Multiple ionization of liquid water by swift heavy ions

M.E. GALASSI; P.D. FAINSTEIN; B. GERVAIS; R.D. RIVAROLA

Belfast, Irlanda

Conferencia; 13th International Conference on the Physics of Highly Charged Ions (HCI),; 2006

The processes of multiple ionization (MI) and fragmentation ofwater molecules by ion impact are present in many subfields ofphysics, chemistry, and radiobiology. For example, the mostimportant processes occurring during irradiation of biologicaltissue with fast heavy ions are the electronic ionization andexcitation of the target molecules. In particular, ionization ofwater molecules produces an indirect effect on the DNA moleculesdue to formation of free radicals (water radiolysis), whichprovokes biological damage. Thus, an accurate theoreticalcomputation of single and multiple ionization cross sections isvery important for modelling purposes. Previous calculations of MI cross sections (MICS) of water byheavy ions were made using a Monte Carlo approach, including oneadjustable parameter determined by comparison with experimentaldata [1]. However, the computation of this parameter is moredifficult when experimental results are not available. In the present work we determine theoretical MICS of liquid waterin the framework of the Independent Particle Model (IPM). So, thecomputation of single particle probabilities as a function of theimpact parameter is necessary. To obtain these probabilities, wehave used two different methods: the Exponential Model (EM) andthe Continuum Distorted Wave - Eikonal Initial State (CDW-EIS)approximation [2]. Within the Exponential Model, it is assumedthat the molecular orbitals present a spherical form (centered inthe oxygen atom). The single-electron emission probability foreach shell are described by exponential functions$p_i( ho)=p_{i0}e^{- ho/r_i}$, where $p_{i0}$ is the ionizationprobability from the shell extit{i} at $ ho = 0$ and $r_i$ isthe radial expectation value. The $p_{i0}$ values are chosen asadjustable parameters to reproduce CDW-EIS cross sections. Theradii are calculated taking into account the binding energies ofthe corresponding orbitals for liquid water. Details on themolecular CDW-EIS approximation are described in reference [2]. Wehave checked that the use of any of these methods does not changestrongly the proportion of multiple ionization. When doubleionization is considered it is shown that ejection from twodifferent orbitals dominates the reaction, being largelypreferable than emission of the two outer-shell electrons. Theoretical MICS were employed as input data in a Monte Carlo codeto study water radiolysis by ion impact [3,4]. A very gooddescription of experimental radiolytical yields was obtained.