Kinetics of bromine release during the photodecomposition of lead and silver bromide

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Kinetics of bromine release during the

photodecomposition of lead and silver bromide

S. Rushbiook Williams, L. Barr

To cite this version:

S. Rushbiook Williams, L. Barr. Kinetics of bromine release during the photodecomposition of lead and silver bromide. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-293-C6-296.

�10.1051/jphyscol:1980674�. �jpa-00220112�

JOURNAI- DE PHYSIQUE Colloque C6, supplkment au no 7, Tome 41, Juillet 1980, page C6-293

Kinetics of bromine release during the photodecomposition of lead and silver bromide

S. J . C . Rushb~ook Williams and L. W. Barr

Scottisl~ School of Non-Destructive Testing, Paisley College of Technology, High Street, Paisley, Renfrewbhire PA1 2BE, Scotland

Abstract. - During an investigation of anionic diffusion in lead and silver bromide, the kinetics of bromine release into vacuum under irradiation from ^U.V. light was observed.

Both materials display an initial linear release with time ( I ) which then changes into a fidependence.

Both materials exhibit a temperature sensitivity to the release kinetics but differ in the~r response.

These results will be discussed with reference to current ideas on AgBr and PbBr, photodecomposition theories.

1. Introduction. - The anion half of the photode- composition of silver bromide has received less atten- tion than the commercially important cation reaction.

Hamilton [I], however, was able t o observe photo- hole mobility by the emulsion reaction close to silver bromide grains, forming, what was interpreted as bromine atoms, molecules or bromogelatin com- plexes.

Malinowski (21 in reviewing current experiniental data, states that the photohole moves to the surface as a vacancy hole complex and their dissociates.

For lead bromide a similar lack of attention is given to the anion, but Schoonman [3] sums up current experimental data and proposes detailed kinetics of the arrival of the photoholes at the surface, but no data is available to support these ideas.

In both materials, the release of a bromine atom or molecule is necessary for photodecomposition to con- tinue. The measurement of this release will lead to data on the photohole arrival kinetics. Reported here are experiments that measure directly this bromine release from both materials during photodecomposi- tion in vacuum.

2. Experimental. -The isotope Br8' is generated by neutron irradiation of samples of lead and silver bro- mide single crystals. After an anneal and etch to remove radiation damage, the samples were placed in the apparatus detailed in figure 1.

It was established that the carbon traps all the bro- mine released and also that once trapped, no further movement of the tracer takes place within the char- coal so not affecting the counting geometry.

Tracer activity is monitored via the well scintilla- tion counter and an automatic data counting system.

Standard radiation corrections are applied for

U.V. ^LIGHT PUMP

Fig. 1.

-

radioactive decay, background and counter dead time.

Temperature of the crystal was monitored by a thermocouple in close proximity to the crystal and temperature was varied by a pumped fluid system in the heater jacket.

Ultra violet irradiation was from an unfiltered high pressure discharge lamp and via the specfrasil disc on to the crystal.

The behaviour of the apparatus was as predicted in that the activity in the charcoal remained zero until the U.V. light was switched on when the activity increased. This increase stopped whenever the irradia- tion stopped. Within the confines of the detection system, all changes took place instantaneously.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1980674

C6-294 S. J . C . RUSHBROOK WILLIAMS AND L. W. BARR

To enable the release to be studied in detail, the irradiation intensity was kept low and thus print out saturation of the crystals did not occur for many hours.

3. Results and discussion. - The release of bromine in both lead and silver bromide can be characterised by three regions. The first is a period where the release is linearly proportional to time. This changes into a region where the release is proportional to the square root of time and finally, after a considerable exposure period, the experimental arrangements tend to, and finally, saturate as print out becomes dominant within the crystal.

The initial release, detailed in figure 2, is characte- ristic of a fresh crystal. By measurement of the speci- fic activity of the sample, the initial region accounts

Fig. 2. - Count rate R against time in hours for AgBr.

for between tcn and fifty monolaycrs of bromine released. The value seems independent of light inten- sity or temperature. The inaccuracy of the figure being in the difficulty of determining the surface area of the crystal and counting efficiency of the detection system.

Attemps were made, after the linear release had changed into the square root of time release to re- establish the initial kinetics by switching the light off for periods of up to three hours. No effect was notic- ed. However, periods in excess of thirty hours may be necessary for the re-establishment of a space charge layer if (he destruction of it is causing the ini- tial linearity.

