Spl Beta Research Articles

Development of scintillating fiber imager

A scintillating fiber imager (SFI) has been developed for restriction landmark genomic scanning (RLGS) technology. Test measurements have been carried out with genomic linkage map labelled with /sup 32/P (/spl beta/-ray emitter) and /sup 58/Ni beam at 90 MeV/nucleon as well. The system offers real time acquisition and an easy quantification of RLGS data compared to film-based autoradiography, and it is proved to be a powerful position sensitive detector for measurements in nuclear physics.

Field-emission enhancement of Mo-tip field-emitted arrays fabricated by using a redox method

This work has improved the emission characteristics for Mo-tips of field-emitter array (FEA) through a reduction-oxidation (redox) process. The maximum emission current of the 1600 tips array significantly increased from 23.2 /spl mu/A to 2.36 mA with the turn-on voltage decreasing from 70 V to 48 V after the redox treatment. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) indicate that the main factor improving electron field emission characteristics for the Mo-tips is the increase in enhancement factor (/spl beta/) and, possibly, number of emission sites of Mo films.

Turn-on voltage investigation of GaAs-based bipolar transistors with Ga/sub 1-x/In/sub x/As/sub 1-y/Ny base layers

The fundamental lower limit on the turn on voltage of GaAs-based bipolar transistors is first established, then reduced with the use of a novel low energy-gap base material, Ga/sub 1-x/In/sub x/As/sub 1-y/N/sub y/. InGaP/GaInAsN DHBTs (x/spl sim/3y/spl sim/0.01) with high p-type doping levels (/spl sim/3/spl times/10/sup 19/ cm/sup -3/) and dc current gain (/spl beta//sub max//spl sim/68 at 234 /spl Omega///spl square/) are demonstrated. A reduction in the turn-on voltage over a wide range of practical base sheet resistance values (100 to 400 /spl Omega///spl square/) is established relative to both GaAs BJTs and conventional InGaP/GaAs HBTs with optimized base-emitter interfaces-over 25 mV in heavily doped, high dc current gain samples. The potential to engineer turn-on voltages comparable to Si- or InP-based bipolar devices on a GaAs platform is enabled by the use of lattice matched Ga/sub 1-x/In/sub x/As/sub 1-y/N/sub y/ alloys, which can simultaneously reduce the energy-gap and balance the lattice constant of the base layer when x/spl sim/3y.

A new lateral field emission device using chemical-mechanical polishing

A polysilicon lateral field emission device using chemical-mechanical polishing (CMP) is proposed and experimental results on the first prototype are reported. In this method, dry oxidation process determines the interelectrode gap. Thus, it is relatively easy to form electrode gaps with dimensions less than 1 μm. Also, the process allows for good uniformity and reproducibility in controlling the interelectrode gap. The turn-on voltage of the fabricated device with interelectrode gap of 3500 /spl Aring/ is as low as 5.4 V and the emission current is as high as 9 μA at 9.3 V. From the Fowler-Nordheim (FN) equation, field emitting area (/spl alpha/) and field enhancement factor (/spl beta/) are estimated to explain the low turn-on voltage and the high emission current. The emission current fluctuation is about /spl plusmn/4% for 25 min.

Scaling of accelerating gradients and dephasing effects in channel-guided laser wakefield accelerators

Future wakefield accelerator (LWFA) experiments are expected to operate in the short pulse resonant regime and employ some form of laser guiding, such as a preformed plasma channel. Performance of an LWFA may be characterized by the maximum axial electric field E/sub m/, the dephasing length L/sub d/, and the corresponding dephasing limited energy gain W/sub d/. Dephasing is characterized by the normalized phase slippage rate /spl Delta//spl beta//sub p/, of the wakefield relative to a particle moving at the velocity of light. This paper presents analytical models for all of these quantities and compares them with results from simulations of channel-guided LWFAs. The simulations generally confirm the scaling predicted by the analytical models, agreeing within a few percent in most cases. The results show that with the proper choice of laser and channel parameters, the pulse will propagate at a nearly constant spot size r/sub M/ over many Rayleigh lengths and generate large accelerating electric fields. The spot size correction to the slippage rate is shown to be important in the LWFA regime, whereas /spl Delta//spl beta//sub p/, is essentially independent of laser intensity. An example is presented of a 25-TW, 100-fs laser pulse that produces a dephasing limited energy gain in excess of 1 GeV.

