[DOUGLAS] 18-RAYS FROM RADIUM 121 



area under the curve, as some of the rays are stopped or absorbed, and 

 others are scattered through angles greater than 90°. 



It is of interest to note that the exponential law of absorption 

 requires that, to a first approximation, the rate of decrease of area 

 under the curve should be constant, due to the combined effects of 

 absorption and scattering through angles greater than 90°, and it 

 may be remarked again that this is proved not to be the case, the 

 area actually decreasing more rapidly as the thickness of absorber is 

 increased. 



As the area diminishes and v approaches vo there will come a time 

 when even the fastest particles have been slowed down so much that 

 they cannot escape complete absorption, hence a range must exist, 

 and that thickness of absorber may be termed the "effective range" 

 which makes the whole curve shrink finally to Vq. The "actual range" 

 which is not directly obtainable experimentally will be referred to 

 later. 



The determination of the range in different substances was made 

 by the following method : 



A 10 cm. cube electroscope, the base of which consisted of one 

 sheet of aluminium foil (.004615 gms/cm^) and one sheet of paper 

 (.00848 gms/cm^), was mounted on the pole pieces of an electro- 

 magnet. The active material was placed 6 cm. below the electro- 

 scope. The magnetic field was sufiiciently strong to deflect between 

 40 per cent, and 50 per cent, of the primary /3-rays unabsorbed, and 

 when their velocity was reduced by about 40 sheets of paper, or its 

 equivalent, complete deflection of the jS-rays took place. For small 

 amounts of absorber^ the intensity with the field off exceeds the 

 intensity with the field on. As the thickness of absorber is increased 

 this excess is diminished until, when 'the range is reached, the in- 

 tensities are the same whether the field be off or on. 



The difficulties encountered in those experiments, as in all those 

 carried out during the course of the investigation, arose in two ways: 



(1) The variability of the natural leak and its continued high value 

 and the extreme sensitivity of the electroscope to air currents in spite 

 of the precaution of placing draught-screens around three sides of the 

 apparatus and protecting its, base by several layers of absorber; 



(2) the comparative weakness of the active material which was used 

 for the majority of the experiments, making accurate measurements 

 very difficult when the reduction of intensity was of the order of 1 in 

 500,000, as has already been explained. 



As a result of these, the exact location of the range was not 

 possible to the degree of precision hoped for, but the extreme limits 



