1
Bull. Soc. Photogr. Imag. Japan. (2017) Vol. 27 No. 1: 1–6

Original Paper

Sensitization of ultra-fine-grain photographic emulsions (II): Combination ofchemical sensitization and halogen acceptors
Ken’ich Kuge, Kazumitsu Morimoto

Abstract:Sensitization methods including chemical sensitization and halogen acceptors (HA) for ultra-fine-grain (UFG) emulsions with different grain sizes and crystal habits were investigated and the desensitizing factors peculiar to UFG emulsions were analyzed. Sulfur-gold sensitization or HA alone gave a small sensitivity increase but the combination of both gave the maximum effect; reduction
sensitization alone had a large effect but the combination with HA decreased the sensitivity from this level. Rehalogenation was
suggested to be an important factor in the desensitization of UFG emulsions. Sensitization effects with exposure intensities andcrystal habits were qualitatively similar between low and high intensities, and {111} and {100} faces, but the former cases showedlarger effects than the latter cases. The sensitivity after sensitizing treatments was proportional to the higher power than the third
one of grain size. The sensitivity of the unsensitized UFG emulsion was also in this proportional relationship, but was much lower
than that of the sensitized emulsions.

Key words:

Silver-salt photographic material, Ultra-fine-grain emulsion, Chemical sensitization, Halogen acceptor, Rehalogenation

1. Introduction
Silver halide photographic materials with ultra-fine-grain (UFG)
emulsions are used for high-resolution recordings such as holograms1). Nowadays, they are also used to record ultra-fine radiation
tracks as they provide high resolution2). Even though UFG emulsions are important for fine-recordings, the photographic characteristics of UFG emulsions are still not understood. For example, thephotographic responses to high-intensity (HI) and low-intensity(LI) exposures are significantly different. Radiation exposure is similar to HI exposure3) while holograms are recorded with long-period
exposure due to the low sensitivity of UFG emulsions. The former
exposure often causes the dispersion of latent image specks (LIS),
which is one factor decreasing sensitivity4, 5). The dispersion of LIS isaffected by chemical sensitization6~11), and these sensitizations arealso affected by the crystal habits of the emulsion grains12~15). These
must be considered when designing photosensitive materials using
UFG emulsions, but their characteristics are still not understoodwell.
It is known that photographic sensitivity to light is proportional
to the volume of silver halide grains or proportional to the third
power of the grain size5). It was also reported that the sensitivity toradiation was proportional to the 1.5th power of the projected area of
the grains16). On the other hand, there are reports that the sensitivity
is proportional to higher than the third power of the grain size in thesize region including ultra-fine grains17,18). This suggests that there
are some desensitizing factors that enhance the sensitivity decrease,
Received 19th, January 2017; Accepted 22nd, April 2017

*Graduate School of Advanced Integration Science, Chiba University**1-33, Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

particularly in UFG emulsions.
To increase the intrinsic sensitivity of photographic materials,
some chemical sensitization methods are usually applied19, 20).
Among these methods, sulfur-gold (SG) sensitization has high sensitivity and effectively recovers the sensitivity loss due to the dispersion of LIS and HI exposure4,21~23). Previously, we investigated thesensitivity increase of UFG emulsions by SG sensitization and thehalogen acceptor (HA) of sodium nitrite; we found that the halogen
acceptor gave a large sensitivity-increase for UFG emulsions especially at LI exposure, while the SG sensitization did not18). It is
known that HAs are effective in removing rehalogenation24~28). A
hole arriving at the surface turns to an adsorbed halogen atom andthis attacks a silver atom in the LIS to cause rehalogenation. In
UFG emulsions, holes will easily turn to halogen atoms due to thelarge surface-volume ratio, and rehalogenation proceeds rapidly.
Therefore, reduction (R) sensitization can also restrain rehalogenation, as already reported14, 19, 20, 29). Moreover, there is a patent that R
sensitization is useful to increase in the sensitivity of UFG emulsion
for hologram30), although the mechanism of sensitization is not discussed. However, we did not try the R sensitization in UFG emulsions at that time.
Following the previous investigations18) we tried to expand themethods to increase the sensitivity of UFG emulsions. We used both
R and SG sensitizations, and also used acetone semicarbazone andsodium nitrite as HAs. We then compared the effects by thesemethods and investigated the additivity of these methods. We also
analyzed the difference with low and high intensities, the size-de-Bull. Soc. Photogr. Imag. Japan. Vol. 27 No. 1 (2017)

pendence, and the effect of crystal habit of the grains to clarify thedesensitizing factors peculiar to UFG emulsions.

