The exposure of the hybrid detector of the Pierre Auger Observatory

The Pierre Auger Observatory is a detector for ultra-high energy cosmic rays. It consists of a surface array to measure secondary particles at ground level and a fluorescence detector to measure the development of air showers in the atmosphere above the array. The "hybrid" detection mode combines the information from the two subsystems. We describe the determination of the hybrid exposure for events observed by the fluorescence telescopes in coincidence with at least one water-Cherenkov detector of the surface array. A detailed knowledge of the time dependence of the detection operations is crucial for an accurate evaluation of the exposure. We discuss the relevance of monitoring data collected during operations, such as the status of the fluorescence detector, background light and atmospheric conditions, that are used in both simulation and reconstruction.Comment: Paper accepted by Astroparticle Physic

Anomalous behaviour of the low-temperature heat capacity for KCl crystals with heavy univalent impurities

De faibles concentrations d'impuretés lourdes et monovalentes Cs+ et I- ont pour conséquence une augmentation de la chaleur spécifique de KCl à basse température. Ce comportement anormal a été expliqué à l'aide d'une théorie utilisant une fonction de Green, pour de faibles concentrations. Les calculs ont été effectués grâce à un modèle de perturbation localisée qui inclut un changement de masse sur le site de l'impureté aussi bien qu'un changement des constantes de forces radiales avec les plus proches voisins. Un accord raisonnable est obtenu avec les données expérimentales. La contribution du défaut de masse est de 80 % du changement de la chaleur spécifique ; la contribution restante est due aux changements de constante de force. Les contributions les plus importantes au changement de chaleur spécifique sont due aux modes impairs de symétrie F1u. La partie réelle du dénominateur de résonance dans la représentation irréductible F1u donne une fréquence quasi localisée. La valeur trouvée de la fréquence de résonance dans le cas de KCl : I- est voisine de celle obtenue précédemment par l'étude de la conductivité thermique du même système.The effect of low concentrations of heavy univalent Cs+ and I- impurities on the low-temperature heat capacity of KCl gives an enhancement in the heat capacity. This anomalous behaviour has been explained on the basis of a low concentration Green's function theory. A localized perturbation model which includes mass change at the impurity site as well as changes in the nearest neighbour radial force constants has been employed in performing the calculations. Reasonable agreement with the experimental data is obtained. The mass defect contribution is found to be 80 % of the changed specific heat and the remaining contribution is due to force constant changes. The main contributions to the changed specific heat is caused by odd parity F1u-symmetry modes. The real part of the resonance denominator in F1u-irreducible representation gives a quasi localized frequency. The present value of the resonant frequency in case of KCl : I- is seen to be similar to those obtained earlier in the study of thermal conductivity of the same system

Magnetohydrodynamic rotating flow of a generalized burgers' fluid in a porous medium with hall current

This study concentrates on the unsteady magnetohydrodynamics (MHD) rotating flow of an incompressible generalized Burgers's fluid past a suddenly moved plate through a porous medium. Modified Darcy's law for generalized Burgers's fluid in a rotating frame has been used to model the governing flow problem. The closed form solution of the governing flow problem has been obtained by employing Laplace transform technique. The integral appearing in the inverse Laplace transform has been evaluated numerically. The influence of various parameters on the velocity profile has been delineated through several graphs and discussed in detail. It was found that the fluid is decelerated with increasing Hartmann number M and porosity parameter K. However, for large Hall parameter m, the real part of velocity decreases and the imaginary part of velocity increases