By	
  	
  
Vibha	
  Chaswal,	
  Ph.D.	
  
The	
  ion-­‐chamber	
  should	
  be	
  tested	
  along	
  with	
  
the	
  electrometer	
  and	
  the	
  cable	
  it	
  is...
  Acceptance	
  testing	
  is	
  recommended	
  when	
  

the	
  chamber	
  is	
  back	
  from	
  ADCL	
  before	
  using...
 

PTW	
  0.6cc	
  SN1315	
  farmer	
  type	
  ion-­‐chamber	
  by	
  PTW	
  FREIBERG.	
  Model	
  #	
  
TN	
  30013-­‐13...
  IC	
  set-­‐up:	
  100	
  cm	
  SSD;	
  ion-­‐chamber	
  active	
  

volume	
  at	
  the	
  center	
  of	
  a	
  20cmx2...
The	
  uncertainty	
  in	
  the	
  charge	
  collection	
  due	
  to	
  
stem	
  effect	
  should	
  be	
  less	
  than	
  ...
 	
  	
  	
  	
  	
  stem	
  effect	
  slightly	
  greater	
  than	
  0.5%	
  but	
  5cmx30cm	
  has	
  greater	
  
penumbr...
 	
  	
  	
  Collect	
  charge	
  readings	
  using	
  both	
  bias	
  polarities	
  
with	
  all	
  other	
  set-­‐up	
  ...
  Which	
  means	
  check	
  the	
  cylindrical	
  symmetry	
  

of	
  the	
  ion	
  chamber’s	
  active	
  volume’s	
  
...
(left)	
  Ion	
  chamber	
  major	
  axis	
  perpendicular	
  to	
  the	
  CAX.	
  (right)	
  Ion	
  chamber	
  major	
  
...
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,...
  Measurements	
  involving	
  ionizing	
  current	
  at	
  2	
  

different	
  voltages	
  are	
  used	
  to	
  assess	
 ...
Collection	
  Efficiency	
  (Pion):	
  	
  
 
 
 

A	
  feel	
  of	
  your	
  ion	
  chamber	
  
Physicist’s	
  extended	
  hand	
  and	
  mind	
  in	
  clinical	
...
The	
  SNR	
  is	
  derived	
  from	
  the	
  same	
  irradiation	
  data	
  set	
  
over	
  the	
  three	
  ion-­‐chamber...
 
1. 
2. 
3. 

 

To	
  quantify	
  the	
  leakage	
  from	
  various	
  components	
  the	
  
following	
  were	
  quan...
Ion chambers Acceptance Testing and Evaluation
Ion chambers Acceptance Testing and Evaluation
Ion chambers Acceptance Testing and Evaluation
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Ion chambers Acceptance Testing and Evaluation

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Ion chambers Acceptance Testing and Evaluation

  1. 1. By     Vibha  Chaswal,  Ph.D.  
  2. 2. 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      
  3. 3.   Acceptance  testing  is  recommended  when   the  chamber  is  back  from  ADCL  before  using   it  for  reference  or  absolute  dosimetry.      
  4. 4.   PTW  0.6cc  SN1315  farmer  type  ion-­‐chamber  by  PTW  FREIBERG.  Model  #   TN  30013-­‐1315  
  5. 5.   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    
  6. 6. 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    
  7. 7.            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    
  8. 8.        Collect  charge  readings  using  both  bias  polarities   with  all  other  set-­‐up  parameters  constant  
  9. 9.   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    
  10. 10. (left)  Ion  chamber  major  axis  perpendicular  to  the  CAX.  (right)  Ion  chamber  major   axis  parallel  to  the  CAX.  
  11. 11. ion chamber response for an orientation perpendicular to the central beam axis
  12. 12. 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)
  13. 13.   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)    
  14. 14. Collection  Efficiency  (Pion):    
  15. 15.       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    
  16. 16. 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      
  17. 17.   1.  2.  3.    To  quantify  the  leakage  from  various  components  the   following  were  quantified:   Ionizations  in  the  ion-­‐chamber  cable  in  the  field   Ionizations  in  the  triax  cable  in  the  field   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.  

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