Microstructure and corrosion behavior of laser surface-treated AZ31B Mg bio-implant material.

Although magnesium and magnesium alloys are considered biocompatible and biodegradable, they suffer from poor corrosion performance in the human body environment. In light of this, surface modification via rapid surface melting of AZ31B Mg alloy using a continuous-wave Nd:YAG laser was conducted. Laser processing was performed with laser energy ranging from 1.06 to 3.18 J/mm2. The corrosion behavior in simulated body fluid of laser surface-treated and untreated AZ31B Mg alloy samples was evaluated using electrochemical technique. The effect of laser surface treatment on phase and microstructure evolution was evaluated using X-ray diffraction and scanning electron microscopy. Microstructure examination revealed grain refinement as well as formation and uniform distribution of Mg17Al12 phase along the grain boundary for laser surface-treated samples. Evolution of such unique microstructure during laser surface treatment indicated enhancement in the corrosion resistance of laser surface-treated samples compared to untreated alloy.

Photocatalytic degradation of rhodamine B, paracetamol and diclofenac sodium by supported titania-based catalysts from petrochemical residue: effect of doping with magnesium.

Three different lots of a residual Ziegler-Natta catalyst slurry (bearing Ti and Mg) obtained from an industrial petrochemical plant were employed as sources for the photocatalyst supported on silica. The effect of additional magnesium (1.0-25.0 wt% Mg/SiO2) on the photocatalytic properties of the doped materials was investigated. Doping the titania-based photocatalyst with Mg results in a shift in the absorption threshold toward the visible spectrum. The optical band gap energy of the bare supported photocatalyst was in the range of 2.5 eV and shifted to 1.72 eV after 25 wt% Mg doping. The systems were evaluated for the photodegradation of one dye (rhodamine B (RhB)) and two drugs (paracetamol and diclofenac sodium) either under ultraviolet (UV) (365 nm - UVA) or visible radiation, separately. Among the evaluated systems, doping with 25 wt% Mg afforded the highest degradation values for the target molecules under UV and visible radiation (i.e. 87%, 60% and 55% of the RhB, paracetamol and diclofenac under UV, respectively, and 82%, 48.3% and 48% under visible irradiation, respectively).

Presence of divalent cation is not mandatory for the formation of intramolecular purine-motif triplex containing human c-jun protooncogene target.

Modulation of endogenous gene function, through sequence-specific recognition of double helical DNA via oligonucleotide-directed triplex formation, is a promising approach. Compared to the formation of pyrimidine motif triplexes, which require relatively low pH, purine motif appears to be the most gifted for their stability under physiological conditions. Our previous work has demonstrated formation of magnesium-ion dependent highly stable intermolecular triplexes using a purine third strand of varied lengths, at the purine•pyrimidine (Pu•Py) targets of SIV/HIV-2 (vpx) genes (Svinarchuk, F., Monnot, M., Merle, A., Malvy, C., and Fermandjian, S. (1995) Nucleic Acids Res. 23, 3831-3836). Herein, we show that a designed intramolecular version of the 11-bp core sequence of the said targets, which also constitutes an integral, short, and symmetrical segment (G(2)AG(5)AG(2))•(C(2)TC(5)TC(2)) of human c-jun protooncogene forms a stable triplex, even in the absence of magnesium. The sequence d-C(2)TC(5)TC(2)T(5)G(2)AG(5)AG(2)T(5)G(2)AG(5)AG(2) (I-Pu) folds back twice onto itself to form an intramolecular triple helix via a double hairpin formation. The design ensures that the orientation of the intact third strand is antiparallel with respect to the oligopurine strand of the duplex. The triple helix formation has been revealed by non-denaturating gel assays, UV-thermal denaturation, and circular dichroism (CD) spectroscopy. The monophasic melting curve, recorded in the presence of sodium, represented the dissociation of intramolecular triplex to single strand in one step; however, the addition of magnesium bestowed thermal stability to the triplex. Formation of intramolecular triple helix at neutral pH in sodium, with or without magnesium cations, was also confirmed by gel electrophoresis. The triplex, mediated by sodium alone, destabilizes in the presence of 5'-C(2)TC(5)TC(2)-3', an oligonucleotide complementary to the 3'-oligopurine segments of I-Pu, whereas in the presence of magnesium the triplex remained impervious. CD spectra showed the signatures of triplex structure with A-like DNA conformation. We suggest that the possible formation of pH and magnesium-independent purine-motif triplexes at genomic Pu•Py sequences may be pertinent to gene regulation.

