Generation of Swift Cluster Ions

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What is a cluster ion?

“Cluster” means “A group of things of the same type that grow or appear close together“, for instance a cluster of grapes, in a dictionary. Here, we define a cluster ion as an ion consisting of more than two atoms. A carbon-60 ion consisting of sixty carbon atoms, soccer-ball-structure is one of typical clusters. Some cluster ions up to MeV energy region can be available using electrostatic accelerators (3-MV tandem accelerator and 400-kV ion implanter) in TIARA. When a swift cluster ion collides with a solid target, they deposit much higher energy at a surface of a target comparing to mono ion. It causes, for example, the non-liner increasing of secondary particles emitted from the surface. The mechanism of interaction between MeV energy cluster ions and a matter is an interest subject of radiation physics. In addition, those increasing secondary particles can be applied for high-sensitive surface analysis, and to give a high energy density to the surface can be applied for surface modification of materials.

Recent topic

Studies on material science and biotechnology for various ion beams with an energy ranging from 30 keV to hundreds MeV are in progress in Takasaki advanced radiation research institute. Understanding of the interaction between an incident ion and a target is indispensable for selection of suitable experimental conditions, i.e., an incident ion species, its energy, and a target, and analyses of experimental result. The interaction for irradiation with mono-atomic ion has been investigated since the early days, and some models have succeeded in explaining most of experimental results. However, various strange phenomena out of such models have been observed for irradiation with a swift cluster ion consisting of a number of atoms: for example, secondary-ion or secondary-electron emission.

When a swift cluster ion impinges on a matter, a lot of atoms are brought into a very small volume simultaneously; thus, vicinal fragment ions interact with each other even after the dissociation of the cluster. In this process, a spatial pattern of the cluster constituents could affect their interactions with a matter. The effect of the spatial pattern on charge-state distribution of fragment ions arising from the dissociation of a cluster passing through a thin foil was investigated to make a new model of the interaction between a cluster ion and a matter.

A new apparatus shown in fig. 1 was developed for the investigation. Ions emerging from a thin carbon foil deflected depending on their charge-state by an electrostatic deflector placed between the foil and a microchannel plate (MCP). A two-dimensional pattern of the deflected ions is observed as luminance points on a fluorescent screen equipped with the MCP. The apparatus enables us to measure a charge-state of each fragment ion of a cluster ion and their spatial pattern just after emerging from the foil simultaneously, as shown in fig. 2. The charge-state distribution of fragment ions of a 3-MeV C₃⁺ was measured for the different pattern, i.e., a linear-chain pattern and an equilateral triangular pattern after emerging the foil.

Fig. 1　Apparatus for simultaneous measurement of charge-state and spatial pattern of a cluster ion

At impingement of a swift cluster ion on a thin foil, its valance electrons are stripped away in a short time, and a rapid explosion process is initiated subsequently by a Coulomb repulsion, which provides a snap-shot of the position of the nuclei in the incident cluster ion. A charge-state of each fragment ion is determined by the position on the MCP on the basis of the deflected angle relevant to the charge depending mainly on an electrostatic field of the deflection plates.

Fig. 2 Two patterns of the correlation of the deflected points and the non-deflection points

The charge-state of each deflected point is identified by the relation between its position and charge distribution on the MCP, as shown in (a). The instance of the pattern (b) and (c) shows for almost equilateral triangular pattern and the linear-chain pattern with almost regular intervals, respectively. The blue round symbol shows original pattern before deflection.

The results are summarized in Table 1. The cluster average charge obtained for the liner-chain pattern is larger than that for the triangular pattern, and the average charge of two edge-position ions in a liner-chain pattern is larger than that of the middle-position ion. The theoretical average charge of the fragment ions of a C₃⁺ ion was calculated considering the effect of nuclear charge of the other neighboring fragments on the binding energy of the valence electrons and the spatial pattern of the fragments; a repulsive Coulomb force among the fragment ions, a wake force induced by each fragment etc. were taken into account in the calculation. Obtained average charges are qualitatively consistent with the experimental results as shown in Table 1.

Although the measurement of the charge-state distribution of fragment ions have been done by other researchers so far, this is the first time of its measurement by classifying the special pattern of the fragment ions. The interaction between a swift cluster ion and a matter correlates with the spatial pattern of the constituents in a cluster, which should be demonstrated as one of the most characteristic of the cluster-matter interaction.

Table 1　Average charges of the fragment ions arising from the dissociation of a C₃⁺ cluster ion passing through a thin carbon foil, estimated experimentally and theoretically.

(a) Comparison of cluster structure   (b) Comparison of atomic-position in a linear-chain pattern