POTENTIALITY OF THE COMPOSITE FULLEREN BASED CARBON FILMS AS THE STRIPPER FOILS FOR TANDEM ACCELERATORS

But there is one very impotent disadvantage. Carbon foils have a limited lifetime due to radiation damage. The rate of damage is proportional to ion beam intensity and ion mass. The factors that limit the lifetime of the foil are: (i) increasing of the film thickness and (ii) tightening of the films in the irradiation area [1] that leads to rupturing of the stripper. As will be shown be¬low all these factors are the consequence of irradiation stimulated structure reconstruction in carbon films.

2 EXPERIMENT

Till end of the 70-th of the last century the main technologies for preparation of free-standing carbon films were arc and electron beam evaporation of graph¬ite. But in 1979 the collaboration of the groups from The Daresbury Laboratory and Atom Energy Agency had resulted in development of the chemical vapour deposition (CVD) technology for preparation of hydro-genated carbon stripper foils. [1-3]. The lifetime of those foils was improved by the factor of order. At the same time the first systematic investigation of the irra¬diation effect on the structure reconstruction in such hydrogenated amorphous carbon films was performed [4, 5]. The conclusion was made that under ion bom¬bardment the thermally activated graphitization process (ordering of the graphite-like structure) takes place. The authors have explained improved radiation resistance of such films by modification of the short-range order with ion bombardment during the deposition process.

Within the 80-th in the Munich technical university researchers made a very comprehensive and complex study of the influence of the short-range order peculi¬arities (such as distribution of the orientation of the graphite-like nanocrystallites, GLN) on the carbon stripper foil radiation resistance [6, 7]. The crystalline structure of graphite is extremely anisotropic. Carbon atoms are very closely packed in basic planes (graphen sheets) by strong covalent bonds. But graphen sheets are bonded together by weak Van-der-Vaals forces. Intera¬tomic distance in graphen sheets is about 0.14 nm while the distance between graphen sheet is more than 0.3 nm. The level of anisotropy was estimated by the ratio of the intensity of the electron diffraction reflections I002/I100 They had proposed and substantiated the following mechanism of the stripper foils destruction under heavy ions bombardment. Usual deposition methods such as arc and electron beam evaporation results in formation of films with structure having preferable orientation of the graphen sheets in GLN parallel to the film plane. Under ion bombardment the dissipation of the ion en¬ergy due to the stopping power leads to heating the car¬bon material in the irradiated region and hence to the graphitization process. Due to existence of the initial texture the graphitized regions also have a preferable orientation. The accumulation of interstitials between graphene sheets and diffusion of vacancies to the boundary of the nanocrystallits lead to the well-known effect of expansion along the c-axis and compression in the base plane.[8]. It is obvious that carbon films having the texture of the c-axis of the GLN perpendicular to the film plain will be compressed and eventually crack. When the orientation of the GLN is isotropic then the effect of the anisotropic changes of the volume of the GLN is compensated.

They had developed the laser ablation technologyfor deposition carbon films and showed that these films have a nearly isotropic structure and improved radiation resistance in comparison with CVD films [6].

In our previous paper it was showed that the level of the isotropy of the amorphous carbon structure could be successively varied by intensity of the ion bombardment during the deposition process [9].

But the nature has given us an almost entirely iso¬tropic carbon material. This is fulleren. Fulleren is a general name of the wide family of the carbon closed molecules that had been revealed about ten years ago [10]. The most stable are C60 and C70 molecules having 60 and 70 carbon atoms respectively (Fig. 1). In solid state this molecules exist in the form of cubic molecular crystals (fullerits). This structure is very isotropic in comparison with graphite-like structure of the usual amorphous carbon films. It is very attractive idea to es¬timate potentiality of a such material for stripper foils. But on the first steps one runs into some hard problems.

 

1. Fullerens have a low sublimation temperature (about 500OC). It makes very simple the evaporation process but it is known that the temperature of the irradiated zone of the stripper foil could reach 1000°C and so pure fullerene films could not oper¬ate at such temperature.
2. Despite a great progress in the technology of fullerene production in large quantities the price of the commercially available fullerens is still very high (about 35-45 dollars per gram for pure C60 powder).

 

 

First problem could be avoided in two ways. It possible to fabricate the is "sandwich" structure: amor¬phous carbon/fulleren/amorphous carbon (Fig. 2). Due to their large size (diameter about 0.7 nm) fulleren molecules could not diffuse to the surface of the film through amorphous carbon and sublimate. To reduce the prices of the films it is possible to use fulleren enriched carbon soot as an evaporated fulleren material.

 

 

Another possible structure is the composite films composed of fulleren agglomerates embeded in an amorphous carbon matrix. Such structure could be pre¬pared by code position of amorphous carbon (arc or electron evaporation) and fullerens (sublimation). But as things turned out the more simple method is available.

We have investigated the structure of the films de¬posited by thermal evaporation of the pure C60 powder in dependence of the deposition rate. Pure C60 powder was sublimated from the tantalum effusion cell at the temperature of about 600OC. One silicon substrate was located at a distance of 4 cm from the tantalum cell. Another substrate was located at a distance of 10 cm from the evaporator. Such a deposition configuration allows us to produce samples deposited under the same conditions but other deposition rate. Deposition rates were about 0.5 |im/min and 0.1 |im/min for the different substrates.