550 Range of a Particles from Radium C and Thorium C. 



■"" extrapolated range" is a definite constant independent of 

 the sensitivity of the apparatus used. 



The values of the " extrapolated ranges " in cm. are shown 

 in Table I. for air at 760 mm. pressure and 0°, 15°, and 20° C. 

 The values given in brackets are those of the ordinary ranges 

 which, though strictly speaking meaningless, are inserted for 

 the sake of comparison. 



Table I. — Ranges in Air. 



0° 15° 20° C. 



6592 6953 7 "074 



[670) (7-04) (7-19) 



4^29 4778 4*61 



1 (4-59) (4-84) (4-93) 



TiiC- 



1167 8-616 8-714 



2 '" (8-27) (8-72) (8-87) 



The error involved in the extrapolated ranges of RaC and 

 ThC 2 is probably less than 1 in 2000 and that in ThC\ less 

 than 1 in 1000. The accuracy in the ordinary ranges is 

 probably of the order of *3 per cent. The range "of RaC in 

 air at 20° is given by Bragg * as 7*14 cm. 



Finally, it has already been noted that the slopes of the 

 straight part of the ionization curves for RaC, T-hCi, and ThC 2 

 agree within the experimental error, their mean being 3"48. 

 Hence for all three types of a particles the ionization curve 

 for a considerable portion near the end of the range, namely 

 between A and C, is represented approximately bv 



I = 3 ' 48 C 77^(B. + f-*):»- eV ' f ^, .... (5) 



where R is the extrapolated range of the a particles in 

 question. It should be noted that this equation contains only 

 one constant characteristic of the type of u particle considered, 

 for the parameter a can be calculated from R . Since this 

 equation holds for a particles of such widely different ranges 

 as RaC, ThCj, and ThC 2 it would appear probable that it 

 holds generally for a particles of any type. 



Summary. 



An investigation has been made of the ionization curves of 

 RaC, ThC i; and ThC 2 in air, particular attention being paid 

 to the end portions of these curves. It is shown that a con- 

 siderable portion of each of these curves is approximately a 

 straight line. 



* Bragg, ' Studies in Radioactivity/ p. 20. 