Performance of a 128 channel analogue front-end chip for readout of Si strip detector modules for LHC experiments

We present a 128-channel analogue front-end chip, SCT 128A-HC, for readout of silicon strip detectors employed in the inner tracking detectors of the LHC experiment. The chip is produced in the radiation hard DMILL technology. The architecture of the chip and critical design issues are discussed. The performance of the chip has been evaluated in detail in the test bench and is presented in the paper. The chip is used to read out prototype analogue modules compatible in size, functionality and performance with the ATLAS SCT base line modules. Several full size detector modules equipped with SCT128A-HC chips has been built and tested successfully in the lab with /spl beta/ particles as well as in the test beam. The results concerning the signal-to-noise ratio, noise occupancy, efficiency and spatial resolution are presented. The radiation hardness issues are discussed.

Can macroscopic oxide thickness uniformity improve oxide reliability?

In this work, we investigated both experimentally and numerically the impact of macroscopic oxide thickness uniformity on Weibull breakdown characteristics for both Weibull parameters, namely, the characteristics times (T/sub 63/) and Weibull slopes (/spl beta/) over a wide range of oxide thickness. A detailed full-scale Monte Carlo analysis is used to examine the breakdown characteristics at low-percentile and its sensitivity to variation of thickness dependence of time-to-breakdown and Weibull slopes. We show that for thinner oxides with very shallow Weibull slopes the impact of thickness variation is drastically reduced as compared to thicker oxides for the same parameters.

Graded-emitter AlGaN/GaN heterojunction bipolartransistors

AlGaN/GaN heterojunction bipolar transistors (HBTs) with a graded emitter-base junction have been grown and fabricated. The epitaxial structures were grown by metalorganic chemical vapor deposition on (0001) sapphire substrates. The HBT devices with an emitter size of 60/spl times/60 /spl mu/m/sup 2/ exhibit well controlled current-voltage characteristics and Gummel plots and achieve a common-emitter current gain in the range /spl beta/=4-10 and a common-emitter offset voltage V/sub CEoff/=4 V at room temperature.

Duality theorems for joint source-channel coding

We consider joint source-channel coding for a memoryless Gaussian source and an additive white Gaussian noise (AWGN) channel. For a given code defined by an encoder-decoder pair (/spl alpha/, /spl beta/), its dual code is obtained by interchanging the encoder and decoder: (/spl beta/, /spl alpha/). It is shown that if a code (/spl alpha/, /spl beta/) is optimal at rate p channel uses per source sample and if it satisfies a certain uniform continuity condition, then its dual code (/spl beta/, /spl alpha/) is optimal for rate 1//spl rho/ channel uses per source sample. Further, it is demonstrated that there is a code which is optimal but its dual code is not optimal. Finally, using random coding, we show that there is an optimal code which has an optimal dual. The duality concept is also presented for the cases of (i) binary memoryless equiprobable source and binary-symmetric channel (BSC), and (ii) colored Gaussian source and additive colored Gaussian noise (ACGN) channel.

Power-law shot noise and its relationship to long-memory α-stable processes

We consider the shot noise process, whose associated impulse response is a decaying power-law kernel of the form t/sup /spl beta//2-1/. We show that this power-law Poisson model gives rise to a process that, at each time instant, is an /spl alpha/-stable random variable if /spl beta/<1. We show that although the process is not /spl alpha/-stable, pairs of its samples become jointly /spl alpha/-stable as the distance between them tends to infinity. It is known that for the case /spl beta/>1, the power-law Poisson process has a power-law spectrum. We show that, although in the case /spl beta/<1 the power spectrum does not exist, the process still exhibits long memory in a generalized sense. The power-law shot noise process appears in many applications in engineering and physics. The proposed results can be used to study such processes as well as to synthesize a random process with long-range dependence.

A study of magnetic particles' behavior by magneto-Cosserat continuum theory

By implementing the magneto-Cosserat continuum theory in FEM scheme, it is possible to simulate magnetic alignment in powders with magnetic anisotropy during compaction in an applied magnetic field. We thereby analyze the relationship between the direction of magnetic field and compaction process, such as cross compaction and parallel compaction. The rotation of the axes is thus calculated at various conditions of applied magnetic field. In our previous study, Cosserat constant /spl beta/ was given arbitrarily. The method to calculate the Cosserat constant /spl beta/ is proposed and its effect on the rotation of the particles' easy axes is discussed. As a demonstration of the developed method, magnetic particles' alignment during compaction in an applied magnetic field is simulated. The calculated results are in good agreement with the experimental ones.

