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A supplementary uniform exposure of photons creates conditions for photographic emulsions to operate at higher levels of sensitivity to imagewise exposures of radiation. Used in combination with active surface development, edge effects for particle track exposures can boost signal-to-noise ratios for weak track signals and reveal a collection of effects never before observed.

photographic emulsionssilver halideexposure effectsconcurrent photon amplification


A new method, using supplemental exposures to light and active surface development, has been developed to increase the signal-to-noise ratio for imagewise exposures of particle tracks. Using this method, new types of particle tracks, not observed before, have been seen in photographic emulsions. It is the object here to introduce the technique and to explore possible mechanisms behind these observations.

In the 1970s, certain groups studied the use of supplemental amplifying exposures for photographic recording. Three research groups: C&C Research @cole_p [@cole], Rome Air Development Center @czwakiel and Rochester Institute of Technology @feather [@kern; @desai] studied a process known as Concurrent Photon Amplification (CPA). The present study has its original roots in the Photo Science group at RIT under Ronald ‘Doc’ Francis.

The technique of CPA was of interest for remote sensing and in general for standard imaging. Photographic speed increases of up to 64 times or 6 stops have been reported for CPA @czwakiel [@kern] so it would be of interest to apply this technique to nuclear track detection and other areas where photographic detectors are used.

Particle Tracks

In the present study, tracks from earlier studies @keith [@keith.elliptical] are analyzed. Tracks were produced using a brief pre-exposure to visible light as an amplification or sensitization of the photographic emulsion. Upon development these exposures produce a uniform background density.

Properties of these new and unusual tracks in photographic emulsions were analyzed and classified. The source of the tracks was unknown. Based on analysis of the tracks, it was suggested that a new type of particle may have been detected. @keith [@keith.elliptical]

In earlier experiments it was found that high levels of pre-exposure can be given to the emulsion and that with an active surface developer, a chemical adjacency effect may occur allowing tracks to be recognized against the high density background.

The most recent study @keith.elliptical found certain particle tracks by simply exposing emulsions with a brief flash from LED lamps.

![Elliptical trajectories. Elliptical particle tracks in photographic emulsion. See Fig [figure~2~] for legend and Fig. [figure~4~] for closeup track detail.](figure_1.eps)


This effect is exemplified in tracks comprised of a central dark track with a white surround as in Fig. [figure~4~].


Study 1

In the first study, in total darkness, monodisperse photographic emulsions were exposed to a brief flash of light (uniform fogging exposure) and then developed after a certain period of time, usually from 2 to 30 minutes. Tracks were noted on the developed emulsions.

The track effect was observed on 6 different film types. Although a number of emulsion types were used, @keith the work focused on Kodak type NTB3 10$mu$ nuclear emulsion and Kodak Ortho Type III film developed using Kodalith RT lith developer.

Following a recommendation from nuclear track technique@beiser, (normally done with thicker emulsions to prevent image spread), development was done without agitation.

Tracks are observed with increasing frequency in monodisperse emulsions as agitation is decreased in development. A fogging pre-exposure enhances observation of the track images.

Unusual tracks appeared on the film that appeared to match the idea of elementary particles traveling through the emulsion, but with characteristics distinct from known particles.

In over 200 exposures of the first study, tracks were observed with or without a fringe or halo effect around a central track of width between 4$mu$m and 200$mu$m.

Study 2

In the second study, film types Arista Ortho Litho 2.0 and Rollei Ortho 25 4x5 sheet film was used with Arista Lith developer, developed by inspection under a red safelight.

In the most recent experiments the film was simply placed on the stage and subjected a photon exposure with or without applied electric or magnetic fields. The photon exposure was between 50$mu$s to 500$mu$s, and the time between removal of film from storage and development was minimized to 1 - 2 minutes. At all times, film was handled carefully with vinyl gloves.

Various arrangements of electric and magnetic fields were used to understand any connection between applied fields and any track curvature.

