Non-exponential Components Research Articles

Redundancy allocation problem of phased-mission system with non-exponential components and mixed redundancy strategy

The redundancy allocation problem (RAP) has been widely studied, which aims to identify the optimal design to achieve the best system indicators, such as system reliability or availability, under certain constraints. This problem has been studied well in the single phased mission systems. But many practical systems, like the manmade satellites or spacecraft, perform a series of tasks in multiple, consecutive, non-overlapping time durations (phases). For these multi-phased systems, dependencies among the phases need to be fully considered. To address this problem, the RAP of the phased mission systems (PMSs) is studied in this paper. Moreover, in order to improve system reliability as much as possible, the mixed redundancy strategy where both active and cold standby components can be simultaneously used in a subsystem is applied. The non-exponential components, which are more practical, are also considered. The Semi-Markov process (SMP) as well as a numerical approximation method are used to deal with the dynamic non-exponential components. Then, an improved Genetic algorithm (GA) is used to determine the types and quantities of active and cold-standby components in each subsystem to optimize the whole system. At last, the propulsion system in a spacecraft is optimized to illustrate the proposed method.

Reliability assessment of multi-state phased mission system with non-repairable multi-state components

• A MMDD algorithm for PMS and a PMS-MMDD model are proposed to model the non-repairable multi-state phased-mission system. • A Markov renewal equation-based method is proposed to evaluate system probabilities with non-exponential distributions. • The comparison to the Monte Carlo simulation method illustrate the computation accuracy and efficiency of the proposed methods. • An attitude and orbit control system in spacecraft is studied as a practical example to illustrate the proposed methods. Phased mission systems (PMSs) like satellites and spacecraft perform their functions over non-overlapping mission periods, called phases. One of the challenges in assessing reliability of PMSs comes from considering the s -dependence among phases, and the consideration on the multi-state behavior of components and systems makes the reliability analysis even more difficult. To effectively address this problem, a multi-state multivalued decision diagram algorithm for PMS and a multi-state multi-valued decision diagram model for phased mission system (PMS-MMDD) method is developed for the reliability modelling of non-repairable multi-state components. Based on the Semi-Markov process, a Markov renewal equation-based method is developed to deal with non-exponential multi-state components and a numerical method, the trapezoidal integration method, is used to compute the complex integrals in the path probability evaluation. A case study of a multi-state attitude and orbit control system in a spacecraft is analyzed to illustrate the proposed PMS-MMDD model and the Markov renewal equation-based evaluation method. The accuracy and computation efficiency of the proposed method are verified by the Monte Carlo simulation method.

Reliability analysis of phased mission system with non-exponential and partially repairable components

Phased mission systems (PMSs) have wide applications in engineering practices, especially in aerospace industry such as man-made satellite and spacecraft. To achieve high reliability in a PMS, certain critical parts in the system are designed to have a redundant architecture, such as cold standby (structural or functional). State-space models such as Markov processes have been widely used in previous studies to evaluate the reliabilities of these systems. But in practice, many real systems consist of mechanical components or mechatronics whose lifetime follow non-exponential distributions like the Weibull distribution. In this type of system, the Markov process is not capable of modeling the system behavior. In this paper, the SMP (Semi-Markov Process) is applied to solve the problem that the components’ lifetime in dynamic systems follows non-exponential distributions. An approximation algorithm for the SMP is proposed to assess the reliability of the PMSs consisting of non-exponential components. Furthermore, the accuracy and calculation efficiency of the approximation algorithm are explored. At last, the reliability assessment of a complex multi-phased altitude and orbit control system (AOCS) in a man-made satellite is presented to illustrate the method.

Optimization redundancy allocation problem with nonexponential repairable components using simulation approach and artificial neural network

AbstractRedundancy allocation is one of the adopted approaches that is used by system designers to improve the performance of systems. In this article, a new model and a novel‐solving method are provided to address the nonexponential redundancy allocation problem in series‐parallel systems with repairable components based on optimization via simulation approach and artificial neural network technique. Despite the previous researches, in this model the failure and repair times of the each component were considered to have nonnegative exponential distributions. This assumption makes the model closer to the reality where most of used components have greater chance to face a breakdown in comparison to new ones. The main aim of this research is the optimization of mean time to the first failure of the system via allocating the best redundant components for each subsystem. Since this objective function of the problem could not be explicitly mentioned, the simulation technique and artificial neural network were applied to model the problem, and different experimental designs were produced using design of experiment methods. To solve the problem, some metaheuristic algorithms were integrated with the simulation method. Several experiments were performed to test the proposed approach, and as the results show, the proposed approach is much more real than previous models, and also the near optimum solutions are promising.

