RADIOACTIVITY 33 



are much greater for /9 than for a rays and it is not surprising 

 that we have been longer in arriving at the same understanding. 



Considering a single /3 particle there does not seem to be 

 much difficulty in picturing its motion. It is a projectile, fired 

 off with a certain initial speed and left to take care of itself. 

 It goes right through atoms as the a particle does but it is 

 continually being swung to this side and that by encounters 

 with centres of electrical force which the atoms contain. The 

 extent to which this takes place depends on the speed to a most 

 surprising extent. The /3 rays from radium will keep a line 

 fairly well in passing through several centimetres of air and 

 many will be found continuing the old line of flight at a metre 

 away from the source. The speed of these rays is just short 

 of that of light, say 2-9 x io 10 cm. per second. Cathode rays 

 which have one-third the speed can only reach a distance of 

 two or three millimetres in air from their source. So we can 

 go on, down to rays which have a speed of io 8 cm. per second 

 or thereabouts (600 miles a second) and these cannot penetrate 

 a single molecule ; they have no power of penetration at all. 

 Neither can they ionise the molecule : it seems as if pene- 

 tration must precede ionisation. Thus the ease with which 

 (3 rays can be handled in comparison to cathode rays is due 

 simply to the triple increase in speed ; they can be made to 

 travel far longer paths and to pass through thicker screens. 



The path of the /3 ray is therefore very irregular and broken. 

 Let us imagine it begun. It goes for a little time through the 

 atoms; it meets with little or no deflection : it has not yet gone 

 close enough to one of the strong centres of force within. But 

 sooner or later the inevitable happens and there is a change 

 in the direction of the movement, more or less severe. Now» 

 does the /S particle lose in speed at such an encounter ? The 

 right answer is not quite clear ; but I think the negative is the 

 safest, for the reason that when /3 particles are thrown against 

 a metal screen large numbers of them are returned and the 

 speed of the returned or " secondary " rays, as they have been 

 called, is not much less than that of the incident. There is a 

 little difference but it may be due to a kind of sorting-out 

 process, the slower ft rays in a bundle being more readily 

 returned. At any rate there is not in general any great loss 

 of speed ; it is to be remembered that if we find a 10 per cent, 

 change of penetrating power on reflection, which is much more 



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