[ RadSafe ] outdoor low dose rate measurements

[KARAM, PHILIP]

Depends on the meter and normal background rates. If you're using an air
ionization chamber I'd say this is not reasonable; with a PIC or a
microR meter it should work. Be careful with the detector, though -
there are meters that claim to measure down to a few microR/hr, but
their accuracy is nothing to write home about. With many of these (the
Thermo PRD is a prime example) you might be able to use them
qualitatively, but not necessarily quantitatively. I'd believe that you
can see and quantify this difference if you're using a PIC or a Bicron
microrem meter - I'd be dubious if you are using an energy-compensated
GM or an air ionization chamber. And as someone mentioned earlier,
survey speed makes a difference as well.

In addition, you need to remember that many detectors are
energy-dependent and that they over-respond to low-energy gammas. Many
microR meters, for example, use an NaI detector that's calibrated to
Cs-137. I participated in conducting a side-by-side comparison using a
1"x1", 2"x2" NaI, and a PIC on a thorium-contaminated site. The NaI
detectors routinely measured 1.5-2.5 times as high a dose rate as did
the PIC because of over-response to the large number of low-energy
photons emitted by Th-232 and decay series nuclides.

Andy


P. Andrew Karam, PhD, CHP
Director of Radiological Operations
NYPD Counterterrorism
(718) 615-7055 (desk)
(646) 879-5268 (mobile)


-----Original Message-----
From: radsafe-bounces at health.phys.iit.edu
[mailto:radsafe-bounces at health.phys.iit.edu] On Behalf Of
JOHN.RICH at sargentlundy.com
Sent: Wednesday, April 03, 2013 3:39 PM
To: radsafe at health.phys.iit.edu
Subject: [ RadSafe ] outdoor low dose rate measurements

Radsafers

I just finished a calculation that's going to require a minimally
trained 
individual (not a radworker)  to notice the difference between 0.03 
mrad/hr and 0.13 mrad/hr on a handheld meter while walking around an 
outdoor storage area. I'm stressing that this is a calculated result 
because it's the maximum calculated difference. The minimum calculated 
difference is between 0.02 mrad/hr to 0.04 mrad/hr. Actual dose rates 
should fall somewhere in between.

Is this a reasonable expectation, or is it asking too much of an outdoor

walk-around with a hand held meter?  BTW, I have no idea what the radon 
conditions are.

thx in advance  - -jmr

John Rich
312-269-3768
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