Tandem Source Research Articles

80 Gbit/s OTDM using electroabsorption modulators

An 80 Gbit/s (8 × 10 Gbit/s) OTDM system, constructed entirely with electroabsorption modulators (EAM) as the active components, is presented. A tandem EAM pulse source with a simple dual frequency drive generates transform limited 4.5 ps pulses. At the receiver, the 10 Gbit/s channels are demultiplexed from 80 Gbit/s in a single step using a low polarisation sensitivity EAM. Error free operation with a sensitivity of –21.5 dBm is achieved.

Dynamics of pear-shaped dimensions and volume of intracavitary brachytherapy in cancer of the cervix: A desirable pear shape in the era of three-dimensional treatment planning

To evaluate the dynamics of pear-shaped dimensions and volume of the intracavitary brachytherapy, and to define a desirable pear-shape in the era of three-dimensional (3D) treatment planning. Since Point A has been used for the dose specification, the pear shape defined the surface enclosed by Point A. This study utilized a new method of evaluating pear-shaped dimensions and its configuration. The pear shape was artificially divided into tandem and colpostat portions for evaluation of its changes. Width, height, and thickness at the tandem portion (Wt, Ht, and Tt) and at the colpostat portion (Wc, Hc, and Tc) were defined, respectively, on the frontal and sagittal plane. To evaluate the dynamics of the pear-shape configuration, 12 variations of applicator geometry and source loading were applied to generate the pear-shape isodose line and dose-volume histogram. When the source strengths in the colpostats were reduced for optimization with the same dose to Point A dose, Wc, Hc, and Tc were decreased, whereas Wt, Ht, and Tt were increased without a change in the overall pear-shaped volume. When the separation of the colpostats was increased without a change in the source strength, Wc was increased, whereas Hc and Tc were reduced without a change in Wt, Ht, Tt and overall pear-shape volume. When the separation of colpostats and distal tandem source were increased, these changes at the colpostat portion were magnified. However, when both colpostat separation and its source strength were increased proportionally, Wc, Hc, and Tc were increased proportionally as well as its volume. The dose specification at Point A is less meaningful without a desirable pear shape encompassing the tumor around the cervix. In the era of 3D treatment planning, understanding the dynamics of the pear shape should improve the individualized dosimetry according to tumor size and location. The relationships between a desirable pear shape and its tumor coverage should establish a more reliable dose specification for cancer of the cervix.

Instrumentation and Fundamental Studies on Glow Discharge—Microwave-Induced Plasma (GD-MIP) Tandem Source for Optical Emission Spectrometry

Instrumentation and fundamental studies on glow discharge–microwave-induced plasma (GD-MIP) tandem source are described in this paper. Mutual effects among parameters, vacuum pressure, discharge current, voltage, and microwave power are examined with different cathode materials. Sputtering rates with and without microwave boost are measured under various experimental conditions. The experimental results demonstrate that the introduction of a microwave plasma will significantly decrease the sampling rate. A possible mechanism for the more uniform erosion obtained with microwave plasma boosting is suggested and discussed. Excitation temperatures are measured with the line-pair method and Boltzmann plot. A considerable increase in the excitation temperature (from about 3500 to 4200 K by using the line-pair method) is found with the addition of microwave plasma boosting. A comparison of the behavior of glow discharge alone and the GD-MIP tandem source is made. Significant enhancements in signal intensities are observed. The experimental results suggest that the excitation temperature plays an important role in signal enhancement.

The graphite furnace and its role in atomic spectroscopy.

The diversity of applications of the graphite furnace is extraordinary, encompassing the fields of physics, thermochemistry, spectroscopy and analytical chemistry. In this respect, the graphite furnace has been used on a continuous basis as a research tool for nearly a century. Following its introduction as an atomization source for atomic absorption spectrometry by L'vov in 1959, its role in atomic spectrometry expanded considerably to encompass analytical applications in emision, fluorescence, absorption and mass spectrometry. In addition to its conspicuous use as an atomization source in these areas, it is frequently employed as a vaporizer when used in the format of combined and tandem sources with other instrumentation. The unique physico-chemical micro-environment which can be attained within the graphite furnace has also been used to advantage in a number of investigations, including the determination of gas- and solid-phase diffusion coefficients of high-temperature metal vapours, the heats of sublimation of refractory metals, fundamental optical constants and the measurement of the heats of desorption of adatoms from high-temperature surfaces. The range of such applications remains to be more fully explored. The attractive features of this source, viz., the high atomization/vaporization efficiency, comparatively long atomic vapour residence times, controllable chemical and thermal environment and its ability to handle high dissolved solids content samples (</=100%) serve to ensure its place in analytical atomic spectroscopy for years to come.

