The reactor, made of Pyrex, is equipped with a vacuum stopcock leading to the analysis system. Two other openings allow the connection with a secondary vacuum pump (Turbomolecular pump, Balzers), and a water vapor flow system that isnít yet operational, but will permit the study of interactions between photodegradation products of solid molecules and the main constituent of the coma : H2O. At about half the depth of the reactor, a stainless steel grid can be settled as a support for the studied organic molecules. A vacuum better than 10-4 can be reached in the reactor. Two UV lamps can be connected : a Xenon lamp (147 nm) and an Hydrogen flow lamp (122 nm).