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ATOMIC ENERGY COMMISSION
STARS IN PHOTOGRAPHIC EMULSIONS PRODUCED BY
PARTICLES ACCELERATED BY THE CYCLOTRON
University of California
Published for use within the Atomic Energy Commission. Inquiries for additional copies
and any questions regarding reproduction by recipients of this document may be referred
to the Technical Information Division, Atomic Energy Commission, P. O. Box E,
Oak Ridge, Tennessee.
Inasmuch as a declassified document may differ materially from the original classified
document by reason of deletions necessary to accomplish declassification, this copy does
not constitute authority for declassification of classified copies of a similar document
which may bear the same title and authors.
Date of Manuscript: June 12, 1947
Document Declassified: July 9, 1947
This document consists of 10 pages.
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I JU S DEPOSITOR',
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STARS IN'PHOTOGRAPHIC EMULSIONS PRODUCED BY
PARTICLES ACCELERATED BY THE CYCLOTRON
By Eugene Gardner
One method we have used to study tile high energy particles from the 184-inch Berkeley
cyclotron is to allow them to strike the emulsion of a photographic plate. The plate is then
developed in the ordinary way, and is viewed under a microscope. The path of a charged
particle can be followed by a trail of developed grains. This method of detecting charged
particles has been used extensively, and there are on the market various kinds of plates,
each having properties which make it suited to some particular type of experiment. We are
using -Nuclear Particle Track Plates made by Eastman Kodak Co. in this country, and
Nuclear Research Plates made by Ilford Limited in England.
The first exposures which we made on the 184-inch cyclotron were made with the circu-
lating beam Inside the tank. The position of the photographic plate is shown in the first
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By placing the plate at di*frent q gneQiag.setrjacac pgrjr!#p T "pnt
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The next slide gives a detail view of the plat, ihdwifi fow the beam enters the edge of
the emulsion. .
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An Eastman plate ep this way is shown-intheati slide..:i
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When this plate was exposed, both alpha particles and deuterons were present in the beam.
The beam entered from the left. The alpha particle range is about one centimeter, and the
heavy darkening at about one centimeter from the left edge is due to the heavy ionization
of the alpha particles when they slow down and stop. The darkening at about two centi-
meters from the edge is due to the deuterons. The tracks lie mostly in the glass, and the
range measured probably corresponds to the range in glass. The full energy available
from the cyclotron is about 400 Mev for alpha particles and about 200 Mer for deuterons.
This plate was exposed at a radius corresponding to 50 percent total energy, so the energy
of the alpha particles shown here is about 200 Mev and that of the deuterons about 100 Mev.
For a plate exposed as heavily as this one, there are so may developed grains that one
can hardly make out individual tracks. However, in a plate with lighter exposure one can
see numerous straight parallel tracks, as shown in the next slide.
4 MDDC 10.1 .:
In still lighter exposures one can get just one track in a field at view. The exposure re-
quired is small, and one of the problems is getting a small enough exposure. The cyclo-
tron normally operates with a pulsed ion source. However, when we expose films we find
that just one regular are pulse is enough to make a plate completely black. We usually
expose films without any are pulse at all by just turning the r. f. on for a half second or so.
We get lighter exposures by lowering the dee voltage. This slide shows 240 Mev alpha par-
ticles in an Eastman plate. The distance from top to bottom of the slide is about 0.2 mm.
The next slide shows a single alpha particle track in an Eastman plate. The track has
been broken up into sections to get it all on one slide.
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Loped grains are close together at the
as one goes away from the end. The
the scale of the photomicrograph.
xt slide shows a single deuteron track
"stopping end" and they thin out rather
parallel lines on the end are 0.01 mm apart.
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The alpha particle and deuteron tracks are similar at the stopping ends, but the grains in
the deuteron track thin out sooner as one goes away from the end. The difference between :
the alpha particle tracks and the deuteron tracks is illustrated in the next slide, where the .
grain densities for six alpha particles and four deuterons have been plotted (Eastman plates):
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The points from the alpha particle tracks fall into one group, and the points from the
deuteron tracks fall into a lower group. By plotting the grain density in this way one can
identify a particle causing a track, If the track stops in the emulsion, and if you have, say,
a millimeter length, you can tell with some certainty whether it was an alpha particle or a
deuteron. On the other hand, if the track does not stop in'the emulsion you do not know at
what abscissa value to plot the density, and it may not be possible to tell what caused the
The main feature of these tracks is that they slow down and stop uneventfully. If you look
at a large number of tracks, once In a while you find a branching, or star formation. We
have investigated the frequency of occurrence Cf stars by looking at large numbers of sec-
tions of tracks, and adding up the lengths of the sections. We find that the mean path length
for making a star is about 40 cm.
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As you follow the tracks along you find that some tracks go out of the emulsion and others
come in, so that you do not, in general, have full length tracks. It is possible, however, to
find sections of tracks at any energy up to the maximum available from the cyclotron by
exposing plates to a beam of that energy and looking at the tracks near the edge where the
particles first strike the emulsion.
The next slide shows a star formed by an alpha particle in an Eastman plate. The track
of the initiating alpha particle has been followed back to the edge of the plate, at which
point it has an energy of 80 Mev.
The next two slides show stars caused by alpha particles of 160 Mev in Eastman plates.
The grain space for the initiating particle is greater than that for the other tracks.
The next slide shows a typical star formed in an Ilford plate by a 200 Mev deuteron. For :'
stars formed by high energy deuterons it is not possible to see the track of the initiating
The next slide shows a rather unusual star, formed in an Ilford plate by a 200 Mev deuteron.
This star has seven prongs, and it is the only one that we have found with this many.
The next slide shows a star in which one proton and two alpha particles come off. The
proton is identified by its wide grain spacing. This is from an Ilford plate bombarded by
200 Mev deutero.s.
It was suggested by Robert Serber and Wendell Horning that we tabulate the number of
2, 3, etc. prong stars and find the relative numbers of each type of star. They also sug-
gested that we find whether the prongs showed radial symmetry, or whether there were
more prongs in the direction of motion of the initiating particles. In a preliminary survey
we found about twice as many prongs in the direction of the beam as in the backward di-
rection. The result of a study of the number of prongs of about 100 stars is shown in the
next slide. These were stars found in Ilford plates which had been bombarded with 200 Mev
In this study we found more 3-prong stars than any other kind, and none of more prongs
than 5. The study is being continued.
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