Re: Gamma surveys (in-situ)

["Frank Helk" <frank.helk@nis-hanau.de>]

Hi Jay,



>Okay, this is probably a stupid question to most of you, but is there a 

>standard distance between detector and surface being measured?  



The most simple answer, especially with Gross-Gamma counting in mind:



"Depends"



To be more precise: It has to be optimized to your measuring goal.



>We have one meter grids and are wanting a kind of secondary reassurance 

>that we've found everything, so we're doing gross gamma counting in addition 

>to all other surveying.  Probe is centered over the square. 



For counting efficiency, you need to be as near as possible, with as less as possible shielding material in between. On the other side, if you are near, even 

little positioning inaccuracies have big effects on the efficiency.



For your one meter grids the size of the probe is to be seen in relation to the grid. If the probe size is small against the grid area, you'll get an overwhelming 

efficiency difference by the location of a possible hot spot. The ideal case would be a probe in the size of a grid field, positioned directly on the area (or very 

close to ...)



I would recommend a series of experiments to evaluate an optimal geometry, i.e. you put the probe centerd over a (inactive, laboratory) area of 1 x 1 m. Then 

you put a calibration source on various points within the area, at least along a diagonal line from the center to an edge. For the measuring points you 

calculate efficiency and detection limit. Repeat for several distances between probe and surface.



The optimum is the maximum distance where you can safely achieve the dectection limit needed, because the geometric effects decrease with the distance. 

That way you'll achieve your detection goal with the most uniform efficiency distribution.



Hope it helps.



Best regards



	Frank





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