Cosmic ray detection with three Geiger detectors

__MATHJAX_DOLLAR__ Research Group: Lucas Brouwer, Keith Harris, Katie Olleman, and Ben Rose

Launch: Whitworth Fall 2010

In this experiment we seek to measure the variation of cosmic ray intensity in the Earth's atmosphere by altitude to a higher degree of accuracy than a similar experiment performed two years ago (see Fig. 1). To detect only the highest energy rays we counted simultaneous coincidences from three stack Geiger counters whereas the previous experiment only used two. In theory we should be measuring only rays with sufficient energy to pass through all counters decreasing the number of false positives inherent to the two counter setup. Our results from this experiment (see Fig. 2) show high energy radiation intensity increasing with altitude as was expected. They also show the radiation curve approaching a maximum near the accepted value of 15,000 m. The data did have a slight dip right where we expected the maximum, for which we present a few possible causes. Overall the data from this experiment does seem to be an improvement over the previous experiment. Our peak is much closer to the accepted value of 15,000 m, and our data contains less noise as a whole.

Background
Cosmic rays were first discovered in 1912 by Victor Hess. In 1936 he was awarded the Nobel Prize in physics for this discovery. Upon initial discovery these rays were thought to be electromagnetic waves. In the 1930s this was disproven when it was discovered they interact with the Earth's magnetic field. This interaction implies these rays are electrically charged and therefore cannot be electromagnetic waves. Cosmic rays are instead high energy particles from space.

This radiation consists primarily of protons and alpha particles, with less than the final 1% being made up of other heavier elemental nuclei. When these high energy particles enter the Earth's atmosphere they interact with the nuclei of air molecules. This sends off a shower of lower energy particles called secondary radiation, as illustrated in Fig. 3. This explains why the amount of high energy cosmic radiation in the atmosphere general increases with altitude. The farther a ray travels through the atmosphere, the more likely it is that it will interact with an air molecule and disappear into a shower of lower energy particles.

However, all the particles in the shower will not always be low energy. The showers from very high energy cosmic rays can be filled with high energy particles as well. In this case a high energy Geiger counter would measure more radiation than the single count that would occur due to the initial high energy particle. For this reason the peak of the atmospheric radiation will not be at maximum altitude, but rather at the maximum of the sum of the primary particles and high energy shower particles. Experimentally this has been shown to be around 15,000 meters, as illustrated in Fig. 4.

Our experiment is seeking to verify the increase of cosmic radiation with altitude, and the maximum peak near 15,000 m. To do this we want to try as hard as possible to only detect high energy cosmic rays, and not weaker and other types of radiation in the atmosphere. To do this we used three Geiger counters stacked on top of each other as our coincidence counter. This configuration stops any low energy radiation in the first or second Geiger counter, so only the highest energy radiation passes through all three. To count cosmic rays using this method we record when all three Geiger counters go off at the same time indicating a single ray has passed through all three.

Experiment Summary: We seek to measure the intensity of high energy radiation with respect to altitude in the Earth's atmosphere.

Hypothesis: The measured high energy cosmic radiation in the atmosphere will increase by altitude and peak near 15,000 m.

Materials

 * Aware RM-60 Geiger counters (3)
 * Parallax Basic Stamp and Board of Education
 * Batteries, lithium ion 9 V (2)
 * NOR gate, 74AC02P2
 * Inverter, HD74LS04P
 * Styrofoam
 * Mylar
 * StratoStar experiment interface module

Pod Construction
The layers were created out of styrofoam and tubes from carbon fiber kite poles. The tubes were glued to the styrofoam using quick dry epoxy.



Pod Packaging




Circuit Diagrams




Data Sheets and Other References

 * Micro Roentgen Radiation Monitor, Aware Electronics RM-60
 * Quad 2-Input NOR Gate, [[media:Datasheet for 74ACT02.pdf|Datasheet for 74ACT02]]
 * Development Board, Parallax Board of Education (USB)
 * Microcontroller, Parallax BASIC Stamp

Software
A program was needed for the Parallax Basic Stamp so it could read in, store, and export the number of counts from the Geiger counters. The Board of Education we used has built-in voltage inputs and outputs that can be specified in the program. We used these to receive signals from the counters and output a digital number of counts to the transmitter. The Basic Stamp runs programs written in the BASIC programming language, so that is what we used.

Program Flow

 * 1) Keep a running total of the counts on pin 15.
 * 2) The flowchart for our program is shown below, and the actual text of the program can be found on the next page.
 * 3) Output six seconds worth of counts to transmitter.
 * 4) Reset counts.
 * 5) Return to step 1 and repeat.

Code
CosmicRaysFinal.bs2 ' {$STAMP BS2} ' {$PBASIC 2.5}

'define variables oneandtwo VAR Byte		'variable to store counts DIRL=225			'outputing on pins 0-7 INPUT 15 			'the logic circuit outputs to pin 15, this is the input to the stamp

'the loop the repeats our code DO COUNT 15, 6000, oneandtwo 	'counts P15 for 6sec and writes it to oneandtwo DEBUG CR,"Counts on P15 for 6sec: " DEBUG DEC oneandtwo 	'outputs the number to computer screen OUTL=(oneandtwo^255) 	'Outputs in an inverted vinary encoding, corresponding to the balloon's transmition LOOP

Geiger Counters and NOR-gate Testing
First we tested to see what the output of one Geiger counter was over a 15 s interval. The outputs were between 1 and 10 counts. Though this was a wide range, most of the data fell around seven. We then tested Geiger counter 1 to both inputs of the NOR-gate. With this test we drained the batteries. Interestingly, when the batteries were low we got values up to 8,000. With a new battery the NOR-gate worked. We then tested both Geiger counters connected to the nor-gate. We got value from 0-2.

