Re: NDT (!?...how about "surveying for shielding adequacy") of diagnostic x-ray rooms

[Douglas Simpkin <dsimpkin@execpc.com>]

Before we all run out with our vials of Tc-99m and NaI survey meters to 
survey our x-ray rooms, consider the following.

1. The NRC licenses institutions for 35.100 & 200 materials with very 
specific uses in mind. SURVEYING X-RAY ROOMS IS NOT ONE OF THEM! While an 
agreement state inspector might be more lenient (so I've heard), our NRC 
inspectors would get ALL bent out of shape if we had to report an incident 
involving a dropped Ci source of liquid Tc-99m being used to survey an 
x-ray room! So make sure you don't violate your NRC license in the pursuit 
of shielding information!

2. You can't assume that the transmission of any monoenergetic gamma ray 
through a shielding barrier is equal to that of a polyenergetic x-ray 
source. Yes, you could take your measured gamma ray transmission and 
guesstimate the barrier thickness x from
		x = -1/mu * ln( Transmission ),
but this only works for narrow photon beams. Data on the broad-beam 
transmission of most gamma emitters (other than Ir-192, Co-60, Au-198, and 
Cs-137) is sparse.

3. I'd also be wary of assuming that the transmission measured by a 
scintillation survey meter gives the same transmission as a proper 
exposure-measuring device (e.g., an ionization survey meter). The response 
to scatter may be grossly different for the two meters. [But then, the 
ionization survey meter needs high incident flux rates to detect the small 
transmitted exposure, which is where the Ci activity sources come in....]

Yes, the subject of surveying x-ray room shielding adequacy will be covered 
in the rewrite of the NCRP-49 report, and no, I don't know when it'll be 
published...

Doug
Douglas J. Simpkin, Ph.D., D.A.B.R.
St. Luke's Medical Center
2900 West Oklahoma Avenue
Milwaukee, WI 53215
phone: (414)649-6457
fax: (414)649-5118
email: dsimpkin@execpc.com

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