Dependence of cosmic ray detection on orientation relative to the Sun

Research Group: Madi Binyon, Craig Russell, and Maria Straight

Launch: Whitworth Spring 2018

''This project conducted research regarding the possible connection between light levels from a phototransistor and amount of coincidence hits recorded by two stacked Geiger counters. The goal of this experiment was to find a correlation between high light levels and an increased amount of Geiger counter readings where both counters are struck with a cosmic ray at the same time. Data was collected throughout the ascent and descent of the flight; the weather balloon reaching a height of 105,000 ft before popping. It was noted during data analysis that phototransistor levels were fairly stagnant during the ascent, making it difficult to examine a correlation between coincidence hits and amount of light recorded. Later, however, descent data was found to show a slight correlation between increased light levels and coincidence hits. In the future, this group wants to focus more heavily on examining the descent data to determine if there actually is a correlation between higher light levels and more coincidence hits gathered.''

Cosmic Rays
Traveling nearly at the speed of light, cosmic rays are produced when high energy particles collide with molecules in the Earth's atmosphere. A web-like pattern is observed as collisions scatter the cosmic rays. These rays are either solar, originating from the sun; galactic, originating from somewhere in space; or anomalous, accelerating towards the inner heliosphere. Each of these rays has its own "finger print" source, and they are categorized as primary or secondary cosmic rays.  The true origin of galactic cosmic rays is unknown--the several magnetic fields they pass through scatter them before they reach Earth--but evidence suggests that they are a product of supernova. 

According to NASA, cosmic rays come from all directions of the sky. Other institutions claim that determining where cosmic rays come from will remain an "unsolved mystery" for some time; however, scientists are currently trying to trace the origin of cosmic rays by examining what they are made of.  NASA has observed that all the natural elements in the periodic table are present in cosmic rays, leaving a wide array of elements to trace back to the origins of the rays. This experiment is expected to have a large quantity of data to analyze because the sun and the stars send a constant stream of cosmic radiation to the Earth, much like a steady stream of rain. 

Theory
This team hypothesizes that in flight, more coincidence hit data will be observed while the top of the box is angled towards the sun rather than away from it. From the direction of the sun, all types of cosmic rays will be gathered: galactic, anomalous, and solar. With this increased amount of cosmic radiation, it is hypothesized that a direct relationship will be established between this and the amount of coincidence hits recorded. By utilizing the phototransistor placed through the top of the box, it will be possible to distinguish with confidence the general vicinity in which the cosmic coincidence hits come from.

Geiger Counter Configuration
As shown in Figure 1, zip ties were chosen as the primary means of securing the Geiger counters. This was found to be the most simple and effective way to keep the instruments from jostling around while in flight. On the right, a view from above into the box is shown. As one can see from this image, the Geiger counters are secured through the styrofoam of the box, adding another layer of safety. In total, eight zip ties were used to keep the counters in place. By stacking the counters one right on top of the other to widen the potential angle for coincidence hits. The Geiger counter tubes have diameters of 15.19 cm and 15.17 cm and lengths of 4.150 cm and 4.109 cm, respectively. These factors caused the widest horizontal distance between the counters to be 4.597 cm. This creates an angle of coincidence of 26.2 degrees from the vertical. By doing this, it is suspected the counters will receive more coincidence hits and more data will be gathered to examine in the end. The counters were placed in the top right corner of the box to allow for the most room possible for the electrical setup. There was a pre-placed hole in the bottom right side of the box, which was planned to be used for electrical purposes, leaving the top right corner available to place the Geiger counters in the pod.

