﻿Absorption 
  of 
  the 
  7 
  Rays 
  of 
  Radium. 
  621 
  

  

  7 
  rays 
  being 
  softer 
  than 
  the 
  primary. 
  Ordinarily 
  we 
  should 
  

   expect 
  the 
  rays 
  to 
  become 
  more 
  and 
  more 
  penetrating 
  and 
  

   ultimately 
  homogeneous. 
  That 
  this 
  is 
  not 
  the 
  case 
  is 
  of 
  

   course 
  explained 
  by 
  the 
  fact 
  that 
  although 
  the 
  softer 
  rays 
  are 
  

   more 
  quickly 
  absorbed, 
  their 
  place 
  is 
  taken 
  by 
  other 
  rays 
  

   produced 
  by 
  scattering. 
  Consider 
  a 
  homogeneous 
  beam 
  of 
  

   7 
  rays. 
  As 
  a 
  certain 
  fraction 
  of 
  these 
  rays 
  is 
  scattered 
  per 
  unit 
  

   mass, 
  the 
  rays 
  coming 
  through 
  any 
  absorption 
  plate 
  would 
  

   become 
  less 
  and 
  less 
  penetrating 
  as 
  the 
  thickness 
  of 
  the 
  plate 
  

   increased. 
  Ultimately, 
  however, 
  the 
  absorption 
  would 
  become 
  

   exponential, 
  as 
  only 
  a 
  definite 
  fraction 
  of 
  the 
  unscattered 
  

   primary 
  rays, 
  which 
  would 
  consequently 
  be 
  unchanged 
  in 
  

   quality, 
  could 
  be 
  scattered 
  per 
  unit 
  mass, 
  and 
  we 
  must 
  reach 
  

   a 
  stage 
  where 
  the 
  production 
  of 
  softer 
  7 
  rays 
  is 
  balanced 
  by 
  

   their 
  absorption. 
  A 
  certain 
  similarity 
  to 
  the 
  absorption 
  of 
  

   ft 
  rays 
  will 
  be 
  noted. 
  In 
  the 
  case 
  of 
  ft 
  rays 
  the 
  slower 
  /3 
  rays 
  

   are 
  produced 
  by 
  the 
  faster 
  /3 
  rays 
  losing 
  velocity 
  in 
  traversing 
  

   matter. 
  One 
  distinction 
  must 
  be 
  drawn, 
  however. 
  In 
  the 
  

   case 
  of 
  /S 
  rays, 
  true 
  absorption 
  only 
  takes 
  place 
  to 
  any 
  great 
  

   extent 
  when 
  the 
  rays 
  become 
  very 
  slow, 
  whereas 
  in 
  the 
  case 
  

   of 
  7 
  rays 
  absorption 
  can 
  take 
  place 
  at 
  any 
  point 
  of 
  their 
  

   path. 
  

  

  The 
  writer 
  * 
  has 
  shown 
  that 
  an 
  exponential 
  law 
  for 
  /3 
  rays 
  

   can 
  only 
  be 
  approximate, 
  and 
  -the 
  question 
  arises 
  as 
  to 
  whether 
  

   the 
  same 
  thing 
  may 
  not 
  be 
  true 
  for 
  7 
  rays. 
  This 
  depends 
  on 
  

   the 
  question 
  : 
  Can 
  7 
  rays 
  be 
  directly 
  scattered 
  ? 
  Crowtherf 
  

   has 
  shown 
  that 
  of 
  a 
  pencil 
  of 
  /3 
  rays 
  every 
  /3 
  ray 
  is 
  scattered 
  

   through 
  a 
  small 
  angle 
  after 
  passing 
  through 
  very 
  small 
  

   thicknesses 
  of 
  matter, 
  and 
  Geiger 
  { 
  has 
  shown 
  the 
  same 
  thing 
  

   for 
  a. 
  rays. 
  If 
  something 
  similar 
  took 
  place 
  in 
  the 
  case 
  of 
  

   7 
  rays, 
  we 
  should 
  have 
  the 
  result 
  thaty 
  rays, 
  as 
  a 
  whole, 
  must 
  

   become 
  less 
  and 
  less 
  penetrating 
  so 
  that, 
  like 
  j3 
  rays, 
  an 
  

   exponential 
  law 
  could 
  only 
  be 
  approximate, 
  the 
  absorption 
  

   ultimately 
  becoming 
  greater 
  and 
  greater. 
  If, 
  on 
  the 
  other 
  

   hand, 
  we 
  start 
  with 
  a 
  beam 
  of 
  7 
  rays 
  and 
  a 
  definite 
  percentage 
  

   of 
  these 
  rays 
  is 
  scattered 
  per 
  unit 
  mass, 
  the 
  remainder 
  

   keeping 
  their 
  direction 
  unchanged, 
  the 
  exponential 
  law 
  can 
  be 
  

   an 
  accurate 
  one. 
  The 
  fact 
  that 
  Russell 
  found 
  the 
  absorption 
  

   in 
  mercury 
  exponential 
  over 
  a 
  range 
  of 
  intensity 
  of 
  360,000 
  

   to 
  1, 
  shows 
  that 
  this 
  is 
  very 
  nearly 
  the 
  case, 
  and 
  this 
  has 
  been 
  

   tacitlv 
  assumed 
  above, 
  although 
  Russell 
  found 
  evidence 
  that 
  

  

  * 
  Grav, 
  Proc. 
  Eov. 
  Soc. 
  A. 
  lxxxvii. 
  p. 
  486 
  (1912). 
  

   t 
  Ciwther, 
  Proc. 
  Roy. 
  Soc, 
  A. 
  lxxx. 
  p. 
  186 
  (1908). 
  

   % 
  Geiger, 
  Proc. 
  Roy. 
  Soc. 
  A. 
  lxxxi. 
  p. 
  174 
  (1908), 
  and 
  A. 
  lxxxiii. 
  

   p. 
  492 
  (1910). 
  

  

  