Controls were made with and without electric and magnetic fields and with and without supplemental uniform exposure to light. In the cases of no uniform supplemental light exposure, no visible tracks were detected.

Detailed characteristics

Both positive and reversed tracks are observed. This is a purely photographic designation not based on observations of particle energy. Photons or electrons should create latent images on the AgBr crystals producing positive tracks, but the mechanism for reversed tracks requires additional study. However it is not totally unexpected that a reversal effect would result from a pre-exposed emulsion.

Interestingly a third category of tracks we will refer to as “evaporated” are observed as track images apparently formed due to the absence of gelatin (or perhaps even plastic base). These tracks are normally only seen under magnification of $> 10x$ and are sometimes without Ag$^0$ (developed silver) being mostly clear except for the optical effects they produce.

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The track effect has been observed on six different film types. The effect is most visible on film types Kodak NTB3 and Kodak Kodalith Type III using lith development. NTB3 is engineered to be electron sensitive and is generally sensitive to both ionizing and non-ionizing radiation from the visible part of the spectrum to gamma and beyond (with a cutoff in the hard UV to X-ray region due to absorption by gelatin). Both NTB3 and Kodalith Type III are monodisperse (all grains about the same size) emulsions. Other film types and development procedures also show the effect. Particularly interesting is the occurrence of tracks using both lith and solution physical development[^1] using both monodisperse and polydisperse[^2] emulsions.

Without magnification, most tracks are essentially invisible in monodisperse and polydisperse emulsions where agitation is used in development. Uniform pre-exposures and no agitation in development greatly enhances observation of the track images. Since agitation in development is the standard technique and high magnification is required to see many of the tracks, it is easy to imagine how these images have largely evaded recognition until now.

Lith development was used with and without agitation and solution physical development was used with agitation. A “border effect” is observed when a track intersects an area of high exposure. A uniform pre-exposure in conjunction with lith development with no agitation amplifies this photographic exposure effect with a photographic development effect called a border or edge effect,@james greatly increasing the visibility of track images.

Image Formation

In the following we refer to the state of photographic development of the track image and the central track image in tracks with a border effect. Track images are formed on photographic film in one of three ways:

  1. <span>Type 1. Excitation of of the AgBr crystals via either ionization or light causing track images via developed AgBr crystals. This is referred to as “positive” or “normal exposure.” This type of track image is seen on the emulsion as a dark line on a lighter background.</span>
  2. <span>Type 2. Reversal of exposed AgBr crystals via an unknown mechanism causing track images via the absence of developed crystals. This is referred to as “reversal” or “bleaching.” This type of track image is seen on the emulsion as a white line on a darker background.</span>
  3. <span>Type 3. Direct action on the gelatin and possibly the plastic base producing track images via the removal and/or deformation of plastic or gelatin. This is referred to as “evaporated.” This category of track simages can be clear, but visible due to the refraction of light and can also be accompanied by developed silver in the repeating patterns.</span>

It is important to note that the mechanism for track type 2 is unknown, but it must be a mechanism that causes a tear-down of latent image. Without the supplementary uniform exposure to light, it would not be possible to see a particle exposure effect that would tear down latent image. This is an effect that has never been observed or discussed prior to our studies. We must seek to understand the mechanism of a particle to tear down latent image.

Edge effects can occur for both track types 1 and 2 so it is possible to have a black central track (type 1) with a white edge effect surround or a white central track with a black edge effect surround. Some images are reversed for analysis. A reversed positive (type 1.) particle track image is generally seen as a white line on a darker background and a reversed (type 2.) particle track is seen as a black line on a lighter background.

Images are labeled in figures as to their type as above with (1),(2) and (3) and context with (O) for original, (R) for reversed. An image with both type 1 and type 2 tracks and reversed context is labeled as “(1:2:R)” and a type 1 track in original context is labeled ``(1:O) See Figs. [figure:microdensity] and [figure:microdensity-a] for an overview of positive and reversed tracks and Section [section:reversed-tracks] for a discussion of reversed tracks.