Initial results of the 2.45 GHz excitation of the superconducting ECR ion source in its high-B modea)

The ‘‘frequency squared scaling’’ law, relied upon to enhance the production of high charge state ions from electron cyclotron resonance (ECR) ion sources, indicates that low frequencies of operation may not be viable for production of high ion densities, and hence for the production of high charge state ions. The superconducting electron cyclotron resonance ion source (SCECR) at the National Superconducting Cyclotron Laboratory is a fully superconducting, hexapole stabilized tandem mirror ECR ion source with a multimode microwave cavity. When operated at its optimized high magnetic field (high-B) mode at 6.4 GHz, the high charge state current output of the SCECR approaches all existing ECR ion sources, including higher requency ECR ion sources (ECRIS). In this study, the SCECR is operated at 2.45 GHz in its high-B mode that is scaled down for that frequency of operation. Initial results with oxygen and argon ion production at 2.45 GHz are presented. It is demonstrated that the SCECR can produce a plasma from which multiply charged ions of up to O7+ and Ar13+ can be extracted with ion current intensities comparable to several higher frequency ECRIS. This study concludes that 2.45 GHz is indeed a viable frequency for production of moderate intensities of multiply charged ions. This may lead to a simpler, and more cost effective microwave apparatus, with a less demanding magnetic field configuration.

Numerical simulation of cesium effects on plasma parameter dependence of H− production in a tandem ion source (abstract)

Effects of cesium vapor injection on H− production in a tandem negative ion source are studied numerically as a function of plasma parameters. Model calculation is done by solving a set of particle balance equations in steady-state hydrogen discharge plasmas. By including H− surface production processes caused by both H atoms and positive hydrogen ions, enhancement of H− production and pressure dependence of H− production observed experimentally are well reproduced in the model. Besides these, electron temperature dependence of H− production is also discussed.

Initial results of the 2.45 GHz excitation of the superconducting ECR ion source in its high-B mode (abstract)a)

The ‘‘frequency squared scaling’’ law, relied upon to enhance the production of high charge state ions from electron cyclotron resonance (ECR) ion sources, indicates that low frequencies of operation may not be viable for production of high ion densities, and hence for the production of high charge state ions. The superconducting electron cyclotron resonance ion source (SCECR) at the National Superconducting Cyclotron Laboratory is a fully superconducting, hexapole stabilized tandem mirror ECR ion source with a multimode microwave cavity. When operated at its optimized high magnetic field (high-B) mode at 6.4 GHz, the high charge state current output of the SCECR approaches all existing ECR ion sources, including higher frequency ECR ion sources (ECRIS). In this study, the SCECR is operated at 2.45 GHz in its high-B mode that is scaled down for that frequency of operation. Initial results with oxygen and argon ion production at 2.45 GHz are presented. It is demonstrated that the SCECR can produce a plasma from which multiply charged ions of up to O7+ and Ar13+ can be extracted with ion current intensities comparable to several higher frequency ECRIS. This study concludes that 2.45 GHz is indeed a viable frequency for production of moderate intensities of multiply charged ions. This may lead to a simpler, and more cost effective microwave apparatus, with a less demanding magnetic field configuration.

Production of a radioactive 18F ion beam for nuclear reaction studies

A two-stage method for generating a radioactive 18F ion beam has been developed. 18F is produced with a medical cyclotron by 11 MeV proton activation of [ 18O]water, then chemically processed off-line for use in a tandem accelerator ion source. Azeotropic distillation reduces the 18O component by 10 5, with a resulting 18O to 18F beam ratio of about 10 3. The average 18F − beam intensity per synthesis is 1 ppA over 120 min from a cesium vapor, sputter negative ion source (SNICS), with a peak intensity of 4.5 ppA.

The kinetics of negative hydrogen ions in discharges

The discovery of large (about 30%) concentrations of negative hydrogen ions in a simple hydrogen plasma and the requirement to produce intense H- ion sources for neutral beam heating of fusion devices has prompted a major effort to understand the kinetics of H- ions in discharges. Here a guide to the development of kinetic models of the hydrogen discharges in ion sources, from simple estimates of rate coefficients to large numerical codes is presented. The results of experimental studies to characterize ion source discharges are also discussed. The modelling and experiments provided the theoretical basis for the successful tandem multi-cusp ion source. Current models generally assume that the primary H- production mechanism is through dissociative attachment of electrons to rovibrationally excited molecules. At present, the degree of agreement between measured and calculated quantities, and even between different models, is mixed. There is concern that the standard dissociative attachment model is inadequate and new mechanisms are being sought. It is apparent that non-perturbative measurement techniques, which can also provide spatially resolved information, are also required.

