NGrozeva: Final Report
From MariachiWiki
Cosmic rays mainly comprise nuclei of atoms that originate from the sun and remains of supernovae outside our solar system. They continuously bombard the earth’s surface at a rate of approximately 1/min/cm^2 and are the primary source of natural background radiation. The earth would probably not be able to support life, however, if it was not for the shielding effect of the atmosphere. Cosmic rays collide with particles as they enter the atmosphere, yielding a cascade of secondary particles, each with only a fraction of the energy of the primary ray. More secondary particles are produced with each subsequent collision in this “air shower.” Thus, the total number of secondary cosmic rays increases with a decline in altitude within the upper atmosphere. When a secondary particle’s potential and kinetic energies, however, become less than or equal to that of atmospheric particles, it can no longer produce other secondary particles. After particles reach this minimum energy, they begin to be absorbed by the atmosphere, reducing their total number. This phenomenon explains the exponential-like rise in the number of cosmic rays detected as altitude increases within the lower atmosphere.
Shruthi and I began our experiment attempting to determine the effect of altitude on the rate of cosmic rays hitting a given surface area, but ultimately, we never answered our initial question. Our set up failed to account for the change in the thickness and density of material that cosmic rays must pass through to reach the scintillation detectors. We consequently deviated from our proposed experimental design to measure the effect of this confounding variable on the coincidence rate, instead. From our data, we were able to conclude that a negative correlation exists between the thickness of building material and the coincidence rate.
In our first experiment, Shruthi and I intended to test our hypothesis that the rate of cosmic rays passing through a given surface area will rise as altitude increases.
We measured the number of coincidences between the two scintillation detectors in Cosmic Chris two times for 120 seconds each on every floor of the physics building. We placed the detectors at an angle of 0 with respect to the ground in order to increase the number of counts and, therefore, lower the statistical error. Each measurement was taken at the same horizontal position, so that only the vertical position, or altitude, changed.
Although our data suggests that there is a relationship between altitude and coincidence rate, it does not support the results of other experiments, which determined a similar correlation, but using larger increments of altitude (1000 m instead of 1000 in.). When we questioned how only a small change in altitude can induce such a large change in rate, we realized that we failed to control for the different amounts of building material. For instance, cosmic rays must pass through more material to reach the basement compared to the upper floors of a building. Since this experiment could not separate the effect of media from that of altitude on the coincidence count, we could not determine how much each factor contributed to the increase in rate.
In our second experiment, Shruthi and I aimed to test our hypothesis that the coincidence rate will rise as the distance from a building increases. This may occur because cosmic rays traveling at an angle can still pass through a building before reaching the scintillation detectors. Moving away from a building decreases the angle with respect to the ground at which cosmic rays can hit the scintillation detectors and not pass through any building material.
Using Cosmic Chris, we measured the coincidence count three times for 120 s each, progressively farther from the physics building.
We cannot conclude anything from the first two data points, since the change in rate is less than the statistical error of each measurement. The difference in rate between the first and third measurements, however, is statistically significant enough to support our hypothesis.
To lower the error and improve our results, we could measure the coincidence count for a longer period of time. To establish a precise relationship between rate and distance from a building, we should perform more trials and actually measure the distance between the building and scintillation detectors rather than simply determine the relative position of the detectors to the building at each measurement.
When we compared the coincidence rate on level P with that outside at the same altitude, we were able to conclude that building materials do lower the coincidence rate.
Like atmospheric particles, particles in a building can absorb the kinetic energies of cosmic rays, which then cease to exhibit translational movement and cannot reach the detectors.
In our third experiment, Shruthi and I aimed to test our hypothesis that the coincidence rate will decline as the thickness of building materials increases.
We stacked two scintillation detectors on top of one another at a fixed distance from each other. We then measured the coincidence count for 120 s when a steel plank and lead blocks were placed between the detectors, and when no material was present.
The data, however, was inconclusive since the error bars of each measurement overlapped. To reduce the statistical error, we measured the coincidence count for 300 s, but the data was still inconclusive.
At this point, we realized that the calculated rate was much lower than it should be (about 30 Hz), but could not discover why.
We improved the results by using Cosmic Chris for the measurements and increasing the thickness of materials. We measured the coincidence count under a relatively thin ceiling, and under a ceiling containing 4 more feet of concrete for 300 s each.
From our data, we were able to conclude that the coincidence rate declines as the amount of material increases.
As a further study, it would be interesting to determine how different materials (e.g. steel, concrete, wood, plastic) affect the coincidence rate differently.
The Cosmic Rays course was wonderful in that it offered a unique hands-on experience and the opportunity to carry out individual experiments and learn through inquiry rather than just follow a prescribed procedure. Guiding our own measurements and interpretations pushed us to think critically about the methods we used and data we obtained. I learned that the process of science necessitates having a certain amount of humility in order to examine one’s experimental methods and change one’s hypothesis or even the entire research question. Most importantly, I learned how essential calculating the statistical error is in interpreting measurements. Otherwise, one could erroneously conclude a non-existing relationship.
NGrozeva
May 1, 2008
