[ RadSafe ] hand held meters

Wed Jan 15 10:08:21 CST 2014

Its a very appropriate choice!  This is a surface leakage survey of a 
very large (4' x 4' x 8') steel (~1/8"- 3/8") and lead (3/8") enclosure 
with a 150 kVp electronic xray source inside in an otherwise 
radiologically clean environment - ie. broad energy range and no 
complications of competing radiations.  The purpose is to verify tight 
construction.  This is NOT an academic whole body radiological 
health/dosimetry study for a research paper!

The detection area of the pancake allows me to survey the ENTIRE SURFACE 
without taking too too long and the detector is adequately sensitive to 
meet the acceptance criteria.

The acceptance criteria is extremely low and well below any regulatory 
limit.  Basically the company for which this cabinet is being made 
wanted ZERO leakage but was eventually convinced that that was 
essentially an unachievable goal or at the very least unverifiable - so 
the limit of 0.05 mR/hr, as indicated with a pancake GM on a surface 
survey, was accepted as a reasonably conservative/achievable lowest 
limit of detection.

I'm SURE I could come up with an instrument and an environment where I 
could detect SOME leakage from the cabinet - no matter how well designed 
or how tightly built.  But that's not the point here.

Again  - avoiding technical issues like micro dosimetry for extremities 
- this is a simple leakage/safety survey as is done in labs everyday and 
not a doctoral thesis.

Over my career I've chosen instruments for surveys from beta to high 
energy accelerator and everything in between with dosimetry evaluations 
from a very low energy x-ray accident to cosmic rays.

The pancake GM IS appropriate for this survey application.

On 1/15/2014 5:01 AM, Robert Atkinson 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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