GM Response

["John R. Laferriere 671-8316" <John.R.Laferriere@dupontpharma.com>]

Radsafers:

Although we always use LSC for "official" wipe surveys
in labs handling Ni-63, we also use a Ludlum model 120
gas flow proportional counter with 0.4 mg/sq.cm. window
for initial in-lab screening of daily operational wipes by 
the lab workers, and have also discovered that a pancake 
GM can be helpful for gross screening for hot spots in 
the lab.  (The lab handles GBq's of Ni-63 on a regular
basis.)

The gas flow was calibrated with a traceable Ni-63 solution
deposited on a plastic substrate to minimize self-absorption,
and is normally in the 0.01 c/d efficiency range. For wipes, 
the efficiency is about half that determined by the standard, 
the drop being due to self-absorption.  We use a scaler with
the model 120, and counting times of a minute or less, for
gross screening.

We recently cross calibrated a pancake GM with the LSC
and a hot wipe, and got about 0.0018 c/d (0.18%) on
the pancake.  Given the lab background, I estimated about
a 50,000 dpm MDA for the pancake/ratemeter combination,
using a near-contact geometry.  Not a great efficiency,
but better than the zero % that we were expecting, and 
good enough to know if there's a real problem developing.
And being able to monitor work surfaces directly is a real
help for lab workers, compared with semi-blind wipes.

Years ago at another company, we used a thin NaI "low energy
probe" to roughly assay mCi quantities of H-3 in a skin puncture
wound, via detection of bremsstrahlung.  I don't recall the
efficiency, but the fact that it could be done at all was
interesting.

John R. Laferriere, C.H.P.
Medical Imaging Division,
DuPont Pharmaceuticals Company
john.r.laferriere@dupontpharma.com
(978) 671-8316

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