Time (sec) 



<io-i* 



10-14 



10-1- 



10-8 



10- 



10-5-10-* 



10-3-10-2 



S. DILLI AND J. M. GREEN 

 Table 1. Chronology of events in radiolysis 



Events 



Primary particles and secondary electrons traverse a molecule 



Molecular vibration and fast molecular dissociations 



Electron capture in molecules in the liquid phase 



Radical moves one space in diffusion in a liquid 



Dielectric relaxation in a liquid — Collision time for thermal electrons 



in a gas at 1 atm — Gas molecule collision time at 1 atm 



Lifetime for radiation de-excitation of an excited singlet state (allowed 



transition) — 



* Forward reaction complete in a y-ray spur in water 



Thermal electron (0 025 cV) captured in a gas (assuming capture 



probability per collision is 1 in 1000) 



♦Forward reaction complete in an a-track in water 



Reaction of radical with a solute in molar concentration 



Lifetime for radiation of triplet state (forbidden transition) 



* The ' forward ' reaction in water is : HoO • 



fragments with their environment inside spurs will be over in perhaps 10"^ 

 sec; any fragments which have reacted will diffuse away from the spurs and 

 react mainly with free electrons and molecules in the medium; interaction 

 between tracks will occur by reaction between diffusing species; such inter- 

 actions may occur in about 10"^ sec in liquids. 



An important point to notice is that, in liquids, charged fragments will be 

 neutralized very quickly by electron capture before any structural changes 

 or reactions can occur. This is not the case in the gas phase, where electron 

 capture may take as long as 10"^ sec and charged fragments are therefore 

 capable of undergoing many reactive collisions before neutralization. For 

 this reason, ion-molecule reactions, which have recently been found able to 

 explain certain gas-phase radiolyses'^, do not seem likely to play a part in 

 the liquid phase. Again, the chance of reformation of the parent molecule 

 by recombination of initial fragments is much higher in the liquid (a 'cage' 

 effect) than it is in the gas phase and radical-radical two-centre reactions 

 are generally more likely in the liquid. 



SYNTHESIS WITH INDUSTRIAL POSSIBILITIES 



Although the range of products, which can be produced on the laboratory 

 scale, is large, serious consideration has been given to a few general processes 

 only. The economic feasibility of a given radiation synthesis is decided by all 

 the coinplex considerations of the chemical industry. The basic factors, 

 which determine the attractiveness of a proposal for an industrial chemical 

 process, are the capital cost of the plant, the operating cost and the return 

 on capital. When these factors are assessed in detail there are four conditions 

 under which radiation may be economically used for chemical syntheses: 



{a) Where the product obtained is of high value and cannot be obtained 

 readily by orthodox methods — A good example on the pilot-plant scale of 

 this category is the recently developed radiation synthesis of complex 

 molecules labelled with tritium i^. The soft /S radiation from ^H itself is used 

 to cause a primary fragmentation of molecules in the gas phase with subse- 



105 