Experimental
Two types of photographic emulsions were used. The first was aseries of emulsions with AgBr0.98I0.02 monodisperse cubic grains,hereafter called EmA. These emulsions were used in our previous
studies18, 31, 32). The edge lengths of these grains were 35, 43, 80, 92,
124, and 148 nm, and we compared the grain-size effect using theseemulsions. The second emulsion consisted of nearly spherical AgBr0.98I0.02 grains and the grain diameter was approximately 40–50
nm. This emulsion, identified as EmB, was also used in our previous
studies18,33).
We subjected these emulsions to SG sensitization or R sensitization. A gold thiosulfate complex {Na3Au(S2O3)2} was used as thesensitizer for the former18, 34) and its quantity was varied from 1 to 15
mmol/molAg. A thioureadioxide {(NH2)(NH)CSO2H} was used as
the sensitizer for the latter and its quantity was varied from 0.1 to 30
mmol/molAg. The emulsions were treated at 60 °C for 60 min
during both sensitization treatments. These emulsions were coatedon a poly(ethylene terephthalate) (PET) base by hand, hence thethickness of coated films had some irregularity.
These unsensitized and sensitized emulsions were treated with
HAs. Sodium nitrite (NO; NaNO2) and acetone semicarbazone{ASC; (CH3)2C=NNHCONH2} were used as HAs. This treatment
was performed using two methods. The first method involved the
addition of the HA to the emulsion just before coating. In the second method, the coated film was immersed in the HA solution before exposure. In the former case, 0.35 mmol/molAg of NO was added to the EmA emulsions, and 0.3-1.2 mmol/molAg of NO or 0.35
mmol/molAg of ASC was added to the EmB emulsion. In the latter
case, a coated film was immersed in the HA solution with a concentration of 0.1 mol/L NO (or ASC) at 20 °C for 3 min.
Sensitivity was measured using a JIS III sensitometer. The originaltungsten filament lamp in the sensitometer was used for the LI
white light exposure (duration 100 s). The intensity in front of thestep wedge was 539 lx and the exposure value was represented withH (lx s). For the HI exposure (duration 10−6 s), a xenon flash lamp
was installed in the sensitometer. As the intensity of the HI exposure was not measured and the exposure value was represented as arelative value (rel. E), exposure values of H for the LI exposure andof rel. E for the HI exposure were not identical.
The exposed films were developed using a D72 developer diluted
1:4. The development temperature was 20 °C and the development
time was 5 min. Normal stop, fixation, and washing treatments were
performed after development. The optical densities (OD) of the developed films were measured with a photographic densitometer andphotographic characteristic curves were obtained. Because thethickness of coated films had some irregularity due to the hand-coating, the measured OD values sometimes varied widely. Therefore, wemeasured the OD at several points and treated the data statistically.
The sensitivity was compared with the reciprocal of the exposure
value that gave OD of 0.1 above fog on a characteristic curve. We set

this small value of 0.1 for the sensitivity measurement because thesensitivity of the UFG emulsions was very low and near the limit ofmeasurement. However, this brought about some uncertainty insensitivity.

Experimental Results
Characteristic curves of the set of EmA emulsions with grains ofdifferent edge lengths with LI exposure are shown in Fig. 1. They
were treated with SG sensitization (5 mmol/molAg) and/or immersion in an NO solution (0.1 mol/L). Sensitivity decreased with decrease in grain size, especially that of the unsensitized emulsion withthe smallest grain decreased drastically to reach the limit of measurement. SG sensitization alone was significantly effective in increasing sensitivity in larger-grain emulsions, while it was not so insmaller-grain ones. HA alone was not very effective, but increased
the maximum density and contrast. A combination of both gave alarge increase in sensitivity, even for the smaller-grain emulsions.
Characteristic curves for the set of EmA emulsions treated with R

2

35nm

Low Intensity Expo.
:U: SG: NOimm: SG+NOimm

43nm

1

0

2

54nm

63nm

80nm

92nm

124nm

148nm

1

OD

2

0

2

1

0

2

1

0
1

2

3

4

1

2

3

4

logH

Fig. 1C haracteristic curves of EmA emulsions with cubic grains of different
edge lengths treated for SG sensitization and/or immersion in sodium nitrite solution (NO) at low-intensity (100 s) exposure.
— : Unsensitization, ---- : SG sensitization, *** : Immersion in NO, : SGsensitization and immersion in NO.

