Cosmic ray detection with two Geiger detectors

Research Group: Amy Brown, Jenna Cunningham, Geoffrey Etter, and Mitchell Lazore

Launch: Whitworth Fall 2008

Pod 4 for the near space balloon research experiment launched by the physics department of Whitworth University collected data about the number of cosmic rays at different altitudes. In order to accomplish this, our group assembled two Geiger counters into an incident counter. Two Geiger counters are necessary in order to distinguish the high energy cosmic rays from the lower energy radiation coming from Earth. Higher energy rays pass through both Geiger counters while low energy rays can only pass through one. By creating software that will only count a ray if it passes through both Geiger counters simultaneously, we can record only the cosmic rays. As the near space balloon ascended after launch it transmitted this data to the command center laptop computer. A graph of our results appears in Fig. 1.

The overall trend, as expected, is that the number of cosmic rays increases with altitude. As the rays collide with the atmosphere they lose energy and no longer register on the Geiger counters. This is true except for the decline above 80000 ft. Above 80000 ft the cosmic rays have enough energy to hit atmospheric particles and give them enough energy to register on the Geiger counters. It's a tree branching or showering effect.

Cosmic rays are energetic particles, usually protons but also helium nuclei and electrons. They come anywhere from our own sun to black holes much farther away. These particles from space are essential to investigating their sources.

Background
Cosmic rays were first discovered in 1912 by Victor Hess. Hess, working for Vienna University, ascended in a hot air balloon to 16,000 feet. At this altitude he measured a very energetic radiation. Because this radiation increases with altitude, it can be assumed that the radiation is coming from a source outside of our atmosphere. Hess earned the Nobel Prize for this discovery.

Through continued research much more has been discovered about cosmic rays. The primary particle that makes up cosmic radiation is the proton. They account for approximately 90% of the incoming particles. Alpha particles account for about 9%, and the remaining 1% is comprised mostly, but not exclusively of electrons. These particles have energies of around 1020 eV. Typical energies of particles produced in labs are significantly lower, such as 1012 to 1013 eV.

A peak in the detection of cosmic rays seems to occur at around 15 km. This suggest that what instruments actually detect isn't always the rays themselves, but the secondary particles produced by the cosmic radiation. As the high energy cosmic radiation particles enter the atmosphere, they collide with particles in our atmosphere. After the collision, there is a transfer of momentum and now there are two particles with high energy. Therefore, after the particles have traveled through the atmosphere, they have excited more particles creating an apparent increase in cosmic radiation.

The question of the origin of the particles can be partially determined by looking at their makeup. Light particles make up approximately 0.25% of cosmic radiation. This is much greater than their relative abundance in the universe. Medium elements are approximately 10&times; more abundant in cosmic radiation than in the universe, while heavy elements are approximately 100&times; more abundant. This suggests that cosmic radiation is being emitted from areas of space that are rich in heavy elements. Additionally it was discovered that as a Soviet space station spun on its axis, the arriving radiation increased every time the detector was pointing at the sun. This suggests that some of the cosmic radiation originates from the sun. An increase was also detected when the sun was experiencing solar flares or solar storms.

There are many particles in the atmosphere that emit radiation. In order to ensure that these were the high energy particles associated with cosmic radiation, we used two Geiger counters. If a particle is low in energy, a single Geiger counter would stop the particle. If the particle is high in energy, the Geiger counter will not stop it and it will be detected by both Geiger counters almost simultaneously. Therefore, the most accurate way to measure the cosmic radiation is to look at the coincidence counts of the Geiger counter (or the time when both Geiger counters registered a particle at the same time).

This experiment measured the cosmic radiation as a function of altitude. We expected to find that cosmic radiation peaked around 15 km as we found in our research.

Software
The code below was written in BASIC Stamp and used to count the number of signals received from the nor-gate. The signal received from the nor-gate is brought in through pin 10. Those signals are stored the variable "number" for intervals of 15 seconds. At the end of the 15 second window, pins 0-7 are used to output the information to, eventually, the ground for data collection. This loop happens indefinitely while the microprocessor is turned on.

Program Flow

 * 1) Signal input through pin 10.
 * 2) Count and store number of signals.
 * 3) Output count after 15 seconds.
 * 4) Return to step 1 and repeat.

Code
'Coincidence counter for Project: HARP

'{$STAMP BS2} '{$PBASIC 2.5}

number VAR  Byte        'Number of coincidence counts DIRL = 255              'Pins 0-7 are outputs INPUT 10                'Input signal through pin 10

DO                      'Indefinite loop COUNT 10, 15000, number 'Count signals for 15 second windows and store in number DEBUG CR,DEC number     'Output number to computer screen OUTL=(number^255)       'Exclusive OR to invert bits of number to correspond to balloon's inverted binary encoding LOOP

Software Testing
Once the BASIC Stamp software coding was completed, we tested the microprocessor circuit and code by putting them together. The software would accumulate and output the signal counts to both the main computer's screen (where the code was built) and to the control pod computer (where the information was eventually received). By comparing the numbers displayed on both computers, we were able to determine that both the circuit and the software were performing as intended. The numbers displayed were the same on every transmission.

Circuit Testing
Using the RM60 Geiger counters from Aware Electronics we used the software that was included to determine if the Geiger counters would individually detect radiation in the laboratory. Once we determined that the Geiger counters were working, we attached them to the circuit consisting of the Parallax microprocessor with counting software, and a AND gate which would output the signals of the Geiger counters only when both received a count, which is the principle of coincidence counting.

During this testing procedure we found that we were getting conflicting results with what we predicted we should get. The output of the AND gate signal was constantly high, which meant that our counters were always receiving some radiation. This was inconsistent with our location in the physics building. It was discovered that the output signal of the RM60s is always high until it receives a radiation count and then it goes low. This meant that in order to transmit and count the coincidence counts we needed a NOR gate, where the output would only be sent to the microprocessor when both inputs were low. This adaptation solved our coincidence count problems, but another problem arose. The output of our NOR gate was sporadically increasing or decreasing, instead of staying consistent with counts. This was quickly determined to be due to incorrectly wiring the NOR gate with an input being used as an output.

Transmitter Testing
The output signal from the microprocessor was broken up into an 8-bit digital signal which was connected to the provided sensor board interface which communicated to the base station through a wireless network. During our testing the signal was successfully sent and received at the base station which showed identical readings to the hardwire testing.

Data and Analysis
The number of average cosmic rays detected by our experiment is dependent on the density of the atmosphere. As you can see by the general trend of our cosmic ray count vs. altitude graph, as altitude increases so does the cosmic ray count. At around 80,000 ft there is a peak in the graph. Altitudes higher than this peak generally register lower counts. As the cosmic radiation travels through Earth's atmosphere it hits the air molecules present. These collisions cause the radiation to lose energy. After a number of collisions the radiation does not have enough energy to register on both of our Geiger counters. Higher altitudes have a lower density of air molecules which means fewer collisions. Fewer collisions translate to more cosmic radiation counts...most of the time. The interesting part of our data is demonstrated in the peak in our graph. Above 80,000 ft the radiation has enough energy that when they collide into the air molecules they impart enough energy that the molecules (or what's left of them) also register on our Geiger counters. Four becomes eight becomes sixteen and so on. The peak forms because there are the optimal conditions for most radiation: namely, the air is dense enough to allow the most number of collisions without depleting the cosmic radiation energy too much.