[ RadSafe ] hand held meters

[Ted de Castro]

I'm doing some consulting now surveying some x-ray enclosures for 
leakage with a pancake GM.  Background seems to vary slightly from visit 
to visit between 0.010 and 0.015 mR/hr - usually the latter. To actually 
resolve that difference with any confidence (statistically speaking) 
I'll do a 10 minute integrated count - 1 minute is insufficient.

For this job 0.05 mR/hr is the acceptance limit.  Even with a SLOW scan 
of 2 inches per second I enter null readings as < 0.02 and that is as 
much as I can confidently attest to at that scan rate. Likewise up to 
the limit and to discern the limit or over limit as an equality I 
localize/maximize and then hold and count.  At these very low count 
rates it takes some experience listening to the clicks and can ONLY (in 
my opinion) be done by listening - and analogue meter is nearly useless 
for this determination and a digital absolutely useless.  ALSO the 
refresh rate for the digital meter should also be considered in 
assessing your maximum scan rate.

SO much for surveys....

On another project I set up at a national laboratory were I worked 
before I retired - I set up energy compensated glass GM detectors - 1 
inch dia x 12 inches active length.  On THAT project I collected 10 
minute count integrations and could easily discern small variations in 
background like seasonal variation or when it starts to rain or is 
raining and when it stops and as the ground dries (no snow in Berkeley - 
usually).  I always figured using these numbers that I could also tell 
when the soil was completely saturated - and ready slide - but never had 
a situation to confirm that.

So - it depends completely on the instrument and how that data is 
aggregated.

I chose GM for these area monitors because although that aren't as 
sensitive as a scintillator or maybe a large Reuter Stokes chamber - 
they are STABLE for use long term in a less than optimal environment 
(unheated/uncooled but dry sheds).

There is a lot to be said for scintillators for surveys - but they are 
more expensive, more finicky and, I feel, show you more than you need to 
see (I don't accept LNTH).  With the GM I find myself in situations 
where "interested parties" looking over my shoulder as I survey are 
comforted that "nothing was detected" - They can see it for themselves! 
- legitimately.  If instead I was using say a 5 inch scintillator - to 
use an extreme  counter example - I'd have almost certainly detected 
"something" albeit insignificantly minimal - BUT still "something" and 
that's enough to leave some "interested parties" "unsettled".  Since in 
this case I use my own instrumentation - a 5 inch scintillator would be 
out of reach anyhow and certainly not cost effective at my rates and 
certainly not necessary to assure meeting acceptance criteria.


On 1/14/2014 12:18 PM, JOHN.RICH at sargentlundy.com wrote:
> radsafers
>
> We're looking for practical experience on using hand held monitors in an
> outdoor setting.
>
> The background gamma dose rates in the area are about 0.02 mR/hr.
> The expected change that we want to see is from about 0.02 mR/hr to 0.04
> mR/hr.
> So the dose rate goes from about 0.02 mR/hr to 0.04 - 0.06 mR/hr. (two x
> background to 3 x background)
>
> The questions are:
> (1) what kind of hand held monitor would be good to see this change (e.g.,
> PIC, GM tube, scintillation detector, etc.)?
> (2) since this is outdoors, how long should the surveyor wait for the
> readings to stabilize after a random spike?.
>
> I asked a similar question earlier,  and the consensus seemed to be that
> making these measurements with a hand held monitor was problematic. My
> personal experience in this area is very limited, but it seems like the
> spikes could reach about 0.01 mR/hr and lasted several seconds.
>
> thanx in advance  - -jmr
>
> John Rich
> 312-269-3768
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