Ken’ich Kuge, Kazumitsu Morimoto3

Sensitization of ultra-fine-grain photographic emulsions (II): Combination of chemical sensitization and halogen acceptors

sensitization (3 mmol/molAg) and/or immersion in the NO solution
are shown in Fig. 2 with LI exposure. In the larger-grain emulsions,fog appeared with R sensitization treatment and we could not obtain characteristic curves. R sensitization alone gave high sensitivity
and increase in contrast, while the combination of HA and R sensitization decreased the sensitivity from that of R sensitization alone.
Characteristic curves for the set of EmA emulsions treated withthe same protocol as in Fig. 1 are shown in Fig. 3 for HI exposure.
Sensitivities of the unsensitized emulsions with smaller grains werebeyond the measurement limit. Although it is known that SG sensitization is effective for HI exposure, the effect was definitive withthe smaller-grain emulsions. The effect of HA alone was small.
Characteristic curves for the set of EmA emulsions treated withthe same protocol as in Fig. 2 are shown in Fig. 4 for HI exposure. R
sensitization alone had high sensitivity. The combination with HA
decreased the sensitivity from this level, similar to the observations
with LI exposure. Behaviors with HI and LI exposures were qualitatively similar, but the sensitivity increase by these methods with HI
exposure was not as drastic as with LI exposure.
The relationships between the edge length and sensitivity for LI
exposure are represented on a logarithmic scale for EmA emulsions
with SG sensitization and/or immersion in NO solution on the left
side of Fig. 5, and those with R sensitization and/or NO immersion
on the right side. Points for these relationships lay mostly on astraight line with similar slope. However, the sensitivity of the unsensitized emulsion with the smallest grains decreased drasticallyand large desensitization appeared, as shown by the deviation fromthe straight line. The slopes of each sensitization method are represented in the lower right of the figures, and they were all larger than
three. These suggested that there were some desensitizing factors insmaller-grain emulsions and these factors worked drastically on theunsensitized emulsion.
The relationships between the edge length and sensitivity for HI
exposure are represented in Fig. 6 in the same way as in Fig. 5. The
points for each sensitization method were on a straight line, but theslopes were all larger than three. The sensitivities for unsensitizedemulsions with smaller grains under HI exposure were very low, andthere was no point at these sizes. These sensitivities were lower thanthose expected from extrapolation of this line. These suggest that adesensitization route peculiar to the UFG emulsion also exists with
HI exposure.
Characteristic curves for the EmB emulsion treated with SG or R
sensitization or the addition of NO before coating, where theamount of sensitizer or additive was altered gradually, are shown in
Fig. 7 for LI exposure. We could not obtain the sensitivity for the

2

35nm

High Intensity Expo.
:U: SG: NO imm: SG+NO imm

43nm

1

0

2

35nm

Low Intensity Expo.
:U:R: NOimm: R+NOimm

43nm
2

1

0

0

54nm

63nm

2

63nm

80nm

92nm

124nm

148nm

OD

2

OD

1

54nm

1

1

0

2

2

3

4

log H

80nm

2

1

1

0

0

2

3

4

0

logH

Fig. 2

Characteristic curves of EmA emulsions with cubic grains of different
edge lengths treated for R sensitization and/or immersion in NO
solution at low-intensity (100 s) exposure.
— : Unsensitization, ---- : R sensitization, *** : Immersion in NO,:Rsensitization and immersion in NO.

0

1

2

0

1

2

log rel.E

Fig. 3C haracteristic curves of EmA emulsions with cubic grains of different
edge lengths treated for SG sensitization and/or immersion in NO
solution at high-intensity (10−6 s) exposure.
— : Unsensitization, ---- : SG sensitization, *** : Immersion in NO, : SGsensitization and immersion in NO.

4

Bull. Soc. Photogr. Imag. Japan. Vol. 27 No. 1 (2017)

35nm

High Intensity Expo.
:U:R: NO imm: R+NO imm

43nm

0

1

0

2

54nm

High Intensity
(b)

(a)

log S (relative)

2

63nm

-1

-2
slope
:U
4.2
: SG
3.7
: NO
3.8
: SG+NO 4.3

-3

OD

1.6

1.8

:U
:R
: NO
: R+NO

2

2.2

1.6

1.8

2

slope
4.2
4.6
3.8
4.0
2.2

log(d/nm)

1

Fig. 6
0

2

0
80nm

1
log rel.E

Logarithmic plot of the edge length of cubic grains in EmA emulsions versus sensitivity for high-intensity (10−6 s) exposure with eachsensitization method. Slope is the inclination of the straight line
through the plotted points.
U: Unsensitization, SG: SG sensitization, R: R sensitization, NO: immersion in NO solution.

