MEASUREMENT OF X-RAYS AND RADIUM 67 



experimental precautions should be adopted in standard measurements: 

 X-ray Tube. — The X-ray tube should be of the hot-cathode type with 

 tungsten target. The focal spot should be as small and as uniform as 

 possible and the target should not be appreciably pitted. Tube current 

 and applied potential shall be kept constant with the requisite degree of 

 accuracy during a test. 



X-ray Beam. — A diaphragm of suitable aperture shall be mounted as 

 near as possible to the X-ray tube so that the X-ray beam utilized shall 

 include a maximum of radiation from the focal spot, and a minimum 

 of off-focus radiation. The uniformity of the radiation field of the test 

 beam shall be verified by ionization or photographic methods. For 

 measurements made at different focal distances, de\iation from the 

 inverse square law should not exceed the specified measurement accuracy. 

 The certainty of this shall be verified by special test. Where, for practical 

 reasons, the inverse square law does not hold, the accuracy of the results 

 is likely to be increased by conducting the measurements as far as possible 

 from the X-ray tube. 



Standard Ionization Ckamber. — A. Chambers for either soft or hard 

 radiation: Chambers used for measurement (in roentgens) of either soft 

 or hard X-rays shall be of the open-air type (free-air, Fasskammer) 

 wherein the measured ions are produced in a column of air which is not 

 bounded by walls. Different forms of such chambers have been found 

 to be satisfactory by the participating laboratories. For full con- 

 structional details the several publications of the laboratories may be 

 referred to. The following general conditions shall be satisfied by 

 standard chambers: 



a. The ionization chamber shall be of such dimensions and so con- 

 structed that: 



1. The spacing between the measuring and the high-potential elec- 

 trodes is sufficient to insure full utilization of the secondary electrons. 



2. The electric field between electrodes is sufficient to insure satura- 

 tion. Chamber forms which make the attainment of saturation difficult 

 are unsuitable. 



3. Distortion of the electric field between the measuring and the high- 

 potential electrodes is avoided as far as possible, for example, by the use 

 of suitable guard electrodes or a guarded field. 



b. A chamber diaphragm (entrant diaphragm, limiting diaphragm) 

 shall be mounted to provide an entrance for the X-ray beam to the 

 chamber and shall conform to the following general conditions: 



1. The material out of which the diaphragm is made shall be of the 

 minimum thickness which will permit not more than one part in a 

 thousand (Jlooo) of the measured radiation to pass through the material 

 itself. 



