564 



SCIENCE. 



[N. S. Vol. XXI. Xo. 5.37. 



sealed tubes, specified iu any last article.* 

 See § 6. 



5. Space Surrounding the X-ray Tube a 

 Plenum of Kadiations.—'S^liile the phos- 

 pherescenee, photographic and electric ef- 

 fects of X-radiation decreases rapidly with 

 the distance D from the tube, the nucle- 

 ating effect {N nuclei generated per cubic 

 centimeter, instantly) is nearlj^ constant 

 over relatively enormous distances. f Thus 

 to give an example among many {8p = 

 25 em.) : 



7) = 6 200 600 6 200 fiOO cm. 



JVX 10-' 88 83 83 79 79 79 



The law of inverse squares Avould predicate 

 a reduction of 10,000 to 1 between these 

 limits; and in fact at 6 cm. the phosphores- 

 cent screen is intensely luminous, at 200 

 cm. very dim, at 600 cm. quite dark, as in 

 the case of any ordinary illumination. 

 The leaves of an electroscope within a glass 

 bell-jar collapse in a time which is directly 

 as the square of the distance from the en- 

 ergized X-ray bulb. The result obtained 

 with nuclei is astonishing; the nuclei-pro- 

 ducing radiation would at first sight seem 

 to be of an extremely penetrating kind, 

 akin to the gamma rays of radium and dis- 

 tinct from the ordinary phosphorescence- 

 producing X-rays. This impression is ac- 

 centuated by the fact that the radiation 

 can not be stopped by lead screens many 

 centimeters in thickness, placed between 

 bulb and fog chamber. The following are 

 typical examples, in which the distance 

 between the lead plates screening the fog 

 chamber and the X-ray tube is D = 600 

 and 200 cm., respectively. N shows the 

 number of nuclei instantly generated be- 

 hind the plates in the two cases. 



* Science, XXT., 275. 



t Supposing that the fog-chamber is not enclosed 

 in impervious metal. In the latter ease, with the 

 lead covering open towards the X-ray bulb only, 

 there is constancy of N within 20 per cent, over 

 G meters. 



Thickness of 



lead screen .14 .28 ..5G .84 1.12 cm. 



.VxlO-= G7 28 28 31 29 31 7G 



.VXIO-" 79 44 48 41 — 44 70 



Again the X-ray tube apparently emits this 

 radiation forward as well as rearward, as 

 if the thin anticathode were quite pervious. 

 I found, for instance, for the radiation of 

 the anticathode at 6 meters from the fog 

 chamber, 



from the front face (tube directed), lYx 10-= = 42 

 from the rear face (tube reversed), -Vx 10-' = 35, 



or 81 per cent, of the former apparently 

 issues from the rear face. Even the re- 

 versal of the current does not stop the 

 radiation, for about 16 per cent, of the 

 normal intensity is still radiated when the 

 concave mirror is made the anode. 



The total efficient radiation may be re- 

 duced to a limit by lead screens a few milli- 

 meters in thickness or less; thereafter it 

 can not be further reduced by lead screens 

 many centimeters in thickness. For in- 

 stance, when the radiation comes from 600 

 cm., a single lead plate (thickness .1-1 cm.) 

 is more than sufficient to reduce the eff'ect- 

 ive radiation to a minimum, which amounts 

 to (somewhat less than) one half of the 

 total intensity, at least when estimated in 

 terms of the nitmber of nuclei produced. 

 If the nucleation comes from 200 cm., one 

 "plate has the same effect, even though a 

 thickness of 400 cm. of air has been re- 

 moved. The thickness, .14 cm., is more 

 than enough to reduce the radiation to the 

 limit in question. This again amoimts to 

 a little more than one half the total in- 

 tensity. At a distance of 5 centimeters no 

 more plates may be needed; but the condi- 

 tions are now too complicated to be de- 

 .scribed here, chiefly because persistent nu- 

 clei are producible. ]\Ioreover 80 per cent, 

 of the total intensity may ultimately escape 

 absorption. Thus the rays from different 

 distances behave alike for the more pervi- 

 ous media (§5^, and in relation to very 

 dense screens. 



li 



