iv
.
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deviations of the measured background and pressure for each pressure inter-
val were then calculated and these results are exhibited in Fig. 1. The
number of observations for each pressure interval is indicated in paren-
theses.
The four Pikes Peak observations are plotted separately as "P",
though they have also been included in the averages. 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 substantially (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 cosmic-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 cm 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 ionization intensity at sea level may be deduced from the experi-
mental work of Clay.
This value is 1.77 ion pairs/cm3-sec (3.1 microroentgens/
hour) compared to Neher'’s valuel® 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 too
large to depend merely on differences in ionization chamber wall thickness
or calibration technique.
Comparing the results of our 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 measurements made at higher alti-
tudes 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
a higher terrestrial radiation component in the mountainous areas of Colorado.
Expressed on an annual basis, our measurements indicate a range of approximately 70 to 175 millirads/year for external environmental radiation dose
rates in populated areas in the United States, with the lower dose rates
prevailing in the more populated eastern and midwestern states.
This
compares with estimates made in the recent report of the National Academy
of Sciences on the biological effects of atomic radiation,+3 which gives
an average annual background dose of about 135 millirads and @ maximum
dose of about 170 millirads in populated areas.