Abstract
The aim of this thesis was to make a combined theoretical
and experimental investigation into the saturation characteristics
of two types of parallel plate ionization chambers - the free -air
chamber and the extrapolation chamber.
Despite the great importance of free -air chambers in
standardising laboratories, little theoretical consideration seems
to have been given to their saturation losses. The theories
surveyed in 1.2 deal with uniform ionization in a parallel gap and
are not directly applicable to a free -air chamber where the X -ray
beam often occupies a small proportion of the collecting volume.
Hubner (1958) appears to be the first to calculate the
recombination occurring in a free -air chamber. He modified
the equation of Von Engel and Steenbeck to take into account the
fact that the ionization did not fill the space between the
electrodes. He claimed that correction factors calculated from
his formula agreed within 0.3 per cent with factors determined
experimentally at the National Bureau of Standards. We found
that these experimental results covered only a limited range of
conditions and as they concerned corrections usually less than
1 per cent, hardly constituted an accurate check of the theory.
We felt that Hi.bner °s treatment could be improved upon by a different choice of recombination formula. Although Shevyrev's
work pointed to Mies equation, we decided to use the derivation
due to Boag and Wilson which was in a more convenient form for
the further modifications we intended to carry out. By
considering the effects of space charge and ion diffusion on the
basic recombination equation, we sought to derive an expression for
the saturation losses in a free -air chamber. We then planned to
check our theory by experiments using a free -air chamber (Greening,
1960) over a wide range of operating conditions.
It was foreseen that it would be a formidable task to set up a theory governing saturation losses in an extrapolation chamber.
Loevinger (1953) quoted an empirical relation for f as a function
of (1/√Vd) obtained from experiments with ß-ray sources for plate
spacings in the range 0.01 to 0.2 cm. and ionization intensities up
to 25 e.s.u. /cm.³sec. This function for f is at variance
with all previously described theories. We would expect the
situation in an extrapolation chamber to be an extremely complex one
since the use of very small air gaps and low ß-intensities is likely
to result in losses from diffusion and initial recombination
superimposed on volume recombination losses. Loevinger's result
does, however, seem surprising since all theories describing these
types of losses contain V to the power 1 if not higher.
It seemed worth-while, therefore, to design and construct a simple extrapolation chamber and make an intensive study of the
variation of ionization current with the various parameters involved.
We planned to discuss the theoretical significance of these
experimental results, making quantitative evaluations where possible.