Ion plasma sputtering as a method of introducing solid material into an electron cyclotron resonance ion source

A direct ion plasma sputtering effect has been observed in an electron cyclotron resonance ion source and developed into a reliable and simple method for producing ion beams from some solid materials. We describe the ion sputtering technique used with the Argonne Tandem Linac Accelerator System Positive Ion Injector Electron Cyclotron Resonance ion source to produce, to date, stable beams of nickel, silver, tellurium, gold, lead, and bismuth and present the results obtained in test cases.

Computer‐aided geometric reconstruction of Fletcher–Suit source positions

A computer-aided method for reconstruction of the source positions of a Fletcher-Suit applicator has been developed. The tandem source positions are determined by digitizing of the tip and two arbitrary points on the shaft from each of two orthogonal simulator films. A colpostat source position is reconstructed by digitizing of a single point on the endcap and three arbitrary points on the cylindrical sidewalls of the colpostat on the films. This computer-aided method considers the true projection geometry and applicator shape and permits localization of the source positions to within a mean error of less than 1 mm. Compared with the conventional method, the new approach (1) is more time efficient because only a few easily identified points are digitized, (2) allows localization when the tandem sources are shifted by a spacer or when colpostat sources are difficult to visualize on the lateral film, and (3) is more accurate than the conventional technique because no manual drawing of source positions on films is involved.

Technically Accurate Intracavitary Insertions Improve Pelvic Control and Survival among Patients with Locally Advanced Carcinoma of the Uterine Cervix

The purpose of this study was to document whether the technical qualities of a brachytherapy application impacts on the outcome of patients with locally advanced cervix cancer treated by definitive irradiation. A previous report from the patterns of care study demonstrated the importance of brachytherapy in the treatment of locally advanced cervix cancer. Locally advanced disease was defined as FIGO stages Ib (if tumor diameter was ⩽4 cm), IIb (if disease was bilateral or involved the lateral aspect of either parametrium), and III. Localization films from 128 patients with locally advanced squamous cell carcinoma of the cervix were reviewed by a radiation physicist and a radiation oncologist with expertise in gynecologic radiotherapy. All patients received external beam irradiation followed by one brachytherapy application (median point A dose = 8040 cGy; range, 4083-10,020 cGy). Brachytherapy parameters assessed were (a) the distance between the right colpostat source and the distal tandem source, (b) the distance between the left colpostat source and the distal tandem source, and (c) the symmetry of colpostat placement. Implants were scored as "ideal" (n = 8) when all three parameters were deemed satisfactory, "unacceptable" (n = 17) when none of the parameters was deemed satisfactory, and "adequate" (n = 41) in all other cases. Significantly improved 5-year local control was seen when comparing ideal and adequate placements to unacceptable placements (68% vs 34%, P = 0.02). A strong trend toward improved 5-year survival was also noted among the group with ideal and adequate implants as opposed to unacceptable implants (60% vs 40%). Multivariate analysis showed that the technical adequacy of the brachytherapy implant was the most important prognostic discriminant of local control. In conclusion, these analyses demonstrate the direct influence of competent technical implant performance on tumor control and even survival. While only a small fraction of implants for cervical cancer are performed poorly in the United States, there is a need for continued emphasis of the principles for proper implant technique.

Identification of cesium effects on enhancement of H− production in a volume negative ion source

Effects of cesium vapor injection on H− production in a tandem source are studied theoretically and parametrically as a function of different surface processes. Model calculation is done by solving a set of particle balance equations in steady-state hydrogen discharge plasmas. By including H− surface production processes caused by H atoms or positive hydrogen ions, enhancement of H− production observed experimentally is reproduced in the model.

The study of a 2.45 GHz plasma source as a plasma generator for the SCECR electron cyclotron resonance ion source

The 2.45 GHz plasma source is a multicusp electron cyclotron resonance source with a tuned single mode microwave cavity. This is a bright plasma source with a well characterized operating mode and plasma parameters [A. K. Srivastava, et al., Rev. Sci. Instrum. 63, 2556 (1992); A. K. Srivastava and J. Asmussen, J. Vac. Sci. Technol. A 11, 1307 (1993)]. The superconducting electron cyclotron resonance (SCECR) is a fully superconducting, hexapole stabilized tandem mirror ion source with a multimode microwave cavity. It is a high charge state ion source with extensive beam diagnostics [T. A. Antaya, Nucl. Instrum. Methods B 40/41, 1024 (1989)]. The coupling of these two sources permits a number of fundamental studies important to both classes of ion sources. The 2.4 GHz plasma source will be used to study the effects of ion or electron (or both) density enhancement on the charge state distribution, total extracted current, and ion energy distributions of the SCECR. The SCECR in turn will be used as a test stand to measure directly the ion distribution and beam characteristics of the 2.45 GHz plasma source. The first results with this hybrid ion source will be presented at this conference.

