[     806     J 
LXXV.   On  the  Ionization  produced  by  a  Rays. 
By  Howaed  L.  Bronson,  Ph.l)* 
THE  following  paper  contains  an  account  of  the  investi- 
gation of  two  entirely  distinct,  but  closely  related 
problems.  The  first  is  an  investigation  to  determine  how 
the  ionization  produced  by  an  a  particle  varies  near  the  end 
of  its  path.  The  second  is  an  investigation  to  prove  definitely 
whether  or  not  radium  B  emits  a  particles  which  are  able  to 
ionize  the  air  for  even  a  short  distance. 
Investigations  by  Bragg  and  Kleeman  (Phil.  Mag.  Dec. 
1904)  and  by  McClung  (Phil.  Mag.  Jan.  1906)  on  the 
ionization  due  to  the  &■  rays  from  radium  C,  have  shown  that 
the  ionization  per  cm.  increases  gradually  with  the  distance 
from  the  source  for  about  6  cms.  and  then  decreases  ex- 
ceedinglv  rapidly,  falling  to  zero  or  very  nearly  to  zero  at  7  cms. 
Rutherford  (Phil.  Mag.  July  1905,  Jan.  1906,  and  April  1906) 
examined  the  photographic  action  of  the  a  rays  from  radium  C, 
but  was  unable  to  get  any  evidence  of  such  action  through 
more  than  about  7  cms.  of  air.  His  calculations,  based  on 
the  amount  of  magnetic  deflexion  of  the  a  particles  which 
had  passed  through  matter  equivalent  in  absorbing  power  to 
nearly  7  cms.  of  air,  show  that  the  a  particles  still  possessed 
about  40  per  cent,  of  their  initial  velocity,  while  the  photo- 
graphic action  was  relatively  very  feeble. 
Now  Townsend  (Phil.  Mag.  Nov.  1903)  has  shown  that  a 
positive  ion  of  mass  comparable  with  that  of  the  a  particle  is 
able  to  produce  fresh  ions  by  collision,  when  its  velocity  is 
undoubtedly  very  much  smaller  than  that  of  the  ex,  particle  at 
the  time  when  it  apparently  loses  its  ionizing  power.  In 
Townsend's  experiments,  however,  the  positive  ion  was  a 
very  inefficient  ionizer  compared  with  the  a  particle,  for  it 
produced  fresh  ions  at  only  a  small  fraction  of  the  total 
number  of  collisions  with  the  gas  molecules,  while  Rutherford 
('  Radioactivity/  p.  434)  has  shown  that  it  is  probable  that 
the  a  particle  at  its  maximum  efficiency  produces  an  ion  at 
practically  every  collision. 
It  would  thus  appear  probable  either  that  the  velocity  of 
the  a  particle  decreases  very  rapidly  when  it  falls  below  about 
40  per  cent,  of  its  maximum,  or  that  its  efficiency  as  an 
ionizer  falls  off  very  rapidly  without  a  corresponding  falling 
off  in  the  velocity.  In  the  latter  case,  w7e  might  expect  to 
find  a  small  ionizing  action  extending  for  a  considerable 
distance  beyond  7  cms.  An  investigation  of  this  point 
therefore  seemed  desirable. 
*  Communicated  by  Prof.  E.  Kutherford,  F.E.S. 
