REU Projects for 2000
Cryogenic Polarized Targets for Investigations of the Nucleus
Advisor: David HaaseThe forces among nucleons depend strongly on the relative spin orientations of the particles. These forces can be studied in experiments that use a beam of polarized neutrons (that is the majority of the magnetic moments are aligned in the same direction) and a target in which the nuclei are similarly aligned. The Polarized Target Group at TUNL produces such targets using low temperatures (T < 1 kelvin), high magnetic fields (2.5 tesla) and some black magic with microwaves. We are looking for an REU student that would like to spend long hours taking data, help in the testing of a new target refrigerator (0.2 kelvin) and test drive a new 2.5 tesla superconducting magnet system. At the end of the summer the student will have learned a lot about nuclear and solid state physics and much practical cryogenics which can be applied in many fields of science and technology.
Stellar Helium Burning
Advisor: C. BruneThe cross section for the reaction 12C + He --> 16O + gamma is crucial for the understanding of many process in massive stars. It greatly affects the ratio of 12C to 16O produced by nucleosynthesis in the star, production of heavier elements in the star, as well as the final fate of the star (i.e., black hole or neutron star). In fact, the cross section of this reaction determines (in part) the ratio of carbon to oxygen on the earth! Unfortunately, the cross section of this reaction is far too small to be measured directly at the energies needed for stellar nucleosynthesis calculations.
We are developing indirect methods for studying this reaction, the low-energy transfer reactions 12C(6Li,d)16O and 12C(7Li,t)16O. Measurements of these reactions provide information which is very useful for determining the 12C+4He --> 16O + gamma cross section at very low energies.
The summer project would involve the installation of a control system for our new ion source, which allows us to accelerate the carbon and lithium beams needed for our measurements. The student would also study the kinematics and detector configurations in order to determine the best experimental arrangement for the measurements. There will be opportunities to participate in several experiments (hopefully including the aforementioned experiment) allowing the student to see nuclear physics in action with "beam on target" and with the associated detectors and data acquisition system.
Few Nucleon Studies Projects
Advisors: Ron Pedroni and Benjamin CroweWe have two main projects that can involve up to three students. The first is the analysis of nd-breakup data, (the nd-breakup reaction uses neutron bombardment to break deuterium nuclei into protons and neutrons) and the second is an accurate determination of ann, the neutron-neutron scattering length. Both of these projects involve increasing our understanding of the nucleon-nucleon (NN) interaction dominated by the nuclear strong force. Although the NN interaction has been studied for over 50 years, theoretical models as yet do not accurately describe experimental results. With better computers, the theorists are able to work with more complicated models and it is the role of the experimenter to provide data to test these new ideas.
Study of nd-breakup data
A large body of kinematically complete nd-breakup data was measured several years ago at TUNL with a 13 MeV neutron beam as part of the thesis project of Ross Setze. Comparison of these data with theory has provided mixed results, with some of the data agreeing with the theory and other data confirming the difference between previous measurements and calculations. Additional measurements have been done at other neutron energies to further explore this reaction. A student will be involved in the analysis of these data. In addition, the student will participate in the experiments carried out by the group during the summer.
Measurement of the neutron-neutron scattering length
An important observable for studying the NN interaction is the scattering length, which is a measure of the strength of the interaction. Unfortunately, the neutron-neutron scattering length cannot be measured directly because we cannot create a simple neutron target. Measurements therefore must involve nuclear systems that complicate the situation. We intend to accurately determine ann to resolve differences in the various published values.
We are developing a method to measure ann at TUNL through the reaction, n + d --> n + n + p, and measuring the emitted proton in the Enge spectrometer, a detection system that separates particle trajectories by momentum for particle identification and energy determination. In previous years, several experiments were carried out which showed that the method will work, though improvements are needed for a successful measurement.