Comparisons with known techniques

CPA was developed and used initially for standard photography where there is a need to maintain image quality along with increases in photographic speed. For applications where constraints on image quality are relaxed or simplified to include only recognition of line and dot images, (the central aim of nuclear track studies), additional sensitivity increases might be achieved by simply increasing a uniform supplementary photon exposure.

But there are differences between typical photographic exposures and exposures to the passage of charged particles which must be taken into account. Salient differences include:

  1. <span>In typical photography multiple photons must be absorbed by an AgBr grain to confer developability, but a single charged particle can expose multiple grains along the track @james</span>
  2. <span>Ionizing radiation will form more internal image than typical photography because of the particle’s traversal through the grain @james</span>
  3. <span>Charged particles create Frenkel defects, which in turn create interstitial Ag ions @barkas</span>
  4. <span>Although it has been pointed out that reciprocity failure does not occur in exposures to ionizing radiation due to lack of more than one particle or photon interaction @james, the exposure time of typical charged particles is very short at about $10^{-15}$s, (i.e. nominal grain size of 0.34$mu$ m at a velocity of $c$ is $3.4times 10^{-7}$ m $/2.99times 10^8$ m/s $simeq 1.1 times 10^{-15}$ s) which is highly susceptible to reciprocity failure. @barkas</span>

CPA should counteract reciprocity failure due to both exposures occurring at the same time and being additive. (in fact this could be one reason why certain particles would not be detectable) That is, particles moving through the emulsion for which reciprocity failure normally occurs would be detected in the CPA-enhanced exposure.

exploitation of edge effect using low-sulfite formaldehyde hydroquinone type ’lith’ development for line and dot images in conjunction with a supplementary uniform exposure to photons.

using no agitation during development, a method to prevent image spread in thick nuclear track emulsions @beiser, intensifies the edge effect due to developer by-products accumulating in areas adjacent to high-exposure regions, which in turn bleaches the adjacent areas.

The main differences between standard nuclear track technique and these techniques are increased background fog, increased signal-to-noise ratios and slightly thinner (more accurate) tracks.

In our studies, tracks have been observed with and without image amplification. In exposures with amplification, the inner track is surrounded by an edge, fringe or halo effect on both sides of the track. The white surround is a chemical adjacency effect.@james With no agitation in development, the oxidation products of the developing agent hydroquinone build-up around the high exposure regions, affecting both size and density of high exposure areas (border effect) and adjacent low exposure areas (halo effect). (See Fig. [figure:microdensity]b). These effects in Fig. [figure:microdensity-a]c and Fig. [figure:microdensity-a]d are possibly due to additional exposure to light@james or other radiation.

![Schematic of three elliptical orbits and the NdFeB disk magnet. The geometry of the LEDs and the permanent magnet causes shadows to be cast. Dotted lines show probable orbit extensions under the magnet with focus locations. ](figure_2.eps)


Concluding remarks

Acknowledgments {#acknowledgments .unnumbered}

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Acknowledgments {#acknowledgments .unnumbered}

The author thanks Ronald Francis for providing me the foundation and initial facilities for this work.

References {#references .unnumbered}

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References {#references .unnumbered}
[^1]: Lith development refers generally to low-sulfite,
formaldehyde-hydroquinone type developers which tend to develop surface latent image. Solution physical development refers to developers that tend to work by dissolving part of the silver halide grain, putting the Ag$^+$ ions into solution and potentially plating complexed Ag$^0$ onto the latent image. Solution physical developers will activate internal as well as surface latent images. This may be important to distinguish between certain types of radiation. X-rays and higher energies penetrate AgBr grains and will create internal latent images whereas visible light tends to create mainly surface latent images.
[^2]: Monodisperse emulsions have all AgBr grains of a similar size and
are generally finer grained, less sensitive to light and higher contrast while polydisperse emulsions have a wide range of AgBr grain sizes, are relatively coarser grained, more sensitive to light with a lower contrast.


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