Re: ED underresponse in pulsed x-ray fields

[welch@CEBAF.GOV (welch)]

We have performed one or two qualitative tests like the one you described at
CEBAF with pulsed beam.  We happen to use the SAIC PD-3 ED.  In our test,
the ED responded surprisingly well, I think due to the positioning of the
dosimeters in a field consisting of mainly scattered radiation.  (In the
test, the beam was   1 GeV, 60-120 Hz, 100 usec on a thick copper target)
However, I have heard from someone at LBL (I think, it's been a while) who
used the same dosimeter in a Mega-rad/hr (instantaneous) field (pulsed as
well) and they responded admirably.
This surprised me because the PD-3 is a GM tube. The saving feature of the
PD-3 may be that in its calibration regime, you do a single point
calibration at a known integrated dose - and you also expose it to a high
dose rate to apply a dead time correction (it's got an EPROM an a really
easy calibration).  If I remember correctly, you can choose the dose rate
for the DT correction, so presumably, you could really blast it during
calibration to set up DT correction properly for Krad-Mrad type rates.  That
doesn't explain all that's going on with the pulsed response though.

If this sounds like a plug, well, take it any way you want, I like 'em.

>     
>     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

KW