Re: FW: Certification of "box" calibrators

[james.g.barnes@ATT.NET]

Glen, et. al.



We used to use a Box Calibrator, but it gave us fits on 

calibration.  One of the central problems we had was that 

one sometimes needed to use larger probes on the 

condenser R meter to get an accurate measurement, but due 

to the size of the probe vs. the volume of the box, you 

ran into all sorts of geometric problems trying to 

determine if your readings were representative or not.  I 

had to smile at John Andrews' statement, "I have always 

fit the data from Landsverk R meters to the curve with a 

polynomial function using as many terms as necessary to 

get the damn thing to fit."  I have had similar 

experiences with them.



Bottom line is that I think the only way you can be sure 

with a box is to use a transfer instrument of the exact 

make and model of the instrument you intend to calibrate 

in the box.  This isn't quite as pretty as a condensor R 

meter readout, but it is the only way I know of that you 

can be sure what the instrument itself is seeing.



On a final note, we retired the box calibrator and set up 

a free range calibrator.  We have been much more 

comfortable with the results, and with clever use of 

inexpensive cameras and shadow shields, the dose to the 

calibration personnel can be easily controlled.



Jim Barnes, CHP

Radiation Safety Officer

Rocketdyne/Boeing

  

> Glen, I believe that with the heavier attenuators you have a virtual source 

> somewhat in front of the real source because of scattering from the 

> attenuator.  I have always fit the data from Landsverk R meters to the curve 

> with a polynomial function using as many terms as necessary to get the damn 

> thing to fit.  

> 

> However, I have never had to calibrate a box calibrator.  I also have seen 

> beaming in a box calibrator from the brass screws used to hold the 

> attenuators to the mechanism.  The attenuation of the brass is significantly 

> different so that there was a non-uniform field in the box with hot spots 

> where the beam penetrated the screws better than the lead.  

> 

> If you want to be precise, you should compare the response of the specific 

> instrument to the beam in the box calibrator and to the beam of a free air 

> calibrator.  The calibration of the free air beam should be well known.  In 

> this way all the scattering in the box calibrator is accounted for.  Once 

> this is done, you should not have to "recalibrate" the box again unless you 

> change something that changes the scattering for a particular detector.

> 

> Personally, I don't much care for box calibrators.

> 

> 

> 

> John Andrews

> Knoxville, Tennessee

************************************************************************

You are currently subscribed to the Radsafe mailing list. To unsubscribe,

send an e-mail to Majordomo@list.vanderbilt.edu  Put the text "unsubscribe

radsafe" (no quote marks) in the body of the e-mail, with no subject line.