In order to study the charge transfer in harpoon reactions we measure the differential cross section of Na + I Na++I-, using a molecular beam set-up . We have build an oven to create iodine atoms, to serve as target for the sodium atoms in the collision.
Dissociated iodine target gas is formed in the oven (4) drawn in Fig.1 in more detail. Actually, the iodine oven is a hybrid between a collision chamber and a secondary beam.
|Figure MESO-m3a-I-F1: iodine oven Schematic drawing on scale of the dissociation oven.|
Each of the tantalum cylinders A and B is formed by a double winding of 0.01 mm sheet, closed by spotwelding while the two cylinders are connected at the bottom by a tantalum ring. The electrical resistance of the two-cylinder system is about 0.15 at high temperature. The cylinders are heated directly by applying a voltage difference of a few volts. The tantalum gas supply C is thus heated indirectly, while the heat contact with the iodine gas is enlarged by a platinum multi-channel array at the end of the pipe. The tantalum heat shield D and the stainless steel heat shield E restrict the heat loss while the upper side of the outer shield is cooled by water. By supplying a power of 300~Watt, the temperature of the inner tantalum cylinder near the collision center can be about 1900C. This temperature is high enough to form an intense beam of dissociated iodine or bromine atoms.
The iodine flow through the oven is restricted because of the limited pumping capacity of the liquid-air cooling trap [(15) in Fig.1 ], placed close to the oven. Moreover, a very low pressure of recombined iodine molecules in the vacuum chamber is required to avoid serious distortion of the atom--atom collision measurement, especially because the total cross section for chemi-ionization of Na + I is only 20 percent of the cross section of Na + I2 . At a limited iodine flow through C, the iodine pressure in the collision region is maximal in this closed-type oven.
At working conditions the iodine pressure in the collision region is estimated to be about $3x 10-4~torr, while the temperature of the oven was about 1200 \circC to ensure complete dissociation.
By closing the inner cylinder with an end cap F and by restricting other leakages, the pressure of dissociated iodine is enlarged by a factor of four as compared to the pressure in the beam of an open source. Because the oven rotates simultaneously with the detector, the slots in the tantalum cylinders and heat shields to the detector are only 7x 2.5 mm2. The entrance slots of the sodium beam are 7x 8 mm2, which enables the measurements of scattered particles up to a laboratory angle of 22 degrees. Other apertures in the heat shields raise the pumping speed for recombined halogen gas between the cylinders.