APPENDIX II
Instrumentation
In this appendix are described the instrument and the technique of measure-
ment. As mentioned in the main portion of this paper, measurements were
made with a 20-liter ionization chamber filled with air at atmospheric
pressure.
The instrument was operated inside an automobile under essentially
identical conditions of vehicle loading and orientation. The ionization
current was measured with a vibrating reed electrometer connected as a
continuously reading voltmeter driving a pen recorder.
Power for the
electrometer and recorder was obtained from an alternating current inverter
operated from the 12-volt automobile storage battery. The entire assembly
was secured to a wooden carrying board as shown in the photograph (Fig. TI-1).
The chamber was improvised from a 20-liter polyethylene carboy having a
3/32-inch wall. To suppress completely the beta response, the chamber was
positioned in a solid 1/8-inch aluminum shield. Including the polyethylene
wall, the gas volume was enclosed by 1.08 g/eme of material, corresponding
to the Feather range of a 2.26 Mev beta particle.
A schematic diagram of the ionization chamber showing the details of the
center electrode assembly is shown in Figure II-2.
The chamber has two guard rings with the top of the center electrode
secured by the guard ring assembly to the carboy. The top and bottom guard
rings are connected together by an insulated wire strung through the stain-
less steel electrode.
For laboratory use one could dispense with the top
assembly but for field use the additional mechanical rigidity is
advantageous.
There is sufficient air leakage in the chamber so that the
chamber maintains atmospheric pressure.
This was verified in the field by
checking the response of the chamber to a weak source against the known
barometric pressure.
As is well known, minute alpha contamination of an ionization chamber at
atmospheric pressure can produce a current which may be of the same order
as the current being measured. For this reason it is essential that the
effect of contamination be measured or that the current produced by alpha
particles be suppressed.
A method of evaluating the alpha current is
furnished by Hess and Vancour.* Essentially, the method depends on the
limited range of alpha particles resulting in the alpha-produced ionization
being independent of the pressure, whereas the lonization produced by
gamma radiation (or charged relativistic particles in the cosmic radiation)
is almost exactly proportional to the pressure.
A frequently used procedure is to pressurize the chamber filling (usually
purified argon) so that the alpha contribution is negligible compared to
the ionization produced by an external radiation field.
4
II-1