Development of hierarchical magnesium composites using hybrid microwave sintering.

In this work, hierarchical magnesium based composites with a micro-architecture comprising reinforcing constituent that is a composite in itself were fabricated using powder metallurgy route including microwave assisted rapid sintering technique and hot extrusion. Different level-I composite particles comprises sub-micron pure aluminum (Al) matrix containing Al2O3 particles of different length scale (from micrometer to nanometer size). Microstructural characterization of the hierarchical composites revealed reasonably uniform distribution of level-I composite particles and significant grain refinement compared to monolithic Mg. Hierarchical composite configurations exhibited different mechanical performance as a function of Al2O3 length scale. Among the different hierarchical formulations synthesized, the hierarchical configuration with level-I composition comprising Al and nano-Al2O3 (0.05 microm) exhibited the highest improvement in tensile yield strength (0.2% YS), ultimate tensile strength (UTS), tensile failure strain (FS), compressive yield strength (0.2% CYS) and ultimate compressive strength (UCS) (+96%, +80%, +42%, +80%, and +83%) as compared to monolithic Mg. An attempt has been made in the present study to correlate the effect of different length scales of Al2O3 particulates on the microstructural and mechanical response of magnesium.

High energy quasi-phase matched optical parametric oscillation using Mg-doped congruent LiTaO(3) crystal.

We report on high energy optical parametric oscillation of 118 mJ output with ~70% slope efficiency in 10 ns duration of 30 Hz operation by using Mg-doped congruent composition LiTaO(3) (MgLT). The periodically poled MgLT device with ~30 microm period for quasi-phase matching (QPM) in 5-mm-thick crystal are prepared. MgLT crystal could become a candidate for high-energy and higher durability material of QPM device, compared to conventional Mg-doped congruent composition LiNbO(3).

Metastable Magnesium fluorescence spectroscopy using a frequency-stabilized 517 nm laser.

We present a laser operating at 517 nm for our Magnesium laser-cooling and atomic clock project. A two-stage Yb-doped fiber amplifier (YDFA) system generates more than 1.5 W of 1034 nm light when seeded with a 15 mW diode laser. Using a periodically poled lithium niobate (PPLN) waveguide, we obtained more than 40 mW of 517 nm output power by single pass frequency doubling. In addition, fluorescence spectroscopy of metastable magnesium atoms could be used to stabilize the 517 nm laser to an absolute frequency within 1 MHz.

MANIPULATION OF THE DOSE-RESPONSE OF COMPOSITE GLOW PEAK 5 IN THE THERMOLUMINESCENCE OF LiF:Mg,Ti (TLD-100) VIA OPTICAL EXCITATION POST-IRRADIATION: POTENTIAL FOR IMPROVED DOSE-RESPONSE LINEARITY BEYOND 1 Gy.

Many dosimetric applications and especially those involved in clinical dosimetry are hampered by the supralinearity of TLD-100 which begins at a level of dose of 1 Gy. This research investigates the effect of optical excitation following irradiation on the dose-response. It is expected that this will lead to a more linear dose-response, however, irrespective of the hoped-for linearity, the theoretical/kinetic simulations of the effect of optical excitation will further enhance our understanding of the thermoluminescence mechanisms, especially the role of spatially correlated trapping and luminescent centers. In the following, the various stages carried out in these investigations are discussed and preliminary results presented.

Long-term fade study of the DT-702 LiF: Mg,Cu,P TLD.

LiF thermoluminescent dosemeters (TLDs) are used by the US Navy to record radiation exposure of personnel. The Model DT-648 LiF:Mg,Ti TLD has been replaced by a new Model DT-702 LiF:Mg,Cu,P TLD. The DT-648 was used for many years and has undergone extensive testing to identify its pre- and post-irradiation fade operating characteristics. Studies have shown that the addition of copper increases the thermoluminesence sensitivity of the TLD for improved low-level radiation monitoring. This study evaluates various fading characteristics of the new copper-doped dosemeter using current equipment for processing of TLDs and calibrating to a National Institute of Standards and Technology standard source. The 57-week study took place at the Naval Dosimetry Center, Bethesda, MD, USA. TLDs were stored for various lengths of time before and after being exposed to a National Institute of Standards and Technology calibrated radiation sources. TLDs were then processed using current US Navy instructions and the resulting dose compared with the calibrated exposure. Both loss of signal and loss of sensitivity were evaluated. The results of this study have shown that the DT-702 TLD has no statistically significant change in sensitivity or change in signal with up to 57 weeks of pre- or post-irradiation time. The results of this study will increase the accuracy of exposure record keeping for the Navy and will allow longer issue periods. This will increase flexibility with international and domestic shipping procedures, as well as reduce workload requirements for dosimetry processing.

