Re: Efficiency Determination

["Scott Richards" <sr996@poppa.fab.albany.edu>]

I think it is time to start thinking realistically about surveys that 
are taken in a research laboratory. Let me tell you about my last 
inspection that we had by our state agency. These guys wanted us to 
determine the efficiency for ALL isotopes for all of our detection 
instruments. This included meters, LSC, etc. Now, on top of that they 
wanted me to determine the different efficiencies for each of the 
different types of scintillation coctail that we use.  This all 
because they felt our efficiencies needed to be more accurate. FYI, 
we already have ludlum give us efficiencies for each instrument for 
P-32, C-14 and S-35 each time the instrument is calibrated.  Well, 
to say the least, I told them I didn't agree with them and that there 
are so many inefficiencies mainly dealing with survey techniques that 
it was not necessary to take these extra steps.  This is after I 
observed the inspectors taking wipes that range from 10 sq.cm. to 200 
sq.cm. 

As I told the inspectors, I am very aware that there are specific 
contamination limits that must be used to determine when actions 
should be taken. But really, people, with laboratory personnel, these 
surveys turn go, no-go surveys instead of quantitative surveys.  I can 
see possibly having one or two instruments with efficiencies that are 
more accurate, but not every single instrument. 

> OK, so we get an efficiency determination that is fairly accurate in the
> calibration lab.  Now we have to make real world measurements and have even
> more problems than have already been discussed.
> 
> 1.   In the research lab, we may have a variety of radionuclides in use in a
> given area.  Do we use P-32 or C-14 efficiencies for some otherwise
> unidentified contamination?  S-35 or Cr-51 efficiencies?  Do we need to wait
> until we have wipe sample counts with radionuclide identification before we
> report the survey meter results?

No, you can easily do a quick determination by covering up the probe 
with a piece of plastic that will shield out C-14 or S-35.
> 
> 2.   In making an actual measurement, it makes a great deal of difference
> for a GM pancake probe whether the contamination is in a small spot (all of
> the radioactivity is under the detector) or spread out over 100 cm^2 or more
> (only about 20% of the radioactivity is under the detector).  The tiny drop
> of spilled activity is quite common in the research environment.

Another inefficiency.......
> 
> The accuracy of the attempt to measure dpm goes down fast when using a
> portable survey meter for operational work.  I am, therefore, an advocate of
> reporting survey meter results in net cpm.  The more hazardous nuclides
> (such as P-32) will be counted with a higher efficiency than most of the
> less hazardous beta emitters (such as C-14, S-35, etc.).  Thus, 250 net cpm
> of P-32 will represent a lower activity than 250 net cpm of S-35, provided
> that the physical characteristics of the spills are similar.  We have used
> cpm values for our survey meter contamination action levels for many years,
> and this policy has survived a number of state inspections.
> 
> Frank E. Gallagher, III, CHP    
> Manager, Radiation Protection,
>   and Radiation Safety Officer
> University of California, Irvine
> Environmental Health and Safety     |    Phone:  (714) 824-6904
> 300 University Tower                |    Fax:    (714) 824-8539
> Irvine, CA   92697-2725             |    E-mail: fegallag@uci.edu
> 
> 
Scott Richards, RRPT
Radiation Safety Officer
University at Albany, SUNY
1400 Washington Ave., Chemistry B-73
Albany, NY  12222
518-442-3497
Fax: 518-442-3783
SR996@poppa.fab.albany.edu