FW: Certification of "box" calibrators

[glen.vickers@EXELONCORP.COM]

> -----Original Message-----

> From:	Vickers, Glen 

> Sent:	Tuesday, November 20, 2001 12:12 PM

> To:	Radsafe (E-mail)

> Subject:	Certification of "box" calibrators

> 

> Group,

> 

> Here's something refreshing.  I'm looking for more detailed information on

> box calibrators.

> 

> What type of transfer instrument do you use to certify your box

> calibrator.  Our specific irradiator is a Shepherd Model 89 which spans

> from 0.1 mR/hr to 1200 R/hr.  It's difficult to find a single instrument

> that can span such a wide range.  We use MDH-1015C with multiple probes,

> but even with the low rate probe, the leakage currents can be a

> substantial portion of the signal you're trying to measure.  Integrated

> counts over time are necessary when we get down to a single sig fig and

> this only exaggerates leakage current losses.  We always make sure we have

> 2 sig figs to generate a smooth curve fit.

> 

> At what rate should the source fall off?  Should it be x^-2 or a little

> slower like x^-1.95 due to build up and backscatter?  When the curve falls

> of more rapidly than x^-2, then I would suspect a loss mechanism in the

> instrument.  I've noticed situations where the rate of fall off increases

> as the dose rate decreases.  I'm thinking this is where leakage current

> might come in.  The fix current rate loss becomes more significant as the

> dose rate and current of interest declines.  It appears to be a smooth

> function as you increase the attenuators such that you could probably

> calculate the loss by comparison of the curve fit exponentials and provide

> corrections.  Self-correcting data?  I think so.  I can resolve in my mind

> how the curve fit could easily be corrected, but how about the

> y-intercept.  How much should it be modified?My old professor was right

> when he said your detailed mathematical modeling skills fade as you start

> working in the real world.

> 

> Here's some real data.  Notice the rate at which the exponent falls off

> increases as the attenuation increases.

> 

> Atten	Exponent

> 0	-1.9942

> 2	-2.0571

> 4	-2.1209

> 8	-2.1894

> 10	-2.2855

> 20	-2.2255

> 40	-2.4928

> 80	-2.5452

> 100	-2.5592

> 200	-2.8082

> 

> 

> Anybody do any MCNP simulations of a box calibrator?  Personally, I like

> some of the work done by Canberra, where they'll model a geometry, verify

> with a couple of measurements, and provide a greatly simplified

> calibration process.  I believe that once you modeled a box calibrator,

> you could crunch your MCNP curve fits against the source Ci content and

> come up with less error than the typical use of a transfer instrument.

> 

> Any good references?  None of the standard textbooks have the answers in

> the back.  I know I'm asking questions in the right direction, I'm looking

> to firm up my ability to diagnose and correct anomolies in the

> certification process.

> 

> Glen Vickers

> 





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