Modifications to the detector are currently being developed by the nuclear astrophysics group from UNC, and will be available for implementation on our experiments this summer. In every measurement, the most important procedure is to separate the data of interest from the background signals. In this case, there is a high background from neutron induced charged particle reactions taking place with the various materials in the Enge spectrometer system. Our goal is to find appropriate ways to minimize the background during the summer data-taking runs. Work also needs to be done on the Monte Carlo simulation code used in analyzing the ann data.
Radiative Capture Group Projects
Advisors: Henry Weller, John Kelley, and Ronald TilleyThe Radiative Capture Group at TUNL consists of three faculty members, two post-doctoral researchers and a number of graduate students and visiting researchers. The group is involved in research using ion beams at TUNL and gamma-ray beams at the Duke Free Electron Laser Laboratory (DFELL).
Development of an apparatus to probe the internal structure of the nucleon.
We are presently pursuing a measurement of the electric and magnetic polarizabilities of the proton via measurements of high-energy gamma-rays that are Compton scattered from protons in a cryogenic target at the DFELL. The polarizabilities provide fundamental information on the internal structure of the nucleons (made of quarks and gluons) and the use of polarized gamma-rays will permit a more accurate determination of the polarizabilities than has been previously possible. The results will be used to test the implications of Quantum Chromodynamics (QCD), the most fundamental theory of the nuclear force.
A student would be involved in the assembly and testing of plastic anti-coincidence shield detectors. They will surround four large NaI main detectors and will be used to reject unwanted cosmic-ray induced background events. The student will also participate in the installation of the detector support assembly, installation and calibration of the NaI detectors, and the development of Monte Carlo simulations that will estimate the count rate of the experiment and the primary sources of background.
Experiments with Deuterium to Understand Nuclear Forces and Big Bang Nucleosynthesis
The Radiative Capture Group at TUNL is also studying the deuterium system from the perspectives of radiative capture (formation of deuterium from a free proton and neutron) and the inverse process: photodisintegration (gamma-ray excitation and breakup of the deuteron into a neutron and a proton). As the simplest nuclear system, the deuteron can give significant insight into nuclear physics processes via studies of the properties of the nuclear strong force and measurements of fundamental sum rules. In addition, a detailed knowledge of the system at energies corresponding to stellar temperatures on the order of 50 million degrees has important implications for our understanding of stellar evolution and the formation of heavy nuclei during the period immediately following the Big Bang.
A student would participate in measurements of deuterium photodisintegration at the DFELL using linearly polarized gamma-rays, and would help to develop Monte Carlo simulations that will be used to determine the ratio of the electric to the magnetic strength involved in the photodisintegration process. In addition, the student would participate in developing a 200-500 keV pulsed polarized neutron beam at TUNL, along with the target and detector set-up, for use in the complementary neutron-proton radiative capture experiment.
How chaotic are nuclei? Evaluation of the nuclide 28Si as a candidate for complete spectroscopy.
Advisors: Gary MitchellThe motivation of this project is to determine the characteristics of the level statistics and the transition strength distributions (i.e., the degree of chaoticity) in this nuclide. Over a wide energy range, 28Si has energy level states with the quantum property of zero isospin; T = 0 states. (In analogy with the two spin-states of an electron, "spin-up" and "spin-down", isospin is associated with the two nucleon-states, where "isospin up" is a proton and "isospin down" is a neutron.) This nucleus would be particularly interesting to compare with the striking results for 26Al and 30P where states with different total isospin coexist at all energies. In studies of these collections of mixed-isospin states, the statistical distributions were dominated by the breaking of the approximate symmetry isospin.
An REU student would be involved in the first step of this project which is to evaluate the present status of information on 28Si and to devise a game plan for future experiments. The student will learn about nuclear level schemes and quantum assignments, and the experiments to measure the properties of a nucleus. In addition, the student would participate in the ongoing experimental studies at the High Resolustion Laboratory.
A study of pulse processing in germanium based detectors to search for exotic particles.
