ED underresponse in pulsed x-ray fields

[BYRPV@ccmail.ceco.com]

     Radsafers,
     
     We recently performed X-ray radiography at Byron using a 6 MeV 
     electron accelerator (MINAC 6) with a tungsten target as the X-ray 
     source.  I anticipated that the electronic dosimeters we use on site 
     would underrespond but was surprised to measure a 90% underresponse.  
     This effect occurred with EDs placed in line-of-sight with the MINAC 
     and well out of the beam.  The effect was not observed in areas where 
     the beam or the leakage (effective energy of 2.5 MeV) had been 
     attenuated by concrete walls or floors.  In these cases, the EDs were 
     in lower dose rate fields and responded accurately.
     
     The experiment was not set up with research grade rigor but merely 
     consisted of EDs placed side by side with ion chamber self reading 
     dosimeters (SRDs) at various locations in the vicinity of the MINAC.  
     
     There are several factors which probably worked in concert to produce 
     the underresponse.  The MINAC 6 delivers 4 usec pulses at a rate of up 
     to 200 per second.  This equates to a duty cycle of approximately 
     0.0008 meaning the peak dose rate is approximately 1250 times the 
     effective dose rate.  The EDs underrespond to high dose rates (ie. 
     thousands of rem/hr).  Also, the electronics may have been fooled by 
     the pulsed nature of the radiation.
     
     This is just an informative message.  If anyone wants further 
     information or has had similar experience, I invite comments.
     
     Paul Vitalis
     ALARA Health Physicist
     Byron Nuclear Power Station
     4450 N. German Church Rd.
     Byron, IL 61010
     
     Byrpv@ccmail.ceco.com
     
     Any opinions expressed are mine alone and do not necessarily represent 
     those of my employer.