The effects of ionisation density on the glow curve structure of LiF:Mg,Ti (TLD-100): the behaviour of composite glow peak 5 in 'slow-cooled' material.

The effects of ionisation density on the structure of the glow curve of LiF:Mg,Ti (TLD-100) are briefly reviewed and discussed within the framework of the spatially correlated TC/LC model and localised recombination. The effects of 'slow-cooling' on the structure of composite peak 5 following low-ionisation density beta/gamma irradiation are described and analysed in both 'slow-cooled' and 'normally cooled' samples. It is demonstrated that 'slow-cooling' using a cooling rate of 30 degrees C h(-1) increases the relative intensity of glow peak 5a to composite glow peak 5 from approximately 0.1 to approximately 2, thereby greatly improving the precision of measurement of the ratio 5a/5. The improved precision removes a hurdle impeding the development of the ionisation density discrimination properties of the peak 5a/5 nanodosimeter.

A COMPARATIVE STUDY OF THE DOSIMETRIC FEATURES OF α-Al2O3:C,Mg AND α-Al2O3:C.

A comparative study of the dosimetric features of α-Al2O3:C,Mg and α-Al2O3:C relevant to thermoluminescence dosimetry is reported. A glow curve of α-Al2O3:C,Mg measured at 1°C/s after beta irradiation to 1 Gy shows two subsidiary peaks at 42°C (labelled as I) and 72°C (II) and the main peak at 161°C (III) whereas a glow curve of α-Al2O3:C measured under the same conditions shows the main peak at 178°C (II') and a lower intensity peak at 48°C (I'). Apart from these ones, there are several other peaks at temperatures beyond that of the main peak in both α-Al2O3:C,Mg and α-Al2O3:C. However, the latter are not included in this study. We report a comparative quantitative analysis of dose response and fading of peaks I, II and III of α-Al2O3:C,Mg and peaks I' and II' of α-Al2O3:C. Analysis shows that the dose response of peaks I and III is sublinear within 1-10 Gy whereas that of peak II is superlinear within 1-4 Gy followed by a sublinear region within 4-10 Gy. In comparison, the dose response of peak I' is superlinear within 1-4 Gy followed by a sublinear region within 4-10 Gy whereas that of peak II' is sublinear within 1-4 Gy followed by a superlinear region within 4-10 Gy. As regards to fading corresponding to 1 Gy, peak I is very unstable and fades within 300 s, peak II is more stable and takes up to 43200 s to fade. In comparison, peak III fades down to 30% of its initial intensity within 2400 s. Interestingly, between 2400 and 800 s, the intensity fades by 17% only. Regarding fading in α-Al2O3:C, peak I' fades within 600 s whereas peak II' shows an inverse fading behaviour up to 64800 s. The rate of fading for peaks I, II and III in α-Al2O3:C,Mg was found to decrease with increase in dose. However, no such behaviour was observed in α-Al2O3:C. The fading in both samples is discussed on the basis of a charge hopping mechanism.

New Al2O3:C,Mg crystals for radiophotoluminescent dosimetry and optical imaging.

Optical and dosimetric properties of a new radiophotoluminescent material based on aluminum oxide doped with carbon and magnesium (Al2O3:C,Mg) and having aggregate oxygen vacancy defects are presented. The Al2O3:C,Mg crystals are characterized by several new optical absorption and emission bands. It is suggested that the main optical properties of this material are due to the formation of aggregate defects composed of two oxygen vacancies and two Mg-impurity atoms. Radiation-induced optical absorption bands are centered at 335 and 620 nm and produce fluorescent emission at 750 nm with a 75 +/- 5 ns lifetime. The dose measurements are performed by illumination of the Al2O3:C,Mg crystal with 335 nm or 650 nm light and by measuring the intensity of the 750 nm fluorescence. The detector material is insensitive to room light before and after the irradiation and the traps are stable up to 600 degrees C. A dose measurement range between 5 mGy and 200 Gy, suitable for therapeutic radiology applications, was demonstrated. The short luminescent lifetime and nondestructive readout is favorable for imaging applications.

Measurements of the optical density and the thermoluminescent response of LiF:Mg,Ti exposed to high doses of 60Co gamma rays.

