Cosmic ray detection with partially-shielded Geiger counters

Research Group: Jon Barta, Keenan Brownsberger, and Cass Busch

Launch: Whitworth Fall 2014

The experiment conducted was designed to measure the change in cosmic ray intensity within Earth's atmosphere at various altitudes through detection of radiation counts from two partially shielded Geiger counters. By making several changes to similar experiment designs done in 2008, 2010, and 2012, this experiment endeavored to reduce unwanted noise radiation and better ensure all counts were cosmic rays. Specifically, our changes include using two Geiger counters to measure consecutive counts, and encasing all but the top side of the stacked Geiger counters in lead to provide shielding from non-cosmic sources of radiation. Our collected data and results agree with both our hypothesis and the accepted scientific theory since the cosmic ray intensity was shown to increase with altitude until it peaks at approximately 16 kilometers and then decreases again as it goes to higher altitude.



Background
The existence of cosmic rays was discovered by Victor Hess in 1912. His studies from 1911 to 1913 were published in the Proceedings of the Viennese Academy of Sciences, which earned him a Nobel Prize in 1936. With the discovery of this new phenomena came intrigue and continual study by scientists from that date onward. A soviet physicist, Sergey Vernov, was the first person to study cosmic ray intensity at high altitudes by using an air balloon. The experiment we conducted and explain here is similar to that of Vernov's.

Cosmic rays are high energy radiation that originate from outer space. Scientists originally hypothesized that cosmic rays came from supernova. However, research has shown inconsistencies that cause the true origins of cosmic radiation to still be unknown. About 90 percent of cosmic rays that strike Earth's atmosphere are protons, about 9 percent are alpha particles, and the remaining rays are mostly electrons with a small amount yet diverse amount of other particles. Since most cosmic rays are charged particles, the Earth's magnetic field acts as a shield that deflects most rays from hitting Earth, but some still make it through the atmosphere. These high energy rays that originate from space are classified as primary cosmic rays. Primary rays collide with particles in the Earth's atmosphere and create a shower of smaller and less energetic particles called secondary cosmic rays. We expect most of our radiation counts to be from secondary rays. From previous research, the intensity of cosmic rays increases with altitude until about 17 kilometers at which point the intensity peaks and then steeply declines at higher altitudes. We sent our air balloon up to approximately 30 kilometers. The first twelve kilometers make up the troposphere which is the most dense layer of the Earth's atmosphere, containing almost all atmospheric water vapor and about 80 percent of its mass. From 12 kilometers to 50 kilometers is the stratosphere, which has air on average about 1000 times thinner than the troposphere. We expected less secondary rays at higher altitude due to the thin air which decreases the chances for primary rays to have a collision.

The other theory utilized in our project is radiation shielding. Different types of radiation react different with different materials, but generally speaking, heavy and denser materials are better at absorbing radioactive particles. Thus lead is often used to shield from radiation since it has a heavy nuclei and a dense solid state. We used a thin sheet of lead in hopes of shielding lower energy radiation particles in the atmosphere to reduce noise in the data.

Hypothesis: Our experimental cosmic ray detection setup with sheilding will reduce the number of low energy radiation particles striking our Geiger counters that primarily originate from Earth. Thus, the data should improve upon the experiments of previous years and closely demonstrate the characteristics of accepted cosmic radiation data: increasing intensity until around 16-17 kilometers where cosmic radiation intensity peaks, and then it decreases with additional altitude.

