Delineation of tracks of heavy cosmic rays and nuclear processes with in large silver chloride crystal

Childs, C. B.

Slifkin, L. M.

English

NASA Technical Reports Server

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TECHNICAL NOTE 
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DELINEATION OF TRACKS OF HEAVY COSMIC 
RAYS AND NUCLEAR PROCESSES WITHIN 
LARGE SILVER CHLORIDE CRYSTALS 
Char les B. Childs 
University of North Carolina 
and 
Goddard Space Flight Center 
Greenbelt, Maryland 
and 
Lawrence M. Slifkin 
University of North Carolina 
Chapel Hill, North Carolina 
NATIONAL AERONAUTICS AND SPACE ADMINISTRATION 
WASHINGTON March 1963 
/" 
https://ntrs.nasa.gov/search.jsp?R=19630003839 2020-03-24T06:09:40+00:00Z
DELINEATION OF TRACKS OF HEAVY COSMIC 
RAYS AND NUCLEAR PROCESSES WITHIN 
LARGE SILVER CHLORIDE CRYSTALS 
by 
Charles B. Childs 
University of North Carolina 
and 
Goddard SPace Flight Center 
and 
Lawrence M. Slifkin 
University of North Carolina 
SUMMARY 
Tracks of energetic charged particles, such as heavy primary 
cosmic rays and the products of nuclear collisions, have been made 
visible within the interior of large , transparent crystals of silver 
chloride. The tracks are delineated by photoelectric formation of 
metallic silver along them. This technique may be useful as a conven-
ient and distortion-free method for the study of heavy primaries and 
fission fragments. 

