereas titanium interstitials occupy channel positions. Defect migration energies and paths are also considered; interstitials are more mobile than vacancies, with a low predicted oxygen interstitial migration energy of around 0.3 eV. We also calculate the threshold displacement energy ( E d ) for each atom type in SrTiO 3 perovskite using molecular dynamics simulations, by introducing a primary knock-on atom with a range of energies (20–250 eV) in principal crystallographic directions at 300 K. We find that all atom types are most easily displaced via direct replacement sequences on their own sublattices, which are extensive for Sr atoms due to focusson processes acting along channels. The weighted average threshold displacement energies (for use in TRIM-type calculations) are 50 eV for oxygen, 70 eV for strontium and 140 eV for titanium atoms. These computed energies for O and Sr are comparable to experimentally-derived values in perovskites, whereas the E d for Ti is much higher; it is expected that the value reported here is more accurate due to experimental difficulties in distinguishing different types of defects.

Pulsed neutron differential die away analysis for detection of nuclear materials

Abstract: Differential die away analysis (DDAA) is a very sensitive technique for detecting the presence of fissile materials such as 235U and 239Pu. DDAA is based on conventional pulsed neutron source interrogation, employing typically low cost 14 MeV (d, T) generators. In DDAA, a neutron generator produces repetitive pulses of neutrons that are directed into an inspected cargo. As each pulse passes through the cargo, the neutrons are thermalized and absorbed. The population of thermal neutrons decays with the diffusion decay time of the inspected medium (the so called thermal neutron “die-away” time) on the order of hundreds of microseconds. If SNM is present, the thermalized neutrons from the source cause fissions that produce a new source of fast neutrons. These fast fission neutrons decay with a time very similar to that of the thermal neutron die away of the surrounding cargo. The sensitivity of DDAA for a given source of neutrons is greatly affected by the size, geometry, density and composition of the inspected cargo. The sensitivity is also affected by the reflection of neutrons from surfaces such as the ground, nearby walls or nearby cargo; these effects were studied. The ability of DDAA to detect enriched uranium sample in dense hydrogenous cargo, such as paper and wood is shown.