Since the discovery of radioactivity, due to the
photographic action of the emitted radiations, by
Becquerel in 1896, the photographic plate has been
an important tool in the detection of electrons.
Although overshadowed by electronic counting devices,
it still plays an important role, since the developed
image gives a more detailed distribution of the
radioisotope in the specimen under observation. The
technique of autoradiography utilises the photographic
action of all the emitted ionizing radiations
for locating the radioactive material in a sample,
and was first used by Lacassagne and Lattes (l ) in
1925.
The general procedure is to introduce the active
isotope into the system and to select the specimen to
be studied, which is then placed in contact with a
suitable photographic material and left for exposure.
After processing, the location of the radioactive
material can be deduced by studying the image, but
the latter is not sharp since it is difficult to
achieve intimate contact between the specimen and
recording film. This method gives autoradiographs
with a resolution of 50 to 100 microns(2) . An
improved method was achieved by Evans(3), who floated
sections of his material on to a photographic plate
which was then dried out and left for exposure. The
resolving power of this method was estimated to be
5 to 6 microns. Belanger and Leblond(4) obtained
similar resolution by coating sections with liquefied
emulsion at 37° C.
These methods suffer from many disadvantages.
The activity has to be firmly fixed in the emulsion
so that subsequent treatment will not leach it out;
also there is a danger of artifacts caused by
diffusion and pressure, and the inactive substances
in the specimen often render the emulsion grains
developable. In the stripping plate technique described
by pe1c(5) and others, the emulsion is mounted
on a thin support (cellulose esters have been used)
and the latter shields the emulsion from abrasion and
chemical action, but this advantage is gained at the
expense of resolution.
There have been many refinements of the method.
Berriman, Herz, and Stevens(6) using a new, fine grain
emulsion 4 microns thick on top of ordinary emulsion
have obtained a resolution of 200 lines per mm.
Gomberg(7) has developed a method called wet process
autoradiography and claims a resolution down to 1
micron, but there is a corresponding loss in resolution
when a protective coating is used to shield the one
micron thick sensitized layer, formed on the surface
of the specimen, from direct interaction with the
chemicals used in forming the film.
None of these methods employs direct magnification,
although Fink(8) mechanically enlarged the
specimen between lead sheets in a rolling mill, before
the autoradiograph was taken. Optical magnification
of the image is employed, but when this is greater
than 10x, the silver grains appear as groups of hazy
smudges, irregularly distributed, and no longer give
a true picture. The only way to get direct magnification
is to use a focusing method, in which the
electrons emitted from the specimen pass through a
suitable magnetic field and form an image on the
photographic plate some distance from the specimen.
In this method the whole system is in vacuo.
Emission microscopes, in which the object constitutes
the source of electrons, have been used for a long
time, but these did not employ a radioisotope as a
source of electrons. According to Lawrence(0) ,
sections have been placed in magnetic fields in an
attempt "to pull the /3 -rays straight out and get
real cell definition" but magnetic fields are not
intense enough for this.
In 1947, Marton and Abelson(10) described a
method called tracer micography in which monoenergetic
internal conversion electrons from a radioactive
source were focused by a magnetic lens, producing a
magnification of 1.6x. With their apparatus, using
a 1 milli curie per mm2 source of Ga67 at a numerical
aperture of 0.04 radians, satisfactory blackening of
a plate was obtained after a 1 hour exposure. They
obtained a best resolution of 30 microns, and proposed
after-acceleration of the electrons to reduce
exposure time, spherical aberration, and possibly
chromatic aberration.
At the same time, a similar instrument was
developed in Edinburgh(11), (12) with which a resolution
of 5 to 10 microns was obtained at a magnification
of 7x. An attempt has been made to improve this
instrument, and to study the principles on which its
operation depends; also to evaluate the potential
of such an instrument in the field of autoradiography.
The work involved is described in the following
chapters
