Towards cross sections for positrons scattered by uracil molecules

We report measurements of relative total and positronium (Ps) formation cross sections as well as results from using the Ps annihilation ratio spectroscopy (PsARS) method~[1] for low energy ($<30$eV) positrons ($e^+$s) scattered by uracil (an RNA nucleobase) molecules. In our experiments, $e^+$ beam passes through the scattering cell with heated uracil powder. These measurements are the first step in an experimental study of positron interactions with biological molecules. The interest in $e^+$ interactions with such molecules is boosted by possible relevance to positron emission tomography (PET), an important diagnostic tool in medicine. It has been recognized that Ps formation is a ``subtle aspect of PET''[2] and it has been found that the Ps yield in water and organic solvents ranges from 38 to 70\%[3]. A study of $e^+$ scattering on uracil molecules is a beginning in a detailed study of a variety of possible interactions of $e^+$s with a live organism. Our preliminary results indicate large relative importance of Ps formation compared with other scattering channels at energies close to the threshold of Ps formation. [1]W.E. Kauppila et al., Phys. Rev. Lett. 93, 113401 (2004). [2]M.D. Harpen, Med. Phys. 31 (1), 57-61 (2004). [3]P. Castellaz et al., J. Nucl. Radiochem. Sci. 3, R1-R7 (2002).