Phototransistor Tube
To detect sunlight, a simple tube was designed, as shown in Figure 2, to control the filtering of the sun’s rays and house the phototransistor. This phototransistor tube has a radius of 11.57 mm and a height of 84.07 mm. As seen in the CAD drawing, there is a small base on the bottom of the tube to prevent the tube from slipping out of the top of the box. There is also a hole through the middle of the base which the phototransistor will fit through snugly, assisting with the goal of keeping the phototransistor as vertically straight as possible. This added function will guarantee when examining the data that the phototransistor’s orientation will not have changed drastically throughout the flight. With the second figure, an onlooker can see how the tube’s design sticks out slightly past the top of the box. This feature was added in order to narrow the potential angle for sunlight to enter the tube. By doing this, confidence was added in the data and a hard and fast distinction was created in regards to the directionality of light rays received.



Phototransistor Tube Washer
The washer was created to maintain a vertical angle throughout flight of the phototransistor. The team was worried that when coincidence counts were recorded, the light levels being recorded would be inconsistent if the phototransistor was jostling around too much. To fix this, a washer was 3D printed that would just barely fit over the phototransistor and slide into the tube comfortably, keeping the device as straight as possible throughout the flight. When the pod was recovered, the phototransistor was found exactly where it was placed, giving the impression that the washer and phototransistor tube did their jobs.



University Flight Computer
Taking a place in the bottom of the box is a specially made Whitworth University Launch Computer, as seen in Figure 3. This device was chosen because it was the most convenient solution at hand to hold the breadboards and organize the wires. By placing the flight computer on the longer side of the box, a secure hold was created to reduce the chance of any wires or important parts being misplaced during flight. By placing this next to the pre-cut hole in the side of the box, an easy access port was created to insert and remove the SD card. During flight, the hole was plugged to reduce the amount of outside access to the inner contents of the project.

Launch Cameras
Once the pod was all ready to launch, it was noted by Dr. Larkin how much room was left inside. Because of this, launch cameras were added to the pod to document the flight. One camera was placed on the lid, as shown in Figure 5. A hole as cut through the lid to film an upwards view of the launch. The pod was located second to the bottom, so a solid view of the balloon was collected. Two other cameras were placed through the bottom and through one side of the pod, as shown in Figure 4. By using three cameras, a large amount of footage was obtained from several different angles.

Electrical Design
The electrical design, shown in Figure 6, for the project included the LPC controller, an accelerometer, a phototransistor, a microSD card mount, and two Geiger counters.

Phototransistor
A phototransistor is a device that is able to sense light levels and transmit a numerical reading back to the group. The phototransistor acts as a resistor; when light hits the device, it lowers the resistance, sending a larger voltage back to the MBED, giving the group a larger reading. The phototransistor was also connected to an OP AMP, model TLC 272, which amplifies the voltage in order to receive more detailed readings.

The op amp was setup as a transimpedance amplifier, with the collector leg of the phototransistor was connected to the inverting input (-IN A) on connection 2, and the emitter leg was connected to ground.

Accelerometer
The accelerometer used was a model ADXL 345 one a Sparkfun breakout board. The ground and VCC pins connected to the ground and V-out on the mbed. SDA, SDO, and SCL were connected to the SPI pins 11 (mosi), 12 (miso), and 13 (sck) on the LPC. This could have been shared with the SD card mount, as SPI can support multiple devices, however, for simplicity the two devices were separated. The CS connection was wired to pin 21, a PwmOut pin.

SD Card Mount
The SD mount was connected to ground and V-out with it's ground and VCC pins. DI, DO, and SCk were connected to the SPI pins 5 (mosi), 6 (miso), and 7 (sck) on the LPC. CS on the SD mount was connected to pin 8, a digital out.

Geiger Counters
The Geiger counters were connected to the LPC via a cut phone cord with four connections and a RJ11 connector. A yellow, green, red and black cable can be found inside the connector. The outside cables (black and yellow) were soldered together and connected to ground. The middle two wires, red and green, are either signal or power. To determine this, orient the connection so that the clip is facing down and the cord is pointed away. The second wire from the left will be the signal wire. For the two Geiger counters used, the signal wires were attached to pin 29 or pin 30.

