All research projects are supported by the National Science Foundation

Spring 2004

MIDN Christoper (Jack) Hathaway `04

In the spring of 2004, Jack did some incredible work to integrate the Naval Academy's new Clover germanium detector with the nuclear physics lab. I will give a summary of his work, put you can download his entertaining PowerPoint presentation here. Pictures of the Clover detector are shown below.

The Clover has four separate germanium crystals within the casing which detect gamma rays. An advantage of the Clover over other germanium detectors is that contains four times as much germanium (which makes it more efficient for detecting gamma rays), but because the crystals are separated from each other, the detector does not lose its resolution. In order to maximize the results from the Clover, Jack had to write some code to perform an Add-back routine. When a high-energy gamma ray enters one crystal, it may not deposit all of its energy into that crystal. Instead a lower energy gamma ray may leave and enter a neighboring crystal. This process (called Compton scattering) is demonstrated here (thanks to Dave Campbell!). If this occurs, we can add back the two energies to get the correct energy of the original gamma ray. After a few wrong starts, Jack successfully did this and then checked that the high-energy efficiency working in this Add-back mode was indeed better than if we used the Clover as four separate detectors. Jack used ⁵⁶Co, ⁶⁰Co, and ¹⁵²Eu sources to perform this experiment, and he was successful.

UPDATE: Jack is currently learning how to fly F-18’s in California.

Fall 2004

MIDN Billy Mohr `06

In the fall of 2004, Billy got to go on a field trip to Argonne National Lab and use the best gamma-ray spectrometer in the world – Gammasphere. A picture of Billy in front of Gammasphere is shown. The experiment produced high-spin states of ¹⁷¹Ta, where we were looking for evidence of triaxial deformation. After learning how to operate the data acquisition system and the basics of scanning data stored on 8 mm tapes, Billy returned to the Academy and learned how to process a large data set (~250 GB worth of information). Billy determined the calibration and efficiency of Gammasphere for our experiment. He then determined the average recoil velocity of the ¹⁷¹Ta nuclei through analysis of the observed Doppler shift in the gamma rays. Once this was accomplished, the data was sorted into a database, from which a coincidence cube was extracted. Billy analyzed this cube and added substantial amounts of new gamma rays in the level scheme of ¹⁷¹Ta. While doing this, he learned about the shell structure of the nucleus, the pairing between nucleons, and the electromagnetic characteristics of gamma rays. Billy even began an analysis of the rotational alignment properties of the different decay sequences. All of his work was put together in a nice PowerPoint file which you can download. This work was presented at the National Conference of Undergraduate Research in April of 2005.

UPDATE: Billy earned his Master’s degree in nuclear physics from the University of North Carolina and will begin flight school soon.

Spring 2005

Midn Timothy Fitgerald `06

In the spring of 2005, Tim picked up on the Clover project where Jack Hathaway left off. We want to determine the electromagnetic character of gamma rays emitted by excited nuclei with the Clover. First, Tim had to construct a new target chamber for use of the Clover with the U.S. Naval Academy tandem accelerator. He successfully completed this project in about a month and picture of him installing the chamber is shown. Next we ran an experiment of bombarding protons on magnesium oxide foil. Magnesium emits a high-energy gamma ray that is an electric quadrupole. Tim is edited the code developed by Jack Hathaway to sort the Compton scattered data into two spectra: one spectrum has gamma rays that scattered parallel to the reaction plane, the other has gamma rays that scattered perpendicularly to the plane. Whether the gamma ray emitted by magnesium favors scattering parallel or perpendicular will signify if it is dominated by an electric or magnetic character. Tim gave a 10 minute presentation to the faculty at the end of the semester.

Fall 2005

Midn Timothy Fitzgerald and Billy Mohr

Unfortunately, the accelerator at Yale University had some major problems, so Tim and I were not able to perform an experiment on ¹⁶⁷Ta that we had planned. However, I had just participated in a Gammasphere experiment with my colleagues at Florida State University in the summer. High-spin states were populated in 161Tm to search for evidence of a wobbling sequence which would indicate stable triaxial deformation in the nucleus. While the FSU folks took the lead on ¹⁶¹Tm, there was a good amount of data on the odd-odd nucleus ¹⁶⁰Tm. So, Tim got a chance to sort the data into a Blue database and build his own gamma-gamma-gamma coincidence cube. This took some time as we battled the extra information in the data stream resulting from the use of a spinning target wheel during the experiment. Then it was time to start setting gates and Tim quickly extended the two known bands to much higher spins (from 26 to 44). In addition, two new bands were found and linked into one of the previously established bands. At this point, the semester ran out and Tim gave a 20 minute presentation to the faculty. He also agreed to come back the following semester to interpret the configurations of these bands.

