A Dynamic Programming Approach to Adaptive Fractionation

We conduct a theoretical study of various solution methods for the adaptive fractionation problem. The two messages of this paper are: (i) dynamic programming (DP) is a useful framework for adaptive radiation therapy, particularly adaptive fractionation, because it allows us to assess how close to optimal different methods are, and (ii) heuristic methods proposed in this paper are near-optimal, and therefore, can be used to evaluate the best possible benefit of using an adaptive fraction size. The essence of adaptive fractionation is to increase the fraction size when the tumor and organ-at-risk (OAR) are far apart (a "favorable" anatomy) and to decrease the fraction size when they are close together. Given that a fixed prescribed dose must be delivered to the tumor over the course of the treatment, such an approach results in a lower cumulative dose to the OAR when compared to that resulting from standard fractionation. We first establish a benchmark by using the DP algorithm to solve the problem exactly. In this case, we characterize the structure of an optimal policy, which provides guidance for our choice of heuristics. We develop two intuitive, numerically near-optimal heuristic policies, which could be used for more complex, high-dimensional problems. Furthermore, one of the heuristics requires only a statistic of the motion probability distribution, making it a reasonable method for use in a realistic setting. Numerically, we find that the amount of decrease in dose to the OAR can vary significantly (5 - 85%) depending on the amount of motion in the anatomy, the number of fractions, and the range of fraction sizes allowed. In general, the decrease in dose to the OAR is more pronounced when: (i) we have a high probability of large tumor-OAR distances, (ii) we use many fractions (as in a hyper-fractionated setting), and (iii) we allow large daily fraction size deviations.Comment: 17 pages, 4 figures, 1 tabl

Quantum shot-noise at local tunneling contacts on mesoscopic multiprobe conductors

New experiments that measure the low-frequency shot-noise spectrum at local tunneling contacts on mesoscopic structures are proposed. The current fluctuation spectrum at a single tunneling tip is determined by local partial densities of states. The current-correlation spectrum between two tunneling tips is sensitive to non-diagonal density of states elements which are expressed in terms of products of scattering states of the conductor. Thus such an experiment permits to investigate correlations of electronic wave functions. We present specific results for a clean wire with a single barrier and for metallic diffusive conductors.Comment: 4 pages REVTeX, 2 figure

Effect of incoherent scattering on shot noise correlations in the quantum Hall regime

We investigate the effect of incoherent scattering in a Hanbury Brown and Twiss situation with electrons in edge states of a three-terminal conductor submitted to a strong perpendicular magnetic field. The modelization of incoherent scattering is performed by introducing an additional voltage probe through which the current is kept equal to zero which causes voltage fluctuations at this probe. It is shown that inelastic scattering can lead in this framework to positive correlations, whereas correlations remain always negative for quasi-elastic scattering.Comment: 5 pages latex, 5 eps figure

Motor Vehicles: Are they emerging threats to Lake Victoria and its environment?

Lake Victoria and its basin supports more than 30 million people, while its fishes are exported the world over. This second largest fresh water body is however experiencing stress due to eutrophication, sedimentation, declining levels and more recently the motor vehicle sector. This contribution examines the general pollution from motor vehicle and gives an in-depth analysis of motor vehicle washing along the lakeshore. The results indicate the water samples from the motor vehicle washing and urban runoff points to be slightly acidic (i.e., average pH of 6.7) and average Total at these points. The conductivity for the motor vehicle washing points averaged at 150 S/cm, while the urban runoffs point was more varied ranging from below 150 S/cm to over 400 S/cm (average 301 S/cm). A positive correlation coefficient of more than 0.7 is obtained between the total daily count of vehicles and each of the water quality parameter tested. This signifies a strong correlation between motor vehicle related activities and the pollution of the lake. In general, the motor vehicle industry is found to have a noticeable negativeeffect on the lake

Observation of discrete time-crystalline order in a disordered dipolar many-body system

Understanding quantum dynamics away from equilibrium is an outstanding challenge in the modern physical sciences. It is well known that out-of-equilibrium systems can display a rich array of phenomena, ranging from self-organized synchronization to dynamical phase transitions. More recently, advances in the controlled manipulation of isolated many-body systems have enabled detailed studies of non-equilibrium phases in strongly interacting quantum matter. As a particularly striking example, the interplay of periodic driving, disorder, and strong interactions has recently been predicted to result in exotic "time-crystalline" phases, which spontaneously break the discrete time-translation symmetry of the underlying drive. Here, we report the experimental observation of such discrete time-crystalline order in a driven, disordered ensemble of $\sim 10^6$ dipolar spin impurities in diamond at room-temperature. We observe long-lived temporal correlations at integer multiples of the fundamental driving period, experimentally identify the phase boundary and find that the temporal order is protected by strong interactions; this order is remarkably stable against perturbations, even in the presence of slow thermalization. Our work opens the door to exploring dynamical phases of matter and controlling interacting, disordered many-body systems.Comment: 6 + 3 pages, 4 figure

Local densities, distribution functions, and wave function correlations for spatially resolved shot noise at nanocontacts

We consider a current-carrying, phase-coherent multi-probe conductor to which a small tunneling contact is attached. We treat the conductor and the tunneling contact as a phase-coherent entity and use a Green's function formulation of the scattering approach. We show that the average current and the current fluctuations at the tunneling contact are determined by an effective local non-equilibrium distribution function. This function characterizes the distribution of charge-carriers (or quasi-particles) inside the conductor. It is an exact quantum-mechanical expression and contains the phase-coherence of the particles via local partial densities of states, called injectivities. The distribution function is analyzed for different systems in the zero-temperature limit as well as at finite temperature. Furthermore, we investigate in detail the correlations of the currents measured at two different contacts of a four-probe sample, where two of the probes are only weakly coupled contacts. In particular, we show that the correlations of the currents are at zero-temperature given by spatially non-diagonal injectivities and emissivities. These non-diagonal densities are sensitive to correlations of wave functions and the phase of the wave functions. We consider ballistic conductors and metallic diffusive conductors. We also analyze the Aharonov-Bohm oscillations in the shot noise correlations of a conductor which in the absence of the nano-contacts exhibits no flux-sensitivity in the conductance.Comment: 17 pages, 8 figure

Place branding of seaports in the Middle East

This paper analyses seaports’ brand personalities as a means of understanding similarities and differences of these important locations and their relationship with their host place image. Drawing upon Aaker’s (J Mark Res 34:347–356, 1997) brand personality construct, the study presents lexical analysis from the websites of nine seaports in the Middle East. Each seaport’s website is content analysed, and the brand personality is measured using Aaker’s (1997) framework and Opoku’s (Licentiate Thesis, Lulea University of Technology, ISSN, 1402-1757, 2005) dictionary of synonyms. Findings show that seaports have developed a level of isomorphism upon particular dimensions of brand image; however, the findings also show the most distinctive seaports were linking their seaport to their place brand. In particular, the findings show only the Port of Jebel Ali has a clear and distinctive brand personality and to a lesser extent the Ports of Sohar, Shahid Rajee and Khor Fakkan. The research has important management implications of branding for public diplomacy and demonstrates seaport brand positioning in relation to place branding, used to inform public communication and marketing