The longest and predominant region of photode- composition is when the bromine released is propor-

Flg. 3..- Count rate squared (Rq against time for AgBr.

this region that the temperature measurements reported below were made.

As the temperature is varied the rate of release varies. Here the two materials show different cha- racteristics, as shown in figure 4. The lead bromide can be characterised by an equation of the form

where dR*/dt represents the release rate, Ri a cons- tant containing many terms related to photographic

I

I

2 0 4 0 8 0 8 0

T E M P E R A T U R E OC

tional to the square root of time, Or as plotted in 1:ig. 4. - Slope of relcased count rate bquared against time,

figure 3 count rate squared against time. It is within plotted for various temperatures. x ⁼ AgBr, = PbBr?.

KINETICS 01' RI<OMINE RELEASE DURING THE PHOTODECOMPOSITION OF LEAD C6-205

DISCUSSION efficiency light intensity and spectral distribution

and terms relating to the radioactivity detection system. k and T have their usual meaning. It is of interest that the value of 0.16 compares well with a value for the photographic efficiency - temperature dependence noted by Verwey [4] while observing lead formation during irradiation. ⁴

Silver bromide is not so easily described. The broad peak around room temperature is too poorly defined by our present data to justify further analy- sis. R2

Studies of oprical density as a function of time [4, 21 for lead and silver bromide yield information ³ about the catonic half of the reaction.

For Silver bromide this is proportional to time while for lead bromide both linear and root time dependence can occur depending on light intensity.

By stopping the experiments and measuring opti- cal dcnsitic\ a root time release of bromide is observ- 2

cd for coniparable points in the photo decornposi- tion for both substance even above and below the critical light level observed by Verwcy in lead bro- mide.

Finally as the print out image builds up on the surface and internal surfaces the photographic pro- cess saturates (51 and this is show;z in figure 5 .

Question. ^-- L. SLIFKIN. 30 seconds. An irradiation of two hours was the IS the process greatly affected by changing the defect sh 0' test irradiated crystal examined.

populations, say by doping with CdH ?

Question. - P . W. M. JACOBS.

1

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. .;/." ^. .

-7.

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- 2 .

f *

1.

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i

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2 4 6 8 1 0 1 2

Reply. - S. RUSHBROOK WILLIAMS.

None of the results reported here were taken close TIME hours

to this point in an attempt to reduce any recombi-

nation reactions. ^{I ) ? . 5 . - Count rate} \ q ~ ~ a ~ c t l H-' clgall!\r ~ I ~ I I C I'ur AgRr.

We have observed no such variation despite heavy dopant concentrations. The release kinetics in the

f i

region seem very immune to crystal variations.

Question. - Z . MORLIN.

Some years ago we made similar experiments with ZnS and CdS, and found metallic Zu i.e. Cd preci- pitates in the grain boundaries. The first precipitates could have been observed electro-microscopically already after 30 seconds.

Reply. - S. J . C. RUSHBROOK WILLIAMS.

We too observed grain boundary and dislocation decoration by cationic precipitates, though most of the metal appeared to be formed at the surface. We certainly did not examine the crystals as early as

Did you d o optical or electron microscopy to determine if silver nuclei are forming at grain boun- daries ?

Reply. - S. J. C. RUSHBROOK WILLIAMS.

Yes, we did optical microscopy. There was build up of silver or lead along grain boundaries. There was also considerable build up on the surface of the crystals.

Question. - F . GRANZER.

Do you observe precipitation of photolytic silver during bromine release ?

Reply. - S. RUSHBRWK WILLIAMS.

Yes : Most of these investigations lead to print-out made with a directly measurable increase in optical density.

C6-296 S. J . C . RUSHBROOK WILLIAMS A N D L. W . BARR

References

[ I ] HAMILTON, J . F., BRADY, L. E. and HAMM, F. A., J . Appf. [3] SCHOONMAN, J., in Phvs~cs of Electrolytes, Vol. 1 (Academic

Phys. 27 (1956) 874. Press), 1972.

[Z] MALINOWSKI, J., Pholo. Sci.

+

[S] CHILDS, C. and SHIFKIN, L., Bull. Amer. Phys. Soc. 6 (1961) 52.