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  540 
  ] 
  

  

  XLII. 
  Notes 
  on 
  ft 
  and 
  y 
  Rays. 
  By 
  J. 
  A. 
  GrRAY, 
  D.Sc* 
  

  

  IN 
  this 
  paper, 
  some 
  remarks 
  are 
  made 
  on 
  two 
  effects 
  of 
  

   scattering 
  on 
  ionization 
  curves 
  and 
  also 
  on 
  the 
  measure- 
  

   ment 
  of 
  intensity 
  of 
  7 
  rays. 
  They 
  are 
  given 
  here 
  as 
  they 
  

   have 
  some 
  bearing 
  on 
  the 
  discussion 
  in 
  the 
  next 
  paper. 
  

  

  In 
  many 
  cases, 
  when 
  taking 
  absorption 
  curves 
  of 
  ft, 
  7, 
  and 
  

   X 
  rays 
  it 
  is 
  found 
  that 
  the 
  ionization 
  produced 
  by 
  the 
  rays 
  

   can 
  be 
  approximately 
  expressed 
  by 
  the 
  relation 
  I 
  = 
  I 
  e~^ 
  x 
  , 
  

   where 
  I 
  represents 
  the 
  ionization 
  and 
  x 
  the 
  thickness 
  of 
  the 
  

   absorbing 
  material. 
  It 
  might 
  appear 
  at 
  first 
  sight 
  that 
  rays 
  

   which 
  were 
  such 
  that 
  their 
  quality 
  was 
  unchanged 
  when 
  they 
  

   passed 
  through 
  matter 
  would 
  give 
  such 
  an 
  ionization 
  curve, 
  

   but 
  this 
  is 
  not 
  necessarily 
  the 
  case 
  as 
  the 
  following 
  con- 
  

   siderations 
  show. 
  

  

  Schmidt 
  f 
  has 
  worked 
  out 
  a 
  theory 
  of 
  the 
  absorption 
  of 
  

   ft 
  rays 
  in 
  which 
  it 
  is 
  assumed 
  that 
  in 
  passing 
  through 
  a 
  

   layer 
  of 
  absorbing 
  material 
  of 
  thickness 
  da, 
  a 
  beam 
  of 
  ft 
  rays 
  

   loses 
  a 
  fraction 
  adx 
  by 
  absorption, 
  ftdx 
  by 
  scattering. 
  If 
  we 
  

   assume 
  the 
  scattered 
  rays 
  to 
  be 
  of 
  the 
  same 
  quality 
  they 
  will 
  

   also 
  be 
  absorbed 
  and 
  scattered 
  to 
  the 
  same 
  extent. 
  If 
  R 
  is 
  

   the 
  intensity 
  of 
  the 
  rays 
  passing 
  through 
  a 
  plate 
  of 
  thickness 
  

   x, 
  r 
  the 
  intensity 
  of 
  the 
  rays 
  " 
  reflected 
  " 
  from 
  the 
  same 
  

   plate, 
  then 
  

  

  1— 
  P 
  2 
  

  

  R 
  = 
  e 
  -nx 
  (\\ 
  

  

  P 
  

  

  ' 
  l_p2 
  <? 
  -2 
  j 
  uA 
  J 
  - 
  * 
  /» 
  ■ 
  ; 
  * 
  W 
  

  

  ji 
  and 
  P 
  being 
  connected 
  with 
  a 
  and 
  ft 
  by 
  the 
  equations 
  

  

  Kl-P) 
  B 
  _ 
  2pV 
  

  

  When 
  a 
  is 
  large 
  enough, 
  

  

  (1) 
  reduces 
  to 
  R= 
  (1 
  - 
  P 
  2 
  >"^, 
  

  

  the 
  ionization 
  curve 
  becoming 
  exponential, 
  and 
  

  

  (2) 
  becomes 
  ? 
  , 
  = 
  P, 
  

  

  so 
  that 
  P 
  represents 
  the 
  fraction 
  reflected 
  from 
  a 
  "thick 
  " 
  plate. 
  

  

  It 
  is 
  only 
  when 
  P 
  2 
  is 
  small 
  compared 
  with 
  unity 
  that 
  the 
  

   ionization 
  curve 
  is 
  exponential. 
  

  

  Schmidt's 
  equations 
  apply 
  approximately 
  to 
  the 
  trans- 
  

   mission 
  of 
  the 
  ft 
  rays 
  of 
  radium 
  E 
  through 
  lead. 
  Results 
  

   are 
  given 
  below 
  (Table 
  I.) 
  of 
  the 
  relative 
  readings 
  obtained 
  

  

  * 
  Communicated 
  by 
  Prof. 
  H. 
  T. 
  Barnes, 
  F.R.S. 
  

   t 
  Schmidt, 
  Ann. 
  d. 
  Phys. 
  xxiii. 
  p. 
  671 
  (1907). 
  

  

  