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«STUDENT RESEARCH PAPERS SUMMER 2011 VOLUME 22 REU DIRECTOR UMESH GARG, PH.D. REU Student Research Papers – Summer 2011 University of Notre Dame – ...»

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NSF/REU

RESEARCH EXPERIENCE

FOR UNDERGRADUATES

IN PHYSICS

STUDENT RESEARCH PAPERS

SUMMER 2011

VOLUME 22

REU DIRECTOR

UMESH GARG, PH.D.

REU Student Research Papers – Summer 2011

University of Notre Dame – Department of Physics

Student Research Title Page No.

Andrew Arend Thickness Measurements and Isotopic University of Wisconsin Identification of Cadmium, Tin, and Tellurium 1 Targets Molly Ball Monmouth College Correction Methods for Project GRAND Data 11 Brittany Batman Manchester College A Simulated Galaxy Redshift Distribution 21 Christopher Bell University of Notre Dame Numerical Methods in Quantum Field Theories 31 Anna Czerepak Pt-Ir Tip Etching Techniques for Scanning 42 University of Illinois Tunneling Microscopy Erin Dahlstrom Examination of the Validity of Statistical Models for the 12C + 12C Fusion Reaction at Sub-barrier Rice University 52 Energies Joshua Ferguson Butler University Double Chooz and Neutrino Detection 62 Quinn Hailes Measuring the Half-Life of 60Fe Data Analysis Hampton University 72 John Hardin Analyzing Potential Tracking Algorithms for the University of North Carolina Upgrade to the Silicon Tracker of the Compact 83 Kevin McDermott Muon Solenoid University of Notre Dame Robert Heitz Recoil Mass Separator Hysteresis Measurements 101 Virginia Tech and Germanium Detector Development i Student Research Title Page No.

Janie Hoormann Magnetic Fields in a Supernovae-driven, Baylor University Stratified Disk 111

Justin Kelly Magnetotransport of Topological Insulators:

Northern Arizona University Bismuth Selenide and Bismuth Telluride 120 Ruben Medina

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of nucleosynthesis. The p-process is thought to be responsible for the origin of the 35 most neutron deficient stable even-even isotopes between 92,94Mo and 196Hg [1]. The nature of the p-process is still under debate but currently the most favored interpretation is gamma-induced photodisintegration of stable nuclei. There are significant challenges in understanding these reaction rates, particularly near the Cd, Sn, and Te isotopes. In preparation for measuring some of the relevant rates we characterized a set of 54 thin foil targets of Cd, Sn, or Te. The targets had Al or C backings and Al or Ta frames.

Using Particle Induced X-ray Emission (PIXE), we determined the elemental composition of each target by comparing the observed x-ray emission lines with that of the characteristic emissions of the relevant elements. While PIXE is a useful technique for identifying the elemental compositions of the foils, we concluded that it is not an accurate method of measuring foil thickness. We next used Rutherford Backscattering with a 12C3+ beam to determine the isotopic composition and thicknesses of the targets. A Silicon detector was placed at 150° with respect to the beam direction to detect the backscattered 12C particles. A mixed 148Gd 241Am α-source was used to ensure a good energy calibration.

Analyzing the energy at the leading edge of the energy spectrum of the scattered particles allowed identification of the specific isotopes in each sample. The width of the scattered 12C peak, indicating the maximum energy loss of 12C in a target, yielded the thickness of the target when compared to calculations. The target thicknesses range from about 87 to 913 µg/cm2.

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The nature and site of the p-process is still a major enigma in our understanding of nucleosynthesis. The p-process is thought to be responsible for the origin of the 35 most neutron deficient stable even-even isotopes between 92,94Mo and 196Hg [1]. The nature of the p-process is still under debate but currently the most favored interpretation is gamma-induced photodisintegration of stable nuclei. Alternative possibilities include proton capture (p,γ) by stable heavy nuclides. Proton capture now seems unlikely because of the increasing coulomb barrier associated with heavier elements. The chief ways to compensate for high coulomb barriers are increased kinetic energy i.e. increased temperature or very proton rich environments [2]. Neither condition is present in sufficient magnitude in stars to adequately explain the p-process.

