L. H. GRAY 



taken by irradiating a stack of thin slips of Perspex in a broad beam is shown 

 in Figior 3. It can be seen that when the machine is operated at 1 -5 MeV 

 a layer of Perspex 3 mm thick, or about 0-4 g/cm^ of any similar material, 

 lying about the peak of the curve, received a dose lying within + 10 per cent 

 of the mean. The maximum dose which can be delivered in a single pulse 

 close to the window, when using the electron beam without external focusing, 

 is of the order of 10^ rad and the mass of material receiving this dose is about 

 0-25 g. This corresponds to a maximum dose rate during the 2 [j-sec pulse of 

 some 5 x 10^° rad/sec. For certain purposes it is desirable to have an even 

 more concentrated electron beam and auxiliary quadrupole focusing 

 magnets have therefore been constructed, following a design due to M. C. 

 Crowley-Milling. Two permanent magnet quadrupole lenses, each of which 

 brings the beam to a sharp line focus, are combined at the appropriate 

 spacing to give a reasonably good point focus. With this arrangement, about 

 50 per cent of the beam current can be made to pass through a circle of 2 mm 

 diameter at the exit window and the dose in a single pulse at this point is then 

 of the order of 10^ rad to a mass of about 0-01 g. '*'' 



Analytical Systems 

 Light and u.v. spectroscopy 



Spectrographic examination of the living cell is rendered difficult by the 

 large amount of unspecific scattering which usually results from differences 

 in refractive index between the cell and the medium in which it is suspended. 

 Nevertheless, certain pigment molecules having relatively high molecular 

 extinction coefficients have been extensively studied in yeast, bacteria, and 

 ascites tumour cells. Quantitative studies of the proportion of each member 

 of the cytochrome enzyme chain which is in the oxidized and reduced state at 

 a given time, have been carried out by the use of the very beautiful double 

 beam method of Britton Chance'^''. It is evident that by means of single 

 microsecond pulses it is quite possible to irradiate the cell when the cyto- 

 chrome system is in known intermediate Redox conditions between those 

 corresponding to aeration and to complete anoxia. It is not impossible that 

 molecular species generated by the action of ionizing radiation might be 

 studied spectroscopically by methods similar to those employed by Britton 

 Chance, and that short-lived species might be studied by flash spectroscopy. 

 So far, our own experience of flash spectroscopy is limited to a pi-eliminary 

 investigation of the kinetics of the oxidation of ferrous sulphate, about which 

 a good deal is already known from the analysis of steady states. For this 

 purpose. Dr. Boag^^ has developed a light source which goes a long way to 

 fulfilling most of the desirable characteristics, namely that the duration of the 

 flash should be of the order of a few microseconds, that the spectrum should 

 be continuous extending well into the u.v., that the source should have high 

 intrinsic brilliance for efficient utilization of the light in an optical system, 

 and adequate output for photographic recording of a spectrum with a single 

 flash. In Boag's design an energy of about 1 J is stored in a titanite 

 (specific inductive capacity s.i.c. ~ 1,000) capacitor, charged to 12 kV. 

 When connected through a trigger spark gap in air to uranium electrodes in 

 a water filled metal envelope with a quartz window, an underwater spark is 

 formed which lasts about 10 [xsec and gives a strong continuous spectrum. 



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