The WASA 4π multidetector system, aimed at investigating light meson production in light ion collisions and η meson rare decays at the CELSIUS storage ring in Uppsala is presented. A detailed description of the design, together with the anticipated and achieved performance parameters are given.

A pellet target produces micro-spheres of different materials, which are used as an internal target for nuclear and particle physics studies. We will describe the pellet hydrogen behavior by means of fluid dynamics and thermodynamics. In particular one aim is to theoretically understand the cooling effect in order to find an effective method to optimize the working conditions of a pellet target. During the droplet formation the evaporative cooling is best described by a multi-droplet diffusioncontrolled model, while in vacuum, the evaporation follows the (revised) HertzKnudsen formula. Experimental observations compared with calculations clearly indicated the presence of supercooling, the effect of which is discussed as well.

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