Dependence of cosmic ray detection on Geiger counter shielding

Research Group: Sarangi Handun Pathirana, Connor Lynch, and Levi Russell

Launch: Whitworth Spring 2018



''The goal of our experiment was to find the most effective shielding material against cosmic radiation in the Earth's atmosphere with the gain in altitude. We encased two Geiger counters; one with Delrin and the other with Lead. The weather balloon reached a height of 105,000 ft before the weather balloon burst. No complications with recording of data had occurred during ascend, bursting of the balloon or descent of the pods and useful data that could be analysed were recorded. Based on the data that was collected, Delrin was more effective at lowering the number of counts compared to Lead.

Background
Victor Hess, an Austrian-American Physicist, first discovered cosmic rays when he conducted a historic balloon flight, during which he measured ionization in the atmosphere. As a result of his experiment, Hess found that ionization in the atmosphere was roughly three times the ionization at sea level, a phenomenon he attributed to radiation entering the atmosphere from above--radiation now called cosmic rays.

Cosmic rays are high-energy particles that arrive from outer space, composed primarily of protons and the nuclei of hydrogen (89%), helium (10%), and heavier nuclei (1%). Cosmic rays are believed to be produced by different activities in space which includes flares and coronal mass ejections on the Sun, black holes and supernovae in the Galaxy. The atmosphere and magnetic field of the Earth shields about 99% of the cosmic rays from the surface of the Earth, thus, we are not significantly harmed by the cosmic radiation activity.

However, cosmic rays are hazardous to astronauts traveling out in space, beyond Earth's atmosphere and magnetic field. Cosmic rays can damage structures and break apart molecules in living cells. It may increase the risk of getting cancer, health issues with the central nervous system and degenerative diseases. Due to the Space Age that we live in, there is a greater need to find methods to protect astronauts and spacecrafts from galactic cosmic radiation.

The objective of our project is to to determine the relative effectiveness of materials; lead and Delrin (plastic) with regards to their ability to protect crafts from cosmic rays within near-Earth orbit. We expect that the more dense the material, the more effective it will be in blocking higher energy radiation. Based on our hypothesis, our results should show that at the same thickness, lead will block out more radiation than Delrin (plastic). The proposed project method to find the dependence of cosmic ray detection on Geiger counting shielding is to send two Geiger counters to near space, each encased in a different form of radiation shielding. The first of these will be surrounded by a layer of lead. The second of our Geiger counters will be encased in Delrin (plastic). Both casings were not of the same thickness due to weight restrictions. Although the lead was thinner compared to Delrin, it weighed about three times as much as the Delrin. Also due to our limitations in weight, we will not be able to send a control (a Geiger counter that is not encased) within our capsule. We will be using the data results from another group that will be sending a Geiger counter that is not encased. During flight our Geiger counters will transmit data to our microcontroller, which will then record the radiation experienced by each of our counters, showing, in theory, the effectiveness of each material as radiation shielding.

Mechanical Design
To most accurately measure the effects that different types of radiation shielding have on cosmic rays, we utilized two Geiger counters, lead plating, polyoxymethylene (Delrin) plating, and a Mbed microprocessor, among other necessary tools such as a microchip to store data and a battery pack to power our Mbed. The first Geiger counter was shielded with lead, while the second was shielded with Delrin, both materials were secured with duct tape because using epoxy or glue was unnecessary. To mitigate the risks of data loss during flight, both Geiger counters were secured to our pod with zip ties and connected to the Mbed with a telephone wire. The Mbed was secured and connected to the battery pack and Geiger counters secured to the pod.

To protect our data we needed to avoid numerous problems during flight, lest they lead to variance and uncertainty within our results. First, we needed to avoid the Geiger counters, especially those with shielding, reflecting cosmic rays into the other Geiger counters, as this could trigger false counts and invalidate collected data. To avoid this eventuality, we organized our Geiger counters within the pod in a way such that their angle of coincidence was as low as possible, thus minimizing the likelihood of false counts. Second, we needed to mitigate the risks that invalidated the data collected by previous groups that conducted similar experiments. Foremost among there were mistakes in wiring, which caused (1) a split in the ground wire connecting the Mbed to the battery pack and (2) a movement of the Geiger counters' ground wire from the ground bus to the hot bus. To prevent these two issues we slightly bent the wires going into the breadboard and zip tied cardboard over the breadboard, thus minimizing the chance of wires coming loose and causing a loss of data. Further, we secured the rest of our materials in a way that minimizes the risk of failure during the violent events of the flight.

Electrical Design



 * 6 V power pack 4 AA Energizer Ultimate Lithium Batteries
 * ~800 mAh capacity
 * Approximately 143 mA current draw


 * 5.59 hr theoretical run time

Our electrical design utilized a breadboard, Mbed, wires, and a battery pack to power to all necessary systems within our pod, including two Geiger counters and our Mbed. To connect each element of our electrical system, we used jumper cables to minimize the risk of a disconnect throughout the course of the flight. To connect our Geiger counters to the Mbed, we used telephone cables, two new sets soldered by us as we found past cables to be unreliable. As we were creating the cable we found that the color for the power and data lines are different for each end of the telephone wire. This means for one set of wires the data could be the white wire, while the other sets of wires could have data as green. To power the Mbed and our system we used a battery pack containing, 4 AA lithium batteries, connected to the breadboard with the VIN and VO pins. All in all, this was a system that ran flawlessly when tested prior to launch with minimal chance of error during flight.