CONTENTS 
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . i 
INTRODUCTION .......... . . . . . . . . . . . . . . . . . . . . . . .. 1 
EXPERIMENTATION. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 2 
OBSERV ATIONS ................ . . . . . . . . . . . . . . . . .. 3 
CONCL USION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 
ACKNOWLEDGMENTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7 
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 7 
iii 
INTRODUCTION 
DELINEATION OF TRACKS OF HEAVY COSMIC 
RAYS AND NUCLEAR PROCESSES WITHIN 
LARGE SILVER CHLORIDE CRYSTALS* 
by 
Charles B. Childs t 
University of North Carolina 
and 
Goddard SPace Flight Center 
and 
Lawrence M. Slifkin 
University of North Carolina 
The study of cosmic ray particles by means of photographic emulsions has been reviewed 
by Powell, Fowler, and Perkins (Reference 1). Photographic emulsions, while representing a very 
powerful and useful technique of study, do have disadvantages such as high sensitivity to the exact 
processing conditions and distrotion of tracks resulting from swelling and drying of the gelatine. 
Thus, exploration for other recording methods seems worthwhile. 
This paper describes the development of one such alternative technique-the delineation of 
tracks throughout the interior of single crystals of silver chloride. Crystals as large as about 
1. 5 x 1.0 x 0.5 cm have been employed; and the use of larger crystals would also be feasible. Pre-
liminary reports have been published elsewhere (References 2 and 3). The tracks are made visible 
by "decoration" with print-out silver, and have been reproduced without distortion. 
It is perhaps of interest to briefly review the photographic process as normally employed, and 
compare it with the physical processes involved in the formation of print-out silver within a large 
crystal. In an emulSion, the cosmic ray particle registers its passage through the silver halide 
microcrystal by the liberation of electrons which ultimately combine with silver ions to form me-
tallic specks on the surface of the microcrystal (References 4 and 5). During subsequent chemical 
development, these specks catalyze the reduction of the entire crystal to metallic silver. In the 
interior of a large crystal, however, the decomposition products (silver and a halogen) have little 
chance of reaching the surface before recombination; moreover, any silver specks formed in the in-
terior cannot interact with a chemical developer applied to the surface. The registration and magni-
fication of the trajectories of particles through large crystals must therefore rely upon other 
phenomena • 
• This work was supported in part by the Office of Scientific Research, U. S. Ait Force; and the Army Research Office; and was published 
in substantially the same form in Rev. Sci. InS/T. 34(1):101-104, January 1963. 
tPresently at University of Illinois, Urbana, Illinois. 
1 
__ J 
The passage of a charged particle through matter produces local vibrational and electronic ex-
citation, resulting in intense heating of a cylindrical region about the track (see Reference 6). In a 
crystal as plastic as silver chloride the expansion of this hot region deforms the surrounding mate -
r ial, and the cylinder is in turn deformed as itis rapidly quenched (within about 10 -11 second) to room 
temperature (see References 7 and 8). The track of a charged particle is thus surrounded by a cy-
lindrical region of severely deformed matter. It should be possible to decorate this track by applying 
the techniques already developed for the study of dislocations in crystals. 
During the past decade there has been an extensive development of methods for making disloca-
tions visible by the deposition of matter of some sort on them. The technique of preCipitation of a 
solute from a supersaturated solid solution (see Reference 9) presumably could not be applied to 
cosmic ray track delineation, because annealing the crystal would probably decrease the damage in 
the track. Hedges and Mitchell (Reference 10), however, were able to decorate dislocations in Ag Br 
at room temperature by photolyzing the regions near the crystal surface. Moreover, Haynes and 
Shockley (Reference 11) showed that with pulsed photoresponse techniques it is possible to sweep 
photoelectrons many millimeters through AgCl. Each electron trapped in the interior then gives rise 
to an atom of metallic silver, by virtue of the ability of interstitial silver ions to migrate to the 
trapping sites. A combination of these two techniques has been shown to be capable of delineating 
imperfections throughout large crystals (see Reference 12), and it is this process that has been em-
ployed in the present work. 
EXPERIMENTATION 
The specimens employed in this work were in the form of slabs, cut from Harshaw Chemical 
Company AgCl single crystal discs. It was found that only those specimens for which the polyvalent 
metal impurity content was less than six parts per million (as gauged from the ionic conductivity) 
gave good decoration. The careful removal (by means of polishing papers and cloths moistened with 
3% KCN solution) of 2 to 3 mm from one of the broad surfaces was necessary to produce a relatively 
strain-free surface through which the photoelectrons could subsequently be driven. The crystals 
were annealed in air on finely powdered silica for 14 hours at 425°C, and returned to room tempera-
ture at a rate of no more than 12°C/ hour. They were again etched with KCN solution. 
The crystals were mounted in plexiglass holders which also carried nuclear photographic emul-
sions. The strain-free face of each crystal was parallel to an emulsion slab, and separated from it 
by only a 1/4-mil sheet of Mylar. The positioning of the emulsion and crystal was well defined, so 
that after separation of the two it would be possible to transform from coordinates based on the 