Advisors: Art Champagne and Christian IlliadisCurrent cosmological theories predict that the universe contains mass, or heavy particles that we have not yet identified. What is this "dark matter" made of? There are a number of candidates postulated to fill this role, and the experimenter "votes" for or against a proposed particle by performing experiments to detect these particles directly or indirectly. Before we can build an experiment designed to detect non-baryonic dark matter, (particles not composed of quarks) we have to learn how to distinguish between real events from the particles of interest and background events. One way to do this is to look at the pulse shapes produced in a detector for different kinds of interactions. This pulse-shape analysis will have applications to nuclear astrophysics experiments as well.
The student will learn about particle detectors, and particle interactions with matter. We hope to gain enough experience with digitized signals from the germanium detector so that we can develop some methods for rejecting unwanted events. Results of this study will provide vital information on which the experiment to measure non-baryonic dark matter will be designed.
Optimizing the computer control system for LENA, the Laboratory for Experimental Nuclear Astrophysics
Advisors: Art Champagne and Christian IlliadisThe LENA system is currently under the last phase of construction and is nearing the testing stage. The laboratory uses 2 low-energy accelerators coupled to a single target and is designed for the measurements of reaction cross-sections (an indicator of the probability or strength of an interaction) at the low energies of astrophysical interest. Both accelerators and the beam-transport system are controlled by a PC running LabView. LabVIEW is widely used in both industry and academia as a program development application that relies on a graphical programming language. Since LabVIEW is based on graphic symbols and icons, little previous experience in programming is required to become skillful in its use.
Although the basic framework of LabVIEW modules exists for LENA, we need to refine the control system as the accelerators are brought on line. The REU student would be expected to learn how to run the LENA accelerators, become familiar with the systems, and then work towards optimizing the control system. In addition, the student will become familiar in the construction and implementation of LabVIEW modules, or "Virtual Instruments".
Development of new plasma jet ionizer systems for spin polarized beams
Advisors: Sergio Lemaitre and Tom CleggUsing a testbench installed in the Low Energy Nuclear Astrophysics (LENA) lab at TUNL, we are developing and testing new hardware to produce an intense plasma jet of hydrogen or deuterium ions of very low energy, <1 eV. This would not be possible (because the ion beam would blow up under influence of its own internal space charge) except for the fact that these ions in the plasma are accompanied by an equal number of electrons with opposite charge.
We seek to make the most intense plasma jet possible of these ions, and then allow it to drift along an axial magnetic field over a distance of approximately 30 cm. Our intent is to place within this drift length a long cylindrical cell within which we ultimately may store 'polarized' atoms of hydrogen or deuterium, e.g atoms prepared with all of their nuclear spins aligned similarly with respect to the magnetic field.
The idea is that this cell, filled with 'spin-polarized' ions would become a target for the drifting ions in the plasma. If polarized H atoms are stored, and if unpolarized D+ ions reside in the plasma jet, then charge-exchange reactions will occur to produce polarized H+ ions, leaving behind unpolarized D atoms. The probablity for this ionization process at these sub-eV ion energies is one of the very largest observed in nature for such charge-exchange reactions.
We are currently studying the primary plasma jet. We expect in the coming summer to be installing additional systems which will allow us to fill the cell with unpolarized atoms for our initial tests. Our first goal is to measure experimentally the efficiency for the charge-exchange reaction, thereby assuring ourselves that if we later fill the cell with 'spin-polarized' atoms, we would be able to obtain the expected output polarized ion current. Estimates suggest that we may be able to improve by as much as a factor of 10 on the currents now available for experiments at TUNL, thereby extending the range of measurements that can be done at this lab.
Projects students can participate on this summer include:
- Helping install and debug systems to make the unpolarized atoms which will be fed to the storage cell;
- Fabricating and installing electronics systems needed, e.g. interlocks, computer interfacing, and
- Developing simple software packages which will allow computer monitoring and control of new experimental systems.