Possibility of X-ray spectral diagnostics of a plasma in the tracks of fast multicharged ions
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MA DIAGNOSTICS
Possibility of X-ray Spectral Diagnostics of a Plasma in the Tracks of Fast Multicharged Ions I. Yu. Skobelev Joint Institute for High Temperatures, Russian Academy of Sciences, Izhorskaya ul. 13/19, Moscow, 125412 Russia Received July 9, 2007
Abstract—A plasma model of the relaxation of a medium within the tracks of heavy ions in condensed matter is proposed that is based on solving time-dependent radiative collisional kinetic equations with the initial condition corresponding to a medium’s state described by the classical model of multiple ionization of the target atoms by the field of fast multicharged ions. It is shown that the plasma model allows one to describe X-ray spectra recorded in the interaction of ion beams with condensed targets. An X-ray spectral method for plasma diagnostics is proposed that is based on the plasma model. The results obtained can also be used to study the initial stage of the formation of defects in solid bodies under the action of individual fast heavy ions. PACS numbers: 52.70.La DOI: 10.1134/S1063780X08030082
1. INTRODUCTION The interaction of individual fast heavy ions with condensed matter leads to the formation of tracks in solids (see, e.g., [1, 2]) and is accompanied by the emission of X-ray spectral lines due to the decay of autoionization states of multicharged ions of the target material. The emitted lines carry information on the state of matter in the interaction region and can be used to diagnose the matter there. Up to now, the recorded spectra were interpreted based on the classical model of the relaxation of an isolated ion (see, e.g., [3–6]). The essence of this model, which will be referred to as the atomic model, consists in the following. It is assumed that, in the interaction of an incident individual ion with a target atom, the Coulomb field of the ion multiply ionizes the atom to produce excited ions with the spectroscopic symbol Z. Some of these ions have at least one vacancy in the K shell of electrons. The K vacancy is then filled due to a radiative transition from higher (L, M, …) shells, so an X-ray photon is emitted. The main assumption of the atomic model is that, in the interaction with the field of an incident ion, the formation of an excited (autoionization) ionic state Z can result in the emission of only one spectral line due to a radiative transition in the excited ion. In the atomic model, the intensities of the observed X-ray lines are simply proportional to the product of the cross sections for the corresponding multiple ionization processes and the branching factors AZ/(AZ + ΓZ), where AZ is the radiative transition probability and ΓZ is the autoionization probability. Hence, in the atomic model, the observed X-ray spectra are completely governed by the interaction of an incident ion with an atom of the medium and contain
no information on the state of the medium after the passage of the exciting particle; i.e., the atomic model fully ignores the fact that, in the plasmalike state resulting from the initial ionization of matt
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