Solving the redundancy allocation problem of k-out-of-n with non-exponential repairable components using optimization via simulation approach

In this article, a new model and a novel solving method are provided to address the non-exponential redundancy allocation problem in series-parallel k-out-of-n systems with repairable components based on Optimization Via Simulation (OVS) technique. Despite the previous studies, in this model, the failure and repair times of each component were considered to have non-negative exponential distributions. This assumption makes the model closer to the reality where the majority of used components have greater chance to face a breakdown in comparison to new ones. The main objective of this research is the optimization of Mean Time to the First Failure (MTTFF) of the system via allocating the best redundant components to each subsystem. Since this objective function of the problem could not be explicitly mentioned, the simulation technique was applied to model the problem, and di erent experimental designs were produced using DOE methods. To solve the problem, some meta-Heuristic Algorithms were integrated with the simulation method. Several experiments were carried out to test the proposed approach; as a result, the proposed approach is much more real than previous models, and the near optimum solutions are also promising.

The reproducibility of different metabolic markers for muscle fiber type distributions investigated by functional 31P-MRS during dynamic exercise

PurposeThe objective of the study was to investigate the reproducibility of exercise induced pH-heterogeneity by splitting of the inorganic phosphate (Pi) signal in the corresponding 31P-MRS spectra and to compare results of this approach with other fiber-type related markers, like phosphocreatine/adenosine triphosphate (PCr/ATP) ratio, and PCr-recovery parameters. Material and methodsSubjects (N=3) with different sportive background were tested in 10 test sessions separated by at least 3 days. A MR-compatible pedal ergometer was used to perform the exercise and to induce a pH-based splitting of the Pi-signal in 31P-MR spectra of the medial gastrocnemius muscle. The PCr recovery was analyzed using a non-negative least square algorithm (NNLS) and multi-exponential regression analysis to estimate the number of non-exponential components as well as their amplitude and time constant. The reproducibility of the estimated metabolic marker and the resulting fiber-type distributions between the 10 test sessions were compared. ResultsThe reproducibility (standard deviation between measurements) based on (1) Pi components varied from 2% to 4%, (2) PCr recovery time components varied from 10% to 12% and (3) phosphate concentrations at rest varied from 8% to 11% between test sessions. Due to the sportive activity differences between the 3 subjects were expected in view of fiber type distribution. All estimated markers indicate the highest type I percentage for volunteer 3 medium for volunteer 2 and the lowest for volunteer 1. ConclusionsThe relative high reproducibility of pH dependent Pi components during exercise indicates a high potential of this method to estimate muscle fiber-type distributions in vivo. To make this method usable not only to detect differences in muscle fiber distributions but also to determine individual fiber-type volume contents it is therefore recommended to validate this marker by histological methods and to reveal the effects of muscle fiber recruitments and fiber-type specific Pi concentrations on the intensity ratios between the splitted Pi-components.

Scintillating Metal Organic Frameworks: A New Class of Radiation Detection Materials

AbstractThe detection and identification of subatomic particles is an important scientific problem with implications for medical devices, radiography, biochemical analysis, particle physics, and astrophysics. In addition, the development of efficient detectors of neutrons generated by fissile material is a pressing need for nuclear nonproliferation and counterterrorism efforts. A critical objective in the field of radiation detection is to develop the physical insight necessary to rationally design new scintillation materials for specific applications. However, none of the material types currently used in has sufficient synthetic versatility to exert systematic control over the factors controlling the light output and its dynamics. Here we describe a spectroscopic investigation of two stilbene-based metal-organic frameworks (MOFs) we synthesized, demonstrating that they emit light in response to ionizing radiation, creating the first completely new class of scintillation materials since the advent of plastic scintillators in 1950. This highly novel and unexpected property of MOFs opens a new route to rational design of radiation detection materials, since the spectroscopy shows that both the luminescence spectrum and its timing can be varied by altering the local environment of the chromophore within the MOF. Therefore, the inherent synthetic flexibility of MOFs, which enables both the chromophore structure and its local environment to be systematically varied, suggests that this class of materials can serve as a controlled “nanolaboratory” for probing a broad range of photophysical and radiation detection phenomena. In this presentation we report on the time-dependent fluorescence and radioluminescence of these MOFs and related structures. Multiple decay characteristics have been observed for some materials under study, including fast (ns) exponential and slow (microsecond) non-exponential components. We interpret the results in terms of the electronic states, crystal structures, intermolecular interactions, and transport effects mediating the luminescence. The potential for particle discrimination schemes and large scale production of MOFs and will be discussed.

