﻿910 
  Prof. 
  Rutherford 
  and 
  Mr. 
  Richardson 
  : 
  Analysis 
  of 
  

  

  a 
  or 
  /3 
  rays 
  in 
  the 
  material 
  on 
  which 
  the 
  active 
  matter 
  

   is 
  deposited. 
  

  

  Since 
  thorium 
  C 
  does 
  not 
  emit 
  any 
  appreciable 
  7 
  radiation, 
  

   the 
  results 
  of 
  these 
  experiments 
  thus 
  indicate 
  that 
  thorium 
  D 
  

   emits 
  only 
  one 
  penetrating 
  type. 
  This 
  conclusion 
  was 
  con- 
  

   firmed 
  by 
  examining 
  the 
  7 
  radiation 
  from 
  pure 
  thorium 
  D 
  

   obtained 
  by 
  recoil 
  on 
  an 
  aluminium 
  plate. 
  Although 
  the 
  

   effects 
  were 
  too 
  small 
  and 
  the 
  period 
  of 
  transformation 
  too 
  

   short 
  for 
  accurate 
  measurements, 
  the 
  results 
  clearly 
  indicated 
  

   that 
  no 
  appreciable 
  amount 
  of 
  soft 
  radiation 
  was 
  present 
  

   under 
  those 
  conditions. 
  

  

  Analysis 
  of 
  the 
  rays 
  from 
  Thorium 
  (B 
  + 
  C 
  + 
  D). 
  

  

  In 
  order 
  to 
  determine 
  the 
  penetrating 
  power 
  of 
  the 
  rays 
  

   given 
  out 
  by 
  thorium 
  B, 
  it 
  was 
  necessary 
  to 
  use 
  sources 
  

   containing 
  the 
  active 
  deposit 
  in 
  equilibrium. 
  We 
  have 
  

   seen 
  above 
  that 
  two 
  sources 
  were 
  prepared 
  for 
  this 
  purpose. 
  

   When 
  the 
  radiothorium 
  source 
  was 
  employed, 
  the 
  preparation 
  

   was 
  contained 
  in 
  a 
  watch-glass 
  and 
  covered 
  with 
  goldbeater's 
  

   skin 
  to 
  prevent 
  the 
  escape 
  of 
  the 
  emanation. 
  The 
  absorption 
  

   curve, 
  in 
  aluminium, 
  was 
  determined 
  up 
  to 
  a 
  thickness 
  of 
  

   11 
  cm. 
  The 
  curves 
  obtained 
  are 
  shown 
  in 
  figs. 
  1 
  & 
  2 
  

   (PL 
  XIX.). 
  These 
  curves 
  were 
  analysed 
  by 
  the 
  method 
  

   described 
  in 
  previous 
  papers 
  *. 
  It 
  will 
  be 
  seen 
  that 
  the 
  

   curves 
  can 
  be 
  separated 
  into 
  four 
  exponential 
  types 
  corre- 
  

   sponding 
  to 
  groups 
  of 
  rays 
  of 
  penetrating 
  powers 
  yu,= 
  160, 
  

   fi 
  = 
  32, 
  yu- 
  = 
  0'36, 
  and 
  /jl 
  = 
  0*096. 
  The 
  last 
  group 
  is 
  of 
  course 
  

   that 
  due 
  to 
  thorium 
  I), 
  and 
  the 
  remaining 
  three 
  are 
  given 
  

   out 
  by 
  thorium 
  B. 
  

  

  Fig. 
  1 
  shows 
  the 
  analysis 
  of 
  the 
  soft 
  types 
  of 
  radiation. 
  

   The 
  logarithm 
  curve 
  lor 
  the 
  second 
  type 
  of 
  radiation 
  yu,= 
  32 
  

   is 
  shown 
  in 
  the 
  figure. 
  The 
  points 
  lie 
  on 
  a 
  straight 
  line 
  

   showing 
  that 
  this 
  radiation 
  is 
  absorbed 
  exponentially. 
  Fig. 
  2 
  

   brings 
  out 
  in 
  a 
  similar 
  way 
  the 
  exponential 
  absorption 
  of 
  the 
  

   penetrating 
  type 
  of 
  radiation 
  /a 
  = 
  - 
  096, 
  and 
  also 
  the 
  relative 
  

   effect 
  in 
  the 
  experiments 
  due 
  to 
  the 
  penetrating 
  radiation 
  from 
  

   thorium 
  B, 
  viz. 
  /x 
  = 
  0'36. 
  This 
  latter 
  radiation 
  probably 
  

   corresponds 
  to 
  that 
  described 
  by 
  Marsden 
  and 
  Wilson 
  t 
  

   and 
  by 
  Hahn 
  and 
  Meitner 
  J, 
  and 
  noted 
  in 
  papers 
  recently 
  

   published. 
  

  

  * 
  Rutherford 
  & 
  Richardson, 
  Phil. 
  Mag. 
  xxv. 
  p. 
  722 
  (1913). 
  

   f 
  Marsden 
  & 
  Wilson, 
  Phil. 
  Mag. 
  xxvi. 
  p. 
  354 
  (1913). 
  

   + 
  Meitner 
  & 
  Hahn, 
  Phys. 
  Zeit. 
  xiv. 
  p. 
  873 
  (1913). 
  

  

  