﻿Retardation 
  of 
  Alpha 
  Rays 
  hy 
  Metals 
  and 
  Gases. 
  605 
  

  

  at 
  a 
  definite 
  distance 
  from 
  the 
  ionization-chamber, 
  it 
  was 
  

   found 
  that, 
  when 
  the 
  metal 
  sheet 
  was 
  moved 
  away 
  from 
  the 
  

   polonium 
  towards 
  the 
  ionization-chamber, 
  the 
  ionization 
  

   increased. 
  This 
  increase 
  in 
  the 
  ionization 
  was 
  attributed 
  to 
  

   the 
  alpha-particle 
  having 
  a 
  greater 
  velocity 
  (or 
  range) 
  upon 
  

   entering 
  the 
  chamber 
  when 
  the 
  sheet 
  was 
  near 
  the 
  chamber 
  

   than 
  it 
  had 
  when 
  the 
  sheet 
  was 
  at 
  a 
  distance 
  from 
  the 
  

   chamber. 
  Hence 
  the 
  metal 
  sheet 
  did 
  not 
  cut 
  down 
  the 
  

   range 
  of 
  the 
  particle 
  so 
  much 
  when 
  the 
  sheet 
  was 
  at 
  a 
  

   distance 
  from 
  the 
  polonium 
  as 
  it 
  did 
  when 
  near 
  the 
  polonium 
  . 
  

   As 
  a 
  preliminary 
  to 
  more 
  extensive 
  experiments 
  by 
  this 
  

   method 
  two 
  tests 
  were 
  made 
  to 
  ascertain 
  whether 
  a 
  scattering 
  

   of 
  the 
  rays 
  could 
  explain 
  the 
  increase 
  in 
  the 
  ionization 
  

   observed 
  when 
  the 
  metal 
  sheets 
  were 
  moved 
  away 
  from 
  the 
  

   polonium 
  towards 
  the 
  ionization-chamber. 
  

  

  First 
  test. 
  — 
  Any 
  marked 
  scattering 
  of 
  the 
  rays 
  by 
  the 
  foils 
  

   would 
  change 
  the 
  shape 
  of 
  the 
  cone 
  of 
  rays, 
  and 
  especially 
  

   the 
  form 
  of 
  the 
  top 
  portion 
  of 
  the 
  cone. 
  The 
  slope 
  of 
  the 
  

   top, 
  or 
  nearly 
  horizontal 
  portion, 
  of 
  the 
  Bragg 
  ionization- 
  

   curve, 
  as 
  well 
  as 
  the 
  value 
  of 
  the 
  maximum 
  ionization, 
  depends 
  

   upon 
  the 
  form 
  of 
  the 
  cone 
  of 
  rays 
  arriving 
  at 
  the 
  ionization- 
  

   chamber. 
  Thus, 
  if 
  scattering 
  of 
  the 
  rays 
  exists 
  to 
  a 
  very 
  

   marked 
  degree, 
  it 
  might 
  be 
  expected 
  that 
  differences 
  between 
  

   the 
  slope 
  and 
  form 
  of 
  the 
  two 
  Bragg 
  curves 
  obtained 
  with 
  

   and 
  without 
  the 
  metal 
  foil 
  over 
  the 
  polonium 
  could 
  be 
  readily 
  

   detected. 
  With 
  polonium 
  as 
  the 
  source 
  of 
  rays, 
  numerous 
  

   determinations 
  of 
  the 
  Bragg 
  curves, 
  both 
  with 
  and 
  without 
  

   the 
  various 
  foils 
  over 
  the 
  polonium, 
  were 
  made. 
  A 
  study 
  of 
  

   these 
  curves 
  showed 
  them 
  to 
  run 
  parallel 
  to 
  each 
  other 
  and 
  

   to 
  give 
  the 
  same 
  value 
  of 
  the 
  maximum 
  ionization. 
  The 
  

   effect 
  of 
  putting 
  the 
  foils 
  over 
  the 
  polonium 
  was 
  merely 
  to 
  

   diminish 
  all 
  the 
  ordinates 
  of 
  the 
  curves 
  by 
  the 
  same 
  amount. 
  

  

  Second 
  test. 
  — 
  An 
  iris 
  diaphragm 
  whose 
  circular 
  opening- 
  

   could 
  be 
  adjusted 
  to 
  any 
  desired 
  diameter 
  between 
  ^ 
  and 
  

   Ot} 
  cm., 
  was 
  constructed 
  of 
  thin 
  sheets 
  of 
  brass 
  and 
  placed 
  

   directly 
  below 
  the 
  ionization-chamber. 
  The 
  centre 
  of 
  the 
  

   opening 
  of 
  the 
  diaphragm 
  was 
  directly 
  below 
  the 
  centre 
  of 
  

   the 
  ionization-chamber. 
  With 
  the 
  source 
  of 
  rays 
  (radium 
  C^ 
  

   at 
  such 
  a 
  distance 
  from 
  the 
  ionization-chamber 
  that 
  the 
  

   chamber 
  cut 
  the 
  top 
  portion 
  of 
  the 
  Bragg 
  curve, 
  the 
  ioniza- 
  

   tion 
  was 
  measured 
  for 
  various 
  distances 
  of 
  the 
  metal 
  sheets 
  

   above 
  the 
  source 
  of 
  rays 
  ; 
  first 
  with 
  the 
  diaphragm 
  entirelv 
  

   open 
  and 
  then 
  with 
  the 
  opening 
  in 
  the 
  diaphragm 
  of 
  such 
  

   diameter 
  as 
  to 
  just 
  limit 
  the 
  geometrical 
  beam 
  of 
  rays, 
  or 
  to 
  

   cut 
  off 
  the 
  edge 
  of 
  the 
  beam. 
  For 
  any 
  given 
  position 
  of 
  the 
  

   sheet 
  above 
  the 
  source 
  of 
  rays, 
  the 
  ionization 
  was 
  always 
  

  

  