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ATOMIC ENERGY COMMISSION
CLOUD CHAMBER STUDY OF THE PARTICLES IN THE NEUTRON
BEAM OF THE 184" CYCLOTRON
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,
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: July 11, 1947
Document Declassified: September 9, 1947 US DEPOSITOR
This document consists of 6 pages.
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CLOUD CHAMBER STUDY OF THE ,ARTICLES IN THE NEUTRON
BEAM OF THE 184 CYCLOTRON
By Wilson Powell
The 200 million electron volt deuterons made by the 184-inch cyclotron strike an inter-
nal target and produce a beam of neutrons which come out through the 1.25 inch thick steel
wall of the vacuum chamber of the cyclotron. This beam of neutrons drops to half-value at
an angle of five degrees from the tangent to the deuteron beam striking the target. A large
Wilson cloud chamber was placed in the neutron beam at a point 35 feet from the target.
The recoil protons produced by the neutrons in matter surrounding the cloud chamber were
observed with the purpose of finding the maximum energy of the protons and thereby the
maximum energy of the neutrons in the neutron beam.
The circular cloud-chamber 15 3 /4 inches in diameter is equipped with a pulsed mag-
netic field of 9400 oersteds produced by a pair of Helmholtz coils. The cloud chamber tim-
ing circuit turns on the field, expands the chamber, turns on the cyclotron oscillator and
pulses the arc source of deuteron ions in the proper sequence. The tracks produced in the
chamber are very sharp and free from turbulence so that the energies of 200 Mev protons
can be determined to within plus or minus ten percent.
A double camera 52 inches from the cloud chamber with lenses separated by 4.5 inches
takes two pictures of the chamber. These are reprojected together on a ground glass screen
the plane of which can be rotated and placed at proper distance to recombine the tracks. The
projector is indentical with the camera so that the reprojected tracks are life size. The
curvatures of the tracks are measured directly on the ground glass by matching them with
circles drawn on lucite. The error in measurement for a track amounts to plus or minus
five percent for an arc of two meter radius and twelve inches in length.
The films were examined with the purpose of finding the highest energy particles. Out
of approximately 8000 tracks four were discovered which had energies between 150 and 200
Mev. A thorough examination of all the photographs has not yet been made and at present
the high energy protons are being sent through several centimeters of lead as a check on
these pictures. It is quite possible that more particles of this energy will be found as the
Qualitatively it is possible to account for these high energies by the following argument.
When the deuterons strike the target the proton is knocked off and remains inside the
cyclotron. The remaining neutron continues on and passes through the cloud-chamber. How-
ever the neutron may, at the instant the proton is stripped off, possess a velocity with re-
spect to the center of mass of the deuteron. This velocity can be in such a direction as to
add to the velocity corresponding to half the kinetic energy of the deuteron (which is 195
Mev.), and give a neutron with considerably more energy than 1/2 x 195 Mev.
- 1 -
Thus, although at first sight these results appeared difficult to explain it is reasonable
to hope that more detailed calculations and more accurate measurements will be in agree-
Cloud Chamber and magnet assembly.
Projector used in analyzing cloud chamber pictures.
- 3 -
Three stars in oxygen at 1/2 atmosphere pressure. The stars have 5, 4 and 3 prongs. No
A proton scatter in which a short recoil track is visible. Magnetic field of 9,000 oersteds.
- 4 -
A 4 particle star in oxygen. Magnetic field of 9,000 oersteds.
First high energy tracks found in the cloud chamber photographs. Visible only after
some search. If a proton, this track has an energy of 150 Mev 15%. Energy of the deuteron
beam in the cyclotron was 170 Mev. Magnetic field of 9,000 oersteds.
- 5 -
This picture contains a track which, if proton, has an energy of 190 Mev 1 15%. Energy
of the deuteron beam in the cyclotron was 190 Mev. Gas used in the chamber was oxygen,
and the 4 particle star shown is a common type in oxygen.
This exceptionally good .ligh energy track crosses the chamber completely. If a proton,
it has an energy of 165 Mev t 15%. energy of the deuteron beam in the cyclotron was 190
Mev. Magnetic field of 9,000 oersteds.
UNIVERSITY OF FLORIDA
3 1262 08907 9536
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