Battery Pack
A battery pack was attached to the back of the University Flight Computer to provide power to the pod. The battery used was a Tenergy Lipo 7.4V Battery Pack with PCB. The night before the launch, the battery pack was charged to full power to prepare for the upcoming flight.

Code
The code was written in C++ in the mbed browser interface. The code writes data to a tab-separated values (TSV) formatted .txt file. This allowed the data to be analyzed using Excel or Mathematica.

Geiger Counter
Using a weak radioactive source, strontium-90, experiments with the Geiger counters showed more counts per minute that background radiation. In the presence of the source, the counts per minute ranged from 100-120. Observed background radiation in the Eric Johnston Science Building ranged from about 15-35 counts per minute.

Phototransistor
Light levels for the phototransistors were tested inside the lab almost every week, but it was obvious these values would not be equivalent to what would be detected outside. To see how sensitive the phototransistor was to real sunlight, the pod was taken outside one sunny Tuesday. By using PuTTY, an open-source terminal emulator, phototransistor light levels were recorded, ranging in values from 0.246 to 0.803, with 0.803 being recorded when the box was directly facing the sun and 0.246 being recording when the phototube was covered by a hand.

Code
The flight code was tested using a function generator and an oscilloscope to test the upward limit of how many counts per minute the code could process. The oscilloscope indicated that a Geiger counter would show a voltage drop lasting about 50 microseconds when detecting a hit. The function generator was set to simulate this by outputting pulses with voltage drops lasting 50 microseconds. The function generator was connected to both of the wires for the two Geiger counters so that all the inputs would be coincidence hits. The code was tested for 600 counts per minute, 60000 counts per minute, 60000 counts per minute, and 600000 counts per minute.

600 counts per minute

6000 counts per minute

60000 counts per minute

600000 counts per minute

Since the expected maximum for the flight was only about 1000 counts per minute for a single Geiger counter, it was concluded that the code would have no problems processing the input from the Geiger counters. The observed counts per minute were higher than expected for the 60,000 and 600,000 tests. This may have been due to the code taking time to run the calculations and recording counts for longer than one minute.

Data and Analysis
Data was collected every time a Geiger counter detected a hit, and every minute the data for single hits and the data for coincidence hits were each averaged and their values written to the file on the SD card. The code collected and wrote the following data: Time(s), SingleX, SingleY, SingleZ, SingleP, CoincX, CoincY, CoincZ, CoincP, Counts1, Counts2, CoincHits. "Counts1" and "Counts2" refer to the counts per minute detected from Geiger counter 1 and Geiger counter 2, respectively. "CoincHits" is the number of hits per minute when both counters detected a hit within 50 microseconds of one another, indicative of a relatively vertical cosmic ray. "SingleX" refers to the x-value from the accelerometer averaged in the last minute from data collected every time only one Geiger counter detected a hit. Similarly, "CoincZ" refers to the z-value from the accelerometer averaged from data collected every time in the last minute that the two Geiger counters simultaneously (within 50 microseconds) detected a hit. "SingleP" and "CoincP" are the averaged values from the phototransistor.

The raw data can be found here:

Geiger Counter Data


As hypothesized, the Geiger counters detected more counts per minute as the altitude of the balloon increased, as shown in Figure 7. Similarly, Figure 8 shows that the number of coincidence hits per minute also increased with altitude. This is due to the decrease in the atmospheric radiation shielding. Contrary to the original hypothesis, the ratio of coincidence hits to counts decreased with altitude. It was hypothesized that as altitude increased, the reduced atmosphere would cause a decrease in the scattering of cosmic rays. It was expected that a higher percent of coincidence hits would be observed at the higher altitudes, indicating that more cosmic rays were coming from the vertical direction, especially from the direction of the sun. However, as altitude increased, the percent of coincidence hits decreased, as shown in Figure 9.

