Re: Question about contamination levels vs. exposure rates

["David Whitfill" <DWhitfil@kdhe.state.ks.us>]

I did some response testing of a GM pancake probe using a PTB beta standard
set (Sr/Y-90, Tl-204, Pm-147). Distances from the source to detector
distance was 200 mm for Pm-147 and 300 mm for Tl-204 and Sr/Y-90.

Here are the actual measured values:

Pm-147 (beta max 224 keV) 16, 000 cpm, 2.75E-03 mrad/s ===> 1.62E+03 cpm
per mrad/h
Tl-204 (beta max 766 keV) 70,000 cpm, 1.72E-02 mrad/s ===> 1.13E+03 cpm per
mrad/h
Sr/Y-90 (beta max 2,270 keV from Y-90, 546 keV from Sr-90) 400,000 cpm,
2.07E-01 mrad/s ===>5.37E+02 cpm per mrad/h

AVG = 1.10E+03 cpm per mrad/h
STDS = 5.42E+02 cpm per mrad/h

For thumbrule purpose using 1,000 cpm per mrad/h is probably good enough
(or 10,000 dpm per mrad/h with an efficiency of 0.1 count per
disintegration). It is not unusual to require eye protection for areas with
100 mrad/h beta as measured with an ion chamber where mrad/h= CF (OW-CW)
with the CF typically equal to 4 mrad/mR and the open window and closed
window readings of course in mR. This would be one hot smear!

DJWhitfill

Opinions expressed are mine and do not reflect official policies or
positions of the Kansas Department of Health and Environment.


                                                                                                
                    "Winters, Mike"                                                             
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                    02/09/00 08:30 AM                                                           
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Are there any thumb rules for Beta exposure rates as they relate
contamination levels?

We are trying to pick an arbitrary contamination level in dpm at which we
would instruct our technicians to obtain an open window dose rate.  Many
places use 100,000 dpm as the threshold for doserate.  What do you use? If
different, how did you derive your level?

Thanks

Michael  S. Winters
USEC Health Physics
Paducah Gaseous Diffusion Plant
Mailstop 7115
5600 Hobbs Road, Paducah, KY, 42001
ph  270-441-6943
fax 270-441-6113
email: wintersms@pgdp.usec.com

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