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RE: Measurement uncertainty of home radiation monitors
IMHO, GM's are quite reliable and trustworthy. The ones in 'toys' often
being exactly the same you find in 'high class' instruments.
The discharge process in GM's is based on a well understood physical effect,
and depends on wall thickness and composition, gas composition, and applied
voltage.
As long as wall and gas stay the same (that is, the tube does not leak), you
have the same sensitivity if the voltage stays the same. That makes GM's
quite interchangeable.
Voltage dependency is (correct me if I am wrong) some 1% to 3% for a 5% V
change around the center of the plateau. It is simple and inexpensive to
implement voltage regulation well below 1%, so that is not a problem in any
modern instrument.
Temperature effects are minimal, and nobody - as far as I know - accounts
for them in GM electronics (if somebody does, please let me know).
In conclusion, GM's do not drift - as long as you stay below a few billion
total counts, say the manufacturers. At low count rates, they are
intrinsecally linear.
OTH, as Mark hints, GM's preferred mode of failure, leakage, is evidenced in
an increase in dark count rate. Power supply failures usually result in no
HV - no counts - but sometimes in ringing or poor regulation, or spurious
counts in case of spark discharges (humidity).
Now, I am the first to say that periodic checks and (maybe, infrequent)
calibrations are needed; a number of accidents were traced to malfunctioning
detectors. A meter can be kaput and still report background counts: that can
happen (has happened) especially in PMT (scintillator) assemblies: when the
NaI or the plastic are gone (detached), you still get the PMT background.
Particularly important, users should be aware that Geigers read zero when
flooded with sufficiently strong sources. Same for scintillators, although
in some cases manufacturers correct that there.
The calibration issue is quite different with scintillator or semiconductor
detectors: there, you have pulses of varied height, and your count rate
depends on the threshold for peak discrimination, which is kind of delicate,
especially if you consider that, for example, in a 12 stages PMT pulse
height is proportional to applied voltage to the power of 12. You must
calibrate periodically and with low energy gammas (Am241) if you want to use
scintillators to monitor environmental radioactivity. Unless you do
spectroscopy, scintillators are a waste of money and a constant worry.
So, how precise is a GM? in my opinion, 5% is a quite conservative figure,
IF you perform a calibration once with a known and weak source, and IF you
only measure gammas. If you don't calibrate, I would trust 'nominal'
manufacturers data for gamma sensitivity to be within 10% of true values.
Betas and alphas absorption depends too much on window thickness, and that
is too uncertain.
In my experience, the problem in calibration is not in getting a known
source, is in shielding well enough to be able to make a zero reading. I am
under the impression that in many if not most cases calibration is made
<assuming> that detector background is zero (that is, is made on one point
only). I have read GM manufacturers specifying device backgrounds at level
equivalent to << 10% of normal environmental background, so that COULD be a
cause of systematic overreporting (or variance) of environmental
radioactivity levels. In other words, part of reported count rate, at low
levels, could be instrumental, and not accounted for by calibration (LNT is
wrong again!). Now that I think about it, I have never seen firmware
allowing to subtract a fixed (variable) instrumental background from a count
rate.
And how accurate is a GM? one sigma is the inverse of the square root of the
counts you have got. For 1% standard deviation accuracy you must collect ten
thousand counts.
And if I have confused again precision with accuracy, please correct and
forgive, but don't shoot me. And sorry if I make it look complicated, it
seems so simple...
Marco
Dr Marco Caceci
Principal
LQC s.l.
Noorderbrink 26
2553 GB Den Haag
The Netherlands
Tel + 31 70 397 5653
http://radal.com
-----Original Message-----
From: owner-radsafe@list.vanderbilt.edu
[mailto:owner-radsafe@list.vanderbilt.edu]On Behalf Of mark.hogue@SRS.GOV
Sent: 11 March, 2002 12:25
To: radsafe@list.vanderbilt.edu
Subject: Measurement uncertainty of home radiation monitors
There have been a number of links posted to hobby-level radiation monitors
on the web.
...........
How much confidence can we have in these monitors? I'd like to think that
the monitors will work fine until the tube fails and that failure will be
obvious. But if that's true, why do we have to do so many calibrations and
source checks on monitors in industry? (I have seen a monitor failure at a
nuclear facility result in a steady 300 cpm and that was not an obvious
failure. That was a G-M tube with a count rate meter and I think the fault
was in the tube.)
More to the point, is the original calibration valid in the range of 5 to 20
microR/hr that is the normal reported reading? The vendor claims (below) an
accuracy of +/-5%, but is imprecise in what that means (e.g. range of the
monitor?). Or, should we believe that a reading of 5 microR/hr is accurate
to +/- 0.25 microR/hr?!
...........
Mark Hogue
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