Billy decided to tackle a new project that would contribute to the search for wobbling bands in the A = 165 region. A wobbling band decays through ΔI = 1 transitions that are dominated by an E2 component (rather than the usual M1 character). As discussed above in Tim Fitzgerald’s first project, the electromagnetic character of a gamma ray can be determined by a polarization measurement. Although Gammasphere does not contain clover detectors, many of the detectors have been electrically split such that one may determine whether the gamma ray hit on the left or right side of the detector. This was done primarily to well with Doppler correction to effectively reduce the opening angle of the detector. This information is written as a low-resolution signal called the “side channel” energy. We can use this side channel information to also do a poor-man’s polarization measurement. If both sides of the detector fired, this likely means the gamma ray Compton scattered parallel to the plane. If only one side fired, either the transition was completely absorbed in the detector when it first hit, or it Compton scattered perpendicular to the plane. Although this method is not nearly as sensitive as a clover, we may gain some idea of the electromagnetic character of a given transition. Billy was able to lay the ground work for codes that use a double gate and project the resultant side channel information. However, as his research course was only a one hour course, we were not able to determine whether this is a reliable polarization method. More work will be performed in the future.

Spring 2006

Midn Timothy Fitzgerald and A.J. Storrs (`07)

Tim continued to look through the data for new bands in ¹⁶⁰Tm. Although he found many candidates, all of these sequences were later associated with other nuclei formed in the reaction (such as ^158,159Er). We then focused on interpreting the bands that Tim found last semester. Tim learned about the Nilsson model and determined possible proton and neutron orbitals near the Fermi surface for ¹⁶⁰Tm. He determined the h_11/2 and g_7/2 protons were likely candidates along with i_13/2 and h_9/2 neutrons. Next, he learned how to calculate aligned angular momentum and made plots of these values versus rotational frequency for the bands in an Excel spreadsheet. This gave us our first clues to which orbitals were responsible for the bands. For instance, the first i_13/2 neutron crossing was blocked in one of the bands, which indicates this neutron is likely involved in the configuration. Tim then determined the branching ratio for each of the bands, and then calculated the B(M1)/B(E2) ratios. These experimental values were then compared with theoretical values to positively identify the two strongest bands as having the h_11/2 proton coupled to the i_13/2 and h_9/2 neutrons, respectively. Once again, Tim presented his research to the faculty at the end of the semester.

A.J. Storrs was responsible to determine whether any new information on the nucleus ¹⁵⁹Tm could be mined out of the same data Tim Fitzgerald was working on. This nucleus is part of the famous chain of N = 90 nuclei that lie between vibrational and well-deformed nuclei. A.J. was able to find the two strongest bands in ¹⁵⁹Tm, but could not extend them to higher spin. There were not enough statistics to allow for new information. However, he too learned how to calculate aligned angular momentum and determine the likely configurations of the bands based on these plots.

Fall 2006

Midn Alexander (Sandy) Ludington `07 – Bowman Scholar

In August 2006, Sandy and I took a trip to Yale University to investigate the products of the ⁵¹V + ¹²⁰Sn reaction. This reaction is of interest as one of its main products is ¹⁶⁷Ta. The isotones of ¹⁶⁷Ta (¹⁶³Tm and ¹⁶⁵Lu) have strongly deformed bands which are believed to be triaxial. However, only ¹⁶⁵Lu shows evidence for wobbling bands. Stefan Frauendorf (a nuclear theorist at Notre Dame) has postulated that the density of proton levels is a key factor as to whether wobbling will be observed or not. To test his theory, ¹⁶⁷Ta must be investigated to look for signs of wobbling. However, the reaction leading to high-spins states in this nucleus had never been run before. So the optimal energy for producing ¹⁶⁷Ta was not known. Therefore, Sandy and I performed an excitation function of the reaction with the help of the Yale professors, plus FSU and Yale grad students.

The reaction was run at three different beam energies (228, 235, and 240 MeV) and nine clover detectors (see picture of Sandy standing in front of the array) were used to measure the gamma rays emitted by the excited nuclei. Sandy brought the data back to the Academy and sorted through over 3400 spectra to check for gain shifts. He corrected these shifts, performed an energy calibration and determined the energy dependent efficiency for the detectors, and calculated the proper Doppler shift. Sandy then built coincidence matrices for the data taken at the three different beam energies. By measuring the intensity of the gamma rays produced by each nucleus, we could determine the relative production of each at the different energies. These relative abundances were then compared with calculations from a program called PACE. It was found that good agreement between experiment and theory achieved when the calculated values were shifted by 7 MeV. We now know that 235 MeV appears to be the optimum beam energy for a proposed experiment at Gammasphere. Sandy delivered a fantastic talk at the end of the semester and his powerpoint presentation can be downloaded here.

