690 



TABLE 755.— COMBINATION OF LEAD SHIELD THICKNESS AND DISTANCE 



FOR ADEQUATE PROTECTION FOR EXPOSURES TO DIFFERENT 



AMOUNTS OF RADIUM, NOT EXCEEDING 8 HOURS PER DAY 



Workers with radioactive materials must observe certain precautions to avoid being 

 burned by the emitted radiations. Tables 749, 751, 754, 755, taken from the National 

 Bureau of Standards Handbook H 23 on Radium Protection, give some of the necessary 

 precautions. These precautions are for radium ; if some other radioactive product is being 

 worked with, care must be taken to increase these precautions if the materials are more 

 active than radium. See Table 732. 



The a-rays are much more easily stopped than the /3- or 7-rays. The most energetic 

 o-rays are stopped by an ordinary sheet of paper or a sheet of aluminum .06 mm thick. 

 The /3-rays are stopped by a few millimeters of aluminum, while many of the 7-rays will 

 penetrate a block of lead a number of inches thick. 



TABLE 756.— CONSTANTS FOR CATHODE-RAY SPEEDS IN MATTER 



Cathode rays whose direction of motion is perpendicular to the direction of a uniform 

 magnetic field (H) describe a circular path of radius (r) according to the formula cor- 

 rected for relativity change of mass of electron. 



Hr=1704[/3(l-/3 2 )- 1/2 ] 



where H is expressed in gauss and r in cm. 



When cathode rays impinge on matter they are deflected from their original direc- 

 tion of motion. These deflections grade all the way from 180° "reflections" to the "diffu- 

 sion" corresponding to deflections through very small angles. The large-angle deflections 

 are ordinarily comparatively infrequent. However, when the substance struck by the 

 cathode rays is crystalline, certain directions may be preferred by the deflections. Here 

 the beam of cathode rays behaves as though it consisted of a train of waves of wavelength 

 X, = 0.02426//3, where X« is in angstroms. The preferred directions for the "reflected" 

 cathode-ray beams may be calculated from the Brasjg formula (see Siegbahn's "X-ray 

 Spectroscopy"). The simple Bragg formula is quite limited in application here, however, 

 since refraction in the crystal is very appreciable for the cathode-ray beams. In general, 

 the cathode rays which have been deflected bv matter will have lost speed, but the rays 

 which have undergone these "preferred" deflections remain of the same speed as the 

 primary cathode beam. 



Cathode rays lose speed on penetrating matter. The losses of speed by individual 

 cathode particles grade from complete stoppage to no loss of speed. The maiority of the 

 cathode particles, however, lose speed according to the relation (Thomas-Whiddington- 

 Bohr law) 



/3 4 — /3 4 = ax 



where /3o is the initial speed, and /3 the speed after traversing a path length x in the mate- 

 rial {x to be measured in cm along the actual curved path), and a is a constant roughly 

 equal to 6.5/> where p is the density of the material in g/cm s . A convenient form for the 

 expression is the following. Note that the two forms are not equivalent except at very 

 low speeds (experiment has not yet decided between the two) : 



IV —V^ — bx 



where V and V are the initial and final "equivalent voltages" (see above) of the cathode 

 rays, in kv, and b is a constant roughly equal to 40 X 10V- A tabulation of experimental 

 values of a and b for various materials follows : 



Material a 



Beryllium 12. 



Aluminum 17. 



Copper 56. 



Silver 66. 



Gold 138. 



Moist air, 76 cmHg 18° C 



SMITHSONIAN PHYSICAL TABLES 



0062 



.75 X 10" 



1.1 " 

 3.6 " 



4.2 " 

 9.0 " 



.44 X 10 s 