emulsion to those based on the crystal. 
These holders , wrapped in Mylar and in black plastic tape to produce a light-tight package, were 
flown in a balloon for 9 hours at an altitude of 108,000 feet. The orientation of the package during the 
flight was such that the emulsion slab was above the crystal, and both were inclined 30 degrees to the 
horizontal. 
Some three months after the flight, the specimens were subjected to the internal decoration pro-
cedure; photoelectrons created at the strain-free surface by an ultraviolet mercury lamp were swept 
2 
into the crystal by an electric field. Although AgCl is an ionic conductor at room temperature, the 
relaxation time of an internal electric field in AgCl is hundreds of microseconds, as compared with 
a photoelectron lifetime on the order of microseconds. Thus, if the ultraviolet radiation is applied 
in pulses a few microseconds in length, and spaced at about one millisecond, and if the sweeping field 
is pulsed synchronously with the light flashes; the photoelectrons may be displaced before the ionic 
conductivity can produce appreciable relaxation. Various mechanical and electronic arrangements 
to provide these synchronous pulses have been described by Haynes and Shockley; Webb; Hamilton, 
Hamm, and Brady; and Sliptitz (References 11, 13, 14, and 15). The apparatus employed in this work 
(Reference 16) applies a potential difference of 2 kv across the crystal, and obtains the ultraviolet 
flashes from a General Electric BH-6 lamp at a repetition rate of 1000 per second. Approximately 
109 photoelectrons per cm2 of crystal surface are produced in each flash. 
In a crystal several millimeters thick, most of the photoelectrons are usually trapped within the 
crystal at the sites of imperfections or impurities. Mobile interstitial silver ions with jump fre-
quencies of almost lOll / sec (Reference 17) migrate to trapped electrons to form metallic silver 
atoms. Repeated electron trapping and neutralization result 
in the formation of specks of silver about one micron in 
size. At the intensity and pulse repetition rate used in this 
work, an exposure of two hours is sufficient to form a col-
loidal distribution within the crystal. A typical crystal, as 
seen by scattered light, is shown in Figure 1. Observation 
under a conventional microscope shows that (ifthe material 
is sufficiently pure) the Ag specks delineate dislocations 
and the tracks of energetic charged particles. 
OBSERVATIONS 
A one-to-one correspondence between the tracks of 
heavy (carbon and above) primary cosmic rays in the emul-
sions and in the crystals was established. Every heavy 
particle track in an emulsion was also observed in the cor-
responding crystal, at the expected location and orientation. 
Moreover, there was a correspondence between track widths 
in the emulsions and widths of the same tracks in the crys-
tals. Figure 2 shows one such track inside a crystal, and 
Figure 3 shows a portion of another track in the emulsion 
along with its continuation in the crystal. No tracks attrib-
utable to protons, alphas, or light primaries have been 
found in the crystals; apparently the denSity of energy re-
leased by particles of low charge and high velocity is too 
small to produce much thermal strain in the crystals. 
Control experiments were performed on many crystals 
Figure l-Dark field photograph of silver 
ch loride crysta I after exposure to synchro-
nous electric field and ultraviolet light. 
The crystal is 4.8mm high and lOmmwide. 
The lines visible in the photograph are 
grain boundaries. 
3 
which were not flown at high altitudes, and no patterns at-
t ributable to cosmic ray tracks were ever observed in these. 
It was always possible to follow the tracks of the heavy 
primaries completely throughout the 5-mm thickness of the 
crystals, as shown in Figure 4. The apparent increase in 
track width in the photographs of track segments below 
about 3000J..L is due to the scattering of light within the crys-
tal. This was demonstrated by viewing the same track 
through the opposite surface of the crystal. 
Only one primary track was found which ended within 
the crystal; a large increase in track density was apparent 
near the end of its range. 
In addition to the straight tracks of the primary radi-
ation, twelve events with two or more secondary particles 
were found. Examples of these are given in Figures 5 
through 9. They were apparently induced by fast light pri-
maries, since the tracks of the incoming particles are not 
visible. 
CONCLUSION 
It is thus established that the internal print-out tech-
nique is capable of recording the tracks of energetic 
charged particles, provided that the energy loss density is 
great enough. Therefore, at relativistic speeds, only highly 
charged particles are seen; alpha particles are recorded if 
t hey move slowly enough to release sufficient energy to 
damage the crystal thermally. For example, this technique 
has been used to detect nuclear disintegrations produced by 
1.55 Bev protons, in which more than twelve secondary 
particles had ranges greater than 1800 J..L which is equiva-
lent to about 3-1/ 2 mm of emulsion (Reference 15). 
Further experiments will be necessary in order to 
evaluate the potential utility of this technique. The question 
of whether the track width in the crystal is a reliable 
measure of energy loss rate-and hence, of the charge of 
r elativistic particles-must be settled. Extensive com-
parison of tracks in crystals and emulsions would then 
permit a calibration of the method. The effects of crystal 
purity and the possible use of AgBr as well as Agel should 
4 
~-----------40p------------~ 
Figure 2-Heavy casmic ray partie Ie track 
inside the crystal afFigure 1. 
, . 
• 
a • 
~-------- 50 p--------~ 
b 
Figure 3-Tracks of the same casmic ray 
particle (a ) in the adjacent emuls ion and 
(b) 72 IL belaw the crystal surface. 
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\ 
