[ RadSafe ] Fwd: hand held meters

[Dan McCarn]

Hi John:

Hand-held devices are very sensitive to the geometry of the sampling
conditions. For instance, if you are in a swale or depression, the meter
will sense a higher level than if the ground is flat. This gave rise to
many misunderstood measurements related to e.g. craters made by artillery.
If a reading is made inside the crater, it will be always be higher than
the soil surrounding the crater.

In my experience, it takes a full "field-season" for a newby geologist to
understand what he is actually measuring, and to take into account the
geometry of the measurement conditions.  I recommend that your people leave
the meter on all the time so that they can get a basis for understanding
what they are measuring,

Measurements will also differ according to the height above the soil that
an instrument "sees" the surrounding gamma values.  Difference will also be
present in soils that have been used for agriculture from the radium &
radium progeny from the phosphate fertilizers spread on the soil.  SInce I
was "looking" for uranium ores, my measurements included background as well
as elevated rates for rocks that had elevated levels of natural
radionuclides (K-U-Th Series).

My ears can still hear the 1000s of hours that my scintillation counter was
active during field work. I'd average about 140 days each year in the
field, so I became very accustomed to hearing artifacts. Temporal spikes
are not uncommon, so set the time constant on a longer integration time to
smooth these features out.

Also, measurements with meters will vary slightly according to the ambient
temperature of the meter; but please guard against thermal shocks since it
is possible to crack the NaI crystal that way.

Best!

Dan ii

Dan W McCarn, Geologist
108 Sherwood Blvd
Los Alamos, NM 87544-3425
+1-505-672-2014 (Home – New Mexico)
+1-505-670-8123 (Mobile - New Mexico)
HotGreenChile at gmail.com (Private email) HotGreenChile at gmail dot com


On Tue, Jan 14, 2014 at 3:51 PM, Chris Alston <achris1999 at gmail.com> wrote:

> Hi John
>
> I think that we need to ask you to be a little more specific as to
> your application.  So, for instance, what would you be looking for?
> E.g., interesting variations in background, deposits of uranium ore,
> subject of your choosing here_____________________.
>
> Generally speaking, NaI scintillometers are much more sensitive and
> faster than GM meters; but the time to "return to background" from a
> random spike is essentially immediate, if you are listening to an
> audio output.  Of course, you cannot tell that until you perceive the
> longer interval between clicks, or the lower frequency of a tone.
> Then, on many meters, one can select the response time constant of the
> visual indicator of dose rate or event count rate, on others it is
> preset for each range of the instrument.  A "short" time constant
> might be 0.5 - 2 seconds, a "long" one might be 5 - 22 seconds.  In
> this context, "response time" usually means something along the lines
> of how long it takes for a meter's needle to go from 10% to 90% of a
> final reading.
>
> Hope this helps a little.  It sounds to me like you might want to demo
> a couple of instruments, to see what works for you.
>
> Cheers
> cja
> ---------- Forwarded message ----------
> From:  <JOHN.RICH at sargentlundy.com>
> Date: Tue, Jan 14, 2014 at 3:18 PM
> Subject: [ RadSafe ] hand held meters
> To: radsafe at health.phys.iit.edu
> Cc: EDWARD.L.MARTIN at sargentlundy.com
>
> 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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