7
ute also to the opacity of the air.
Identification of continuous ab=
sorption and of the absorbing structure of negative ions (so far only
postulated by theory) is made difficult by the experimental Mmitations
and particularly by the fact that one looks at the fireball through a
non-equilibrium, continuously changing gamma-ray, neutron and x-ray
field.
The gases in this field have in addition to their absorbing char=
acteristics also emissive properties, the emission originating from excited states and from recombination processes.
The latter ones are prob-
ably quite strong in the space close to the bomb where,
for a short time, ion densities of the order of 107
> to 10°19 ions, per cc
occur (normal air has appr. 5 X 1019 molecules per cc).
Furthermore
the actual temperature profile at the edge of the fireball is probably
not very sharp nor well defined,
One looks therefore into a rather uneasy atmosphere and the existence of such non-equilibrium conditions and inhomogeneities present diff-
icult problems in optical studies.
In spite of this it seemed, however,
well worth the effort to enhance our knowledge of the physics of the expanding fireball and to study in particular the characteristics of the
plasma located at the edge of the fireball.
In order to simplify the problem it is desirable to separate the
effects of gamma-rays and neutrons from those of black body type radiation including soft x-rays,
The first steps in this direction were
°
taken by the optics teams during operations Tumbler ~ Snapper, Upshot Knothole and to a minor extent during Castle in so called chord experin=ents,
In order to become
independent of the bomb as a light source,
auxiliary light sources of known characteristics were set up and fired
in the vicinity of the bomb.
A
ge
Observation of such sources through the