﻿Notes 
  on 
  j3 
  and 
  y 
  Rays. 
  541 
  

  

  when 
  these 
  ravs 
  are 
  absorbed 
  by 
  lead. 
  Lead 
  foils 
  were 
  used 
  

   of 
  thickness 
  r 
  00152 
  cm. 
  

  

  Table 
  I. 
  

  

  Ionization. 
  

  

  Number 
  of 
  foils. 
  

  

  Percentage 
  transmitted 
  

   by 
  each 
  foil. 
  

  

  100 
  

  

  

  

  46 
  

  

  1 
  

  

  46 
  

  

  26-6 
  

  

  2 
  

  

  57 
  

  

  16-9 
  

  

  3 
  

  

  63 
  

  

  11-2 
  

  

  4 
  

  

  66 
  

  

  7"4 
  

  

  5 
  

  

  07 
  

  

  5-02 
  

  

  6 
  

  

  08 
  

  

  3-40 
  

  

  7 
  

  

  68 
  

  

  The 
  scattering 
  of 
  the 
  /3 
  rays 
  is 
  so 
  marked 
  in 
  lead 
  that 
  it 
  

   matters 
  little 
  whether 
  the 
  /3 
  rays 
  are 
  initially 
  parallel 
  or 
  not. 
  

   The 
  numbers 
  agree 
  very 
  well 
  with 
  the 
  formula 
  (1) 
  

  

  11 
  = 
  100 
  

  

  1-P' 
  

  

  1-P 
  1 
  

  

  ■2/ax 
  

  

  fJ.X 
  

  

  where 
  P 
  = 
  0'70 
  and 
  <?-°- 
  0015 
  ^ 
  = 
  0'68. 
  

  

  The 
  third 
  column 
  shows 
  that 
  the 
  first 
  foil 
  only 
  transmits 
  

   46 
  per 
  cent, 
  of 
  the 
  rays, 
  while 
  when 
  the 
  ionization 
  becomes 
  

   exponential, 
  the 
  same 
  foil 
  transmits 
  68 
  per 
  cent. 
  Conse- 
  

   quently, 
  when 
  P 
  is 
  large 
  the 
  ionization 
  curve 
  I^o*?"^ 
  does 
  

   not 
  signify 
  rays 
  (/3 
  or 
  7) 
  the 
  quality 
  of 
  which 
  is 
  unaltered 
  

   by 
  transmission 
  through 
  matter. 
  

  

  Por 
  example, 
  Crowther 
  * 
  hns 
  found 
  that 
  when 
  homogeneous 
  

   /3 
  rays 
  are 
  tested 
  by 
  platinum 
  the 
  ionization 
  curve 
  is 
  

   approximately 
  exponential. 
  As 
  a 
  " 
  thick 
  " 
  layer 
  of 
  platinum 
  

   wonld 
  reflect 
  about 
  70 
  per 
  cent, 
  of 
  the 
  /3 
  rays 
  tested, 
  we 
  see 
  

   that 
  the 
  rays 
  cannot 
  remain 
  homogeneous 
  ; 
  a 
  fact 
  which 
  

   Crowther 
  f 
  had 
  previously 
  proved 
  and 
  which 
  has 
  been 
  well 
  

   brought 
  out 
  by 
  recent 
  experiments 
  of 
  W. 
  Wilson 
  %. 
  The 
  

   reason 
  why 
  the 
  ionization 
  does 
  not 
  decrease 
  more 
  quickly 
  at 
  

   first 
  is 
  because 
  the 
  decrease 
  in 
  ionization 
  is 
  almost 
  entirely 
  due 
  

   to 
  scattering, 
  there 
  being 
  very 
  little 
  absorption. 
  Scattering 
  

   also 
  accounts 
  for 
  the 
  rays 
  becoming 
  heterogeneous 
  after 
  

  

  * 
  Crowther, 
  Proc. 
  Roy. 
  Soc. 
  A. 
  lxxxiv. 
  p. 
  226 
  (1910). 
  

   t 
  Crowther, 
  Proc. 
  Cainb. 
  Phil. 
  Soc. 
  1909. 
  

   X 
  W. 
  Wilson, 
  Proc. 
  Roy. 
  Soc. 
  A. 
  lxxxvii. 
  p. 
  310 
  (1912). 
  

   Phil. 
  Mag. 
  S. 
  6. 
  Vol. 
  26. 
  No. 
  153. 
  Sept. 
  1913. 
  2 
  O 
  

  

  