ULTRAVIOLET SPECTROSCOPIC TECHNIQUE 133 



detector, instead of direct use of a thermal detector, is frequently found 

 advantageous for reproducing the geometry of an irradiation experiment, 

 or for reasons of economy and convenience. 



In ultraviolet, the photochemical decomposition of uranyl oxalate is 

 widely used as an actinometer (Leighton and Forbes, 1930; Forbes and 

 Heidt, 1934; Bowen, 1946; Launer, 1949). The quantum yield for this 

 decomposition has been carefully measured at a series of wave lengths 

 from 208 to 434 m/x (Leighton and Forbes, 1930; Brackett and Forbes, 

 1933). The initial oxalate content of the stock solution, and the residual 

 oxalate after irradiation, are measured by permanganate titration and the 

 decomposition determined by difference. From the quantity of oxalate 

 decomposed and the quantum yield, the number of photons absorbed can 

 be calculated, and from this figure, the incident intensity determined, 

 knowing the absorption of the solution and making appropriate correc- 

 tions for such factors as reflections at cell windows. It is convenient to 

 use a solution of sufficient concentration to absorb all the incident radia- 

 tion; adequate stirring must be provided to equalize the irradiation of all 

 volumes of the solution. The photochemical reaction has been shown 

 to have nearly unity temperature coefficient. 



A rather unusual photochemical transformation of certain triphenyl- 

 methane dyes (Lifschitz, 1919; Lifschitz and Joffe, 1921; Harris et al., 

 1935) from a colorless to colored form has been used as a means of com- 

 paring ultraviolet intensities in the spectral region 2400-3200 A (Calvert 

 and Rechen, 1952; Harris and Kaminsky, 1935; Weyde, 1930; Weyde et 

 al., 1930; Miyake, 1949). The quantum yield for the transformation has 

 been shown to be unity over this spectral region and is independent of 

 temperature (Weyde and Frankenburger, 1931; Calvert and Rechen, 

 1952). This actinometer can be used at considerably lower light inten- 

 sities than can the uranyl oxalate. Under appropriate conditions, the 

 transformation is quantitatively autoreversible, permitting the same 

 solution to be used repeatedly (Weyde, 1930; Weyde et al., 1932). 



PHOTOGRAPHIC DETECTORS 



The photographic plate or film has a number of advantages as a radia- 

 tion detector. It is a sensitive detector and can integrate the radiation 

 received. Furthermore, it is unique ^ among ultraviolet radiation detec- 

 tors in that it is a two-dimensional detector, thus permitting an entire 

 spectrum or microscopic image to be recorded in one exposure. While 

 it cannot be regarded as a precision instrument for the measurement of 

 radiation, with appropriate auxiliary techniques, accuracies of the order 

 of ± 3 per cent can be obtained. 



Since individual plates or films vary unavoidably in sensitivity and in 

 contrast, for quantitative work it is necessary to standardize each plate 



^ With the potential exception of the image orthicon tube. 



