| As shown in earlier figures, a
photographic emulsion consists of a myriad of tiny crystals of
silver halide--usually the bromide with a small quantity of
iodide--dispersed in gelatin and coated on a support. The
crystals--or photographic grains--respond as individual units to the
successive actions of radiation and the photographic developer.
The photographic latent image may be defined as that
radiation-induced change in a grain or crystal that renders the
grain readily susceptible to the chemical action of a developer.
To discuss the latent image in the confines of this siterequires
that only the basic concept be outlined. A discussion of the
historical development of the subject and a consideration of most of
the experimental evidence supporting these theories must be omitted
because of lack of space.
It is interesting to note that throughout the greater part of the
history of photography, the nature of the latent image was unknown
or in considerable doubt. The first public announcement of
Daguerre's process was made in 1839, but it was not until 1938 that
a reasonably satisfactory and coherent theory of the formation of
the photographic latent image was proposed. That theory has been
undergoing refinement and modification ever since.
Some of the investigational difficulties arose because the
formation of the latent image is a very subtle change in the silver
halide grain. It involves the absorption of only one or a few
photons of radiation and can therefore affect only a few atoms, out
of some 109 or 1010
atoms in a typical photographic grain. The latent image cannot be
detected by direct physical or analytical chemical means.
However, even during the time that the mechanism of formation of
the latent image was a subject for speculation, a good deal was
known about its physical nature. It was known, for example, that the
latent image was localized at certain discrete sites on the silver
halide grain. If a photographic emulsion is exposed to light,
developed briefly, fixed, and then examined under a microscope (see
the figure below), it can be seen that development (the reduction of
silver halide to metallic silver) has begun at only one or a few
places on the crystal. Since small amounts of silver sulfide on the
surface of the grain were known to be necessary for a photographic
material to have a high sensitivity, it seemed likely that the spots
at which the latent image was localized were local concentrations of
silver sulfide.
Electron micrograph of exposed, partially developed, and fixed
grains, showing initiation of development at localized sites on the
grains (1µ = 1 micron = 0.001 mm).
It was further known that the material of the latent image was,
in all probability, silver. For one thing, chemical reactions that
will oxidize silver will also destroy the latent image. For another,
it is a common observation that photographic materials given
prolonged exposure to light darken spontaneously, without the need
for development. This darkening is known as the print-out image. The
printout image contains enough material to be identified chemically,
and this material is metallic silver. By microscopic examination,
the silver of the print-out image is discovered to be localized at
certain discrete areas of the grain (see the figure below), just as
is the latent image.
Electron micrograph of photolytic silver produced in a grain by
very intense exposure to light.
Thus, the change that makes an exposed photographic grain capable
of being transformed into metallic silver by the mild reducing
action of a photographic developer is a concentration of silver
atoms--probably only a few--at one or more discrete sites on the
grain. Any theory of latent-image formation must account for the way
that light photons absorbed at random within the grain can produce
these isolated aggregates of silver atoms. Most current theories of
latent-image formation are modifications of the mechanism proposed
by R. W. Gurney and N. F. Mott in 1938.
In order to understand the Gurney-Mott theory of the latent
image, it is necessary to digress and consider the structure of
crystals--in particular, the structure of silver bromide crystals.
When solid silver bromide is formed, as in the preparation of a
photographic emulsion, the silver atoms each give up one orbital
electron to a bromine atom. The silver atoms, lacking one
negative charge, have an effective positive charge and are
known as silver ions (Ag+).
The bromine atoms, on the other hand, have gained an electron--a
negative charge--and have become bromine ions (Br-). The
"plus" and "minus" signs indicate, respectively, one fewer or one
more electron than the number required for electrical neutrality of
the atom.
A crystal of silver bromide is a regular cubical array of silver
and bromide ions, as shown schematically in the figure below. It
should be emphasized that the "magnification" of the figure is very
great. An average grain in an industrial x-ray film may be about
0.00004 inch in diameter, yet will contain several billions of ions.
A silver bromide crystal is a rectangular array of silver (Ag+)
and bromide (Br-) ions.
