From: jgd@rsiatl.UUCP (John G. De Armond)
Newsgroups: sci.energy
Subject: Re: Aircraft radiation exposure
Message-ID: <1535@rsiatl.UUCP>
Date: 1 Mar 90 06:40:57 GMT

jans@tekgvs.LABS.TEK.COM (Jan Steinman) writes:

><jgd@rsiatl.UUCP (John G. De Armond)>
><Every time I fly, which used to be at least twice a week, 2 Victoreen pocket
>chambers go with me...

>De Armond's instruments are not sensitive enough.  Background at my house, as
>measured on my Radalert, is 13-16 counts per minute.  On one cross-county
>flight at an announced altitude of 33,000 feet, I measured 245 counts per
>minute, a second flight (at 41,000 feet) measured 429 counts per minute.  These

But sensitivity is not the issue in dosimetry.  Absorbed dose and tissue
equivalence are the major concerns.  My pocket chambers are designed with
tissue-equivalent walls and are ionization chamber based instruments which
means that they are linear over a wide range of energy.  A GM detector is
a particularly bad choice for measuring high energy cosmic radiation, as the
walls partially evacuated interior lack the density to host ionization
events.

As most any 1st year H-P student knows, GM tubes should NEVER be use for dose
measurements regardless of the scale calibrations sometimes applied out of
tradition mostly.  A GM tube can be used as a transfer standard as long
as the source, geometry, and shielding are exactly the same as the
calibration source.

>The Radalert uses a quenched LND 712 GM tube,calibrated to a Cesium-137 source
>to 982 CPM per mr/hr.  Therefore worst flight exposure (from an admittedly
>tiny sample) was ~< 0.44 mr/hr, about 33 times the earth-bound level,
>but still a far cry from 100 mr/hr!  Total exposure on a four-hour flight
>would then be about 1% of the EPA yearly limit for the population as a whole.

Not only is the 712 a poor tube for the application but a Cs-137 calibration
is a very poor choice for environmental radiation measurements that desire
to correlate even remotely with dose.  The degraded spectrum at the earth's
surface tends to peak around 300 kev which would indicate a Ba-133 calibration.
At high altitudes, high energy particles, Bremstrahlung and cosmic radiation
push the average energy >>1 mev.  Cs-137 is commonly used because Cs-137 is
the predominant long-lived fission product to be dealt with in nuclear
plants and because it has a nice half-life.

>Funny, my Radalert saturated (>19999 counts) on one trip through an airport
>X-Ray machine!  I did not time the exposure precisely, but assuming it was a
>generous five minutes between resetting the Radalert and opening my briefcase,
>the exposure rate was *at least* 4 mr/hr.

No, you simply measured >19999 counts.  The 712 tube is very sensitive at
X-ray energies.  The actual dose delivered is very low - as an ionization
chamber or pocket dosimeter would have illustrated.

A little knowledge is a dangerous thing.

John

Newsgroups: sci.energy,sci.electronics
From: John De Armond
Subject: Re: geiger counter ques
Message-ID: <bxftfs_@dixie.com>
Date: Mon, 08 Mar 93 06:21:00 GMT

dannyb@panix.com (Daniel Burstein) writes:

>A quick (and no doubt embarrassing) question:

>We're all familiar with the standard geiger counter.  question: I realize
>it does a good job at picking up Beta, but does it register Gamma in any
>sort of reliable way?

>thanks

>danny

Not at all embarrasing.  Lots of misunderstanding regarding GM tubes.

The GM detector will detect gamma rays, though the efficiency varies
widely depending on the structure.  Very few gamma actually interact
with the fill gas.  More common is the gamma interacting with the
housing in a manner that kicks out secondary electrons or soft X-rays.
Thus, the design of the housing heavily influences the efficiency.

Unfortunately, and despite the fact that most GM counters also have a
dose scale, nothing regarding dose can be inferred from the count rate.
This is because the GM tube registeres the same count in response to a
very weak gamma as to a very energetic one.  A GM counter's scale CAN be
calibrated in terms of mR/hr for a single type of gamma spectrum.  Civil
Defence-type detectors are typically calibrated against Cs-137
radiation, a gamma at 0.667 MEV.  This is convenient because the
radiation environment post bomb with old fallout tends to have its
average energy around 0.650 MEV.  Same for the degraded background
radiation typical in normal workspaces of a nuclear plant.  This does
NOT hold, however, for measuring a point source - even a Cs-137 one -
and for radioactive sources with energies widely different from the
calibration point.  It also does not hold in a mixed beta/gamma
environment.

John