Comparison of photoresponsive drain conduction and gate leakage in n-channel pseudomorphic HEMT and MESFET under electro-optical stimulations

Photoresponsive drain conduction and gate leakage characteristics of n-channel PHEMT and MESFET are comparatively reported as a function of electro optical stimulation (V/sub GS/, V/sub DS/, P/sub opt/; /spl lambda/=830 nm). Both in PHEMT and MESFET, a strong nonlinearity of drain photoresponse (R) with P/sub opt/ was observed and can be modeled empirically as R=/spl kappa/P/sub opt//sup -/spl beta// (/spl kappa//sub PHEMT//spl Gt//spl kappa//sub MESFET/) where model parameters /spl kappa/ and /spl beta/ accommodate differences in device/epitaxial structures and electrical biases. Gate leakage current was linearly increasing with P/sub opt/ while it was independent of V/sub DS/ in both PHEMT and MESFET. However, I/sub G/ was a strong function of V/sub GS/ in PHEMT while it was almost independent in MESFET due to suppressed modulation of photoresponsive depletion width with heavy channel doping. Photonic gate response (R/sub pG/), on the other hand, was observed to be constant in MESFET while it was a strong function of electrical bias in PHEMT.

Effects of O/sub 2/ rapid thermal annealing on the microstructural properties and reliability of RF-sputtered Ta/sub 2/O/sub 5/ films

The microstructural properties and reliability of sputtered Ta/sub 2/O/sub 5/ films treated by various temperatures of rapid thermal annealing (RTA) in O/sub 2/ atmosphere have been systematically investigated. Analytical results revealed that whenever the RTA temperature was >650/spl deg/C, the noncrystallinity of as-grown Ta/sub 2/O/sub 5/ film would be effectively improved from an amorphous phase to the /spl beta/-Ta/sub 2/O/sub 5/ phase. Leakage current measurement indicated that leakage current decreases with increasing annealing temperature in a low RTA temperature range (/spl les/650/spl deg/C) and, contrarily, increases with increasing annealing temperature in a high RTA temperature range (650 to 950/spl deg/C). The former result was asserted in that reducing pinholes and oxygen vacancies played key factors. However, the latter result arose due to significant Si diffusion into the Ta/sub 2/O/sub 5/ film, causing a leaky transition layer distributed along the grain boundary to form the leakage path. Finally, the time-dependent dielectric-breakdown experiments revealed that 950/spl deg/C O/sub 2/ RTA treated Ta/sub 2/O/sub 5/ film possessed the superior crystallinity, creating less interfacial hole trap states at the junction of Ta/sub 2/O/sub 5//Si and exhibiting the best long-term reliability.

Reliability sampling plans for lognormal distribution, based on progressively-censored samples

This paper presents reliability sampling plans for the lognormal distribution based on progressively censored samples. In constructing these sampling plans, large-sample approximations to the best linear unbiased estimators of the location and scale parameters are used. For some selected progressive censoring schemes, reliability sampling plans are tabulated for p/sub /spl alpha// and p/sub /spl beta// to match MIL-STD-105. While in general, variable-sampling plans require smaller sample size when compared with attribute-sampling plans, the ordinary complete and right-censored life test experiments are special cases of the progressively censored experiment. Hence, the progressively censored reliability sampling plans in this paper are widely applicable. General application of the procedure is discussed, and two examples are provided.

The capacity-cost function of discrete additive noise channels with and without feedback

We consider modulo-q additive noise channels, where the noise process is a stationary irreducible and aperiodic Markov chain of order k. We begin by investigating the capacity-cost function (C(/spl beta/)) of such additive-noise channels without feedback. We establish a tight upper bound to (C(/spl beta/)) which holds for general (not necessarily Markovian) stationary q-ary noise processes. This bound constitutes the counterpart of the Wyner-Ziv lower bound to the rate-distortion function of stationary sources with memory. We also provide two simple lower bounds to C(/spl beta/) which along with the upper bound can be easily calculated using the Blahut algorithm for the computation of channel capacity. Numerical results indicate that these bounds form a tight envelope on C(/spl beta/). We next examine the effect of output feedback on the capacity-cost function of these channels and establish a lower bound to the capacity-cost function with feedback (C/sub FB/(/spl beta/)). We show (both analytically and numerically) that for a particular feedback encoding strategy and a class of Markov noise sources, the lower bound to C/sub FB/(/spl beta/) is strictly greater than C(/spl beta/). This demonstrates that feedback can increase the capacity-cost function of discrete channels with memory.

