I-125

[Larry_R_Sanders_at_~BMI4@battelle.org]

     I wish to thank everyone who responded to my request for information 
     on source jigs for calibrating ion chambers.
      
     I have another series of questions which revolve around the use and 
     detection of I-125.
     
     We are currently beginning to use I-125 as a radiotracer in studies. 
     Most of the bugs are worked out for the use portion of the study. The 
     only problem that we are having is with detection.
     
     Our license bounds us to reach an LLD of 300 dpm. This 300 dpm limit 
     is for fixed contamination. That is, we can't release items or 
     materials which contain fixed contamination greater than 300 dpm. The 
     source for this limit being Reg Guide 1.86 and Policy and Guidance 
     Directive FC 83-23," Termination of Byproduct, Source and Special 
     Nuclear Materials Licenses." Decay in storage is currently not an 
     option at our facility.
     
     Can anyone tell me where the 300 dpm limit for releasing of I-125 
     contaminated material came from, or the basis for this limit? It seems 
     I-125 and I-129 require the same release limit as Ra-226, Ra-228, 
     Th-230, Th-228, Pa231, and Ac-227. Other isotopes from Iodine have a 
     significantly higher release limit.  
     
     We currently have a LEG-1 (Low Energy Gamma detector) calibrated to 
     I-129 which sees the 300 dpm, but only when static readings are taken. 
     I am under the impression that GM detectors such as the Ludlum Model 3 
     with a pancake probe will see the I-125 conversion electrons, but not 
     at the required LLD. I can't calibrate the Ludlums with I-129 because 
     the detector can't see the gammas. 
     
     The I-129 source which I ordered to calibrate the LEG-1 detector, 
     ESP-2 instrument was NIST traceable to 377 Bq. That is the actual 
     activity amount of I-129 was 377 Bq. This however, was placed in a 
     plastic backing and cover and sealed. The plastic backing was 2.7 mm. 
     When we tried to calibrate the instrument we were not seeing 377 Bq 
     because of attenuation of the low energy gammas through the plastic. 
     The source manufacturer stated he could not tell us the emission rate 
     of the source, only the actual activity of the material. He also 
     stated, the plastic could attenuate up to 10 % of the gammas but not 
     to hold him to that. We did our own attenuation experiments and got 
     attenuation of up to 50 %, but this was after emission through the 
     plastic. Based on this we assumed a 10% attenuation. 
     
     
     Does this seem reasonable ? Has anyone else run into this problem?
     
     The next issue we had to tackle was the actual emission spectrum of 
     the I-129 source. Apparently, the source emits low energy gammas with 
     an intensity of around around 79%. I know the beta emission intensity 
     is 100%. I'm not sure what the other 21% emission entails.
     
     Can anyone tell me what the other 21% is?
     
     We now have a need for obtaining a backup instrument and detector for 
     researcher and radiation safety use. Instead of purchasing another LEG 
     we are trying to lease one. Does anyone know where I could lease an 
     LEG detector or equivalent? I have tried GTS but they have no 
     detectors which will meet the required LLD?         
     
     I apologize for the length of this message, but I believe these are 
     valid questions to ask and my resources are limited except for this 
     medium. Thank you for your patience. Please respond to my E-Mail 
     unless you feel the information can be used by other RADSAFE users.
     
     Larry R. Sanders. 
     Radiation Technical Support
     Battelle Memorial Institute
     Larry_R_Sanders_at_~BMI4@battelle.org