180 RADIATION BIOLOGY 



Secondary chambers of the thimble type have been adapted to the 

 measurement of 3- to 15-kv X rays, although their accuracy is question- 

 able in many cases (Glasser and Portmann, 1928; Holthusen and Braun, 

 1933; Klistner, 1928). Wall effects become relatively large and even the 

 thinnest of materials absorbs a large percentage of the radiation, thus 

 preventing its contribution to the ionization within the chamber. "The 

 accurate use of such thimble chambers is so beset with uncertainties that 

 it is usually more satisfactory to use a small open-air standard chamber 

 for biological X-ray measurements at such low voltage." This sentence, 

 quoted textually from a review written in 1934 (Taylor, 1936), applies, 

 essentially without modification, to the situation existing today. 

 Although in the interim of seventeen years several papers have been pub- 

 lished on measurement of lightly filtered radiation from X-ray tubes 

 operated at low voltage (Oosterkamp, 1950; Thoraeus, 1941; Seemann, 

 1949; Quimby and Focht, 1943; Lea, 1941), only one has reported in 

 detail on the calibration of thimble chambers in this region. ^^ 



Day (1949), as a result of extensive investigations on the quality 

 dependence of a commercially available dosimeter in filtered and unfil- 

 tered beams of X-ray tubes operated from 10 to 200 kv, reports correction 

 factors of considerable magnitude which, if neglected, would introduce 

 errors quite intolerable for quantitative investigation. These corrections 

 diminish in magnitude when kilovoltage and filtration are increased; the 

 latter appears important in practice, particularly when the radiation 

 emerges through a thin beryllium window, even though the voltage 

 applied to the tube may be as high as 200 kv. Although it is realized 

 that the difficulties of proper design and construction are many, the 

 persistent lack of thimble chambers useful in this range proves disappoint- 

 ing, for it hinders a more extensive use of a type of radiation which is 

 eminently suited to many radiobiological studies on small organisms 

 because of its availability at high dosage rates and of its reasonable cost.^^ 



1^ In order to avoid misrepresentation of this statement, it is to be understood that 

 by "thimble chamber" is here meant a chamber of finite vokime which is (1) devoid 

 of a hmiting diaphragm, and is (2) equally sensitive to radiation, independently of the 

 direction from which it may come. Although several portable chambers have been 

 designed to replace a standard open-air chamber for roentgen measurements of radia- 

 tion in limited enclosures, and most of them have been shown to be wave-length- 

 independent down to the very soft region, none of them obeys both criteria (1) and (2) 

 in a strict sense. 



^^ The user of soft X rays, with access to commercial thimble chambers only, may 

 improve the correction factor of his chamber by interposing between his material 

 and the radiation source a filter of bakelite equal to the thickness of the wall of the 

 thimble chamber, and by measuring the dose without the filter. This procedure, 

 though logical, requires experimental confirmation with the open-air chamber; it can 

 be applied only in the presence of moderate dosage-rate gradients, wherein it might 

 provide a simple means of attaining a first approximation. The best procedure, 

 as mentioned above, consists in the use of open-air chambers or their equivalent when- 

 ever spatial dimensions allow it. 



