rate was only about 50% for 70 keV X-rays and only about 18%
for 134 kev 144ce gamma-rays.
Since under normal conditions
only a small fraction of the external gamma dose rate is from
gamma-rays of energy less than 150 keV,* the error in the
total field dose rate due to energy dependence is in most
instances no more than 1 or 2%.
The angular response of the chamber was also determined
experimentally and a correction factor +2% was adopted to
account for the slightly greater response for gamma-rays
entering perpendicular to the chamber axis.
In order to investigate recombination and wall effects, an
experiment was carried out in a constant-gamma radiation
field of about 60 ur/hr.
The chamber pressure was reduced
in steps from 1000 psi down to less than 50 psi.
The
resultant linear plot of ionization current versus pressure
indicated that essentially no current was being lost due to
recombination.
The extrapolated intercept for zero gas
filling indicated a wall effect due to an excess of electrons:
from the wall causing ionization in the gas of less than 1%
of the total current.
These quantities must be known to
theoretically predict the response of the chamber to gamma
radiation.
Measurements in deep mines have substantiated calculati
showing that the ionization produced by radioactive contamination of the chamber walls and electrodes is negligible
compared to the ionization current obtained per unit dose
rate
at our operating pressure
and volume.
The ionization current per ur/hr for gamma rays can be
calculated from the known volume and pressure.
This calcula
tion also requires a prior determination of the ionization
to be expected in argon relative to air, which requires an
accurate knowledge of the W values for the two gases and the
mass energy transfer coefficients for the energy gamma rays
in guestion.
Also, the effective attenuation of the wall
(~.90), which was estimated experimentally by measuring the
dose rate from a collimated source inside steel cylinders of
varying thickness, enters into the calculation.
Considering
all the uncertainties involved in these quantities as well
as an uncertainty of several percent in the filling pressure
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