The absorption spectra and glow curves of LiF:Mg,Ti exposed to high doses of 60Co gamma rays as well as the optical density curves and thermoluminescent (TL) response as a function of dose were measured. Absorption peaks around 442 nm (2.8 eV), 307 nm (4.0 eV) and 248 nm (5.0 eV) were observed. The optical density is a linear function of dose up to 2500 Gy and then sublinear for the 5.0 eV band, and for the 4.0 eV band it is sublinear with a filling constant of 2.1 x 10(-4) and 6.2 x 10(-4) Gy(-1). The dose response of the 2.8 eV band is linear from 2500 Gy to 10 kGy with a slope of 1.0 x 10(-5) Gy(-1). Once the absorption spectra were obtained, the glow curves and the TL response were obtained. In the dose range, 290-10,000 Gy, the TL response of peaks 5 and 7 was supralinear-sublinear.

Properties of the 4.45 eV optical absorption band in LiF:Mg,Ti.

The optical absorption (OA) and thermoluminescence (TL) of dosimetric LiF:Mg,Ti (TLD-100) as well as nominally pure LiF single crystal have been studied as a function of irradiation dose, thermal and optical bleaching in order to investigate the role of the 4.45 eV OA band in low temperature TL. Computerised deconvolution was used to resolve the absorption spectrum into individual gaussian bands and the TL glow curve into glow peaks. Although the 4.45 eV OA band shows thermal decay characteristics similar to the 4.0 eV band its dose filling constant and optical bleaching properties suggest that it cannot be associated with the TL of composite peaks 4 or 5. Its presence in optical grade single crystal LiF further suggests that it is an intrinsic defect or possibly associated with chance impurities other than Mg, Ti.

Search for ionisation density effects in the radiation absorption stage in LiF:Mg,Ti.

Optical absorption (OA) dose-response of LiF:Mg,Ti (TLD-100) is studied as a function of electron energy (ionisation density) and irradiation dose. Contrary to the situation in thermoluminescence dose-response where the supralinearity is strongly energy-dependent, no dependence of the OA dose filling constants on energy is observed. This result is interpreted as indicating a lack of competitive process in the radiation absorption stage. The lack of an energy dependence of the dose filling constant also suggests that the charge carrier migration distances are sufficiently large to smear out the differences in the non-uniform distribution of ionisation events created by the impinging gamma/electron radiation of various energies.

Theoretical and experimental investigation of the f(D)max-(S/S0)max anomaly in LiF:Mg,Ti.

Previous investigations in the framework of the Unified Interaction Model (UNIM) have demonstrated the difficulty in theoretically predicting both f(D)max and [S(Ds)/S(0)]max with the same set of UNIM parameters. This anomaly is re-investigated and an alternative explanation, based on experimental measurements combined with UNIM analysis, is given. This explanation is based on the assumption that N(LC), the total number of available luminescent centres is changed by the sensitisation dose or anneal or both. Allowing N(LC) to increase as the sensitisation dose is increased, a good fit to both f(D) and S(Ds)/S0 is obtained.

Optical excitation of LiF:Mg,Ti following alpha and beta irradiation.

It is demonstrated experimentally that optical excitation of irradiated LiF:Mg,Ti (TLD-100) by 4 eV photons has the same effect for both alpha particle (high-ionisation density) irradiation and photon/electron irradiation. In both cases, peak 5a converts to peak 4 causing peak 4 to increase following the bleach. Such an observation is consistent with the major premise of track structure theory that radiation effects following heavy changed particle (HCP)/neutron irradiation are due exclusively to the interaction of the secondary electrons created by the HCP slowing down.

The application of LiF:Mg,Cu,P to large scale personnel dosimetry: current status and future directions.

