[ RadSafe ] hand held meters
robert8rpi at yahoo.co.uk
Wed Jan 15 07:01:30 CST 2014
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?
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
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:
> 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
> 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
> You are currently subscribed to the RadSafe mailing list
> Before posting a message to RadSafe be sure to have read and understood the RadSafe rules. These can be found at: http://health.phys.iit.edu/radsaferules.html
> For information on how to subscribe or unsubscribe and other settings visit: http://health.phys.iit.edu/
You are currently subscribed to the RadSafe mailing list
Before posting a message to RadSafe be sure to have read and understood the RadSafe rules. These can be found at: http://health.phys.iit.edu/radsaferules.html
For information on how to subscribe or unsubscribe and other settings visit: http://health.phys.iit.edu/
More information about the RadSafe