A crystal of silver bromide in a photographic emulsion
is--fortunately--not perfect; a number of imperfections are always
present. First, within the crystal, there are silver ions that do
not occupy the "lattice position" shown in the figure above, but
rather are in the spaces between. These are known as interstitial
silver ions (see the figure below). The number of the interstitial
silver ions is, of course, small compared to the total number of
silver ions in the crystal. In addition, there are distortions of
the uniform crystal structure. These may be "foreign" molecules,
within or on the crystal, produced by reactions with the components
of the gelatin, or distortions or dislocations of the regular array
of ions shown in the figure above. These may be classed together and
called "latent-images sites."
"Plain view" of a layer of ions of a crystal similar to that of
the previous figure. A latent-image site is shown schematically, and
two interstitial silver ions are indicated.
The Gurney-Mott theory envisions latent-image formation as a
two-stage process. It will be discussed first in terms of the
formation of the latent image by light, and then the special
considerations of direct x-ray or lead foil screen exposures will be
covered.
When a photon of light of energy greater than a certain minimum
value (that is, of wavelength less than a certain maximum) is
absorbed in a silver bromide crystal, it releases an electron from a
bromide (Br-) ion. The ion, having lost its excess
negative charge, is changed to a bromine atom. The liberated
electron is free to wander about the crystal (see the figure below).
As it does, it may encounter a latent image site and be "trapped"
there, giving the latent-image site a negative electrical charge.
This first stage of latent-image formation--involving as it does
transfer of electrical charges by means of moving electrons--is the
electronic conduction stage.
Stages in the development of the latent image according to the
Gurney-Mott theory.
The negatively charged trap can then attract an interstitial
silver ion because the silver ion is charged positively (C in the
figure above). When such an interstitial ion reaches a negatively
charged trap, its charge is counteracted, an atom of silver is
deposited at the trap, and the trap is "reset" (D in the figure
above). This second stage of the Gurney-Mott mechanism is termed the
ionic condition stage, since electrical charge is transferred
through the crystal by the movement of ions--that is, charged atoms.
The whole cycle can recur several, or many, times at a single trap,
each cycle involving absorption of one photon and addition of one
silver atom to the aggregate. (See E to H in the figure above.)
In other words, this aggregate of silver atoms is the latent
image. The presence of these few atoms at a single latent-image site
makes the whole grain susceptible to the reducing action of the
developer. In the most sensitive emulsions, the number of silver
atoms required may be less than ten.
The mark of the success of a theory is its ability to provide an
understanding of previously inexplicable phenomena. The Gurney-Mott
theory and those derived from it have been notably successful in
explaining a number of photographic effects. One of these
effects--reciprocity-law failure--will be considered here as an
illustration.
Low-intensity reciprocity-law failure (left branch of the curve )
results from the fact that several atoms of silver are required to
produce a stable latent image. A single atom of silver at a
latent-image site (D in the figure above) is relatively unstable,
breaking down rather easily into an electron and a positive silver
ion. Thus, if there is a long interval between the formation of the
first silver atom and the arrival of the second conduction electron
(E in the figure above), the first silver atom may have broken down,
with the net result that the energy of the light photon that
produced it has been wasted. Therefore, increasing light intensity
from very low to higher values increases the efficiency, as shown by
the downward trend of the left-hand branch of the curve, as
intensity increases.
High-intensity reciprocity-law failure (right branch of the
curve) is frequently a consequence of the sluggishness of the ionic
process in latent-image formation (see the figure above). According
to the Gurney-Mott mechanism, a trapped electron must be neutralized
by the movement of an interstitial silver ion to that spot (D in the
figure above) before a second electron can be trapped there (E in
the figure above); otherwise, the second electron is repelled and
may be trapped elsewhere. Therefore, if electrons arrive at a
particular sensitivity center faster than the ions can migrate to
the center, some electrons are repelled, and the center does not
build up with maximum efficiency. Electrons thus denied access to
the same traps may be trapped at others, and the latent image silver
therefore tends to be inefficiently divided among several
latent-image sites. (This has been demonstrated by experiments that
have shown that high-intensity exposure produces more latent image
within the volume of the crystal than do either low- or
optimum-intensity exposures.) Thus, the resulting inefficiency in
the use of the conduction electrons is responsible for the upward
trend of the right-hand branch of the curve.
In industrial radiography, the photographic effects of x-rays and
gamma rays, rather than those of light, are of the greater interest.
