Calibration and Measurement Procedure
Calibration of the ionization chamber is done with a 1.021 milligram
radium source in an 0.2 mm wall platinum needle. The needle is placed
in an aluminum source holder 1.8 mm thick. The radium equivalent of this
needle has been determined by the National Bureau of Standards and a value
of 0.84 rhm has been taken for the dose rate output® of the radium.
The
source is positioned 2 meters away from the geometric center of the
chamber and the dose rate estimated by simple inverse square.
No correction has been made for the absorption of the air between the source and
chamber which is perhaps slightly overcompensated by scatter from walls,
floors, and neighboring objects in the laboratory.
After zeroing the electrometer, the chamber is operated with no collecting
potential.
The resulting trace varies in value from zero to a reading
corresponding to about fifty percent of the laboratory background.
reading is generally stable for very long periods.
The
When it changes, it
does so with a time constant measured in hours. It is radiation sensitive
but to an extent which is less than one percent of the calibration sensitivity of the chamber with collecting potential, and in the opposite
direction. The precise behavior of this spurious current is complex
though we find that a new chamber, having an aquadag coated lucite center
electrode, does not exhibit this "zero" current to the same extent. It is
probably identical to what Ohmart has called the "radicelectric effect."4
After the zero reading, a 22.5-volt collecting potential is applied and the
background reading determined.
A background trace is taken for approximately five minutes.
The difference between the zero and background readings
constitute the required measurement.
pr/br
A calibration factor for the chamber of 0.498 Hy
amperes
(STP) or 2.09 x 10719 r/hr-
has been obtained. The latter value depends, of course, on the correctness
of the glass-sealed Victoreen resistor used in conjunction with the vibrating
reed electrometer.
The value of the resistor used in these measurements
was 9.6 x 10 1 ohms as furnished by the manufacturer and verified in this
laboratory. Using the roentgen as equivalent to 1 esu/cm » one obtains
the easy conversion that 1 wr/hr is equivalent to 0.576 ion pairs/cm -sec.
With the calibration factor above, this corresponds to a sensitive volume
of 22.7 liters in reasonable agreement with the geometrical volume of 21.3
liters. Because of wail effects, ionization chamber theory would predict
only an approximate equality between geometric and ionization volumes.
Because of the substantially isotropic nature of the radiation measured, an
alternative and possibly superior method of calibration is to position the
source in a number of points around the chamber and average the calibration
factors so obtained.
This method of calibration would correct in some measure
for the shielding of terrestrial radiation furnished by the components
surrounding the ion chamber, but also would result in an overestimation of the
ecsmic-ray contribution which is not shielded significantly by the surrounding
apparatus.
At this stage in our work we do not feel that this degree of
refinement in the calibration constant is indicated considering the grosser
errors inherent in field measurements of this type.
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