II 



MEASUREMENT OF X-RAYS AND RADIUM 



Lauriston S. Taylor 



Department of Commerce, National Bureau of Standards, Washington, D. C. 



Production of X-rays. The measurement of X-ray quality. Secondary methods of 

 expressing X-ray quality. Standard measurement of X-ray quantity or intensity. 

 Recommendations concerning X-ray standards adopted by the National Laboratories: 

 Measurement of quality — Measurement of the roentgen — Calibration of dosemeters. 

 Types of standard chambers. Standard measurement of ionization currents. Secondary 

 ionization chambers: Method of calibration. Types of thimble chamber roentgenometers. 

 Measurement of X-ray output. Comparison of X-ray generators. Measurement of 

 very low and very high voltage X-rays: Grenz rays (3 to 15 kv.). Ultra-high voltage 

 X-rays, Radium measurement. 



THE PRODUCTION OF X-RAYS 



X-ray Tubes. — As pointed out in other chapters, X-rays are produced 

 by the sudden stoppage or deceleration of fast moving electrons. In 

 practice, this effect is brought about in an evacuated tube having two 

 electrodes between which a high potential difference, E volts, may be 

 applied. Any electrons in the space between the electrodes are thus 

 moved toward the positively charged electrode or target, with a velocity 



V, which is given approximately by the relation v = 5.95'VE' X 10^ 

 cm. /sec. Upon striking the target, the electron is brought to rest in a 

 relatively short time and an X-ray produced which may have a minimum 



o 



wave-length Xj^in = 12354. -^ E, where X is expressed in Angstroms 

 (10-8 pjn ) 



In the ordinary high-vacuum tube, the number of free electrons in 

 the interelectrode space is negligibly small. In the older type of gas 

 X-ray tube, electrons in sufficient number were released from the cathode 

 by the bombardment of positive ions produced in the electrical discharge. 

 However, it was difficult to maintain steady gas pressures within the tube, 

 and hence the positive ion and electron currents were unsteady and uncer- 

 tain. In the present-day Coolidge tube (9), electrons are produced by 

 means of a hot cathode in a tube so highly evacuated that the small 

 amount of residual gas plays no important part in its operation. The 

 electrons released from the filament by thermionic emission are then 

 accelerated in the electric field and a steady space current ensues. 



43. 



