P. Antoranz

SIMULTANEOUS MULTIWAVELENGTH OBSERVATIONS OF THE BLAZAR 1ES 1959+650 AT A LOW TeV FLUX

We present the resultsfroma multiwavelength campaignonthe TeVblazar1ES 1959+650, performed in2006May. Data from the optical, UV, soft- and hard-X-ray, and very high energy (VHE) gamma-ray (E > 100 GeV) bands were obtained with the Suzaku and Swift satellites, the MAGIC telescope, and other ground-based facilities. The source spectral energy distribution (SED), derived from Suzaku and MAGIC observations at the end of 2006 May, shows the usual double hump shape, with the synchrotron peak at a higher flux level than the Compton peak. With respect to historicalvalues,duringourcampaignthe sourceexhibiteda relatively highstateinX-raysand optical, while inthe VHEbanditwasatoneof thelowestlevelsofarrecorded.Wealsomonitoredthesourceforfluxspectralvariability onatimewindowof 10daysintheoptical-UVandX-raybandsand7daysintheVHEband.Thesourcevariesmorein the X-ray than in the optical band, with the 2Y10 keV X-ray flux varying by a factor of � 2. The synchrotron peak is locatedintheX-raybandandmovestohigherenergiesasthesourcegetsbrighter,withtheX-rayfluxesaboveitvarying more rapidly than the X-ray fluxes at lower energies. The variability behavior observed in the X-ray band cannot be

High-frequency modeling of GaN/SiC blue light-emitting diodes

We report on this work a model to accurately predict the electrical behavior of double-heterostructure GaN/SiC blue light-emitting diodes up to microwave frequencies. A procedure to extract the series resistance (R-s) from the reflection coefficient is suggested. This procedure offers the advantage of using measurements without any bias current and therefore the obtained values of R-s are influenced neither by the device heating nor by inaccuracies in the calculation of the ideality factor. The junction capacitance and conductance measured in the range 1 kHz-10 MHz shows two different relaxation mechanisms, and the total capacitance can be fitted very accurately to a double Lorentzian function. Blue light-emitting diodes and lasers based on gallium nitride (GaN) semiconductor compounds represent one of the most important breakthroughs in electronics and optoelectronics of recent years. The combination of silicon carbide (SiC) and GaN has recently enabled low-cost blue-emitting diodes to be introduced in industry

Testbench to characterize pixels of the Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescope

Photomultipliers have a wide range of applications, from nuclear medicine to nuclear physics. In particular, they are commonly used in high energy physics and astrophysics. The Major Atmospheric Gamma-ray Imaging Cherenkov (MAGIC) telescope is an example of their applications. This work presents a test system for the characterization of the transient response of the photomultipliers used as fast light detectors in the telescope. The bias of the photomultiplier is described, taking into account its effects in the response to fast light pulses. Two methods are used to generate these pulses: 1. a specifically designed pulse generator, and 2. a plastic scintillator. For the former case, we also calculate the anode charge we expect in this setup to cross-check it with the actual measurements. (c) 2006 Society of Photo-Optical Instrumentation Engineers.

Design, modelling and testing of electro-optical transmitters for the central pixel of the MAGIC telescope camera.

In this work we have built an electro-optical system for the transmission of low frequency analogue signals through optical fibre. The main goal was to achieve minimum pulse distortion with maximum dynamic range. The system has been used in the framework of the MAGIC telescope experiment for the transmission of the analogue output from a photo-multiplier dedicated to optical observation of astrophysical objects, in particular pulsars. The received signal polarizes an infrared LED (λ=850 nm), which converts the pulse into an optical analogue pulse. The electro-optical pulse is transmitted by means of a multi-mode optic fibre and finally amplified and filtered by the optical receiver. The whole system has been tested using a pulse generator resembling the type of pulsed signal we expect from pulsars, that is with period of about tens of milli-seconds and few milli-seconds wide. The system was calibrated in order to: a) obtain a fixed relation between the received pulse and the final data and b) enhance the dynamic range and low distortion. In what follows, we show the behaviour of the optical transmitter under different pulse shapes, amplitude and frequencies up to several hundred Hz. The electro-optical system has been mounted on the MAGIC telescope and tested successfully with the observation of the pulsed optical signal from the Crab pulsar.