We used a NOR-gate because the last group found out that the Geiger counters output a constant high signal and when it detects something it outputs a low signal. In reality the NOR-gate is acting like a standard AND-gate, if the output was a high signal.

Battery Testing
We were having issues with battery consumption. We first thought that the whole circuit was drawing too many Amps, so we built a connector to power the stamp by two 9 V batteries in parallel. This did not solve the issue. We later determined that the issue was with the NOR-gates. So we switch back to the NOR-gate used in the previous Cosmic Ray (2008) experiment. Later we found out that the nor-gates that we ordered needed to have all unused pins at VCC or GND. The product information for the unused nor-gate follows in this section.

Circuit and Software Testing
Since very few cosmic rays penetrate to ground level, when we had each of the three Geiger counters attached to an input of our logic circuit, we would get outputs of zero counts. In order to check that the logic and software was working, we connected one Geiger counter to all three inputs. This way we were able to get numerical readings assuring us that the logic gates and the software on the stamp were working properly.

Transmitter Testing
We also tested the transmitter in the same way as the software. We made the circuit think one Geiger counter was three, so we could transmit non-zero values. We transmitted by the 8-bit digital signal. This was connected to the output of the Parallax Basic Stamp. Before we tested with the actual transmitter and computer receiver, we build an LED circuit to see if the Stamp outputted any signal. All tests were successful.

Data and Analysis
The balloon rose through two layers of the atmosphere, the troposphere and the stratosphere. The troposphere extends from the earth’s surface to 7000 or 17000 meters depending on whether one is at the poles or the equator, respectively. The second layer of the atmosphere the balloon entered was the stratosphere. This extends from the troposphere to about 51000 meters. The boundary between the troposphere and the stratosphere, which is called the tropopause, roughly occurs over Washington at 10000 to 15000 meters.

As the balloon traveled through the troposphere, there was a general increase in cosmic ray frequency. However, two local minimums appear: one at ~10000 m and one at ~14000 m. Then, as the balloon journeyed upward through the stratosphere the cosmic ray frequency dropped off. The general trend of increasing frequency through the troposphere and decreasing frequency through the stratosphere matches what has been experimentally shown according to Hyperphysics. However, no local minimums are demonstrated in the Hyperphysics article. We believe the local minima we observed to be an anomaly. The first local minimum (~10000 m) occurs when the intensity of the radiation first begins to show a significant increase by altitude. The second local minimum (at altitude ~12000 m) occurs when the intensity of the radiation first changes from increasing to decreases by altitude. We believe there may be some correlation between changes in the slope of the intensity graph and the local minimums observed. Perhaps these are areas of significant change in pressure or places where the air is particularly turbulent. Primary cosmic rays produce showers of lower energy, short-lived particles when they interact with atoms in the atmosphere. Because of this, regions of high intensity radiation are most likely areas of high density secondary cosmic rays (showers). Thus above such areas there are probably a high concentration of events causing primary cosmic rays to broken apart into secondary rays. Since the transition from the stratosphere to troposphere is a local maximum of cosmic ray frequency versus altitude; and since above such heights a significant number of primary cosmic rays decay to secondary cosmic rays, there must be a space not far above the tropopause where the primary rays readily decay to secondary rays. Thus there must be some property of this volume in the lower stratosphere which facilitates cosmic ray decay.

Similarly to how there appears to be a set of environment parameters which causes an increase in cosmic rays, there seems to be the potential for an localized characteristics which decrease ray frequency. This is demonstrated by our two local minima which are not usually found experimentally. While they may simply be the result of a poor environment for data collection, perhaps due to turbulence, they also leave the possibility for an abnormally low ray frequency at a given altitude. This would result from having fewer showers which limits the number of cosmic rays to primary rays instead of the numerous secondary arrays. Thus it is possible that we encountered a special situation within the atmosphere which naturally impedes the decay of primary cosmic rays to secondary cosmic rays.

Comparing our results to those of the cosmic ray group of 2008, we may note several points. Their data also shows an unusual local minimum, which their data says occurs around 15000 m. However, their data peaks around 25000 m. Thus it appears to be off by a factor of 5/3. Thus looking at the overall trend of their data it is possible that their abnormal local minima occurred earlier and may match our local minima at 10000 m. However, their data is not particularly conclusive. It is difficult to read as it obscured by noise. In comparison, we were able to sufficiently eliminating obstructive noise.

Our data shows that as one travels upwards through the troposphere the frequency of cosmic rays increase. However, our data also shows places where this upward trend may be disturbed along the way. After one has passed on into the stratosphere, cosmic ray frequency decreases more smoothly than it increased in the troposphere. Thus there is a general or universal event at the boundary between the troposphere and the stratosphere which causes an increase in cosmic rays, which we believe is due to primary rays decaying into a shower of secondary rays. There also appears to be the potential for localized events in the troposphere to cause similar decay. However, we must also admit that these unexpected local minima may simply be the result of experimental error. Overall, there is more to be explored as what exactly causes these changes in the frequency of cosmic rays is yet to be determined.