2

1

:U
: 1m SG: 5m SG: 15m SG

1

0

0

1
log rel.E2

0.5

Fig. 4

Characteristic curves of EmA emulsions with cubic grains of different
edge lengths treated for R sensitization and/or immersion in NO
solution at high-intensity (10−6 s) exposure.
— : Unsensitization, ---- : R sensitization, *** : Immersion in NO, : R
sensitization and immersion in NO.

0

OD

-1

0.5

Low Intensity-2:U: 0.1 mR
: 0.3 mR
:1m R
: 3 mR

1

(b)

(a)

log S (relative)

0
-3

:U
: 0.3 NO add: 0.6 NO add: 1.2 NO add

1

-4

0.5
slope
:U
3.9
: SG
4.6
: NO
3.9
: SG+NO 4.3

-5
1.6

1.8

2

slope
:U
3.9
:R
4.4
: NO
3.9
: R+NO 4.5
2.2

1.6

1.8

2

0
2.2

log(d/nm)

Fig. 5

 ogarithmic plot of the edge length of cubic grains in EmA emulLsions versus sensitivity for low-intensity (100 s) exposure with eachsensitization method. Slope is the inclination of the straight line
through the plotted points.
U: Unsensitization, SG: SG sensitization, R: R sensitization, NO: immersion in NO solution.

unsensitized emulsion, due to its low sensitivity. SG sensitization
showed a small increase in sensitivity; increase in the amount of sensitizer caused significant fog. R sensitization showed a large effect ofsensitization with an increase in the amount of sensitizer. Additionof NO showed a larger increase in sensitivity than SG sensitization,
but did not reach the level of R sensitization. Increase in the amountof NO did not cause a significant increase in sensitivity. The effect of
R sensitization on the EmB emulsion was much larger than that on

2

3

logH

4

Fig. 7C haracteristic curves of EmB emulsions with octahedral grains after
different sensitization methods at different levels and at low intensity (100 s) exposure.
Top: SG sensitization, center: R sensitization, bottom: addition of NO solution before coating.

the EmA emulsions with similar grain size.
We added NO or ASC to the EmB emulsions treated with SG or
R sensitization before coating or immersed the coated films in solutions containing them to investigate the combined effects. Characteristic curves of these materials with LI exposure are shown in Fig.
8. The figure on the left shows the results of SG sensitization and thefigure on the right shows the results of R sensitization. As with the
EmA emulsions with small grains, sensitization effects by SG sensitization or HA alone were not large but the combination of both
gave the largest effect. Immersion treatment gave a larger effect thanKen’ich Kuge, Kazumitsu Morimoto

Sensitization of ultra-fine-grain photographic emulsions (II): Combination of chemical sensitization and halogen acceptors

1.5
: SG
: SG+NO add: SG+ASC add: SG+NO imm: SG+ASC imm

OD

1

:R
: R+NO add: R+ASC add: R+NO imm: R+ASC imm

0.5

0

2

3

4

2

3

4

logH

Fig. 8

Characteristic curves of EmB emulsion with octahedral grains after
different sensitization methods and combination of them at low intensity (100 s) exposure.
Left: SG sensitization, right: R sensitization.
NO: sodium nitrite, ASC: acetonsemicarbazone, add: addition of HA solution before coating, imm: immersion of coated film in HA solution.

addition. This increased amount of sensitivity was much larger thanthat in the EmA emulsions. R sensitization alone also showed a large
effect, even though this did not reach the level with combined SG
sensitization and NO; the combination with R sensitization and
HA decreased the sensitivity, as with the EmA emulsions. Generally
the effect of sensitization method on the EmB emulsion was similar
to that on the EmA emulsions but quantitatively larger; this was one
main difference between the EmB and EmA emulsions.

