[ RadSafe ] hand held meters

[KARAM, PHILIP]

John -

Very good advice. It's also worth mentioning that many of the NaI-based microR meters are energy-dependent, so their readings can be off substantially. We did a test along with NYS DOH an USEPA, using a NaI-based detector side-by-side with a pressurized ion chamber at a site contaminated with thorium. The NaI detectors (we used two or three different brands) all came in with readings that were about half of what the PIC registered. The reason, of course, is that the NaI detectors over-responded to the low-energy gammas from the Th decay series.

So I concur - a PIC is a far better way to measure low levels of radiation compared to either GM or NaI. 

Andy

-----Original Message-----
From: radsafe-bounces at health.phys.iit.edu [mailto:radsafe-bounces at health.phys.iit.edu] On Behalf Of Dixon, John E. (CDC/ONDIEH/NCEH)
Sent: Thursday, January 23, 2014 9:54 AM
To: The International Radiation Protection (Health Physics) Mailing List
Cc: EDWARD.L.MARTIN at sargentlundy.com
Subject: Re: [ RadSafe ] hand held meters

I have looked over most of the posts on this article on RADSAFE. If you are looking for low background gamma radiation, use a simple stable instrument. NaI probes are quite sensitive and can exhibit wide swings in their count rates and obtaining spectra does not seem to be your goal. I recommend the one instrument which has not received attention here: the pressurized ion chamber (PIC). There are several manufactures (Fluke, Thermo-Eberline, etc.). Fluke's can accommodate very low dose rate levels (uR/hr) and they are digital with slower response times; however, they give you pretty stable final readings without the count rate swings a NaI system might give you. If you are looking to rapidly identify a source of the dose rate level in question, use a NaI to find where the source is and then use a PIC to quantify the dose rate level. Wait times for this instrument vary, but I believe one minute should be sufficient for a reading. Averaging 3 readings might also be a good idea.

Regards,
John Dixon

-----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: Tuesday, January 14, 2014 3:19 PM
To: radsafe at health.phys.iit.edu
Cc: EDWARD.L.MARTIN at sargentlundy.com
Subject: [ RadSafe ] hand held meters

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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