ACTION OF IONIZING RADIATION ON SIMPLi: ORGANIC COMPOUNDS 



energy. However, at this stage the activated or charged molecule tends to 

 decompose into smaller fragments, both charged and uncharged. 



(^) 

 CsH*^ — > c^h; + C3H; (5) 



(^) 



or C5H+, > C,H+ + CH- {6) 



(7) 



Both the above mechanisms probably occur, but it is seen that if these 

 processes do occur, every bond in the pentane molecule is capable of being 

 broken. The individual contribution of the above reactions has still to be 

 determined and although equations (2) to (7) are foi'mally similar, it must 

 be remembered that bond dissociation energies are not the same in charged 

 and uncharged molecules. 



A further complicating factor in these primary studies is that pure com- 

 pounds must be irradiated, and as soon as some of the compound has been 

 decomposed the compound is no longer pure. The effects of these impurities 

 on the radiolysis can be considerable and they will appear as secondary 

 effects. Before any of the products are present in detectable amounts, con- 

 siderable doses are required and this means that secondary effects could be 

 considerable with sources of high intensity. 



A technique that can give direct information about the primary processes 

 is that using scavengers. A scavenger is a compound which will react with 

 molecular fragments to produce recognizable compounds or to remove 

 selected fragments from the system. It is assumed, in some cases without 

 proof, that the scavenger is not itself decomposed in the radiolysis and does 

 not interfere to a marked extent with the radiolysis (for example, by electron 

 capture). Iodine has been used as the scavenger^- ^ in this study, as by the 

 use of gas chromatography the resulting alkyl iodides may be analysed^'^-'. 

 This gives the relative proportions of the free radicals formed. 



The essential process in the use of iodine is the quantitative reaction of 

 iodine with all free radicals present: 



R- + I2 — > Ri + r {8) 



It is assumed that this reaction has zero activation energy® and also that 

 the iodine free radical is incapable of initiating further reaction, combining 

 only with another iodine atom. 



The analysis of the products after the irradiation of an iodine-hydrocarbon 

 mixture is performed by gas chromatography. The sensitivity of the tech- 

 nic|ue is very greatly increased by using iodine labelled with ^^^I at a specific 

 activity of about 100 (jlC per ml. of hydrocarbon liquid. The eluted alkyl 

 iodides are detected by a Geiger-Miiller counter. Most analyses are 



88 



