The use of cavity ringdown spectroscopy (CRDS) for atomic absorption measurements in a 27-MHz low-power argon inductively coupled plasma (ICP) is described. These results are used to demonstrate the utility of CRDS for both plasma diagnostic and analytical measurements. In these experiments, an aqueous solution of lead was introduced into a modified torch designed to enhance the ICP conditions for atomic absorption measurements. Absorption intensity characteristics of the lead 283.3-nm absorption line as a function of observation height and lateral position in the plasma were recorded for three different ICP powers (700, 500, and 200 W). The radial distribution of the ground-state lead atom density was derived from Abel inversion of the lateral measurements. At the novel 200 W operating condition, spectral line shapes vs. height and lateral position were fitted to Voigt profiles. Line-of-sight values of the gas kinetic temperature and electron density at different plasma locations were estimated from Gaussian and Lorentzian broadening components, respectively. The results are discussed and compared with those from other methods. The unique flexibility of CRDS for atomic and ionic absorption measurements in an ICP and the potential application of the ICP-CRDS technique for analytical measurements are demonstrated. Analytical results are compared with theoretical estimates of the lead detection limit.

This paper presents a stochastic algorithm for iterative error control decoding. We show that the stochastic decoding algorithm is an approximation of the sum-product algorithm. When the code's factor graph is a tree, as with trellises, the algorithm approaches maximum a-posteriori decoding. We also demonstrate a stochastic approximations to the alternative update rule successive relaxation. Stochastic decoders have very simple digital implementations which have almost no RAM requirements. We present example stochastic decoders for a trellisbased Hamming code, and for a Block Turbo code constructed from Hamming codes.

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A modified Gradient Descent Bit Flipping (GDBF) algorithm is proposed for decoding Low Density Parity Check (LDPC) codes on the binary-input additive white Gaussian noise channel. The new algorithm, called Noisy GDBF (NGDBF), introduces a random perturbation into each symbol metric at each iteration. The noise perturbation allows the algorithm to escape from undesirable local maxima, resulting in improved performance. A combination of heuristic improvements to the algorithm are proposed and evaluated. When the proposed heuristics are applied, NGDBF performs better than any previously reported GDBF variant, and comes within 0.5 dB of the belief propagation algorithm for several tested codes. Unlike other previous GDBF algorithms that provide an escape from local maxima, the proposed algorithm uses only local, fully parallelizable operations and does not require computing a global objective function or a sort over symbol metrics, making it highly efficient in comparison. The proposed NGDBF algorithm requires channel state information which must be obtained from a signal to noise ratio (SNR) estimator. Architectural details are presented for implementing the NGDBF algorithm. Complexity analysis and optimizations are also discussed.Comment: 16 pages, 22 figures, 2 table

Inductively coupled plasma cavity ringdown spectroscopy (ICP-CRDS) is applied to isotopic measurements of uranium. We have successfully obtained the isotopic-resolved spectra of uranium at three different atomic/ionic transition lines, 286.57, 358.49, and 409.01 nm. Of the three lines, the largest isotope shift of approximately 9 pm was measured at the 286.57 ionic line. Isotopic-resolved spectra were recorded in ratio of 1:1 (235U/238U, 2.5 micrograms/mL) and at the natural abundance ratio of 0.714% (235U/238U, 1.25 micrograms/mL 235U). The smallest measurable isotope shift of approximately 3 pm was determined for the 409.01 nm ion spectral line. Detection limits (DL) were obtained under optimized ICP operating conditions to be in the range of 70-150 ng/mL, except for the 238U component of the 286.57 nm line (300 ng/mL). This latter result was determined to be due to a strong, previously unreported, absorption interference from the argon plasma. The 235U isotope component (DL 70 ng/mL) was found to be unaffected. This work demonstrates the applicability of ICP-CRDS for uranium isotopic measurements. The potential of development of a field-deployable, on-line uranium isotope monitor using plasma-CRDS is discussed.

We are exploring sensitive techniques for elemental measurements using cavity ring-down spectroscopy (CRDS) combined with a compact microwave plasma source as an atomic absorption cell. The research work marries the high sensitivity of CRDS with a low-power microwave plasma source to develop a new instrument that yields high sensitivity and capability for elemental measurements. CRDS can provide orders of magnitude improvement in sensitivity over conventional absorption techniques. Additional benefit is gained from a compact microwave plasma source that possesses the advantages of low power and low-plasma gas flow rate, which are of benefit for atomic absorption measurements. A laboratory CRDS system consisting of a tunable dye laser is used in this work for developing a scientific base and demonstrating the feasibility of the technique. A laboratory-designed and -built sampling system for solution sample introduction is used for testing. The ring-down signals are monitored using a photomultiplier tube and recorded using a digital oscilloscope interfaced to a computer. Lead is chosen as a typical element for the system optimization and characterization. The effects of baseline noise from the plasma source are reported. A detection limit of 0.8 ppb (10(-)(10)) is obtained with such a device.

We have been exploring innovative technologies for elemental and hyperfine structure measurements using cavity ring-down spectroscopy (CRDS) combined with various plasma sources. A laboratory CRDS system utilizing a tunable dye laser is employed in this work to demonstrate the feasibility of the technology. An in-house fabricated sampling system is used to generate aerosols from solution samples and introduce the aerosols into the plasma source. The ring-down signals are monitored using a photomultiplier tube and recorded using a digital oscilloscope interfaced to a computer. Several microwave plasma discharge devices are tested for mercury CRDS measurement. Various discharge tubes have been designed and tested to reduce background interference and increase the sample path length while still controlling turbulence generated from the plasma gas flow. Significant background reduction has been achieved with the implementation of the newly designed tube-shaped plasma devices, which has resulted in a detection limit of 0.4 ng/mL for mercury with the plasma source CRDS. The calibration curves obtained in this work readily show that linearity over 2 orders of magnitude can be obtained with plasma-CRDS for mercury detection. In this work, the hyperfine structure of mercury at the experimental plasma temperatures is clearly identified. We expect that plasma source cavity ring-down spectroscopy will provide enhanced capabilities for elemental and isotopic measurements.

Cold vapor cavity ringdown spectroscopy has been successfully applied to the detection of elemental mercury. Using an absorption cell 0.18 m in length, detection limits of 0.027 and 0.12 ng were obtained using peak area and peak height measurements, respectively. For the peak area measurement, this corresponds to a gas phase concentration of less than 25 ng m 23 . For comparison, using a similar absorption cell, standard AAS yielded a Hg detection limit (peak height) of 9 ng, (gas phase concentration of ~ 8.3 mg m 23 ).