MEASUREMENT OF X-RAYS AND RADIUM 65 



of air ionized is then taken as that of a cyUnder of cross sections equal to 

 the area of D and length equal to the effective length of the collector 

 electrode C (the distance a to a'). 



Secondary Effects. — To insure full utilization of the X-ray flux through 

 the chambers two other factors must be taken into consideration: (a) 

 The spacing between C and H must be sufficient to permit the recoil 

 electrons e to expend their total energy in producing ions before striking 

 the plates. For a narrow (1 cm.) beam of 200-kv, (peak) X-rays a spacing 

 of 10 cm. between C and H is sufficient. This spacing varies with the 

 exciting X-ray voltage, although for practical purposes it does not appear 

 to increase indefinitely with increasing voltage. (6) Precaution must also 

 be taken that sufficient electric field be provided between C and H to 

 produce saturation, that is, pull all the ions to C or H before an appreci- 

 able number are lost by recombination. For most X-ray flux densities 

 encountered in practice, field strength of 100 to 150 volts/cm. is sufficient. 

 For very high ion concentrations, greater fields may be required. 



Since the ionization current is a function of the mass absorption of 

 X-rays by air, correction must be made for the air temperature and 

 pressure as stipulated in the definition. Accordingly, the air volume is 

 reduced to that under normal conditions, 0° C. and 76 cm. Hg pressure, 

 by Charles's and Boyle's law. 



In further satisfaction of the definition of the roentgen, secondary 

 effects from the walls and plates of the chamber are reduced to a minimum 

 by making them of low-atomic-number material, carbon or aluminum. 

 Thus any tertiary X-rays excited by secondary X-rays from the main 

 beam in striking the plates are of such long w^ave-length as to be quickly 

 absorbed before having an opportunity to produce an appreciable number 

 of ions in the measuring volume. Following out the procedures given 

 above, the intensity of an X-ray beam as measured in terms of roentgens 

 is given by the following formula: 



/ i ^ T 760 (^. 



where i is the current to the collector electrode measured in electrostatic 

 units, I the effective length of the collector electrode, a the area of the 

 Hmiting diaphragm in square centimeters, T the absolute temperature, 

 and P the pressure in millimeters. Where the current is measured in 

 amperes, the result must be multiplied by 3^ X 10~^. 



RECOMMENDATIONS CONCERNING X-RAY STANDARDS ADOPTED BY 



THE NATIONAL LABORATORIES 



As a result of an intercomparison of their standards (1, 46), the 

 National Laboratories of the United States, England, Germany, and 