Mechanical
The defining feature that improves upon the pod design and concept of the past cosmic ray detection pods is the addition of lead shielding. The purpose of the lead in this experiment is to absorb low energy rays that approach the Geiger counters. We had our lead covering the Geiger counters across all sides and the bottom. The purpose of covering these dimensions is to eliminate the possibility of counting any incoming low energy rays originating from Earth. The top was left open to the sky to let any incoming cosmic rays from across the universe easily strike the Geiger counters. Between the two Geiger counters we placed a ~1" solid chunk of styrofoam as a separator to reduce the angle range at which incoming rays could have possibly struck both Geiger counters. This method was another way of reducing the number of noise counts that could have come in at low angles relative to the horizontal plane of the pod. The styrofoam also acts as the placeholder of a potential third Geiger. We deem a third Geiger counter as unnecessary because if a cosmic ray is on path to go through the first and third Geiger counters, the middle Geiger counter will inevitably be struck as well. Hence we see the third Geiger counter as mere extra weight added to the pod.

Experiment Materials

 * Lead sheeting (1/64" thick)...................... 28.00 Dollars
 * 7402N Quad 2-Input NOR Gate................. 5.91 Dollars (Datasheet)
 * Aware RM-60 Geiger counters (2).............. --.-- Dollars
 * Arduino UNO microcontroller board........... --.-- Dollars (Datasheet)
 * Lithium ion 9V Batteries (2)........................ --.-- Dollars
 * StratoStar sensor interface module........... --.-- Dollars
 * Assorted Wires.......................................... --.-- Dollars
 * Breadboard............................................... --.-- Dollars
 * Pod........................................................... --.-- Dollars
 * Styrofoam................................................. --.-- Dollars

Pod Materials

 * Styrofoam
 * Mylar
 * Carbon Fiber Kite Poles (x2)
 * Epoxy
 * Zip-Ties

Dimensions of the pod (with lid) are 16.5 cm x 16.5 cm x 20.5 cm



Configuration
Each Geiger counter cord had four small wires inside of it. The four wires are Vsup, Vout, GND, and NC. The GND and NC wires must be touching in order for the Geiger counter to properly send data. We had the two input signals, Vout, from the Geiger counters go to one of the four possible NOR gates in the 7402N chip. The output of the used NOR gate went to digital pin 2 on the Arduino. The Vsup of the two Geiger counters and the Vcc of the 7402N NOR gate were connected to the 5V power supply on the Arduino. The two 9V batteries were put in parallel and connected to the power input jack on the Arduino. All components that needed to be grounded in the circuit were connected to the ground on the Arduino. We had an 8 byte signal be transmitted through our transmitter. The eight wires were connected individually into digital pins 3-10. The schematic circuit diagram can be seen in Fig. 2.



NOR
The NOR gate chip is a digital logic gate. The NOR gate is designed to have two inputs feed into the gate. When the two inputs are in an uninterrupted state they continually read "high" so that the gate is continually "closed". When one input reads "low" and the other reads "high" the gate still remains "closed". To "open" the gate both signals must read "low" simultaneously. In application to the project, when there is no radiation being detected by the two Geiger counters the two input signals will read "high" which means the NOR gate will be "closed". When low energy radiation hits one Geiger counter we expect the mass of the Geiger counter to slow or stop the radiation so that the second Geiger counter receives a delayed signal or no signal at all. Since one gate will be read "low" and the other will read "high" the gate will remain "closed". When high energy cosmic radiation hits one Geiger counter we expect it to easily travel through the Geiger counter's body and hit the other Geiger counter almost simultaneously since cosmic rays generally travel at relativistic speeds. The simultaneous inputs delivered by both Geiger counters will be "low" which will "open" the gate.

Software
A code was written to record the number of times the NOR gate was "opened" every thirty seconds, and pass along that number through the digital outputs to the transmitter.

Code Flow
 Define all the connections If a simultaneous count is detected, increase the count reading by 1. Run a loop:  After 30 seconds, read the count reading number as bits. To each digital output, output a high if the bit=0, low if bit=1. Reset count reading to zero. Repeat.  

