with a vibrating reed electrometer, connected as e continuously reading
voltmeter, driving a pen recorder. To shield completely against beta
radiation, the chamber was mounted in an aluminum container so that,
including the polyethylene wall, the gas volume was enclosed by 1.08 g/ex®
of material, corresponding to the Feather range of a 2.26 Mev beta
particle.
As is well known, minute alpha contamination in an ion chamber at atmospheric pressure can produce an ion current which may be of the same
order as the ion current being measured. For this reason it is important
that the effect of the contamination be measured or that the alpha-produced
current be suppressed. Several different methods have been used by
previous investigators.
In our measurements we have resorted to a tech-
nique which relies on the difference in electric fields necessary to
effect total collection of ion pairs produced by particles of low and high
specific ionization; i.e., electrons from gamma or cosmic-ray interactions
and alpha particles, respectively. The details of the instrumentation
end technique are described in Appendix II.
Readings were taken at one hundred fifty-four locations in nineteen states,
between New York and Utah. The natural environmental radiation levels
encountered ranged from a low of 8.4 microroentgens/hour along the
Pennsylvania Turnpike to a high of 38.6 microroentgens/hour at the summit
of Pikes Peak (Alt. 14,110 ft.).
A summary of the dose rates measured in
the principal cities along the route is given in Table I and a complete
tabulation of all measurements in Appendix I.
A map of the itinerary Is
shown at the beginning of Appendix I.
OF the major cities listed, Denver had the highest natural background
with an average of 18.5 +1.5 microroentgens/hour, @ level almost twice
that found in eastern and midwestern cities.
These measurements were made during part of the period of Operation
Plumbob, the 1957 series of United States continental weapons tests at
the National Test Station in Nevada, and these tests influenced certain
of the measured values. Elevated levels were encountered in eastern
Arkansas (26.0 - 50.2 microrcentgens/hour) and in the Black Hills of
South Dakota (22.0 - 33.8 microroentgens/hour). That the initial elevated
levels at these two locations were attributable to fresh fallout was
demonstrated by the reduction in the measured levels by 50 - 75 percent
upon resurvey about three weeks later. A resurvey of the Denver area
almost three months later furnished results essentially identical with
the earlier survey.
In general, one finds that the background radiation level increases as a
Function of decreasing barometric pressure. This is shown in Fig. 1, for
which the data have been reduced in the following way. Where the radiation
levels were demonstrably elevated from local sources, they were renovei
feonr consideration. The remaining one hundred thirty measurements ‘rere
cl-.ssified according to the barometric pressure at the time of measvuvereit in intervals of one inch of mercury. The average values and standard
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