Thermalization of energetic fluorine-18 atoms for gas-phase reaction with propene

Abstract

The gas-phase reactions of radioactive /sup 18/F atoms with propene have been studied over the pressure range from 500 to 4000 Torr in mixtures with (SF/sub 6/)/(C/sub 3/H/sub 6/) mole ratios varying from 100 to 1000. The SF/sub 6/ molecules serve both as the /sup 19/F target for the /sup 19/F(n,2n)/sup 18/F fast neutron reaction and as the moderator for removing the excess kinetic energy imparted to the /sup 18/F atoms by nuclear recoil. Moderator/substrate ratios > 100 are large enough that hot atom reactions have been suppressed to yields well below 1%, with almost all /sup 18/F atoms reacting as thermal atoms. Three thermal reactions are dominant with propene, with yields of about 32% for terminal addition to form CH/sub 3/CHCH/sub 2//sup 18/F*; 23% for central addition to form CH/sub 3/CH/sup 18/FCH/sub 2/*; and 45% for abstraction of H to form H/sup 18/F. Terminal addition is favored over central addition for thermal /sup 18/F atoms by a factor of 1.35 +/- 0.05. The competitive thermal reactions of /sup 18/F have also been studied with mixtures of propene and HI in these highly moderated systems. Stabilized C/sub 3/H/sub 6//sup 18/F radicals are assayed as C/sub 3/H/sub 7//sup 18/F after scavengingmore » by reaction with HI, and the relative total reactivity of propene versus HI can be evaluated from the diminution in the yields of these products with increasing (HI)/(C/sub 3/H/sub 6/) mole ratio. The relative reactivity of HI versus CH/sub 3/==CH/sub 2/ toward thermal fluorine atoms is 0.36 +/- 0.03, indicating an absolute reaction rate for fluorine atoms with propene of (1.8 +/- 0.3) x 10/sup -10/ cm/sup 3/ molecule/sup -1/ s/sup -1/.« less