Re: QC for well counters

["Mike Bohan" <mike.bohan@yale.edu>]

        Reply to:   RE>>QC for well counters

To All Radsafers:

We cross calibrate our NaI well counters with Co-57 standards.  It has a similar
energy spectrum and photon abundance to Tc-99m so that only a minor adjustment
is necessary to calibrate to Tc-99m and better yet, it is secondarily tracible
to a NIST standard which wouldn't be possible directly for Tc-99m.  The NRC has
found this method acceptable to document the MDA of a Tc-99m counting system. 
We also cross calibrate Ba-133 for I-131 and I-129 for I-125 for similar
reasons.

In a related matter, Gd-153 also has a similar spectrum to Tc-99m which I
discovered when we received a Gd-153 SPECT Transmission source which was
contaminated with Co-57!  

A NaI based single channel analyser detected the contamination but could not
distinguish the Gd-153 from Co-57.  Fortunately, I had access to a multichannel
analyser and a old (very) Gd-153 Bone Densitometer source around to confirm my
discovery!  If you are receiving SPECT Tranmission sources, I would leak test
them immediately on receipt rather than depend on the manufacturers initial leak
test before putting them into service.  This source was leak tested by the
manufacturer, but was subsequently contaminated because the Gd-153 and Co-57
sources are manufactured in the same area.

Just a reminder, to stay on your toes!?

Mike Bohan                                e-mail: mike.bohan@yale.edu
Radiation Safety Officer              Tele: (203) 785-2950
Yale-New Haven Hospital               Fax: (203) 737-4252
Radiological Physics
20 York St. - WWW 204
New Haven, CT   06504