At the outset it should be stated that the agent that actually
exposes a photographic grain, that is, a silver bromide crystal in
the emulsion, is not the x-ray photon itself, but rather the
electrons--photoelectric and Compton--resulting from the absorption
event. It is for this reason that direct x-ray exposures and lead
foil screen exposures are similar and can be considered together.
The most striking differences between x-ray and visible-light
exposures to grains arise from the difference in the amounts of
energy involved. The absorption of a single photon of light
transfers a very small amount of energy to the crystal. This is only
enough energy to free a single electron from a bromide (Br-)
ion, and several successive light photons are required to render a
single grain developable. The passage through a grain of an
electron, arising from the absorption of an x-ray photon, can
transmit hundreds of times more energy to the grain than does the
absorption of a light photon. Even though this energy is used rather
inefficiently, in general the amount is sufficient to render the
grain traversed developable--that is, to produce within it, or on
it, a stable latent image.
As a matter of fact, the photoelectric or Compton electron,
resulting from absorption or interaction of a photon, can have a
fairly long path in the emulsion and can render several or many
grains developable. The number of grains exposed per photon
interaction can vary from 1 grain for x-radiation of about 10 keV to
possibly 50 or more grains for a 1 meV photon. However, for 1 meV
and higher energy photons, there is a low probability of an
interaction that transfers the total energy to grains in an
emulsion. Most commonly, high photon energy is imparted to several
electrons by successive Compton interactions. Also, high-energy
electrons pass out of an emulsion before all of their energy is
dissipated. For these reasons there are, on the average, 5 to
10 grains made developable per photon interaction at high energy.
For comparatively low values of exposure, each increment of
exposure renders on the average the same number of grains
developable, which, in turn, means that a curve of net density
versus exposure is a straight line passing through the origin (see
the figure below). This curve departs significantly from linearity
only when the exposure becomes so great that appreciable energy is
wasted on grains that have already been exposed. For commercially
available fine-grain x-ray films, for example, the density versus
exposure curve may be essentially linear up to densities of 2.0 or
even higher.
Typical net density versus exposure curves for direct x-ray
exposures.
The fairly extensive straight-line relation between exposure and
density is of considerable use in photographic monitoring of
radiation, permitting a saving of time in the interpretation of
densities observed on personnel monitoring films.
It the D versus E curves shown in the figure above are replotted
as characteristic curves (D versus log E), both characteristic
curves are the same shape (see the figure below) and are merely
separated along the log exposure axis. This similarity in toe shape
has been experimentally observed for conventional processing of many
commercial photographic materials, both x-ray films and others.
Characteristic curves plotted from the data in the previous
figure.
Because a grain is completely exposed by the passage of an
energetic electron, all x-ray exposures are, as far as the
individual grain is concerned, extremely short. The actual time
that an x-ray-induced electron is within a grain depends on the
electron velocity, the grain dimensions, and the "squareness" of the
hit. However, a time of the order of 10-13 second
is representative. (This is in distinction to the case of light
where the "exposure time" for a single grain is the interval between
the arrival of the first photon and that of the last photon required
to produce a stable latent image.)
The complete exposure of a grain by a single event and in a very
short time implies that there should be no reciprocity-law failure
for direct x-ray exposures or for exposures made with lead foil
screens. The validity of this has been established for commercially
available film and conventional processing over an extremely wide
range of x-ray intensities. That films can satisfactorily integrate
x-, gamma-, and beta-ray exposures delivered at a wide range of
intensities is one of the advantages of film as a radiation
dosimeter.
In the discussion on reciprocity-law failure it was pointed out
that a very short, very high intensity exposure to light tends to
produce latent images in the interior of the grain. Because x-ray
exposures are also, in effect, very short, very high intensity
exposures, they too tend to produce internal, as well as surface,
latent images.
Many materials discolor on exposure to light--a pine board or the
human skin, for example--and thus could conceivably be used to
record images. However, most such systems reset to exposure on a
"1:1" basis, in that one photon of light results in the production
of one altered molecule or atom. The process of development
constitutes one of the major advantages of the silver halide system
of photography. In this system, a few atoms of photolytically
deposited silver can, by development, be made to trigger the
subsequent chemical deposition of some 109
or 1010 additional silver atoms,
resulting in an amplification factor of the order of 109
or greater. The amplification process can be performed at a time,
and to a degree, convenient to the user and, with sufficient care,
can be uniform and reproducible enough for the purposes of
quantitative measurements of radiation.