e 
" 
b • 
c 
• 
td 
...--- 50 J.L ----t 
Figure 4-Cosmic ray particle track (a) in the emulsion, and within the crystal at depths of (b) 
500 fJ- i (c ) l000 fJ- i (d) 1500 fJ- i (e) 2000 fJ- i (f) 2500 fJ- i (g) 3000 fJ- i (h) 3500 fJ- i (i) bottom surface, 
4860 fJ- . The apparent increase in track diffuseness below about 3000fJ- is due to scattering of light 
in the crystal. 
5 
( O"l 
'" 
~ 
. 
' \ 
1-----IOOfl-----l 
Figure 5-Two secondary particles produced in a.collision at a 
depth of 1560f.L be I ow crysta I surface. Note the increase in the 
track densities near the ends of the ranges. This Figure and 
those following are composites of several photographs taken at 
different depths. The faint, irregu lar lines in the Figures are 
the edges of the individual photographs. 
~ 
1------ 100 fJ 
Figure 6-A five-prong star produced 1440 f.L below 
the crystal surface. 
l _____________ _ 
~. ' . 
. ~, 
1001-1 
Figure 7-A star with eight prongs, two of which are only 
lightly decorated, produced 1500 f.L below the crystal surface. 
• • 
L-____ 100 fI 
Figure 8-A six-prong star observed at a depth of 
1500f.L below crystal surface. 
l 
be investigated. Finally, elimination of 
the dislocation background by the re-
crystallization method of Bartlett and 
Mitchell (Reference 19) seems promising. 
These investigations are currently in 
progress. 
H the method does indeed prove to be 
reliable, several specialized applications 
suggest themselves. The collection of 
statistics on heavy primaries by means 
of prolonged exposures from recoverable 
satellites would now be simplified, and 
without the fogging normally produced by 
- ---
~-----------IOO~------------~ 
Figure 9-A three-prong star produced at 1740J.L below the 
crystal surfoce. A poorly decorated segment, such as that seen 
in the center of the long track of this figure, is quite uncom-
mon; the reason for this lack of decoration is not understood at 
present. 
the high incidence of lighter particles. Heavy particles in solar flares could Similarly be studied. 
Moreover, in any collision recorded in the crystal, angles could be measured rather accurately, since 
the development process does not distort the medium. Finally, this technique seems well-suited to 
the study of fission fragments, either by exposure of pure crystals to particles from high-energy ac-
celerators (References 18 and 19), or by exposure to slow neutrons of crystals doped with fissionable 
material. 
ACKNOWLEDGMENTS 
One of the authors (L. S.) derived much benefit from early discussions with Drs. E. Ney, 
P. Freier, and J. Naugle at the UniverSity of Minnesota. Also, the authors sincerely appreciate the 
generous assistance of Drs. E. Ney, M. W. Friedlander (Washington University), and C. Fichtel 
(Goddard Space Flight Center) in flying crystals and processing emulsions. 
REFERENCES 
1. Powell, C. F., Fowler, P. H., and Perkins, D. H., "The Study of Elementary Particles by the 
Photographic Method," New York: Pergamon Press, 1959. 
2. Childs, C. B., and Slifkin, L. M., "Detection of Primary Cosmic-Ray Particles in Large Silver 
Chloride Single Crystals," Bull. Am. Phys. Soc. Series IT, 6(1):52-53 February 1, 1961. 
3. Childs, C. B., and Slifkin, L. M., "A New Technique for Recording Heavy Primary Cosmic Radi-
ation and Nuclear Processes in Silver Chloride Single Crystals," I.R.E. Trans. Nuclear Sci. 
NS-9(3):413-414, June 1962. 
4. Mitchell, J. W., "Photographic Sensitivity," Repts. on hog. in Phys. 20:433-515, 1957. 
5. Mitchell, J. W., "The Nature of Photographic Sensitivity," J. Photogr. Sci. 5:49-70, 1957. 
6. Seitz, F., and Koehler, J. S., "Displacement of Atoms During Irradiation," Solid State Physics. 
Vol. 2. New York: Academic Press, Inc., 1956. 
7 
7. J. A. Brinkman, "On the Nature of Radiation Damage in Metals," J. Appl. Phys . 25(8):961-970, 
August 1954. 
8. Harrison, W. A., and Seitz, F., "On the Theory of Radiation Damage," Phys . R ev. 98: 1530, 1955. 
9. Amelincla, S., "The Direct Observation of Dislocation Nets in Rock Salt Single Crystals," Phil. 
Mag. Series 8, 1(3): 269-290, March 1956. 
10. Hedges, J. M., and Mitchell, J. W., "The Observation of Polyhedral Sub-Structures in Crystals 
of Silver Bromide," Phil. Mag. Series 7, 44(349):223-224, February 1953. 
11. Haynes, J. R., and Shockley, W., "Mobility of Electrons in Silver Chloride," Phys. Rev. 82(6):935- ' 
943, June 15, 1951. 
12. Childs, C., and Slifkin, L., "Room-Temperature Dislocation Decoration Inside Large Crystals," 
Phys. R ev. Letters 5(11):502-503, December 1, 1960. 
13. Webb, J. H., "Ultrashort Light and Voltage Pulses Applied to Silver Halide Crystals by Turbine-
Driven Mirror and Spark-Gap Switch," J . Appl. Phys. 26(11):1309-1314, November 1955. 
14. Hamilton, J. F., Hamm, F. A., and Brady, L. E., "Motion of Electrons and Holes in Photographic 
Emulsion Grains," J. Appl. Phys. 27(8):874-885, August 1956. 
15. Siiptitz, P., "Die Eigenschaften von Photoelektronen in Silberchlorid bei Zimmertemperatur," 
Z. Physik 153(2):174-185,1958. 
16. Childs, C. Master's Theses, University of North Carolina, 1959. 
17. Seitz, F., "Speculations on the Properties of the Silver Halide Crystals," Revs. Modern Phys. 
23(4):328-352, October 1951. 
18. Childs, C., and Slifkin, L., "Detection of Nuclear Disintegrations Produced by 1.55-Bev Protons 
in Silver Chloride Single Crystals," Phys. R ev . Letters 9(8):354-355, October 15, 1962. 
19. Bartlett, J. T., and Mitchell, J. W., "The Generation of Dislocation Loops at the Surfaces of 
Crystals of Silver Bromide," Phil. Mag. Series 8, 5(53):445-459, May 1960. 
20. Baker, E. W., and Katcoff, S., "Fission of Ag and Br in Nuclear Emulsion by 1.0-3.0 Bev Pro-
tons," Phys. R ev. 126(2):729-734, April 15, 1962. 
8 NASA-Langley. 1963 G-311 


DELINEATION OF TRACKS OF HEAVY COSMIC RAYS AND NUCLEAR PROCESSES WITHIN LARGE SILVER CHLORIDE CRYSTALS

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