Optimum extracted H− and D− current densities from gas-pressure-limited high-power hydrogen/deuterium tandem ion sources

The tandem hydrogen/deuterium ion source is modeled for the purpose of identifying the maximum current densities that can be extracted subject to the gas-pressure constraints proposed for contemporary beam-line systems. Optimum useful extracted current densities are found to be in the range of approximately 7–10 mA cm−2. The sensitivity of these current densities is examined subject to uncertainties in the underlying atomic/molecular rate processes. A principal uncertainty remains the quantification of the molecular vibrational distribution following H+3 wall collisions.

Selective deposition of radio-frequency power in volume sources to optimize the tandem configuration for negative ion production

This article is a suggestion for the development of a microwave driven volume source. A high electron temperature plasma is to be generated by coupling microwaves at the electron cyclotron resonance into a closed field line magnetic field configuration to form the discharge chamber of a volume source. As an example, a toroidal magnetic field configuration is examined. Numerous advantages of this scheme are cited, most important of which is the possibility of having a tandem volume source with a very steep plasma temperature gradient.

Applications of laser ablation and electrothermal vaporization as sample introduction techniques for ICP-MS

The field of applications of ICP-MS can be further increased by the use of special sample introduction techniques such as laser ablation (LA) and electrothermal vaporization (ETV). In both cases a tandem source for mass spectrometry is formed by the sample introduction device and the ICP. The first source is specifically designed for the volatilization of a sample and it can be used to introduce selectively only certain parts of a sample into the ICP-MS, based either on local distribution (LA) or volatility (ETV). Applications of LA-ICP-MS are the determination of distribution patterns of minor constituents in solid samples such as ceramics, alloys or hard biological structures. Homogeneity testing in the first two types of samples or determination of distribution patterns of trace elements in the latter can be carried out rapidly with high spatial resolution on a multielement basis. The possibility of on-line separation between fractions of different volatility in a sample with ETV-ICP-MS is demonstrated for volcanic eruption products and other samples.

Isotope effect and electron-temperature dependence in volume H- and D- ion sources.

We present the results of a numerical study of isotope effects in volume negative-ion sources and compare them with experimental results. We have studied both hydrogen and deuterium discharges over a range of discharge current. For basic plasma parameters, the model reproduces the behavior measured in the discharge region of a tandem multicusp source. The theory predicts a local maximum in negative-ion production at an optimal electron temperature ${\mathit{T}}_{\mathit{e}}^{\mathrm{opt}}$\ensuremath{\approxeq}0.6--0.8 eV. This prediction is supported by our analysis of ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ densities measured in a hybrid multicusp source and in the extraction region of a tandem source. We show how the local maximum is related to the temperature dependence of the dissociative attachment rate coefficients. The most significant isotope effect suggested by this modeling effort is a stronger cooling of the vibrational distribution in deuterium due to vibration-translation reactions between molecules and atoms. This vibrational cooling reduces negative-ion production in high-power deuterium discharges below that in equivalent hydrogen discharges.

Inductively coupled plasma–microwave induced plasma tandem source for atomic emission spectrometry

A tandem source has been constructed by linking an atmospheric-pressure inductively coupled plasma (ICP) with a reduced-pressure microwave induced plasma (MIP). An aerosol of sample solution is introduced into the ICP where it is vaporized and atomized. The resulting analyte atoms are then extracted from the ICP, through a sampling orifice and into a moderate-pressure region in which a microwave discharge is supported by a Beenakker-type resonance cavity. It is from this latter discharge that analyte atomic or ionic emission is viewed. This paper examines the analytical characteristics of this pressure-differential tandem source, including optimization studies, background spectra and intensity, noise features, chemical interferences, linear range and detection limits. Experimental results indicate that this novel arrangement provides some of the expected benefits of a tandem source while other figures of merit are only of the same order of magnitude as results obtained by conventional ICP atomic emission spectrometry. Unfortunately, it is found that the microwave plasma adds more background than signal at many elemental emission lines.

Design considerations for a pressure-differential tandem source for use in atomic spectrometry

Factors that govern the likelihood of success of a tandem source formed from two devices that operate at different pressures are examined theoretically and experimentally. The union of an atmospheric-pressure inductively coupled plasma (ICP) with a reduced-pressure microwave induced plasma (MIP) is used to evaluate experimentally the theoretical arguments. For such a tandem source to be attractive, the loss in analyte number density in the lower-pressure second source must be compensated for by an offsetting gain in excitation or ionization efficiency or a reduction in background. These considerations are compared with what is found in the experimental ICP–MIP combination. At present, the ICP–MIP combination offers no improvement in detection limits or in ion to atom ratios over what is found in the ICP by itself. These considerations and comparisons suggest possible avenues for future research into pressure-differential tandem sources.