﻿Recoil 
  Atoms 
  from 
  Thorium 
  C 
  and 
  Actinium 
  C. 
  593 
  

  

  experiments. 
  Again, 
  it 
  was 
  found 
  that 
  at 
  low 
  pressures 
  

   (say 
  01 
  mm.) 
  there 
  was 
  still 
  a 
  considerable 
  amount 
  of 
  

   ionization 
  produced, 
  and 
  it 
  seemed 
  as 
  if 
  the 
  initial 
  portion 
  of 
  

   the 
  curve, 
  when 
  produced 
  towards 
  the 
  axis 
  of 
  ordinates, 
  did 
  

   not 
  pass 
  through 
  the 
  origin. 
  Consequently 
  experiments 
  

   were 
  made 
  with 
  the 
  active 
  plate 
  covered 
  with 
  aluminium 
  

   foil 
  to 
  stop 
  recoiling 
  atoms, 
  and 
  it 
  was 
  found 
  that 
  the 
  varia- 
  

   tion 
  with 
  pressure 
  of 
  the 
  ionization 
  by 
  the 
  a-particles 
  was 
  

   not 
  linear 
  until 
  a 
  pressure 
  of 
  about 
  4 
  mm. 
  had 
  been 
  reached. 
  

   The 
  dotted 
  curve 
  (fig. 
  4 
  A) 
  shows 
  the 
  result 
  of 
  an 
  experiment 
  

   of 
  this 
  nature. 
  It 
  will 
  be 
  seen 
  that 
  the 
  curve 
  cuts 
  the 
  axis 
  

  

  Fig. 
  4. 
  

  

  r 
  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  

  1 
  

  

  \ 
  

  

  

  

  

  

  

  

  \a 
  

  

  

  

  1 
  

  

  \ 
  

  

  \ 
  

  

  

  

  

  

  

  t 
  

  

  / 
  

  

  

  1 
  

  

  

  \ 
  

  

  \ 
  

  

  

  

  

  

  \ 
  / 
  

  

  

  

  4 
  

  

  

  

  \ 
  

  

  

  

  

  ''■' 
  

  

  \b 
  

  

  

  

  "- 
  s 
  m 
  

   ^ 
  

  

  

  

  \ 
  

  

  

  

  

  

  

  \ 
  

  

  

  

  

  

  

  

  

  

  2 
  4 
  6 
  

  

  Pressure 
  (/n 
  mms) 
  — 
  

  

  2 
  3 
  4 
  5 
  

  

  PRESSURE 
  ON 
  MMS) 
  — 
  - 
  

  

  of 
  ordinates 
  (ionization) 
  above 
  the 
  origin. 
  This 
  may 
  be 
  due 
  

   to 
  the 
  production 
  of 
  8 
  rays 
  in 
  the 
  ionization 
  vessel 
  by 
  impact 
  

   of 
  the 
  a-particles. 
  

  

  To 
  estimate 
  the 
  ionization 
  produced 
  by 
  the 
  recoiling 
  atoms 
  

   then, 
  it 
  is 
  necessary 
  to 
  perform 
  two 
  experiments, 
  (a) 
  with 
  

   the 
  active 
  plate 
  uncovered, 
  (b) 
  with 
  the 
  plate 
  covered 
  with 
  a 
  

   layer 
  of 
  aluminium 
  just 
  sufficient 
  to 
  stop 
  recoiling 
  atoms. 
  

   The 
  curve 
  (b) 
  must 
  be 
  plotted 
  subject 
  to 
  a 
  correction 
  for 
  the 
  

   absorption 
  of 
  the 
  a-particles 
  in 
  the 
  aluminium 
  foil, 
  i. 
  e. 
  the 
  

   linear 
  parts 
  of 
  both 
  curves 
  (a) 
  and 
  (b) 
  must 
  be 
  superposed. 
  

   Assuming 
  no 
  ionization 
  by 
  collision 
  at 
  low 
  pressures, 
  the 
  

   difference 
  between 
  the 
  two 
  curves 
  now 
  represents 
  the 
  amount 
  

   of 
  ionization 
  produced 
  by 
  the 
  recoiling 
  atoms. 
  Fig. 
  4 
  is 
  a 
  

   typical 
  example. 
  The 
  dotted 
  portion 
  in 
  curve 
  A 
  represents 
  

   the 
  ionization 
  curve 
  of 
  the 
  a-particles, 
  whilst 
  the 
  full-line 
  

   curve 
  is 
  that 
  due 
  to 
  recoil 
  atoms 
  and 
  a-particles. 
  The 
  

  

  