Development is essentially a chemical reduction in which silver
halide is reduced or converted to metallic silver in order to retain
the photographic image, however, the reaction must be limited
largely to those grains that contain a latent image. That is, to
those grains that have received more than a certain minimum exposure
to radiation. Compounds that can be used as photographic developing
agents, therefore, are limited to those in which the reduction of
silver halide to metallic silver is catalyzed (or speeded up) by the
presence of the metallic silver of the latent image. Those compounds
that reduce silver halide in the absence of a catalytic effect by
the latent image are not suitable developing agents because they
produce a uniform overall density on the processed film.
Many practical developing agents are relatively simple organic
compounds (see the figure below) and, as shown, their activity is
strongly dependent on molecular structure as well as on composition.
There exist empirical rules by which the developing activity of a
particular compound may often be predicted from a knowledge of its
structure.
Configurations of dihydroxybenzene, showing how developer
properties depend on structure.
The simplest concept of the role of the latent image in
development is that it acts merely as an electron-conducting bridge
by which electrons from the developing agent can reach the silver
ions on the interior face of the latent image. Experiment has shown
that this simple concept is inadequate to explain the phenomena
encountered in practical photographic development. Adsorption of the
developing agent to the silver halide or at the silver-silver halide
interface has been shown to be very important in determining the
rate of direct, or chemical, development by most developing agents.
The rate of development by hydroquinone (see the figure above), for
example, appears to be relatively independent of the area of the
silver surface and instead to be governed by the extent of the
silver-silver halide interface.
The exact mechanisms by which a developing agent acts are
relatively complicated, and research on the subject is very active.
The broad outlines, however, are relatively clear. A molecule of
a developing agent can easily give an electron to an exposed silver
bromide grain (that is, to one that carries a latent image), but not
to an unexposed grain. This electron can combine with a silver (Ag+)
ion of the crystal, neutralizing the positive charge and producing
an atom of silver. The process can be repeated many times until all
the billions of silver ions in a photographic grain have been turned
into metallic silver.
The development process has both similarities to, and differences
from, the process of latent-image formation. Both involve the union
of a silver ion and an electron to produce an atom of metallic
silver. In latent image formation, the electron is freed by the
action of radiation and combines with an interstitial silver ion. In
the development process, the electrons are supplied by a chemical
electron-donor and combine with the silver ions of the crystal
lattice.
The physical shape of the developed silver need have little
relation to the shape of the silver halide grain from which it was
derived. Very often the metallic silver has a tangled, filamentary
form, the outer boundaries of which can extend far beyond the limits
of the original silver halide grain (see the figure below). The
mechanism by which these filaments are formed is still in doubt
although it is probably associated with that by which filamentary
silver can be produced by vacuum deposition of the silver atoms from
the vapor phase onto suitable nuclei.
Electron micrograph of a developed silver bromide grain.
The discussion of development has thus far been limited to the
action of the developing agent alone. However, a practical
photographic developer solution consists of much more than a mere
water solution of a developing agent. The function of the other
common components of a practical developer are the following:
The activity of developing agents depends on the alkalinity of
the solution. The alkali should also have a strong buffering action
to counteract the liberation of hydrogen ions--that is, a tendency
toward acidity--that accompanies the development process. Common
alkalis are sodium hydroxide, sodium carbonate, and certain borates.
This is usually a sulfite. One of its chief functions is to
protect the developing agent from oxidation by air. It destroys
certain reaction products of the oxidation of the developing agent
that tend to catalyze the oxidation reaction. Sulfite also reacts
with the reaction products of the development process itself, thus
tending to maintain the development rate and to prevent staining of
the photographic layer.
A bromide, usually potassium bromide, is a common restrainer or
antifoggant. Bromide ions decrease the possible concentration of
silver ions in solution (by the common-ion effect) and also, by
being adsorbed to the surface of the silver bromide grain, protect
unexposed grains from the action of the developer. Both of these
actions tend to reduce the formation of fog.
Commercial developers often contain other materials in addition
to those listed above. An example would be the hardeners usually
used in developers for automatic processors.
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