Branching transport model of NaI(Tl) alkali-halide scintillator

We measure the time dependence of the scintillator light-emission pulses in NaI(Tl) crystals at different temperatures, after activation by gamma rays. We confirm that there are two main nonexponential components to the time decay and find that their amplitude ratio shows Arrhenius temperature dependence. We explain these nonexponential components as arising from two competing mechanisms of carrier transport to the Tl activation levels. The total light output of the NaI(Tl) detectors shows a linear temperature dependence explained by our model.

Scanning ion deep level transient spectroscopy: II. Ion irradiated Au–Si Schottky junctions

Here we introduce a new technique called scanning ion deep level transient spectroscopy (SIDLTS) for the spatial analysis of electrically active defects in devices. In the first part of this paper, a simple theory behind SIDLTS was introduced and factors determining its sensitivity and resolution were discussed. In this paper, we demonstrate the technique on MeV boron implantation induced defects in an Au–Si Schottky junction. SIDLTS measurements are compared with capacitance DLTS measurements over the temperature range, 100–300 K. SIDLTS analyses indicate the presence of two levels, one of which was positively identified as the Ec − 0.23 eV divacancy level. The high sensitivity of SIDLTS is verified and the advantages and limitations of the technique are discussed in light of non-exponential components in the charge transient response. Reasons for several undetected levels are also discussed.

Femtosecond-picosecond fluorescence studies on excited state dynamics of photoactive yellow protein from Ectothiorhodospira halophila

Dynamics of photochemical processes of photoactive yellow protein (PYP) which functions as a photoreceptor for a negative phototactic response of the bacteria has been studied by fs-ps fluorescence up-conversion technique. Fluorescence decay curvs were nonexponential and initial main components have time constants of several hundreds fs to a few ps. The decay processes were interpreted as due to the twisting around vinyl of p-coumaric acid chromophore leading to trans/cis isomerization and were similar to those of bacteriorhodopsin despite the quite different chromophore, except that the reaction is somewhat slower in PYP. Reaction mechanisms were discussed in comparison with those of rhodopsins.

Properties and mechanisms of scintillation in CsGd2F7:Ce3+ and CsY2F7:Ce3+ crystals

The scintillation properties of Ce3+ doped CsGd2F7 and CsY2F7 crystals have been studied at various temperatures. Upon X-ray excitation CsY2F7:5 mol% Ce3+ shows fast ( tau approximately=32 ns) Ce3+ 5d to 4f luminescence with a yield of 1400 photons MeV-1 at room temperature. The most intense Ce3+ luminescence of about 7300 photons MeV-1 is found in CsGd2F7:20 mol% Ce3+. The scintillation decay curve of CsGd2F7:Ce3+ consists of at least two non-exponential components, one of the order of tens of nanoseconds and one in the microsecond region. Both CsY2F7 and CsGd2F7 show a broad STE emission band at around 445 nm, especially at low temperatures. VUV photon excitation measurements at 300 K and low temperatures revealed the presence of two inequivalent types of Ce3+ centres in both CsY2F7 and CsGd2F7. The presence of Gd3+ appears to have a dominating influence on the luminescence characteristics of CsGd2F7:Ce3+, due to the occurrence of both Ce3+ to Gd3+ and Gd3+ to Ce3+ energy transfer as well as energy migration over the Gd3+ sublattice. On the basis of the optical studies, models of the scintillation mechanisms of CsY2F7 and CsGd2F7 are developed which account for the scintillation properties of these crystals.

Energy transfer processes observed in the scintillation decay of BaF2:La

The luminescence decay of BaF2 doped with lanthanum was studied. The decay shows contributions from cross-luminescence (CL) and self-trapped exciton (STE) luminescence. The STE luminescence is quenched in the doped crystals, and shows two non-exponential decay components. Models yielding good fits to the decay data are discussed, as well as the physical relevance of the fitting parameters found.

Photochemical isomerization in the absence of a potential barrier: origin of wavelength-dependent ground-state recovery kinetics

The photophysics and isomerization dynamics in the excited state of the molecule 1,1′-diethyl-4,4′-cyanine have been investigated. Earlier studies have indicated that the isomerization of this molecule is of the barrierless type. As such, it displays some interesting properties of the excited-state relaxation dynamics. In the present work, we investigate in detail the wavelength dependence of the observed ground-state recovery lifetimes, and show that this dependence is due to a dependence on the analyzing wavelength. We believe that the origin of the effect is the admixture of non-exponential fluorescence-decay components into the ground-state recovery signal. This results in apparently faster and non-exponential ground-state recovery kinetics in the part of the absorption spectrum, overlapping with the fluorescence spectrum.