A simple blue light pulse generator with GaN/SiC light emitting diodes for the time response testing of PMTs

A simple and cost-effective integrated synthesizer of fast light pulses has been designed, analyzed and tested for the characterization of the time response of photo-multipliers (PMT). This synthesizer consists of an integrated pulse generator based on Schmidt Trigger Inverters, a broadband matching network and a high speed LED. It enables the generation of pulses as short as less than 10 ns with variable pulse width, amplitude and repetition frequency. In order to accurately know the shape of the pulses applied to the PMT under test, a circuital model of the LED has been developed and verified at frequencies up to 2 GHz. This model accounts for the nonlinear behavior of the LED capacitance as well as the package parasitics. The influence of the mismatch at the different frequency components of the synthesized pulse has been investigated. The pulse transmitter has been used to test the time response of MAGIC telescope pixels.

Toxicity assessment of biological suspensions using the dielectric impedance spectroscopy technique

Abstract Purpose: This article studies the variation of the electromagnetic parameters of a suspension of zebrafish (Danio rerio) embryos to assess its potential applications to toxicological and biomedical research areas. Materials and methods: For this purpose, the dielectric impedance spectroscopy technique is applied to a modified coaxial line enclosing the biological suspension to be characterized in the frequency range from 100 kHz to 100 MHz. The electrical parameters of the suspension under test were obtained by fitting the impedance spectra to the resulted from the simulation of the test fixture using finite elements (FE). Results: Variation of the complex permittivity of the suspensions makes possible to identify viable and non-viable embryos after a toxic exposure, as well as different stages during the blastula period of embryonic development of the zebrafish. Conclusions: The approach presented here, combining experimental and simulation techniques, may provide a basis for a non-invasive method to assess toxicity in any biological suspension.

Broadband dielectric characterization of zebrafish embryo suspensions using the impedance spectroscopy technique

This paper presents the electrical characterization of biological suspensions using the impedance spectroscopy technique combined with simulation methods based on finite elements. For this purpose, a modified coaxial line is used as a test fixture enclosing the biological suspension to be characterized in the frequency range from 100 kHz to 100 MHz. In the study we have used a suspension of Zebrafish (Danio rerio) embryos due to its potential applications to biomedical and toxicological research areas. The electrical parameters of the different suspensions under test are obtained by fitting the experimental impedance spectra to the spectra obtained from simulation of the model of the test fixture using finite elements (FE). The impedance spectra obtained for the suspensions allow the identification of different stages in blastula period of embryonic development of the zebrafish as well as of viable and non-viable embryos. The good results obtained with the combined experimental and simulation techniques may provide a basis for a non-invasive method to monitor the dielectric changes in any biological suspension.

Strategies for shortening the output pulse of silicon photomultipliers

In this work, three strategies for shortening the output pulse of a silicon photomultiplier (SiPM) are reported. The first strategy is passive filtering, where band-pass filtering removes the lowest frequency components in the signal, getting a noticeable reduction in pulse width (a compression ratio of 10: 1 was obtained). In the second place, a reflectometric scheme is proposed where the amplified signal coming from the SiPM is injected into a signal splitter with one of its stubs connected to a short-circuited stub. In the last strategy, the reflectometric part is replaced by an analog subtractor circuit. In this approach, a signal splitter with stubs of different lengths is used. All solutions provide good compression ratios, up to 10: 1. Best pulses obtained are single narrow peaks, with width below 10 ns, preserving the photonic modulation and with good pseudo-Gaussian shape, single polarity and low ringing. The potential of pulse shortening for improving the capability of the detector to resolve single photons is demonstrated by mean of single photon counting patterns. The detection error probability is reduced in one order of magnitude when shortening is used for conditioning the output photosignal.

Limitations of the transmission line theory in the simulation of ultra thin wire conductivities with coaxial resonators

This work aims to illustrate how the transmission line theory may yield to strong errors in predicting the reflection coefficient at the input of a short - circuited coaxial transmission line when the inner conductor is either thin enough or lossy enough to have a measurable DC ohmic resistance. These situations are found in the measurement of wire conductivities at RF frequencies with coaxial resonators. The transmission line theory is based on the assumption that the current flowing through a conductor is located at its surface. However, this hypothesis is not accurate in a number of practical situations. In order to identify some of them, we have made a comparison between the simulations and measurements of the reflection coefficient at the input of a short circuited coaxial resonator where the centre conductor is a thin wire made of either a good conductor (high purity platinum) or a poor one (carbon fibre). The transmission line theory is not able to predict accurately the conductivity of any of these wires at frequencies below 300-500 MHz, where there is a strong frequency dependence of the resistance used in the modelling of the distributed conductor losses. In order to account for this effect, a model is proposed, which is based on modifying the distributed resistance, which account for conductor losses. This model is computationally effortless and was verified with measurements of different materials and with different thicknesses.