Phototransistor and Accelerometer Data
Analysis of the data from the phototransistor and the accelerometer was inconclusive. As seen in Figure 11, there was no discernible difference during the ascent between the light levels measured for single hits and the light levels measured for coincidence hits. This was because there was virtually no variation in the light levels detected during the ascent as the pod was oriented vertically the entire time. After the balloon popped, the phototransistor detected more variation in light levels since the orientation of the pod varied as it descended. During the descent, the light levels measured when coincidence hits were detected were generally higher than those measured for single hits. However, this was a very small difference. Using an op-amp for the phototransistor to amplify this difference might have led to a clearer distinction between the light levels measured for single hits and coincidence hits.

As with the phototransistor data, there was no significant difference between the accelerometer data collected for single hits and the accelerometer data collected for coincidence hits. The differences between the values for single hits and for coincidence hits ranged from 0 to 0.289, 0.319, and 0.819 for the x, y, and z values, respectively. However, the maximum differences were observed when single digits of coincidence hits were recorded, and the average differences between the values were 0.00124, 0.00394, and 0.00584, for x, y, and z values, respectively. This difference is very insignificant, so this data does not provide any evidence for a conclusion about the directionality of cosmic radiation.

What Was Found
As noted above in the data and analysis section, little variation in the phototransistor levels was found during the ascent. This fact limited the ability for this group to attempt to examine a correlation between light levels and amount of coincidence hits recorded; however, further examination of what was thought to be useless descent data revealed much different phototransistor values than was recorded during the ascent. After inspection, there was a correlation noticed between slightly higher light levels and amount of coincidence hits recorded. This fact was discovered extremely late in the process of data analysis, limiting the amount of time available to examine further the extent of this connection. In the next section, potential fixes to the project are identified and possible solutions to problems found are constructed.

Future Investigation
If this experiment was to be performed again, more time would be spent analyzing the descent data. There is not much a group can do about the fact that the pod does not move much during the ascent; this fact, though, could be used as a control to then look at the differing light levels during the descent. While the phototransistor data for the descent was found to be different, the scale between high and low values was small. In a future experiment, a more powerful OP AMP would be used to magnify this distinction and make it easier to examine high versus low light levels and how they correlate to amount of coincidence hits received during flight.

This group, in the future, would like to do more with calibrating and validating Geiger counters. One way to improve the experiment would be to narrow the angle of coincidence; this would prompt a group to receive more meaningful, accurate data concerning where cosmic rays are actually coming from. After the launch, it was considered that the Geiger counter on top could have been getting more low level cosmic radiation counts, and these counts could possibly not be strong enough to reach the bottom counter as well. Further testing is needed to examine this principle, as well as confirming Geiger counter sensitivity, which was not taken into consideration before the flight. While the two Geiger counters received a similar amount of counts throughout the flight, a combination of a more sensitive bottom layer Geiger counter and lots of low level cosmic radiation only powerful enough to strike the top counter could result in skewed data. By using a radioactive source in the lab, Geiger sensitivity can be addressed and recorded. The group is unsure how to go about measuring how strong a cosmic ray is while in flight; this could be a potential topic for a future endeavor.

References/Works Cited
Christian, Dr. Eric R. “Anomalous Cosmic Rays.” NASA, NASA, 7 Apr. 2011, helios.gsfc.nasa.gov/acr.html. 16 May 2018.

Howell, Elizabeth. “What Are Cosmic Rays?” Space.com, Space.com, 11 May 2018, www.space.com/32644-cosmic-rays.html. 16 May 2018.

Swineburne University. “Cosmic Rays | COSMOS.” Swineburne Astronomy Online, astronomy.swin.edu.au/cosmos/C/Cosmic+Rays. 16 May 2018.

U.S.NRC. “Cosmic Radiation.” U.S.NRC, US Nuclear Regulatory Commission, 23 Apr. 2018, www.nrc.gov/reading-rm/basic-ref/glossary/cosmic-radiation.html. 16 May 2018.