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The Pauli exclusion principle states that no two electrons may occupy the same quantum state. Therefore all electrons of an atom must occupy certain discrete energy levels [3]. When an atom is bombarded with ions of sufficient energy, the inner shell (K or L) electrons can be excited into a state of ionization. Outer shell electrons then fall into the newly vacant lower shell orbits. These orbital changes include photon emissions of specific energy levels, typically in the x-ray spectrum. All elements have certain characteristic x-ray emissions, enabling elemental analysis of samples by inducing x-ray emissions. A simple detector apparatus can be used for this analysis. This process is called Particle Induced X-ray Emission (PIXE). PIXE enables only elemental analysis as it is dependent solely upon the atomic number of the target nucleus, and therefore cannot be used to determine isotopic makeup of samples. Additionally, this method is limited to

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hundreds of μg/cm2. PIXE is a powerful non-destructive method of analysis which has applications in physics, biology, materials science, geology, archaeology, and in the art world [4].

2.3 Rutherford Backscattering Rutherford Backscattering is a high kinetic energy elastic collision between an incident ion beam and a target particle. Ions fired at a thin foil have a chance of interacting with and scattering off of the atomic nucleus through a coulomb collision. The chance of scattering (cross-section) of the reaction depends upon the atomic number and isotope, among other things. The cross-section of the reaction is characteristic of each isotope in the target and incident beam and can be used to determine the isotopic composition of the target sample [5].

3 Experimental Methods All experiments were conducted using the FN tandem accelerator at the university of Notre Dame Nuclear Science Laboratory (NSL). Tandem accelerators work by attracting negatively charged ions to a high voltage terminal. At the terminal, the beam passes through a thin foil which strips electrons from the atoms and creates positive ions.

Additionally, because the ions are now of the same charge as the terminal they are repelled from the terminal.

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The incident proton beam of was generated using the tandem FN accelerator at the NSL. The thin foils were analyzed in 9 runs of 6 targets each. At the beginning and end of each run, background was taken by running the beam through an empty target.

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3.1.1 shows the initial data for a Tin target before the removal of background. Figure 3.1.2 shows the same target data after background was subtracted. Background was calculated for each run by averaging the two background measurements. The total number of counted events in the main peak was then normalized so that the background level was scaled to be the same as the background on the initial set of runs. This removed the variable of fluctuation in beam strength. The carbon backings did not absorb as many of the x-rays emitted by the targets as did the aluminum backings. Thus, a further correction was made to targets with aluminum backings. The composition of the targets was determined by the location of the main as well as lesser peaks. Cadmium targets had their largest peak at the same level as the background Argon peak. The lesser peak at 3.1 MeV was used to confirm that the target was not merely background. Tin and Tellurium targets could be easily identified.

Figure 3.1.

1 Tin Target Data on Maestro Software Before Background Subtraction

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3.2 Backscattering The Rutherford Backscattering (RBS) experiment used a mixed 148Gd 241Am alpha source and a silicon detector at 150° with respect to the beam direction. The incident ion beam was 12C3+ at 15 MeV using the tandem FN accelerator at the NSL. The beam energies ranged from 15.003 to 15.005 MeV, calculated using proprietary software and measurements from the accelerator controls. Only 14 of the 54 foils tested in PIXE were tested again in the RBS experiment. Figure 3.2.1 shows data from a sample of Tellurium that was ultimately determined to be 555 µg/cm2 thick 120TeO2. The two alpha peaks are of known energy and therefore easily used for calibration.

The energy loss of the scattered particles was determined from the Full-width Half Maximum (FWHM) of the energy peak. The region of interest of the same data from Figure 3.2.1 is shown below in Figure 3.2.2.