Discussion
Previously we reported that there was intense desensitization on
UFG emulsions due to rehalogenation. HA gave a large sensitivity
increase, especially at LI exposure due to the restraint of rehalogenation while the SG sensitization did not18). This tendency was also
observed in this investigation. Moreover, the maximum density andcontrast due to SG sensitization alone were still low. These suggestthat SG sensitization is not enough to restrain the rehalogenation of
UFG emulsions. It is known that the increase in sensitivity by SGsensitization is due to the increase in the formation efficiency of LIS
and to the induction of developability in smaller sub-image specks19, 21).
Therefore, SG sensitization is also effective at HI exposure4, 21~23),
which causes the dispersion of LIS where many small and undevelopable sub-image specks were formed. However, the small specks
formed under SG sensitization may be rehalogenated easily in the
UFG emulsion by halogen atoms generated simultaneously; thiscauses the small sensitization effect. On the other hand, combination of SG sensitization and HA gave the maximum sensitization
effect in the previous report18) and in this investigation. Therefore,
when HA restrains rehalogenation, small specks formed on SG-sensitized emulsion survive and become developable to cause a large
increase in sensitivity.
It is considered that R sensitization acts as hole traps4, 35) and removes holes on the surface, which has the same matter to adsorbed
halide atoms. Because the ratio of holes on the surface increased
with smaller grains, it restrains both recombination and rehalogenation, causing large sensitization effects in the UFG emulsions. However, the sensitivity could not reach the level of SG sensitization plus

5

HA, because R sensitization cannot make smaller sub-image specks
developable. The sensitization effects were not additive when both R
sensitization and HA were used together. Maybe both had the samefunction of removing halogen atoms, and HA treatment decreased
the high sensitivity caused by R sensitization. One possibility is thatthe adsorption of HA on the R sensitization centers would disturb
their action and decrease the sensitivity from the level of R sensitization alone.
The EmA emulsions consist of cubic grains with {100} faces while
EmB consists of spherical grains, which will be regarded as grains
with {111} faces. The effect of sensitization of the EmB emulsion was
generally larger than those of EmA. There was a remarkable difference between the EmB and EmA emulsions with SG sensitization
plus HA and R sensitization, while SG sensitization alone did notshow a remarkable difference. Birch reported that halogen atoms
were preferentially evolved at {111} faces on cubo-octahedralgrains14). Because this evolution would decrease the number of holes
inside a grain and decrease recombination, the effective removal of
halogen atoms on the surface by HA or R sensitization would increase the sensitivity of the EmB emulsion.
Increase in the amount of R-sensitizer was effective in increasing
the sensitivity, while that of HA was not. HAs are only adsorbed on
the surface and do not form sensitization centers by decomposing
the sensitizer as do SG or R sensitizations. The number of adsorbed
HA will be saturated at adsorption equilibrium and the effect becomes constant thereafter. Consequently, smaller molecules of HA(such as NO) will be favorable for UFG emulsions, as shown in the
results combining SG sensitization and HA in Fig. 8.
Relationships between logarithmic scales of grain size and sensitivity were almost on a straight line, as shown in Figs. 5 and 6. However, this relationship deviated significantly to the lower-sensitivity
side at the unsensitized emulsion. This suggests strong desensitization on UFG emulsions, and this desensitization would be due torehalogenation18). This deviation was recovered by some sensitization
treatments and these treatments would relieve the desensitizing factors working on the unsensitized emulsion. As the slopes of thesestraight lines were all still larger than three, these treatments could
not remove the desensitizing factors with UFG emulsions completely.
Multi-scattering is generally strong at large grains, but weak atfine grains. This causes increase in the absorption of light at larger
grains and this is one possible reason why the slopes were larger than
three. In this case, however, the plotted points will not be on astraight line because the points with larger grains are on the straightline of slope three and the ones with smaller grains deviate from thisline. Moreover, this change would occur with all types of sensitization method because the grain shape itself is not affected by thesemethods. Though the accuracy of the sensitivity measurement wasinsufficient, bending of the line was observed only for the unsensitized emulsion and not others. Consequently, this scattering effectwould not be significant.
We obtained a large sensitivity increase in the UFG emulsion by
combining several sensitization methods. The combination of SG
sensitization and HA revealed the largest sensitivity, although the6Bull. Soc. Photogr. Imag. Japan. Vol. 27 No. 1 (2017)sensitivity was still proportional to the higher power than the third
one of grain size. It was confirmed again that the removal of halogen
atoms is effective in UFG emulsions, and rehalogenation is an important factor causing desensitization.

Acknowledgements
The authors would like to thank S. H. Kimura, Konica Minolta
MG and Y. Watanabe, Konica Minolta Photoimaging for the photographic emulsions used in this work.
This work was supported by JSPS KAKENHI Grant Numbers
26287037 and 15K13476.

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