deviations of the measured background end pressure for each pressure interval were then calculated and these results are exhibited in Fig. 1. The
number of observatiors fcr each pressure interval is indicated in parentheses. The four Pikes Feak observations are plotted separately as "Pp",
though they have also been included in the everages. The point with
barometric pressure, 21.2 inches Hg, has a large standard deviation in
the measured radiation level, being derived from only two observations
which differed substantislly (Pikes Peak Highway, 35.0 microroentgens/hour
and Leadville, Colorado, 23.5 microroentgens/hour).
On the same figure are plotted the adapted ionization chamber measurements
of the intensity of the cosmic radiation alone as reported by Bowen,
Millikan, and Neher? and by Compton.
The most important difference
between these two sets of ¢osmic-ray data is the amount of filtration of
the ion chambers used, the first being thin-walled measurements (0.5 mm
of steel), while Compton's measurements were made with the argon gas
cavity shielded with 5 cm of lead and 2.5 em of bronze in addition to the
steel wall of the chamber.
It should be pointed out that even at sea level the numerical value of the
total cosmic-ray intensity is not something on which there is universal
agreement. Burch, in his critical review, 1 concludes that the best value
for the lonization intensity at sea level may be deduced from the experimental work of Cley. This value is 1.77 ton pairs/cm3-sec (3.1 microroentgens/
hour) compared to Neher's valuel2 of 2.74 ion pairs/cm3-sec (4.8 microroentgens/
hour).
Hess' value! of 1.96 ion pairs/cm3-sec (3.4 microroentgens/hour)
falls between these two. It would appear that the discrepancies are toc
large to depend merely on differences in lonization chamber wall thickness
or calibration techniaues.
Comparing the results of cur measurements with the cosmic-ray data of
Bowen, Millikan, and Neher, it is clear that a substantial part of the
variability in mean outdoor radiation intensities over extensive areas in
the United States is attributable to the variation in the cosmic radiation
intensity with altitude. Most of the me&surements made at higher altitudes were obtained in Colorado, and the shift of the total radiation curve
in Fig. 1 away from the cosmic-ray curve at higher altitudes may be due to
& higher terrestris] radiation component in the mountainous areas of Colorado.
prevailing in the more populated eastern and midwestern states.
Fite en eTROE ER
Expressed on an annual basis, our measurements indicate a range of approximately 70 to 175 millireds/yeer for external environmental radiation dose
rates in populated areas in the United States, with the lower dose rates
This
compares with estimates made in the recent report of the National Acadeny
of Sciences on the biological effects of atomic radiation,13 which gives
“4
weer nen
ce ad
PIONERO phi cent ee cape
apo average annual background dose of about 135 millirads and a maximum
dose of about 170 millireds in populated areas.
.
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