User:NGrozeva
From MariachiWiki
Niya Grozeva is currently a freshman at Stony Brook University double majoring in geology and biology. She was born in Bulgaria and spent most of her childhood in Canada before coming to the U.S. at the age of ten. She currently lives “upstate” in Eastchester, NY with her parents and older brother. Although Bulgarian is her native tongue, her mom frequently reminds her that she should feel ashamed for not being able to speak the language properly. Yet, she loves to visit her family in Bulgaria and fell in love with the Rhodope Mountains during her last trip in the summer of 2007.
Niya has been actively doing yoga for the past three years, but is struggling to find time to practice while in college. She attends meditation classes at the counseling center on campus and encourages students to use this wonderful (and free!) resource. Her other hobbies include keeping up with current events in the NY Times and Daily Show, listening to classic rock, reading web-comics (she highly recommends Sam and Fuzzy), playing frisbee, skiing, and just hanging out with her friends. She is an active member of Stony Brook’s Undergraduate Geology Club and the WISE Student Leadership Council, but still misses some of her former high school clubs. She has already gone back to her high school several times to help the art club paint a mural.
Niya can often be seen attending Geology Open Nights and Our Environment lectures at Stony Brook. She plans to specialize in environmental geology and environmental biology within her two majors to combine her loves of protecting the environment and helping people, since promoting sustainable human practices will ensure sufficient unspoiled resources for present and future generations. Most importantly, she wants to be part of the solution!
Niya is currently taking WSE187: Cosmic Rays & Particle Detectors.
Contents |
April 8, 2008
Since increasing voltage makes a photomultiplier tube (PMT) more sensitive to pulses of light, the efficiency of the detectors and the level of noise produced depend on the voltage across the detectors. Joanna, Veronica and I performed an experiment to determine the voltage at which the detectors are most efficient without overloading the PMT. 3 scintillation detectors were stacked one on top of the other. We measured the number of particles that hit each detector, the number of coincidences between the top and bottom detectors (which we assumed accurately reflected the number of real cosmic rays passing through the middle detector), and the number of coincidences among all 3 detectors, for 60 seconds. Starting at 5.0 V, the voltage was lowered by 0.1-0.2 V for each 60 s interval.
Noise rate and efficiency were calculated as follows:
From our data, we observed that the efficiency is close to 100% and relatively constant from 4.4 to 5.0 V (i.e. efficiency plateaus). After 4.4 V, efficiency declines almost linearly. Noise rate rises exponentially as voltage, and thus sensitivity of the PMT, increases.
April 10, 2008
Factors that may affect how many cosmic rays pass through a scintillation detector:
- Rate- # of particles passing through a given surface area per second
- Orientation- angle of detector with respect to the ground
- Altitude- height of detector above sea level
- Air temperature?- may increase the average kinetic energy of particles
- Atmospheric pressure?
- Humidity/ air pollution?- the more molecules there are in the atmosphere, the more likely that a cosmic ray will collide with a particle and decrease in energy
Just a thought:
Although this idea is not feasible for a class project, I am curious how inter-annual cycles (e.g. the seasons) can affect the rate at which low-energy cosmic rays, whose primary source is the sun, hit the northern hemisphere. For instance, do more cosmic rays pass through the MARIACHI lab during the winter when the earth is closer to the sun or during the summer when it is tilted more toward the sun? (i.e. Does the rate depend more on the earth’s distance or orientation to the sun?) Of course, one cannot control for such variables as temperature and humidity.
To control for the earth’s distance from the sun, one could determine how the rate changes as a function of latitude. Areas located at lower latitudes during the equinox are tilted more toward the sun, and the sun’s radiation, thus, hits the ground at a greater angle with respect to the ground. But again, temperature and humidity are confounding variables.
To determine how the earth’s distance from the sun affects the rate, one could measure the rate at the Tropic of Cancer during the June solstice and at the Tropic of Capricorn during the winter solstice, when the angle of the sun’s radiation, and thus, temperature will be equal. However, because the rates will not be measured at the time, changes in solar activity must be taken into account.
Proposed Experiment
From a previous experiment, we observed that the rate of cosmic rays passing through a scintillation detector increased when the angle of the detector with respect to the ground decreased from π/2 to 0. More cosmic rays, therefore, travel in a direction perpendicular rather than parallel to the ground. However, this information is insufficient to tell us from what direction they originate: the ground or the atmosphere. The purpose of this experiment is to determine what effect altitude, or the height above sea level, has on the rate of cosmic rays that pass through a given surface area.
Materials:
- 2 scintillation detectors connected with cables to a coincidence box, independent voltage supply
- OR Cosmic Chris
Procedure:
- Stack the 2 scintillation detectors one on top of the other so that their entire surface areas overlap. The detectors will always be orientated at an angle of 0 with respect to the ground (i.e. lying horizontally on the ground). Using the efficiency curve obtained last week, set the voltage across the detectors to 4.4 V.
- Measure the number of coincidences per second between the two detectors in the basement of a building and on the top floor of the same building. Make sure that the two locations lie along the same imaginary vertical line (i.e. they have the same horizontal, but not vertical, position).