Circuit and Code
We used the lab-safe radioactive sources owned by the Whitworth Physics Department. We first ran our circuit and analyzed the data flow when no radioactive sources were exposed to the Geiger counters. Since the NOR gate is only opened when both Geiger counters read simultaneous counts and without a source radiation readings from the Geiger counters were very low, no counts were detected. To specifically test the NOR gate circuit, the output of one Geiger counter was connected to both inputs of the NOR gate. Thus when the Geiger counter read a count, the NOR gate would read a simultaneous count and open. The amount of "simultaneous" counts read every 30 seconds was read on the Arduino's serial monitor. A reasonable number was read (around 20-30 counts per 30 seconds). Note that this method of using only one Geiger counter to test the circuit was necessary because the sources used were very weak and so the likelihood of two Geiger counters reading a count at the same time was very low. We let this code run for about 15-20 minutes to make sure the loop repeated continuously without error.

Data Transmission
While testing the circuit and code, we compared the count number read on the Arduino's serial number (every 30 seconds) to the number shown by the transmitter. We let this run for a few minutes and confirmed that every number was transmitted correctly.

Data and Analysis


The balloon rose to an elevation of around 30 kilometers, about half way through the Stratosphere. The data was plotted after averaging each point with the four surrounding points to reduce noise and make the trends more clear. As seen on the graph, the intensity of cosmic rays detected peaked at around 38 counts per minute at an altitude of about 16 kilometers. This makes sense due to the simple fact that cosmic rays originate from outer space. With increased distanced traveled by cosmic rays through Earth's atmosphere there is a corresponding increase in the number of rays absorbed by atmospheric particles. This means there are higher concentrations of cosmic rays at higher altitudes. Nevertheless, the data peaks at around 16 kilometers. This is explained by the detection of secondary cosmic ray particles. Thus, according to our data, 16 kilometers is the altitude where most primary rays have collided with Earth's atmosphere to create secondary rays but the secondary rays have not yet been absorbed. This also accounts for the decrease in radiation intensity at altitudes above the peak because less secondary particle showers are created.

There is a discrepancy between our data and the data of one of the previous Whitworth near space group (2010) that also studied cosmic rays. Their data shows two intensity peaks. The first peak is approximately 14 kilometers and the second peak is approximately 16 kilometers (the same altitude as ours). A far off speculation of the discrepancies could be due to the different experimental setups. We used only two Geiger counters and thus cosmic rays could have lower energy and potentially still be detected whereas the other group used three Geiger counters, hence increasing the threshold energy needed by cosmic rays for detection. Thus it is possible that there is a dip in the intensity of high energy secondary cosmic rays at approximately 15 kilometers. If this is the case, and if lower energy secondary cosmic rays are included in data, the intensity would be seen as rising continuously. Even though this speculation is a possibility, it seems a little unreasonable that simply adding an extra Geiger counter will change the results enough to create a second peak. So the addition of the second peak could have resulted from potential experimental error. We can confirm that our single peak agrees with scientific theory which states that the cosmic ray intensity upon striking Earth's atmosphere should only have one peak at an altitude of approximately 17 kilometers.

The cosmic ray intensity decreased relatively slowly at altitudes above the peak. This slow decrease is most likely due to secondary showers still being created at those higher altitudes, just not as frequently due to the thinner air content.

The effectiveness of the lead sheeting in blocking radiation noise (from Earth) was inconclusive since there was not a two-Geiger counter experiment to act as a standard without lead shielding. This can be something compared against and expanded upon in future experiments. Nevertheless, the data shows strong trends that agree with theory.

Future Improvements
As seen Fig. 3, there are several sets and pairs of data points that overlap at the same altitude either on the way up or on the way down. The reason that the data was received like this was not due to the fact that the balloon floated statically for periods of time. It is because as the balloon was transmitting data it sometimes could not get a GPS reading on were its present location was so it would by default send the same position coordinates from the previous time it transmitted. To resolve a problem like this it would have been a good idea to use a SD card inside the pod that collected the same data as the transmitted data so there is a direct method to collect all the possible data points that were being outputted every 30 seconds.

Data from this experiment can be obtained from the Whitworth Near Space website under the Fall 2014 link. The data is titled "Pod 3 data".