EFFECTS OF EXPOSURE GEOMETRY
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Fre. 2. Cobalt source for 4r geometry exposures, located at the Naval Medical Research
Institute, Bethesda, Maryland.
24 em did not alter the depth-dose curves detectably. The laboratory radiation for
most of the exposures was Co® y-rays. As will be seen, high voltage (250 to 2000
kvp) would Lave served as well for most exposures; however, the use of Co® allowed more direct comparison of the geometry effect with some exposures not
attainable with X-rays (rmg, +, and field exposures), A diagram of the cobalt
generator used for bilateral, crossfire, ring, and 4a exposures is shown in Fig. 2.4
(See reference {0 for a description of the apparatus.)
for essentially all laboratory dosimetry, the same [00-r-capacity Victoreen
thimble chamber and charger-reader were employed. For a few low dose-rate
exposures with the bilateral and ring exposures, a 10-r-capacity Victoreen thimble
chamber, intercalibrated with the 100-r chamber, was used. The chambers were
embedded in a thin, close-fitting plastic shell which was, in turn, inserted into
closely machined holes drilled in the Masonite phantom. Thus, the phantom was
essentially solid during exposure. The same observer took all laboratory measurements, The phantom measurements in the field were made with thin-walled Sieverttype ionization chambers embedded throughout the thickness of the phantom.
For measurement of y-radiation in the fallout field, the «chambers were enclosed
in sufficient copper to exelude @-radiation. The thimble chamber measurements
did not allow accurate characterization of the depth-dose pattern at the surface
and just beneath the surface of the phantom. Since only relative measurements
were used in the phantom measurements, absolute calibration of the chambers
used was not necessary. Curves were not corrected for inverse square falloff, since
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The authors are indebted to Capt. W. E. Kellum, former Commanding Officer of the Naval
Medical Research Institute, and Capt. O. HE. Van der Aue, present. Commanding Officer. for
their cooperetion in making available the cobalt irradiatorfor these studies.