Development and Testing of a Portable Gamma Ray Scintillator

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.

Figure 2. Compton Scattering http://missionscience.nasa.gov/ems/12_gammarays.html

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 4: Centroids Vs. Known Energy

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.

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

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.

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.

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.

Figure 5: Block Flow Diagram

The Electronic Components

Figure 8

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.

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 7: Alpha Payload

Figure 8: Geiger Counter Data

Peter Bloser, Rich Levergood, Nicholas Lajoie, Erich Trickel

Works Cited:

Photo Provided By: NASA

Compton Scattering. N.d. NASA, http://missionscience.nasa.gov/ems/12_gammarays.html. Web. .

Acknowledgements: 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

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.

Figure 6: Power Supply Schematic