Table 1. Cosmic-ray ionization intensity as a
function ofaltitude...
Altitude
Radiation intensity
(fo)
(ur /hr)
Sea Level
3.8
500
1,000
4.1
4.5
4,000
5,000
6,000
8,000
10,000
6.9
7.9
9.0
11.7
14.8
2,000
3,000
5.2
6.0
12,000
14,000
16,000
18.5
22.8
27.7
From the standard atmospheric pres-
Cosmic Radiation
sure-aititude relationship (4) and the.
For measurement of the cosmic
radiation dose rate, the instrument was
flown between altitudes of 4000 and
17,000 feet in a C-47 airplane furnished
by the U.S. Air Force. Measurements
were made over land and water. As
one might expect, no difference was
detected between measurements made
over land and over water, the attenuation of terrestrial radiation at an altitude of 4000 feet being greater than a
factor of 10° (3).
as a continuously reading voltmeter,
driving a pen recorder. It is estimated
that the over-all accuracy of a single
observation is correct to about 1 microroentgen per hour.
_-1o shield completely against beta
radiation, the chamber is mounted in
an aluminum container such that, including the polyethylene wall, the gas
volume is enclosed by 1.08 g of material per square centimeter; this corre-
sponds to the Feather range of a 2.26Mevbeta particle.
The results of the airplane measurements are shown in Fig. 1. A simple
exponential, with the radiation level as
ordinate and barometric pressure as the
abscissa, has been fitted to the data.
The method of least squares furnishes the equation,
log Cp = 2.4595 — 0.0627 (+ 0.0018) P
(1)
where Cp is the measured radiation
level inside the plane in microroentgens
per hour and P is the barometric pressure in inches of mercury. The error
indicated is the standard deviation of
the regression coefficient.
mined; they are listed in Table 1.
The sea-level value was estimated
by extrapolation of the airplane data
to the corresponding pressure (P =
29.92 in.-Hg). The resulting value of
3.8 pr/hr for the cosmic-ray ionization
intensity at sea level is comparable to
values obtained by other investigators.
Neher (5) and Hess (6) obtained values
of 4.7 and 3.4 yr/hr, respectively.
Burch (7) carefully reviewed the earlier
experimental work in arriving at his
estimate of 3.1 pr/hr for the ionization
intensity at sea level, and concluded
that the discrepancies cannot be regarded as altogether resolved. Further
measurements of this important dosimetric parameter would be useful.
Outdoor Environmental Radiation
Measurements in the United States
An experimental determination, based
on observations in about 20 different
locations of varying backgrounds in
New York City, indicates that the
terrestrial radiation as measured by the
100
ionization chamber mounted inside the
automobile is 0.77 + 0.02 (standard
© 4/8/56, OVER WATER
© 4/8/58, OVER LAND
X 1/23/88, OVER WATER
sol-
&
deviation) of the outdoor intensity.
The
V23/58, OVER LAND
first series
of
measurements
made in an automobile—made during
August
1957 (/)—are summarized
here, corrected for attenuation of the
terrestrial radiation component by ‘the
automobile. Measurements madein the
New England states in May 1958 and
in the southeastern states in August
1958 have been corrected similarly.
A portable scintillation detector with
5
'
a sodium iodide phosphor was used for
scanning purposes at locations inacces-
sible to the automobile. This detector
was also turned on and observed con-
tinuously in the automobile between
—
RADIATION LEVEL (MIGROROENTGENS/ HOUR)
above equation, the approximate cosmic-ray dose rates at different altitudes
in the latitude of New York were deter-
points of measurement.
The detector,
though not capable of reading absolute
dose rate, can measure a change in
radiation level of about 0.4 yr/hr.
In all the measurements throughout
the United States, an effort was made
to obtain results which would be repre-
t
14
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2
i
24
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26
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28
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30
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32
BAROMETRIC PRESSURE (INCHES H,)
Fig. 1. Measurements of cosmic radiation ionization intensity made inside a C-47.
904
sentative of the unperturbed natural
background,affected as little as possible
by the occasional substantial variation
in the observed natural radiation levels
produced by localized sources (for ex-
ample, by. granite buildings, brick pav-
ing, and fallout).
SCIENCE, VOL. 131