Observations by Human Subjects on Radiation-Induced Light Flashes in Fast- Neutron, X-Ray, and Positive-Pion Beams. Page: 7 of 23
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light sensations. 5 According to a recent re-
port, proton recoils from 3-MeV neutrons
impinging on the human head during activation
analysis can cause light flashes in the dark-
adapted eye. 6 A suggestion was made by G.
Fazio et al. that the phenomenon observed in
space may be due to light from the Cerenkov
effect that accompanies fast particles.
In order to learn more about light sensa-
tion induced by fast atomic particles, we have
made an initial exploration of visual phos-
phene phenomena due to a beam of fast neu-
trons at the Berkeley 184-inch cyclotron. d
Subsequently, the tests were expanded to n+
mesons from the Bevatron.
Very fast neutrons lose energy by elastic
and nonelastic collisions with nuclei. These
result in heavy ionizing nuclear recoils and
in high speed nuclear spallation fragments.
Although the range of these fragments is
much less than that for primary cosmic ray
particles, they might b-, able to cause quali-
tatively similar biological effects.
Fast Neutron Exposure
A 0.64-GeV proton beam impinged on a
12-cm-thick beryllium target. Fast neutrons
were collimated and channeled by a set of
iron and lead apertures of total thickness of
about 2 meters, shown in Fig. 1. Charged
particles are either deflected away or ab-
sorbed by this arrangement, and the re-
sulting beam consisted mainly of high energy
neutrons in the domain of 20 to 640 MeV. The
majority of the neutrons had energies near
300 MeV. These neutrons form a narrow,
slowly diverging beam that emerges into a
shicided room usually used for meson
studies. Polaroid photographic paper was
used with calcium tungstate intensifier to lo-
calize the beam and to monitor the overall
exposures. The beam size was 6.5 by 5.2
Measurement of the neutron flux density
for neutrons greater than 20 MeV in the beam
was carried out by use of a plastic scintillator
(4 in. diam, 1 in. thick), following prealign-
ment of the beam and determination of its
size. This previously calibrated instrument
utilizes the production of radioactive 11C by
the (n, 2n) reaction from carbon by neutrons
faster than 20 MeV for which the cross sec-
tion is known as function of energy. 9 The
scintillations are counted for C decay im-
mediately after neutron hesm rrprnauro. It
yielded a maximum intensity of '1.04X 106
neutrons cm sec when the primary pro-
ton beam intensity wao maximieCd. The plas-
tic scintillator was used to calibrate the beam
monitor, which is a large scintillating crystal
counter placed in the neutron beam down-
stream from the experiments. This monitor
was used to lower the neutron-beam flux den-
sity by a factor of approximately 100; a level
4 -2 -
near 10 neutrons cm sec was used for
the exposure of the subjects.
The ratio of slow 6v fast neutrons was
measured by activated indium foils.
In addition, a tissue -equivalent liquid,
simulating the human body in composition and
shape, was also exposed to the neutron beam
at high level (1.04X10 neutrons cm sec-).
By converting the C counts obtained to dose
in rem units, we obtained agreement within a
factor of 2 with the plastic scintillator data.
The conversion factor of 5.5 X 10 rem/neu-
tron cm was assumed. A separate deter-
mination of dose was made in a wood phantom
having approximately the geometry of the
head. Landsverk pocket electrometers
(L-50) were used (full-scale deflection, 200
The subjects were dark-adapted by
wearing a combination of green sunglasses
and red x-ray dark-adapting glasses for
more than 2 hours prior to exposure. During
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Tobias, C. A.; Budinger, T. F. & Lyman, J. T. Observations by Human Subjects on Radiation-Induced Light Flashes in Fast- Neutron, X-Ray, and Positive-Pion Beams., report, January 1, 1970; [Berkeley, California]. (https://digital.library.unt.edu/ark:/67531/metadc872480/m1/7/: accessed May 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.