February 2018 49
APPROACHING THE FINAL FRONTIER
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and the solar eclipse was no exception.
Neutrons
The second radiation experiment used a thermal
neutron detector. This detector is about six inches tall
and filled with a clear gel. The gel is on the verge of
boiling at room temperature, and all it needs to boil is
a tiny bit of energy.
The chemical composition of the detector is
such that the gel can absorb thermal or low energy
neutrons. Those captured neutrons provide just
the extra bit of energy that the gel needs to boil.
This results in a bubble appearing every place
that a neutron is absorbed. By the way, this is the
same way the old hydrogen bubble chambers
worked (but the gel doesn’t operate at cryogenic
temperatures).
By counting the number of bubbles inside the
gel, one can determine the number of neutrons that
were detected. Then, after a mission, the gel can be
“reset” and cleared of bubbles by applying a small
amount of pressure to the gel. That pressure squeezes the
bubbles closed so the gel can produce new ones at the
absorption of new thermal neutrons.
Now, neutrons are not a component of cosmic rays.
Cosmic rays are (for the most part) protons or the nuclei
of hydrogen atoms. They’re kicked into high energy states
possibly by the powerful magnetic fields associated with
supernova explosions, and perhaps massive black holes in
the center of galaxies.
Any neutrons detected on the eclipse flight were the
result of cosmic rays colliding with molecules (oxygen and
nitrogen) in the atmosphere. Since the Geiger counter
indicated that the maximum cosmic ray flux occurred at
61,000 feet, I suspect the neutron flux was also greatest at
this altitude.
Air Temperature
Different mechanisms warm the bottom
two layers of the atmosphere. The lowest
layer — the troposphere — is warmed by its
contact with the ground. As we measure
the temperature of the troposphere, we find
that it decreases with altitude, or distance
from the warm ground.
The second layer — the stratosphere
— is warmed by the ozone gas it contains.
The energy of the ultraviolet radiation it
blocks from reaching the surface becomes
thermal energy. We find that as we measure
the temperature of the stratosphere, its
temperature increases with altitude or as we
approach the ultraviolet-rich sun.
In both cases, the sun is the ultimate
source to warm the atmosphere. Therefore,
I was hoping to detect a decrease in
the temperature of either layer of the
atmosphere during the eclipse since it
This is a very typical looking chart of the cosmic ray flux as a function of
altitude. The small drop-off between 43,000 and 45,000 feet looks a lot
like normal random variation in cosmic ray flux. It’s no different from what
is seen in non-eclipse flights.
I purchased two neutron detectors for the solar eclipse mission.
The one on the top was launched into near space where it
recorded 40 neutrons during the 135 minute flight. The one at the
bottom remained on the ground and recorded no neutrons during
the same time. In fact, later experiments indicate that it takes over
a day to detect a single neutron on the ground on average. The
detector is manufactured by Bubble Technology Industries.
