Effusive molecular gas jet target for molecule fragmentation experiments

Compete text of publication follows. Collisions between high energy ions and biological molecules have attracted an increasing attention in recent years. Such collisions often results in molecule fragmentation. Reliable knowledge of these interactions is essential in radiotherapy. As a first step it is important to study the fragmentation of water molecules, since water is the main constituent of tissues in living organs. For measurements with molecules, we developed a special effusive gas jet target in Atomki. It consists of a vertically movable tube through a Wilson-seal and a nozzle, which is easily exchangeable. The nozzle is located at the end of the target tube (see Fig. 1). We designed two types of nozzles: a set of single capillary nozzles with 0,9 - 1,3 mm diameter for the gas flow, and a special nozzle, where the gas flows through the tiny tubes of a microcapillary array (a piece of a micro-channel plate, MCP). All the nozzles are developed for working in the molecular flow range for providing a well collimated effusive gas beam with a target density in the 7 x 1012 cm3 to 1 x 1014 cm3 range, which allows to measure reasonably high count rates, while a low background pressure in the chamber. The molecular flow range is ensured by setting the geometry of the gas flow elements and the pressures. At the nozzle, the type of the flow depends on the main free path of the molecules, and on the length and diameter of the capillary nozzle. The main free path can be controlled by a wad in the target tube and fine tuned by changing the pressure at the entrance of the target system. The wad is a capillary with adjustable length, and an inner wire with smaller diameter. Moreover, there is an other puffer. The pressure at the entrance is regulated by a control valve, which connects the clean water container with a steam pressure of cca. 20 mbar and the input buffer of the gas target with regulated pressure. Acknowledgements. This work was supported by the OTKA Grant No. 73703. I express my thanks to Zoltan Pintye, engineer, for his help in constructing the target system.