82 BIOLOGICAL EFFECTS OF RADIATION 



As already noted, it seems possible that gamma rays may be measured 

 in roentgens with some form of thimble chamber. While many such 

 chambers have been devised, there is not yet uniformity of opinion 

 regarding their use. A number of observers using thimble chambers 

 (16, 30) have experimentally determined the number of roentgens per 

 milligram-hour at 1 cm. from a point source of radium filtered with 

 0.5 mm. Pt, finding a mean value of 8.6 roentgens. 



Primary standardization of radium element is carried out almost 

 exclusively by the national laboratories of several countries, each of which 

 possesses radium standards that have been compared with the standard 

 retained by the International Bureau of Weights and Measures at 

 Sevres, France. The unknown samples are directly compared with the 

 standards by a replacement method, the ionization being measured with a 

 heavily lead-shielded electrometer which serves also as the ionization 

 chamber. 



Since, as noted, the present-day expression of gamma-ray dosage is 

 largely empirical, each measurement is so intimately related to its 

 biological application that matters of filtration and intensity distribution 

 will be discussed in connection with the biological application of radiation. 



REFERENCES 



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2. Behnken, H. Die Roentgendosimetrie. Handbuch der gesamten Strahlen- 

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3. Behnken, H. Die Absolute Bestimmung der Dosiseinheit "1 Roentgen" in der 

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4. Benoist, L. Definition expcrimentale des diverses sortes de rayons-x par le 

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5. BouwERs, A. Measurement of the intensity of X-rays. Acta Radiol. 4: 

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6. BouwERS, A. Over het meten der intensiteit van Roentgenstralen. Dis- 

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7. Braun, R., and H. KtJsTNER. Zur Physik der Fingerhutkammer. Strahlen- 

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8. Christen, Th. Messung und Dosierung der Roentgenstrahlen, aus "Archiv 

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9. CooLiDGE, W. D. A powerful roentgen ray tube with a pure electron discharge. 

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10. DuANE, W., and J. C. Hudson. The measurement of effective wave lengths. 

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11. Failla, G. Criteria for the design of a standard ionization chamber. Amer. 

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12. Failla, G. A new instrument for measuring X-radiation. Radiology 15: 

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13. Failla, G., and P. Henshaw. The relative biological effectiveness of X-rays and 

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