In our measurements we devised a different technique more suited to our
facilities which cepends on the difference in electric fields necessary
to effect total collection of ion pairs produced by particles of low and
high specific ionization; i.¢., electrons from gamma-ray and alpha-particle
interactions respectively.
Figure II-3 shows an ion collestion characteristic for two different
radiation fields measured with our apparatus. The upper curve is for
an external field of 215 microroentgens/hour from a radium source at a
distance of several meters from the chamber.
The lower curve is for the
radiation background for the same position in the Health and Safety
Laboratory. During this measurement the background was about eight
microroentgens/hour. It can be seen that for the elevated radiation field
the saturation potential is reached with a collecting potential of 20 volts
across the chamber. On the other hand, for the background curve, where
the saturation collecting potential must be less than that required for
the higher radiation field, the characteristic shows a pronounced rise from
50 to 300 volts. This rise follows a plateau between 10 and 50 volts.
At 300 volts the measured ionization current is more than fifty percent
greater than the ionization current in the 10- to 50-volt interval.
Figure II-h, which plots the fraction of the ion current at 300 volts
against the collecting potential, shows this effect more graphically.
The explanation for this is simple. While saturation for the ionization
produced by gamma rays is quickly achieved, collection for the alpha
ionization is relatively inefficient and in fact saturation is not reached
up to the maximum collecting potential used. The important feature of the
onves shown in Fig. II-3 and Fig. II-4 is that for a considerable range in
collecting potential, up to about fifty volts, the alpha contribution is
suppressed to the point where it makes a negligible contribution to the
total ion current. This situation prevails provided one reads the minimum
envelope of the output trace.
The character of the trace obtained with different collecting potentials is
shown in Fig. II-5.
The figure shows the trace of the electrometer zero (A),
the trace with no collecting potential applied to the chamber (B), and
the background trace with the different collecting potentials up to
300 volts (C).
One may observe the aipha pulses becoming more prominent
as the collecting potentiel is increased. The saturation curves previously
Gescribed were taken at the lower envelope or minima of these traces.
When runs are made over an extended period (of the order of ten or more
minutes) minima occur with the same amplitude.
One infers that these
minima are associated with the absence of lonization caused by alpha
particles.
Tne general behavior of the phenomena described can be interpreted in terms
of the Jaffré-Zenstra theory of colwmar recombination.© However, the theory
is not sufficiently precise to furnish much more than a qualitative
f 1sc-imtion of the differences in sateration cravccteristics for radiations
~. “ieterent specific Llonizgationas In air-fill.i cracnbers.
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