The simulated dust grain targets to be used in the cosmic dust experiment are grown at UCL in the Electronic Engineering department. Being deposited from a 99% H 2 + 1% CH 4 plasma, the sp 2 :sp 3 bonding ratio of the sample can be controlled and hence a wide range of amorphous carbon surfaces are available. The first targets to be used will be polycrystalline diamond and HOPG (highly oriented pyrolytic graphite) because of their known regular lattices; this keeps theoretical models of the target simple so that the first experimental results can be rigorously analysed. Later targets will be truly amorphous (diamond-like) carbons; the surfaces of these targets will be analysed using a variety of surface science techniques before and after H atom irradiation to carefully monitor and control the experiments.
The carbon targets will be mounted on a commercial cryogenic cold-head, which will cool the simulated dust grains to 10K. The useable surface areas of the targets are 1.5 cm 2 , and they are ~100 m m thick. Before the targets may be used in the experiment they must be flashed to 1300K for a few seconds to drive off any contaminants that remain from their production process. This will be done in situ on the cold-head within the vacuum system to prevent recontamination upon cooling. Since the cold-head sustains considerable damage at temperatures above ~330K, it must be protected from the heat whilst the target is being flashed. To do this, we exploit a peculiar property of sapphire (or ruby). When cold, the thermal conductivity of sapphire is high. Therefore, if we have a layer of sapphire between the cold head and the target, we will be able to cool the target to 10K. When hot, the thermal conductivity of sapphire is low. Therefore, if we heat the target to 1300K (by passing a current through a resistive tungsten plate connected to it), the sapphire will prevent any heat from penetrating through to the cold head. By using such an array of diamond-like-carbon and sapphire and ruby blocks, our target mounting system on the cold head is truly the crown-jewel of the experiment!
Once we have performed experiments using a variety of clean amorphous carbon targets, we plan to proceed to targets coated by icy mantles of different compositions. Silicate targets may also be used. Eventually, it is thought possible to monitor the formation of polyatomic molecules such as H 2 CO by irradiating CO frozen onto an icy dust grain with H atoms. This will allow a myriad of different experiments to be performed to provide essential data for use in astrochemical models.
