William H. Bell

Optorsim: A Grid Simulator for Studying Dynamic Data Replication Strategies

Computational grids process large, computationally intensive problems on small data sets. In contrast, data grids process large computational problems that in turn require evaluating, mining and producing large amounts of data. Replication, creating geographically disparate identical copies of data, is regarded as one of the major optimization techniques for reducing data access costs. In this paper, several replication algorithms are discussed. These algorithms were studied using the Grid simulator: OptorSim. OptorSim provides a modular framework within which optimization strategies can be studied under different Grid configurations. The goal is to explore the stability and transient behaviour of selected optimization techniques. We detail the design and implementation of OptorSim and analyze various replication algorithms based on different Grid workloads.

Evaluation of an economy-based file replication strategy for a data grid

Optimising the use of Grid resources is critical for users to effectively exploit a Data Grid. Data replication is considered a major technique for reducing data access cost to Grid jobs. This paper evaluates a novel replication strategy, based on an economic model, that optimises both the selection of replicas for running jobs and the dynamic creation of replicas in Grid sites. In our model, optimisation agents are located on Grid sites and use an auction protocol for selecting the optimal replica of a data file and a prediction function to make informed decisions about local data replication. We evaluate our replication strategy with OptorSim, a Data Grid simulator developed by the authors. The experiments show that our proposed strategy results in a notable improvement over traditional replication strategies in a Grid environment.

Simulation of Dynamic Grid Replication Strategies in OptorSim

Computational Grids normally deal with large computationally intensive problems on small data sets. In contrast, Data Grids mostly deal with large computational problems that in turn require evaluating and mining large amounts of data. Replication is regarded as one of the major optimisation techniques for providing fast data access.Within this paper, several replication algorithms are studied. This is achieved using the Grid simulator: OptorSim. OptorSim provides a modular framework within which optimisation strategies can be studied under different Grid configurations. The goal is to explore the stability and transient behaviour of selected optimisation techniques.

Charge collection efficiency of irradiated silicon detector operated at cryogenic temperatures

Abstract The charge collection efficiency (CCE) of heavily irradiated silicon diode detectors was investigated at temperatures between 77 and 200xa0K. The CCE was found to depend on the radiation dose, bias voltage value and history, temperature, and bias current generated by light. The detector irradiated to the highest fluence 2×10 15 xa0n/cm 2 yields a MIP signal of at least 15000xa0e − both at 250xa0V forward bias voltage, and at 250xa0V reverse bias voltage in the presence of a light-generated current. The “Lazarus effect” was thus shown to extend to fluences at least ten times higher than was previously studied.

Replica Management in the European DataGrid Project

Within the European DataGrid project, Work Package 2 has designed and implemented a set of integrated replica management services for use by data intensive scientific applications. These services, based on the web services model, enable movement and replication of data at high speed from one geographical site to another, management of distributed replicated data, optimization of access to data, and the provision of a metadata management tool. In this paper we describe the architecture and implementation of these services and evaluate their performance under demanding Grid conditions.

Charge collection efficiency and resolution of an irradiated double-sided silicon microstrip detector operated at cryogenic temperatures

Abstract This paper presents results on the measurement of the cluster shapes, resolution and charge collection efficiency of a double-sided silicon microstrip detector after irradiation with 24xa0GeV protons to a fluence of 3.5×10 14 xa0p/cm 2 and operated at cryogenic temperatures. An empirical model is presented which describes the expected cluster shapes as a function of depletion depth, and is shown to agree with the data. It is observed that the clusters on the p-side broaden if the detector is under-depleted, leading to a degradation of resolution and efficiency. The model is used to make predictions for detector types envisaged for the LHC experiments. The results also show that at cryogenic temperature the charge collection efficiency varies depending on the operating conditions of the detector and can reach values of 100% at unexpectedly low bias voltage. By analysing the cluster shapes it is shown that these variations are due to changes in depletion depth. This phenomenon, known as the “Lazarus effect”, can be related to similar recent observations on diode behaviour.

Review on the development of cryogenic silicon detectors

Abstract In this paper, we report on the performance of heavily irradiated silicon detectors operated at cryogenic temperatures. The results discussed here show that cryogenic operation indeed represents a reliable method to increase the radiation tolerance of standard silicon detectors by more than one order of magnitude. In particular, a 400xa0μm thick “double-p” silicon detector irradiated up to 1×10 15 xa0n/cm 2 delivers a mip signal of about 27xa0000 electrons when operated at 130xa0K and 500xa0V bias. The position resolution of an irradiated microstrip detector, and “in situ” irradiation of a pad detector during operation in the cold are also discussed.

Charge collection efficiency of an irradiated cryogenic double-p silicon detector

Abstract We present results on the measurement of the charge collection efficiency of a p + /n/p + silicon detector irradiated to 1×10 15 xa0n/cm 2 , operated in the temperature range between 80 and 200xa0K. For comparison, measurements obtained with a standard silicon diode (p + /n/n + ), irradiated to the same fluence, are also presented. Both detectors show a dramatic increase of the CCE when operated at temperatures around 130xa0K. The double-p detector shows a higher CCE regardless of the applied bias and temperature, besides being symmetric with respect to the polarity of the bias voltage. At 130xa0K and 500xa0V applied bias the double-p detector shows a CCE of 80%, an unprecedented result for a silicon detector irradiated to such a high dose.

Radiation hard cryogenic silicon detectors

It has been recently observed that heavily irradiated silicon detectors, no longer functional at room temperature, resuscitate when operated at temperatures below 130 K. This is often referred to as the Lazarus effect. The results presented here show that cryogenic operation represents a new and reliable solution to the problem of radiation tolerance of silicon detectors.

Temperature dependence of the charge collection efficiency in heavily irradiated silicon detectors

Abstract Two silicon diode detectors, Al/n + /n/p + /Al, were exposed to fluences of 1.19×10 14 and 2.23×10 15 equivalent 1 MeV neutrons/cm 2 , respectively. After this exposure the detectors were stored at room temperatures for 2 yr (1.19×10 14 ) and six months (2.23×10 15 ). During this time they were thermally cycled around 4.2 K and room temperature a number of times in order to make measurements. The charge collection efficiency is measured to be (at 77 K) 100% for the less severely irradiated detector and 50% for the detector exposed to high levels of radiation. The same results apply to operation at 4.2 K, while no recovery is observed at 195 K. By examining the signal response of the irradiated detectors to α particles, it is shown that some of the radiation damage after reverse annealing is in the form of electron and hole traps, which are either weakly, or not at all, temperature dependent.