A New Simplified System for the Evaluation of BNCT Pharmaceuticals

Abstract

Recent studies (see, for example, [1, 2]) show that large negative ion densities exist in plasma processing discharges, including those of weakly electronegative gases such as SiH4 and CF4. Also, there is strong evidence that the negative ions could be the precursors for particulate formation in processing discharges; see [1], and also other references in [3].Even though it is now well established that large concentrations of negative ions exist in processing discharges, and that they play a crucial role in such discharges, the source of such high negative ion densities has not been clarified. In particular, gases like SiH4 and CH4, which are commonly used in processing discharges, attach electrons only weakly in their ground electronic states, see the references in [3, 4]. Due to the lack of an alternative mechanism, the origin of large negative ion densities in such weakly electronegative gases has been frequently attributed to electron attachment to radicals (molecular fragments) or other byproducts produced in the discharge. This hypothesis had not been tested in direct electron attachment measurements.During the past few years we have observed enhanced negative ion formation via electron attachment to laser-excited, high-Rydberg (HR) states of SiH4 [3, 5], CH4 [4], and C2F2C12 [6], (The HR states are electronically-excited states located close to the ionization potential (IP ) of a moIecule, and have characteristically large cross sections for various processes as well as long lifetimes [7].) These studies clearly showed that many orders of magnitude enhancement in electron attachment rate constants are associated with the HR states compared to the respective ground electronic states. In addition, in the cases of SiH4 [5] and C2F2C12 [6], it was experimentally shown that electron attachment to any laser-produced radicals was much weaker compared to the HR states. In the case of C2F2C12 [6], electron attachment rate constant for the C2F2CI radical was about an order of magnitude larger than the ground 'state of C2F2C12; yet the electron attachment rate constant for the HR states was estimated to be at least four orders of magnitude larger compared to the ground state.. The extremeIy large electron attachment rate constants associated with the HR states of molecules are due to the large polarizabilit ies associated with such states. The polarization potential responsible for the electron capture, Vpol , is given by,