Spring 2007

Midn Alexander (Sandy) Ludington `07 – Bowman Scholar

Sandy continued his research by building the full level scheme of ¹⁶⁷Ta. In fact, he was able to identify all the previously known transitions and even add a few more. This is impressive as we only used nine detectors whereas the previous study used 29 detectors. One transition was of particular interest as it was previously suggested to be a linking transition from the h_9/2 band to the d_3/2 band. Such a transition would have an E1 character, and the earlier study had evidence that it was indeed a dipole; however, their array could determine the electromagnetic character. The Yale array, which uses Clover detectors, allows for polarization analysis which can determine whether gamma rays are electric or magnetic. Therefore, Sandy first constructed a DCO (directional correlation of oriented states) matrix to verify the dipole character of the linking transition. Following this, he spent six weeks coding to properly perform polarization analysis. This took a lot debugging and patience, but he was able to sort the data into perpendicular and parallel scattering. Using the asymmetry ratio , Sandy tested his spectra with known electric and magnetic transitions. As seen in the figure electric transitions have positive asymmetry ratios, while magnetic transitions have negative ratios. Sandy then determined that the linking transition had a positive ratio, and is thus an electric dipole. Sandy also delivered a talk at the National Conference for Undergraduate Research (NCUR 2007) concerning his fall semester research.

Sandy earned a Master’s degree in Nuclear Engineering from the Massachusetts Institute of Technology. He is now a submarine officer.

Fall 2007 & Spring 2008

Midn Evan Seyfried `08 – Bowman Scholar

Evan was given Gammasphere data resulting from a reaction where a beam of ³⁰Si bombarded a ¹⁸¹Ta target. This creates a compound nucleus of ²¹¹Fr which would lead to neutron-deficient, trans-lead nuclei. These nuclei are traditionally under-studied due to the fact 98% of the time, the ²¹¹Fr fissions, which leads to a huge background of gamma rays from the fission products. However, these data were taken with the Washington University HERCULES detector working in conjunction with Gammasphere. HERCULES measures the energy and time-of-flight of the recoils following the reaction. The fission products have significantly different characteristics than the fusion products, therefore, HERCULES can identify the fusion products and their associated gamma rays. Using the gamma-ray data, Evan built the first ever high-spin level schemes for ^205,206,207Fr and ²⁰⁴At. In particular, he found a sequence of low-energy transitions in both the odd-odd nuclei ²⁰⁴At and ²⁰⁶Fr. After doing some reading of papers focusing on nearby nuclei, Evan determined that these were likely “Shears Bands.” As described in his American Physical Society April Meeting talk, Shears Bands result from the closing of the angular momenta vectors from the protons and neutrons onto the total momentum vector. This produces a decay similar to classic nuclear rotation, even though the nucleus is not actually rotating. As stated above, Evan and I went out to St. Louis where he presented his data in a session with graduate student, post-docs, and professor from other universities and national laboratories. A picture of Evan at Gammasphere is shown above.

Evan is now a submarine officer.

Fall 2008 & Spring 2009

Midn Ryan Pifer `09 – Bowman Scholar

Ryan picked up from where Sandy finished for the ¹⁶⁷Ta project. We performed an experiment at Gammasphere to produce high-spin state in ¹⁶⁷Ta so that we could look for evidence of wobbling in this nucleus. The most recent theory suggested that wobbling would NOT likely be observed, but that’s why we do experiments. Ryan sorted the data, and using a coincidence hypercube he greatly extended the level scheme for ¹⁶⁷Ta. Most importantly, Ryan pushed the i_13/2 band to higher spin and found a sequence of transitions feeding into it that had the key signatures of a wobbling band. However, there were some peculiarities that were not quite consistent with a wobbler. With some help from Stefan Frauendorf (Notre Dame), it is our belief that those abnormalities are a result of the i_13/2 structure evolving from a symmetric shape to an asymmetric shape as spin increases. Ryan made a great poster which can be seen here.

Ryan is earning his Master’s degree at the Naval Postgraduate School in Monterrey, CA.

Spring 2010

Midn Eowyn Pedicini `11

Eowyn focused on learning basic nuclear structure models (such as the shell and deformed shell models) as she was preparing to apply for the Trident Scholar program. She also independently learned about asymmetric nuclei and wobbling such that she could present the reasons for our experiment on ^169,171Re. These nuclei have two more protons that the Ta nuclei that previous students explored, but have a neutron number near 94, where wobbling is likely to be centered. We performed the experiments in March at Gammasphere and the data are now at the Academy waiting to be analyzed. Eowyn did a fantastic job with her proposal and her defense of the proposal in front of the Trident committee. She was indeed selected as a Trident and Bowman Scholar! Eowyn will complete her research in the 2011 academic year.