Insulin release at the molecular level: Metabolic-electrophysiological modeling of the pancreatic beta-cells

The role of pancreatic beta-cells is fundamental in the control endocrine system, maintaining the blood glucose homeostais in a physiological regime, via the glucose-induced release of insulin. An increasing amount of detailed experimental evidences at the cellular and molecular biology levels have been collected on the key factors determining the insulin release by the pancreatic beta-cells. The direct transposition of such experimental data into accurate mathematical descriptions might contribute to considerably clarify the impact of each cellular component on the global glucose metabolism. Under these perspectives, we model and computer-simulate the stimulus-secretion coupling in beta-cells by describing four interacting cellular subsystems, consisting in the glucose transport and metabolism, the excitable electrophysiological behavior, the dynamics of the intracellular calcium ions, and the exocytosis of granules containing insulin. We explicit the molecular nature of each subsystem, expressing the mutual relationships and the feedbacks that determine the metabolic-electrophysiological behavior of an isolated beta-cell. Finally, we discuss the simulation results of the behavior of isolated beta-cells as well as of population of electrically coupled beta-cells in Langerhans islets, under physiological and pathological conditions, including noninsulin-dependent diabetes mellitus (NIDDM) and hyperinsulinemic hypoglycaemia (PHHI).

Wiring space and length estimation in two-dimensional arrays

We propose a new global routing area estimation approach for high-performance very large scale integration and multichip modules (MCMs). The objective is to route nets with minimum density of global cells, producing a four-bend routing for each two-terminal net. We propose an approximate upper bound on global cell d/sub H//spl les/2d/sub 0/log(m/(2d/sub 0/)), in an m/spl times/m two-dimensional array, where d/sub 0/ is the estimated lower-bound density. The total wirelength is (2/spl alpha/+/spl beta/)4m/sup 2/d/sub 0//3, where /spl alpha/+/spl beta/=1 and /spl alpha/ is the percentage of diagonal combinations and /spl beta/ is the percentage of adjacent combinations of nets. If /spl alpha//spl les//spl beta/ (this assumption holds since a good placement minimizes the longer wires), then the total wirelength is at most 2m/sup 2/d/sub 0/. By counting on the adjacent and diagonal combinations separately in the cost function, d/sub R//spl les/[4d/sub 0//3]/spl times/log([m/(4d/sub 0//3)]). We verified that the bound obtained are realistic in the worst case. A solution to this problem can be used for quick estimation of necessary wiring space (for standard cell array designs) and difficulty of routing (for gate array designs) in the early design planning stage.

Median-based cyclic polyspectrum estimation

In this paper, median-based estimation methods for the cyclic polyspectrum are proposed. The algorithms do not require a priori knowledge of the /spl beta/ submanifolds, that is, they do not require the knowledge of all of the lower order cycle frequencies of the time-series available for the estimation of the cyclic polyspectrum. Therefore, such methods are particularly useful when the cyclostationarity of the signals under consideration is not completely known. The proposed estimators converge to the theoretical values of the cyclic polyspectrum when the collect time approaches infinity and the spectral resolution becomes infinitesimal. Furthermore, their accuracy is very nearly the same as that of the usual time- and frequency-smoothed cyclic periodogram methods that use a priori knowledge of lower-order cycle frequencies to avoid the /spl beta/ submanifolds.

Optical intensity modulator based on a novel electrooptic polymer incorporating high /spl mu//spl beta/ chromophore

We have synthesized a novel electrooptic (EO) polymer based on a high /spl mu//spl beta/ chromophore incorporating tricyanobutadiene acceptors. A crosslinked polyurethane network was also adopted to enhance its thermal stability. In order to find the optimum poling condition for the polymer, the influence of the electric poling profile on optical characteristics such as the EO effect, thermal stability, and damage was investigated. Then a high-speed intensity modulator using the EO polymer was designed and fabricated. The measured half-wave voltage V/sub /spl pi// was 4.5 V at the wavelength of 1.31 /spl mu/m. Accordingly, the achieved EO coefficient r/sub 33/ was as high as 25 pm/V, and the thermal stability of the poled polymer was as high as 95/spl deg/C. Finally, the modulator was successfully operated up to 40 GHz.

On the asymptotic normality of hierarchical mixtures-of-experts for generalized linear models

In the class of hierarchical mixtures-of-experts (HME) models, in the exponential family with generalized linear mean of the form /spl psi/(/spl alpha/+x/sup T//spl beta/) are mixed, according to a set of local weights called the functions depending on the predictor x. Here /spl psi/(/spl middot/) is the inverse link function. We provide regularity conditions on the experts and on the gating under which the maximum-likelihood method in the large sample limit produces a consistent and asymptotically normal estimator of the mean response. The regularity conditions are validated for Poisson, gamma, normal, and binomial experts.