Aircraft radiation exposure
Jan Steinman
<Every time I fly, which used to be at least twice a week, 2 Victoreen pocket
chambers go with me... To date, I've never seen any exposure over and above
the known drift rate of the the chambers... I have my doubts as to doses as
high as 100 mrem/hour.>
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
were point-to-point averages, including take off and landing, so the actual
air-time exposure was somewhat higher.
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.
(Yes, I know that an ionizing event rate correlates perfectly with mr/hr only
for elements with a given activity. It seems to correlate within a couple
places for most other sources. The cal figure for Cobalt-60, for instance, is
958 CPM per mr/hr.)
<Incidently, another area I like to look at is the good old airport X-ray
machine. Again, no dose has ever registerd on pocket chambers.>
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.
(The Radalert is available from International Medcom, 7497 Kennedy Road,
Sebastapol, CA, 95472, 1-800-257-3825, for about $200. I have no connection
with International Medcom, other than as a satisfied customer. It is available
in kit form for somewhat less. No calibration is required.)
Jan Steinman - N7JDB
Tektronix Electronic Systems Laboratory
Box 500, MS 50-370, Beaverton, OR 97077
(w)503/627-5881 (h)503/657-7703
John F Nixon
>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!
The 100 mr/hr figure appeared in the New York Times article on aircraft
radiation hazards. The article appeared in one of the last 3 Sunday
editions (can't remember which...). Such extreme exposure would occur
only in the case of a polar flight during the arrival of charged particles
from an active solar event. Remember, in that case the plane may be
exposed for a few *hours*, and quite likely will be out of radio contact
with warnings from the ground. I wouldn't mind having aircraft flying at
high latitudes equipped with radiation monitors. A change of a few thousand
feet in altitude can make a significant difference in the total dose.
--
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jni...@atl.ge.com ...steinmetz!atl.decnet!jnxion
John G. De Armond
><j...@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
--
John De Armond, WD4OQC | We can no more blame our loss of freedom on congress-
Radiation Systems, Inc. | men than we can prostitution on pimps. Both simply
Atlanta, Ga | provide broker services for their customers.
emory!rsiatl!jgd | - Dr. W Williams | **I am the NRA**