By    Vibha  Chaswal,  Ph.D.  

  The  ion-­‐chamber  should  be  tested  along  with  the  electrometer  and  the  cable  it  is  going  to  get  used  with    

  For  reproducibility  of  results  the  ion-­‐chamber  should  also  be  tested  against  different  electrometers  and  the  results  should  have  minimum  variability    

  It  is  important  to  be  aware  of  various  sources  of  noise  in  the  system  that  contribute  to  charge-­‐collection  at  the  ion-­‐chamber  electrodes  in  addition  to  charge  collected  due  to  ionizations  by  radiation    

  Acceptance  testing  is  recommended  when  the  chamber  is  back  from  ADCL  before  using  it  for  reference  or  absolute  dosimetry.      

  PTW  0.6cc  SN1315  farmer  type  ion-­‐chamber  by  PTW  FREIBERG.  Model  #  TN  30013-­‐1315  

  IC  set-­‐up:  100  cm  SSD;  ion-­‐chamber  active  volume  at  the  center  of  a  20cmx20cm  field;  solid-­‐water  phantom  with  1.5  cm  build-­‐up  and  5  cm  back  scatter    

  Electrometer:  -­‐300  V  (-­‐100%)  bias      Energy:  6  MV  photon  beam      Procedure:  Record  charge  collection  measurements  for  MU  delivery  ranging  between  2  MU  to  200  MU    

  The  uncertainty  in  the  charge  collection  due  to  stem  effect  should  be  less  than  0.5%  

  This  can  be  checked  by  taking  exposures  using  a  field  size  that  irradiates  just  the  thimble  and  comparing  it  with  charge  collection  reading  taken  when  the  whole  stem  is  in  the  field.    

  Set  up:  100  cm  SSD;  FS  5cmx30cm;  tape  the  IC  in  two  orientations  –  IC  parallel  to  the  30  cm  dim  of  field,  IC  perpendicular  to  it  with  only  thimble  inside  direct  radiation  beam  

           stem  effect  slightly  greater  than  0.5%  but  5cmx30cm  has  greater  penumbra  uncertainties  repeat  test  using  7cmx30cm  or  10cmx30cm  FS  

         stem  effect  is  0.2%,  within  manufacture’s  specifications    

       Collect  charge  readings  using  both  bias  polarities  with  all  other  set-­‐up  parameters  constant  

 Which  means  check  the  cylindrical  symmetry  of  the  ion  chamber’s  active  volume’s  construction    

  Ion  chamber  suspended  in-­‐air  with  the  ion  chamber’s  build-­‐up  cap  on    

  This  provides  same  build-­‐up  from  all  different  directions  of  irradiation    

(left)  Ion  chamber  major  axis  perpendicular  to  the  CAX.  (right)  Ion  chamber  major  axis  parallel  to  the  CAX.  

ion chamber response for an orientation perpendicular to the central beam axis

Ion chamber response for an orientation parallel to the central beam axis (Set up: 10 x 10 field, 100 SAD, 06 MV X-rays, 100 MU, CAX parallel)

 Measurements  involving  ionizing  current  at  2  different  voltages  are  used  to  assess  the  collection  efficiency  of  an  ion  chamber  [Boag  2-­‐volatge  technique]    

  This  is  ‘Pion’  or  recombination  correction  factor  [TG  51]    

  Use  electrometer    100  %  and  50  %  voltage  settings  using  negative  and  positive  bias  (Vh  and  Vl  settings)    

Collection  Efficiency  (Pion):    

  A  feel  of  your  ion  chamber    Physicist’s  extended  hand  and  mind  in  clinical  reference  

dosimetry    JEB’s  way  of  keeping  things  in  head  for  an  easy  reference  

 *Average  nC/cc  MU  for  a  0.6  cc  farmers  chamber  =  0.2092  nC/cc/  MU  *Average  nC/cc  MU  for  a  0.125  cc  farmers  chamber  =  0.1833  nC/cc/  MU    *Average  nC/cc  MU  for  a  0.015  cc  farmers  chamber  =  0.1714  nC/cc  /MU  

OR,  simply  put…  

….the  average  charge  collected  per  cubic  centimeter  of  the  chamber’s  active  volume  for  a  monitor  unit  of  radiation  is  roughly  ~0.2  nC  

So,  expected  charge  collection  for  100  MU  ~  20  nC…..right,  Happy  Physicist!  

*  Averages  derived  from  detailed  linearity  measurements  using  3  chamber  types    

  The  SNR  is  derived  from  the  same  irradiation  data  set  over  the  three  ion-­‐chambers    

  SNR  =  mean  signal  (your  data)/  Std  Deviation  SNR  test  Results  observations:    SNR  generally  increases  with  the  increase  in  Signal      Normalized  SNR  values  show  that  the  SNR  decreases  

with  the  decrease  in  ion  chamber  collection  volume  (reason  I  wouldn’t  try  to  use  a  0.015  cc  ion-­‐chamber  to  assess  doses  at  very  low  signals)  

  At  10  MU  using  0.015  pin-­‐point  -­‐  noise  and  the  detected  signal  are  almost  equal    

  To  quantify  the  leakage  from  various  components  the  following  were  quantified:  

1.  Ionizations  in  the  ion-­‐chamber  cable  in  the  field  2.  Ionizations  in  the  triax  cable  in  the  field  3.  Leakage  when  no  irradiation  but  power  supply  is  on.  

This  was  quantified  with  and  without  the  ion-­‐chamber  being  connected  to  the  electrometer.  

  Leakage  only  due  to  electrometer  electronics  can  be  characterized  by  letting  the  electrometers  run  on  battery  for  a  given  duration  of  time  without  any  connecting  wires  or  ion-­‐chamber.