Expected Results:
- Since the detectors experience a relatively small change in altitude when moving from the basement to the top floor, the coincidence rate should only change slightly. I decided not to measure the coincidence rate at each floor of the building because such small changes in data might not be statistically significant. If cosmic rays do emanate from space, then the coincidence rate will be greater on the top floor of the building, where the detectors possess a greater altitude.
April 15, 2008
Today Shruthi and I performed a trial run of the experiment to see how the expected results compared with the actual data. As outlined in the proposed experimental design, we measured the number of coincidences between the two detectors in Cosmic Chris for 2 minutes on levels S (basement) and D of the Physics building. In order to increase the statistical significance of any difference in the counting rate, we lay Cosmic Chris horizontally, since the rate of particles detected increases when the angle of the detectors with respect to the ground decreases. Measurements were taken 2 m in front of the elevator on the right, with Cosmic Chris lying in the same position relative to the elevator door. 2 measurements were taken in each location.
- Data:
_______ 
Since the change in height between Levels S and D represents only a small change in altitude, we did not expect to obtain a significant difference in the rate between the two locations. So, we were surprised that the rate of cosmic rays passing through Cosmic Chris on Level D was 1.300 times higher than the rate on Level S. We therefore wish to obtain additional measurements on every floor of the building.
Variables to Consider:
Since the atmospheric pressure decreases as altitude increases, we cannot separate the effects of pressure and altitude on the counting rate.
Possible Reason for a Rise in Counting Rate:
As high energy cosmic rays enter the atmosphere, they collide with molecules and produce secondary particles that are lower in energy. These secondary particles in turn collide with more particles forming an “air shower.” Each subsequent collision closer to the earth’s surface, and thus lower in altitude, reduces the energy of the secondary particles. If the energy is low enough, it might not reach the threshold energy of the detector and trigger a count.
Therefore, as altitude increases, the energy of the secondary particles also increases, allowing more to be detected by the scintillation detectors.
Revised Experiment
Purpose:
- To determine how altitude affects the rate of cosmic rays passing through a given surface area.
Materials:
- Cosmic Chris (contains 2 scintillation detectors), stopwatch, meter stick
Procedure:
- Place Cosmic Chris horizontally on the ground (i.e. the detectors are at an angle of 0 with respect to the ground) 2 m in front of one of the elevators on level S of the Physics building.
- Measure and record the number of coincidences between the detectors twice for 120 s each.
- Repeat steps 1 and 2 on levels P, A, B, C and D. Perform the measurements in front of the same elevator and make sure that Cosmic Chris lies in the same position relative to the elevator door.
- Calculate the rate and statistical error for each level.
April 17, 2008
Today we made measurements on levels B and C.
- Data:
____ 
The difference between the counting rates on the two floors was somewhat small and does not appear to account for the larger difference between levels S and D. In setting up the experimental design, we failed to realize that cosmic rays must pass through more and denser material to reach the basement compared to the upper floors of a building. Since particles lose energy when they travel through media, some particles might not possess enough energy upon reaching level S to trigger the scintillation detectors. Therefore, we are measuring the effect of obstacles (in the form of building materials), in addition to altitude, on the counting rate. It would be interesting to see, however, whether we will obtain different data if the same experiment was performed outside. Then we can be sure that building materials will not affect the rate.
April 22, 2008
Today we made measurements on levels P and A:
- Data:
To test how building materials affect the coincidence rate, we took Cosmic Chris outside at the same altitude as level P. We made three measurements progressively farther from the physics building:
____ 
Although the first two measurements yield inconclusive data, there is a statistically significant change in rate between the locations closest and farthest from the building. One reason for this increase in rate is that when one is close to a building, cosmic rays coming at an angle must pass through the building, and thus, lose energy. 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.
A comparison between the rates on level P and outside at the same altitude shows that building materials do effectively lower the energy of cosmic rays, as inferred by the decline in rate:
____ 
April 24, 2008
Today we performed a more controlled experiment to determine the effect of materials on the coincidence rate. Two scintillation detectors were laid horizontally on top of one another at a fixed distance from each other. For the first trial, a steel board and lead blocks with the same surface area as a detector were placed between the two detectors. For the second trial, no material was present between the detectors. The number of coincidences was measured for 120 seconds each. The data obtained, however, was inconclusive, since the change in rate was smaller than the statistical error of each measurement:
To obtain a smaller error, we measured the coincidence count for 300 seconds instead of 120 s. Nevertheless, the change in rate still was not statistically significant:
The steel and lead blocks were probably not thick enough to change the rate appreciably. As a result, we measured the coincidence rate in the MARIACHI lab, where the ceiling is relatively thin, and in a part of the basement overlain with 4 more feet of concrete than in the MARIACHI lab:
____ 
From the data, we can conclude that the coincidence rate decreases as cosmic rays pass through more building material. It would be interesting to determine, however, how much of the change in rate in the previous experiment can be attributed to a change in the amount of material versus a change in altitude. In order to figure this out, we would need to measure the thickness of the roof. Knowing that 4 feet of concrete lowers the coincidence rate by 2.02 ± 0.624 Hz, we could determine what the rate would be on level D if cosmic rays did not pass through the roof and then compare it to the rate outside on ground level (890 in. lower in altitude). Or if possible, we could directly measure the coincidence rate on the roof and compare it to that outside.
