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Development and Testing of a Portable Gamma Ray Scintillator
Peter Bloser, Rich Levergood, Nicholas Lajoie, Erich Trickel
The Physics and Background of Gamma Ray Scintillator:
This Gamma Ray Scintillator uses a Cesium Iodide crystal to indirectly measure the
amount of energy given off by an amount of gamma rays. Gamma Ray Radiation
enters the crystal and interacts with the atoms, which converts the gamma rays into
many optical photons. The optical photons are then detected by a Silicon Photo
Multiplier which is on the inside of the outer aluminum casing shown in Figure 1.
When the gamma rays interact with the
crystal atoms, 3 different possibilities can
happen: Photo absorption, Compton
Scattering, or Pair Production.
Figure 1:
Gamma Ray
Scintillator
Photo Absorption: When a gamma ray photon comes into contact with an atom, its
energy is completely absorbed and causes its electrons to go from its ground state into
an excited state where it will shift to higher energy levels.
Compton Scattering: Similar to Photo Absorption, a gamma ray hits an atom and
transfer energy into it, however not all of the gamma ray’s energy is transferred.
Instead, only some energy is transferred, exciting its electrons a smaller amount. The
remaining gamma ray, which has a lower energy, continues traveling, hitting more
atoms and either repeats the process of scattering or is completely absorbed. As
shown in the picture below, the gamma ray’s path is not stopped when going through
Compton Scattering, instead energy is lost.
The Electronic Components
The other side of this project was making and programming the electronics to
communicate with the Scintillator and record the data that comes flowing out.
We had to create a power supply with voltage regulators, a microcontroller,
and a level shifter so that the UNH Interface Board could talk to our
microcontroller. Figure 5 (bellow) is a block flow diagram of the whole unit
together.
What Went Wrong During Building:
Sources of error during the development of the Gamma Detector Unit were
mainly due to lack of time and old equipment. The interface board used to
communicate the detector to the microcontroller was aged and had failing parts
such as cracked traces. On top of that, the whole creation process came to a halt
due to technical problems with the power supply and interface board. If a newer
interface board had been created, chances of failing parts would have reduced.
When parts did fail, there was not enough time to fix the interface board in time for
the scheduled flight, so the Scintillator was not flown in the payload, instead an
improvision was required and a Geiger counter was flown instead. Figure 7 shows
the payload with the Geiger counter.
Figure 7: Alpha Payload
Figure 5: Block
Flow Diagram
We started with a power supply of 16 batteries and soldered a LM7810 and a
LM7906 voltage regulator onto a small board with Molex connectors. A four
pin cable ran from this to the UNH Board to supply +10V and -6V. Our original
schematic is shown in Figure 6, after the voltage regulator was made we
added a small power LED.
Figure 8
Figure 6: Power Supply Schematic
Similar to Scintillators, Geiger counters detect high energy particles, however,
instead of directly measuring their energy, Geiger counters count the particles that
pass through the Geiger-Mueller tube. Muons are created by cosmic rays that
collide with the Earth’s atmosphere, they are the particle that is most likely to
come into contact and pass through the Geiger counter’s low pressure tube. High
energy particles are more likely to exist the greater your altitude gets. Figure 8
shows the growth in the number of high energy particles increasing with the
altitude.
Figure 2. Compton Scattering
In both Compton Scattering and Photo Absorption, electrons gain energy and move into
the excited state from the ground state. As they return to the ground state, they emit a
photon (usually visible light). These emitted photons then get detected, and that’s how
the Scintillator detects radiation. Gamma rays can be detected and analyzed by
breaking down their energy throughout a medium, (such as a crystal), and detecting the
lower energy photons that remain.
Pair Production: When a gamma ray enters the crystal with enough energy, it can hit
the nucleus of an atom and form both an electron and a positron. Positrons are the
antimatter electron, meaning they have identical mass, yet the opposite charge. These
particles travel throughout the crystal in any direction until they lose all kinetic energy,
and then they collide with a particle oppositely charged. When the two subatomic
particles come together, they annihilate each other, releasing radiation. Similarly to the
other applications, the lower energy photons get detected and read by the Scintillator.
Calibration:
In order to ensure the Scintillator gave off accurate energy values for different
sources, it was set up to read energy levels of know sources such as Cs137. Figure 1
below shows data collected form the Scintillator from Cs137. The first peak in the graph
was disregarded because it was background scatter from Compton Scattering. The
second peak is the energy measured by the Scintillator from the source. The
Scintillator readings were compared to known data for the sources to ensure the same
data was being received. The height of the graph, (called the centroid), was then
plotted with centroids from other sources to make sure readings stayed consistent and
accurate. Figure 3 shows these centroids being plotted, and with this data it can be
concluded that readings from the Scintillator were reasonably accurate.
Figure 3: Cs137 Energy
Figure 8: Geiger Counter Data
http://missionscience.nasa.gov/ems/12_gammarays.html
Figure 4: Centroids Vs. Known Energy
Once we had a powered Interface Board connected to the Scintillator we had
to design a device to receive the signals. For this we used a Parallax
microcontroller for it’s versatility and light power usage. In addition to the
Scintillator the microcontroller had a pressure altimeter, a GPS, a Geiger
Counter, and a compass. Unfortunately only these last three instruments took
measurements at half-second intervals until the voltage on the lithium
batteries dropped bellow the threshold for our microcontroller. When the
microcontroller shut off the voltage on the batteries would recover and when it
did so our flight computer would restart and run for a few seconds before
being cut off by the batteries again. This happened a total of three times, each
with a shorter run time. Also, under Federal law it is illegal to receive GPS
data under certain situations.
When preparing for the flight the UNH Interface board had some technical
difficulties. The delay from these problems was too great for us to get our final
calibrations done before flight day. Even if we would have flown the Scintillator
we would only have gathered data up to a maximum of 50,000ft. This is where
our micro controller’s battery pack began drooping on voltage and the
microcontroller powered down.
However, with our onboard Geiger counter we did collect data that were
exactly what we were hoping for. We got an increasing rate of “hits” from the
counter as our altitude increased. If we would have taken data through our
whole flight, we would have seen an event called the Pfotzer Maximum where
the amount of radiation reaches a peak and then begins to drop as the altitude
increases.
Improvements to be Made:
Next flight, the interface board will be completely redone, giving us new
equipment and bringing down chances of error. It will have a built in voltage
regulator and take the job that the power supply had before. Before, the interface
board only accepted a certain voltage to function, and this problem lead to
technical difficulties. The board also will not need a level shifter to convert signals
for the microcontroller to interpret from the interface board, the new board will be
able to talk to the microcontroller directly, causing us to use less parts.
Acknowledgements:
Works Cited:
Compton Scattering. N.d. NASA,
http://missionscience.nasa.gov/ems/12_gammarays.html. Web. .
Peter Bloser and Richard Levergood for their guidance and
equipment
MMS for funding Project SMART
Friends and Family for making this experience possible and
enjoyable
Photo Provided By: NASA
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