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Integration of the stopping power in kev/µg/cm2 for a range of possible target thicknesses was calculated and interpolated to produce a relationship between energy loss and target thickness. Energy loss in the particle collision was calculated using proprietary kinematics software. Of the 14 foils tested, 2 were determined to contain no heavy elements. Additionally, one of the Cadmium target foils was too thick to obtain an

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analyzing the energy at the leading edge of the peak. Two of the targets tested were known to be 120TeO2. Using these foils as benchmarks, the energy signature of each isotope could be determined.

4 Results and Conclusions The final relevant and adjusted data for all RBS tested targets is shown below in Table 4.1.The hope was that count data from the peaks in PIXE would scale linearly along with the thickness measurements determined through the RBS experiment. The relationship between RBS calculated density and PIXE measured counts for the Tellurium targets is shown below in Figure 4.1.1. Controlling for target backing and framing do nothing to produce a more linear relationship. Further, the different isotopes of Tellurium are not grouped together; although this is not surprising as PIXE is only dependent on the number of protons in the target sample.

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It was determined that PIXE is not a reliable method of gathering data on the thickness of thin foil target. Further RBS experiments must be conducted in order to fully characterize each of the remaining 42 targets. However, the 14 RBS tested targets did provide enough data points to move forward in the main experiment. The Tellurium targets were irradiated in order to better determine the half lives of the unstable isotopes of Tellurium. These results can then be used to improve models of stellar nucleosynthesis and the p-process.

This work was supported by the National Science Foundation under contract number PHY0822648.

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1. E. M. Burbidge, G. R. Burbidge, W. A. Fowler, Fred Hoyle (1957). "Synthesis of the Elements in Stars". Reviews of Modern Physics 29 (4): 547–650

2. A. G. W. Cameron: Nuclear Reactions in Stars and Nucleogenesis. In:

Publications of the Astronomical Society of the Pacific, Vol. 69, 1957, p. 201-222

3. D. Bohm, Quantum Theory, Prentice-Hall, New York, 1951, Chapter 4

4. W. D. Ehmann, D. E. Vance, Radiochemistry and Nuclear Methods of Analysis, Wiley-Interscience, Toronto, 1991, p. 347-355

5. W. D. Ehmann, D. E. Vance, Radiochemistry and Nuclear Methods of Analysis, Wiley-Interscience, Toronto, 1991, p. 355-363

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Project GRAND is a cosmic ray experiment located north of the University of Notre Dame. It consists of 64 huts of proportional wire chambers that make up a 100 m x 100 m detector array. These detectors identify muons produced when cosmic rays reach the Earth’s upper atmosphere. The muon counting rates remain fairly constant, but this is not the case when there are solar flares and coronal mass ejections. An interesting occurrence called a Forbush Decrease was seen in February 2011 in data from the Oulu Neutron Monitor, a lower energy experiment than GRAND. This was caused by a cloud of charged particles and the magnetic fields moving toward Earth which deflect the path of charged cosmic rays that come from outside our solar system and bombard the Earth’s upper atmosphere. GRAND’s data was examined and a similar decrease at the same time as Oulu was found along with additional phenomenon not seen by Oulu, which usually sees more structure than GRAND due to its ability to detect lower energy particles more easily affected by solar activity. Several steps, such as looking at pressure correction and good hut corrections, have been taken to correct GRAND’s data for non-physics effects. An upper air temperature correction remains to be done as well.

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Project GRAND (Gamma Ray Astrophysics at Notre Dame) is a broad air shower array of proportional wire chambers (PWC) that detect secondary muons. Located directly north of the campus of the University of Notre Dame at 41.7°N and 86.2°W at an elevation of 220 m above sea level, the experiment is made up of 64 stations that are arranged in an 8 x 8 grid. Each station contains four PWC pairs with a detection area of 1.29 m2, giving the experiment a total muon detecting yield of 82 m2 [1]. Project GRAND runs two experiments simultaneously. One identifies single tracks of individual muons, while the other identifies extensive air showers, which are when multiple stations are hit in a very small period of time [1]. The experiment is most sensitive to primary energies of greater than 10 GeV with a peak sensitivity

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energy particles from cosmic rays.



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