R \nK^Hi(M.or;if:.\T, mi-.chamsm w tuf c;i:i,i.ri.AR i^rA'F.i. 



The duration of the sjiark could prol^ably be reduced to as Httlc as 1 to 2 

 [isec. Prehniniarv studies* of the kinetics of the fenous sulj^hate oxidation 

 reveal, as exj^ected, a large part of the total yield of ferric iron to have 

 occurred during the post-irradiation period. The maximum dose available 

 at the time in a single pulse from the accelerator used for these studies was 

 3,000 rad. It was evident that considerably larger doses were desirable for an 

 extensive study of the kinetics of radiation-induced reactions in solutions. 

 The work is being continued with the larger pulses available from the elec- 

 tron pulse generator described above. 



When electrons of energy greater than about 0-4 MeV enter water, they 

 srenerate the so-called Cerenkov radiation. This radiation extends as a 

 continuous spectrum throughout the visible and u.v. and is emitted in a 

 forward direction at an angle to the direction of movement of the electron 

 which depends on the electron energy. This light has been ingeniously used 

 by Boag to look for evidence of radicals formed by the decomposition of 

 water. For this purpose the quadrupole focusing magnets referred to earlier 

 were employed so as to concentrate the electron beam proceeding from the 

 accelerator into a fine pencil before it emerged through the aluminium 

 window. Cerenkov radiation is rather weak, and with the optical arrange- 

 ment employed about 50 pulses were needed in order to obtain adequate 

 blackening in a Hilger Intermediate Spectograph (E486). A spectrum has 

 been observed which consisted of groups of absorption bands extending 

 throughout the visible and u.v. to 2300 A at least. The spacing of individual 

 maxima was some 50 cm~^ while groups of bands were separated by about 

 500 cm^^. The spectrum has not been identified. 



Electron spin resonance spectroscopy 



Free radicals were discussed more than 30 years ago as possible inter- 

 mediates in gas-phase photochemical reactions, and the existence of a free 

 radical is generally considered to have been first convincingly demonstrated 

 by Paneth and Hofeditz^^ j^ the course of their study of the decomposition 

 of tetramethyl lead. A little later free radicals were recognized to play an 

 important role as intermediates in ordinary temperature liquid-phase 

 reactions. In 1944 Weiss^'^ put forward the view that radiation-induced 

 reactions in aqueous solution proceed from the initial formation of the two 

 radicals H« and OH«, and from that time the radiation chemistry of liquid 

 systems has been written almost entirely in terms of free-radical reactions. 

 Few, however, of the free radicals which have been postulated have actually 

 been identified independently by physical means. 



The possibility of such identification resides in the fact that a free radical 

 is intrinsically paramagnetic by virtue of the magnetic moment of the un- 

 paired electrons which correspond physically with the chemical free valency. 

 In the absence of any other quantized magnetic moments with which the 

 moment of the free electron may be coupled, a single unpaired electron has 

 a magnetic moment of v 3 Bohr magnetons. Thus, since the molar suscepti- 

 bility xm =X'^h is given by: 



Xm =^' (ao + M-M/3^7^) 

 * Carried out in collaboration with Professor Rotblat, using electron pulses from the 

 St. Bartholomew's Hospital 15 MeV Linear Accelerator. 



292 