LiF:Mg,Cu,P is starting to replace LiF:Mg,Ti in a variety of personnel dosimetry applications. LiF:Mg,Cu,P has superior characteristics as compared to LiF:Mg,Ti including, higher sensitivity, improved energy response for photons, lack of supralinearity and insignificant fading. The use of LiF:Mg,Cu,P in large scale dosimetry programs is of particular interest due to the extreme sensitivity of this material to the maximum readout temperature, and the variety of different dosimetry aspects and details that must be considered for a successful implementation in routine dosimetry. Here we discuss and explain the various aspects of large scale LiF:Mg,Cu,P based dosimetry programs including the properties of the TL material, new generation of TLD readers, calibration methodologies, a new generation of dose calculation algorithms based on the use of artificial neural networks and the overall uncertainty of the dose measurement. The United States Navy (USN) will be the first US dosimetry processor who will use this new material for routine applications. Until June 2002, the Navy used two types of thermoluminescent materials for personnel dosimetry, CaF2:Mn and LiF:Mg,Ti. A program to upgrade the system and to implement LiF:Mg,Cu,P, started in the mid 1990s and was recently concluded. In 2002, the new system replaced the LiF:Mg,Ti and is scheduled to start replacing the CaF2:Mn system in 2006. A pilot study to determine the dosimetric performance of the new LiF:Mg,Cu,P based dosimetry system was recently completed, and the results show the new system to be as good or better than the current system in all areas tested. As a result, LiF:Mg,Cu,P is scheduled to become the primary personnel dosimeter for the entire US Navy in 2006.

Determination of LiF:Mg,Ti and LiF:Mg,Cu,P TL efficiency for X-rays and their application to Monte Carlo simulations of dosemeter response.

The simulation of response of a new passive area dosemeter for measuring ambient dose equivalent H*(10) for photons has been performed using the Monte Carlo code MCNP and experimentally determined responses of LiF:Mg,Ti and LiF:Mg,Cu,P thermoluminescent (TL) detectors for hard-filtered X-ray spectra from 20 to 300 keV and for 137Cs and 60Co gamma radiation. Relative TL efficiency for both types of detectors, determined in experiments with bare detectors and similar Monte Carlo simulations, compared favourably with prediction of microdosimetric models for proposed microdosimetric target sizes in the range of 20-40 nm. The concluding verification experiment showed small deviations between measured and simulated dosemeter energy response values in the range of a few percent.

Confocal fluorescent imaging of tracks from heavy charged particles utilising new Al2O3:C,Mg crystals.

A completely optical, non-destructive imaging of tracks in a fluorescent crystal provides a new way to detect and to assess doses from heavy charged particles and neutrons. The technique combines confocal fluorescent microscopy with a new radiation-sensitive, luminescent material based on aluminium oxide single crystals doped with carbon, magnesium and having aggregate oxygen vacancy defects (Al2O3:C,Mg). Radiation-induced colour centres in the new material have an absorption band at 620 nm and produce fluorescence at 750 nm with a high quantum yield and a short, 75 +/- 5 ns, fluorescence lifetime. Three-dimensional spatial distribution of fluorescent intensity allows one to obtain depth-dose distributions and to discriminate between high- and low-linear energy transfer radiations. Images of single tracks produced by different types of radiation have been obtained. Irradiations with a calibrated 241Am alpha source showed high efficiency for track detection. Thermal neutrons were detected using a nuclear reaction with a 6LiF radiator and production of alpha particles and tritium ions. Fast neutrons were detected using recoil protons produced in a polyethylene radiator installed in front of the crystalline detector. Three-dimensional reconstruction of a recoil proton propagating through the crystal was demonstrated.

Thermoluminescence emission spectra for the LiF:Mg,Cu,Na,Si thermoluminescent materials with various concentrations of the dopants (3-D measurement).

The thermoluminescence (TL) emission spectra from LiF TL materials, called KLT-300 (LiF:Mg,Cu,Na,Si) with various dopant concentrations are measured and analysed. These KLT-300 materials were developed by the Korea Atomic Energy Research Institute (KAERI) to achieve an enhancement of the thermal stability in TL readings. Six types of samples are prepared with different dopant concentrations in the following ranges; Mg (0-0.20 mol%), Cu (0-0.05 mol%), Na and Si (0-0.9 mol%). The spectra measurements are carried out for the six types of samples using a TL emission spectra measurement device. The spectra measurement device consists of a monochromator, photomultiplier tube and temperature control unit to thermally stimulate the samples. The measured data shows the light emission during heating of the sample as a function of temperature and wavelength (three-dimensional TL spectra). The spectra were analysed using a method of deconvolution based on gaussian curve. The wavelength of a main peak of the emission spectra changes depending on the existence of the Cu dopant, while intensity of the spectra rapidly changes with the Cu dopant concentrations. The 385 nm emission is mainly observed in all the spectra from the samples with the Cu dopant, but in those from the samples without the Cu dopant a very weak 401 nm emission is mainly observed. However, any change in the wavelength at a main peak of the TL emission spectra from the sample materials with Na and Si dopants is not observed but that in the intensity at a peak of the spectra is observed.