[ RadSafe ] hand held meters

Wed Jan 15 09:02:52 CST 2014

When you survey for gamma radiation in the field, you often use an
uncollimated single pixel radiation detector, and scan it over a surface.
This provides a "fuzzy" image if one assembles the data into an array.
HASL-300 has useful information that can be applied to this survey.  Figure
3.5 (taken from HASL-300) below shows that for a detector held at a meter
above ground, roughly half of the response is from the area formed by the 3
meter radius.  While HASL 300 says most of the response comes from a radius
of  10 meters to a depth of 30 centimeters, in a mostly uniform field, I
suggest that 3 meter radius and 10 centimeter depth provides at least 50%
of the uncollided flux for photons in the compton dominant energy range at
one meter height.  You can vary the size of the area (or pixel size)
contributing to the flux on the detector by changing the height.  Most
surveys use a detector that is too small (not enough grams of media).  The
surveyors observe count rate fluctuations that are largely statistical, and
not due to variations in ground concentration.  I often designed surveys
using a very large NaI detector where the count rate is sufficient that
there is less than 1% statistical uncertainty in a second.  This permits a
fast scan survey.  Use of a baby buggy or golf bag carrier offloads the
detector weight from the surveyor. One can even collect the spectra and use
NASVD or other smoothing methods to derive ground concentration of various
nuclides.

On Wed, Jan 15, 2014 at 8:01 AM, Robert Atkinson <robert8rpi at yahoo.co.uk>wrote:

> Hi Ted,
> A pancake seems an odd choice for x-ray dose rate measurement. It will
> over-respond to local beta and low energy gamma background. What is the
> energy of the x-ray source? Unless it's very low I'd have thought you would
> have been better off with a detector that does not respond to beta
> particles and low energy gammas. Are you using a compensating filter on the
> pancake?
>
> Robert Atkinson.
>
>
> ________________________________
>  From: Ted de Castro <tdc at xrayted.com>
> To: The International Radiation Protection (Health Physics) Mailing List <
> radsafe at health.phys.iit.edu>
> Sent: Tuesday, 14 January 2014, 23:28
> Subject: Re: [ RadSafe ] hand held meters
>
>
> 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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Joseph J. Shonka, Ph.D.
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