IV.
CONTINUING ENVIRONMENTAL RADIATION STUDIES
Beta Radiation
We are actively engaged in studying the free air
ionization due to B-rays from both natural and fallout
emitters in the soil not only because of its possible
significance to population exposure but also because of
its effect on various types of instrumentation used to
measure gamma dose rates and the possible subsequent misinterpretation of the readings of these instruments.
The
Significance of the latter was impressed upon us when
readings with various NYU thin plastic-walled ionization
chambers at field sites in the New York area were reduced
by 30% or more when the chambers were raised from 40 cm to
130 cm off the ground.
B-ray sensitive instruments such as
thin-walled ionization chambers, geiger counters, and
unshielded NaI detectors are frequently used to measure
gamma ray dose rates in air.
As mentioned before our
instrumentation is designed not to respond to f-rays.
Our first relatively detailed experiment was carried
out over soil at a site at Greenwood Lake, N.Y.
The gamma
ray and cosmic ray free air ionization were determined by
our spectrometric and high pressure ionization chamber
techniques.
Measurements of ionization intensity with an
NYU spherical thin-walled plexiglass chamber (minimum wall
thickness = 250 mg/cm2) were made at a succession of heights
above the ground from 40 cm up to 180 em.**
The gamma plus
cosmic ionization total which was essentially constant over
these heights was subtracted from these readings and the
remaining values of ionization were plotted on semilog paper
as a function of mean detector height.
The resulting points
were well fit by a straight line corresponding to a half-
thickness of 150 mg/cm2 in’air.
Since plexiglass has about
the same mass stopping power for electrons as air, the
chamber wall should roughly be equivalent to 1.6 halfthicknesses and the free air ionization about three times
the measured values (21-6 = 3).
However, the large size
and varying thickness of this chamber makes this approximatio
somewhat tenuous.
The resulting estimates of free air