Code
Current working code for two simultaneous Geiger counters. This code will save the data to the SD card every 5 minutes in case of a power loss during flight.



Validation and Calibration
As the only technological variables in our experiment, the two Geiger counters used were our main priorities in validation and calibration in order to ensure any data collected during flight is not correct and sufficient to base conclusions upon. To do this we first tested the Geiger counters against the known measurement for background radiation—about twenty counts per minute. When we did so, both Geiger counters were consistent with this value, with only minor variations on either side of it among tests. To test the known relationship between distance and the intensity of radiation (R=c/x^2), we placed our radiation source, a small piece of the radioactive material Strontium-90, inside a cardboard holder and used lead blocks to direct its emitted radiation towards our Geiger counter, which was placed at intermittent distances from the source of 0 cm, 10 cm, 20 cm, 30 cm, and 40 cm. At each of these distances we recorded the number of counts recorded by each Geiger counter, both unshielded and shielded, every 15 seconds, which can be seen in Table 1. As can be seen in Figure 7, the unshielded Geiger counters, when put through this experiment, were consistent with our expected values; as they were moved further from the Strontium-90, the radiation experienced by them decreased by a factor of 1/x^2. On the other hand, when one looks at the data for our two shielded counters, as seen in Figure 8, there appears to be a linear relationship between distance and radiation intensity. We believe this is due to the shielding itself and indicates that it is doing its job. At each distance, the radiation that gets through the shielding is so negligible that the variance across distances appears to be insignificant, especially when compared to the data collected from the unshielded Geiger counters. Because all this data coincided with our hypotheses and previously-tested values/relationships, our Geiger counters were validated and lacked a need for calibration.

         

Data and Analysis
The balloon reached an altitude of 105,000 feet during its flight. We were able to collect data all the way up to when the balloon popped and about 20,000 feet while it was falling. The Geiger counters were set up to record all counts it detected every 60 seconds. In Figure 9, you can see the counts for each Geiger counter versus the altitude at the time it detected those counts. The maximum amount of counts happens at the 60-70,000 feet range. The reason the counts start to decline as the altitude reaches 100,000 feet is due to cosmic ray showers. As cosmic rays start hitting the molecules in the atmosphere it creates a chain reaction causing a shower of radiation. In Figure 10, you can see at around 100,000 feet the cosmic ray showers begin and, according to our data, the maximum effect of the showers is around 65,000 feet. In Figure 11 the line represents the ratio of the counts from each Geiger counter. It is a straight line which means that the data from each of the Geiger counters are consistent with each other, meaning none of the Geiger counters were malfunctioning. Although fewer counts were recorded above 65,000 feet it doesn't technically mean it is safer at those altitude levels. The high energy cosmic rays just didn't have enough time to hit molecules in the atmosphere to break up and create a shower. The energy of radiation only matters to an extent, after you reach a high enough energy to break apart DNA bonds then the radiation is dangerous. The goal of radiation shielding material is to protect what you are trying to shield, usually people, so the counts don't tell us everything there is to know about that materials effectiveness at shielding the radiation.

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Conclusion
In our hypothesis, we predicted lead shielding to be a more effective radiation shield than Delrin, but our data indicates that it was less effective, which can be explained by a number of factors. First, the lead shielding we used was half as thick as the Delrin shielding(1.75mm vs 3.40 mm), which alters our data, as thickness is an important factor in radiation shielding. Similarly, the cosmic rays we encountered could have hit the lead and broken into smaller molecules, effectively showering the lead-shielded Geiger counter with radiation and causing it to receive multiple counts from a single particle. Delrin, as a less dense material, likely had similar particles pierce through it, preventing the particle from breaking up and limiting it to a single count. Overall, despite our data appearing to evidence that Delrin is both a cost and weight-effective method of radiation shielding given its lower counts than the lead-shielded Geiger counter, more testing needs to be done to prove this. As mentioned above, there are a number of feasible explanations as to why the Delrin shielding appeared more effective and, understanding those, we can not make such a claim as fact with our data. Further, more testing needs to be done to determine whether or not the cosmic rays that penetrate both lead and Delrin are dangerous, as even a single cosmic ray penetrating shielding, with high enough energy, would be harmful to any life it comes into contact with. As such, testing to determine the energies of the particles that radiation shielding fails to block out is an important step for future developments in the field.

References/Works Cited
Bahadori, Amir, et al. “Measuring Space Radiation Shielding Effectiveness.” ODU Digital Commons, digitalcommons.odu.edu/mathstat_fac_pubs/37/

DeWitt, Joel. OSU EVB RPL | Studies in Cosmic Ray Muons, physics.okstate.edu/rpl/muons.htm

Garner, Rob. “NASA Studies Cosmic Radiation to Protect High-Altitude Travelers.” NASA, NASA, 25 Jan. 2017, www.nasa.gov/feature/goddard/2017/nasa-studies-cosmic-radiation-to-protect-high-altitude-travelers

Hambling, David. “Can Miracle Material Stop Radiation?” Wired, Conde Nast, 4 June 2017, www.wired.com/2008/06/radiation-proof/


 * posted on October 27th, 2016 :: “Different Types of Radiation Shielding Materials.” LANCS INDUSTRIES, www.lancsindustries.com/2016/different-types-radiation-shielding-materials/

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