Re: Use detector physical area, not effective area

[Ted de Castro <tdc@XRAYTED.COM>]

I think there may be some confusion here - I think I may have

mis-understood what was said.



OF COURSE you count the entire area of the probe window and NOT just the

holes in the grid. The protection grid is part of the detection system

and therefore part of its sensitivity assessment.



What I was hearing people say was to use the entire area of the PROBE -

NOT the DETECTOR - which to me includes the structure which supports and

encloses it - ie the solid annular ring that is part of the probe since

it in part of the housing that encloses the detector - but is beyond the

diameter of the detector.



I think this being said that we now are in agreement.



Steve Frantz wrote:

> 

> Ted,

> 

> I'm sorry, I don't agree unless the source is an actual point

> source.

> 

> The difference between physical and effective areas is the metal

> grid covering the detector. Anywhere you put a source, some of

> it will be covered by some part of the grid. If the grid covers

> 20% of the detector, then 20% of the alphas are blocked. So you

> use the physical size since that included the 20% loss.

> 

> If the source is an actual point source, you should measure

> different efficiencies depending on whether the grid happens to

> cover it. They you could easily measure two radically different

> efficiencies.

> 

> Bottom line, if 20% of the calibration source is covered by the

> grid, so is the actual in-field contamination. The grid is part

> of the detector efficiency.

> 

> Other opinions?

> 

> -Stephen Frantz

> -sfrantz@yahoo.com

> 

> --- Ted de Castro <tdc@xrayted.com> wrote:

> > Ok - if the source is the size of the whole detector and is

> > thus

> > effectively a flood source of known Ci/unit area.  BUT when

> > the source

> > is smaller than the effective area only using the effective

> > area makes

> > any sense.  I'd think most calibrations are done with a source

> > smaller

> > than the effective area.

> >

> >

> 

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