REU Projects for 2002
Cryogenic Polarized Targets for Investigations of the Nucleus
Advisors: David Haase and Diana MarkoffThe forces between 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 < 0.5 kelvin), high magnetic fields (2.5 tesla) and some black magic with microwaves.
We are looking for two REU students that would like to spend long hours taking data for a polarized-neutron-on-polarized-deuteron measurement, and help in the construction of a new dilution refrigerator for future measurments of polarized gammas colliding with polarized deuterons. By the end of the summer the students will have learned a lot about nuclear and solid state physics and will have gained much practical cryogenics experience which can be applied in many fields of science and technology.
Evaluation of Nuclear Level Density Measurements and Calculations
Advisor: Gary MitchellThe development of reliable methods for determining reaction rates of all types of nuclei is a large scale project involving international collaborations. Nuclear level densities are important for estimates of nuclear reaction rates in general, and for nucleosynthesis (nuclear reactions involved in the creation of the elements) and reactions involving unstable nuclei in particular. Conventional calculations of level densities are dependent on adjustable parameters. Although these standard approaches work fairly well for a limited subset of nuclei, a more fundamental understanding is required to describe the level densities of many unstable nuclei. Study of these unstable nuclei has recently become important for nuclear astrophysics and radioactive beams experiments. New calculational methods, for example a Shell Model Monte Carlo technique, make explicit predictions concerning level densities, but the data are not precise enough to properly evaluate these methods.
Therefore, in addition to new measurements of level densities, improved analysis methods are very important. We have developed a new method to evaluate level densities. After successfully applying this method to our proton data, we would like to re-examine a carefully selected fraction of the extensive body of neutron data. We plan to analyze the data using two independent methods, the standard one previously used and our new technique. By combining and comparing the two methods, we hope to improve the analysis significantly and obtain more reliable results from existing data without having to re-perform a vast number of experiments.
We invite one student with computer skills to work with us during this summer. The project will be to perform these analyses in collaboration with Professor Mitchell and a graduate student. We anticipate completion of the project by the end of the summer and publication of the results shortly thereafter. While working with us, a student will learn how to apply numerical methods learned in a typical computational class to actual nuclear physics problems. Also, students will acquire experience using conventional statistical (error) analysis that is crucial for a successful physicist. In addition to that, since we will prepare publications in the second half of the summer, a student will obtain hands-on experience with preparing a physics literature paper. It will be an excellent opportunity to improve one's scientific writing skills.
Neutron-Deuteron Analyzing Power at Low Energies
Advisor: Werner TornowThe elastic scattering of polarized nucleons from deuterons at low energies remains the most puzzling and unsolved phenomenon in few-nucleon physics. There remains the longstanding huge discrepancy between data and calculations for the so-called analyzing power, a spin-dependent observable in nucleon-nucleon scattering. In addition, the discrepancy between data and theoretical predictions is different for proton-deuteron scattering and neutron-deuteron scattering at very low energies. This recently discovered new twist to the so-called three-nucleon "analyzing power puzzle" is the subject of this REU project.
In order to help explain this new twist, neutron-deuteron analyzing power data were measured at TUNL in January 2002 at an incident neutron energy of 1.2 MeV. The previous lowest energy data were obtained at TUNL with a neutron energy of 1.9 MeV.
The REU student will analyze the new data at 1.2 MeV using existing computer programs. In addition, the student will be involved in the preparation for a neutron-deuteron analyzing power measurement at an even lower incident neutron energy (0.6 MeV). If scheduling allows, the student will participate in a test experiment during the summer.
Apparatus for Gamma-Ray Induced Reactions with Charged Particle Detection for HIGS
Advisors: Werner Tornow and Gary WeiselThe High-Intensity Gamma-ray Source (HIGS) at the Duke Free-Electron Laser Laboratory (DFELL) provides a worldwide unique source of intense, polarized and monoenergetic gamma rays. At the present time, a detection system capable of detecting charged particles, like protons or deuterons, from gamma-ray induced reactions does not exist at the DFELL. The REU student will reactivate and reconfigure an existing apparatus and data acquisition system originally designed and built at TUNL for alpha-particle detection using incident protons as a probe. Currently, the detection system consists of four large area Si-strip detectors. After successful tests at TUNL, the reconfigured apparatus will be installed at the DFELL for a test experiment. The target nucleus for this test has not been determined yet and may depend on the gamma-ray sensitivity of the Si-strip detectors.
Muon Induced Background in HPGe Detectors
Advisors: Hugon Karwowski and Albert YoungHigh-Purity Germanium-76 (HPGe) detectors have been the favorite detectors for use in the most sensitive searches for neutrinoless double-beta decay (0v2b). In beta-decay, a radioactive nucleus emits an electron and anti-neutrino or a positron and a neutrino. In the commonly observed double-beta decay, two neutrinos are emitted. So far, the double-beta decay in the case where no neutrinos are emitted has not been discovered. If observed, the 0v2b decay provides a means of determining the mass of the electron flavor anti-neutrino. While the atmospheric and solar neutrino experiments clearly show that neutrinos have mass, they cannot determine the masses of the neutrinos involved. Therefore, the 0v2b decay plays a crucial role.
In order to reduce the cosmic-ray induced background, 0v2b experiments must take place deep underground in mines. Although reduced by many orders of magnitude, even deep underground, the cosmic-ray muons contribute to the background. Therefore, this background must be studied carefully in order to identify (and subtract) it in 0v2b decay experiments using HPGe-76 detectors.
The REU student will set-up an experiment at TUNL with a simplified geometry of detectors to measure the background radiation produced by muons interacting with matter in the vicinity of a HPGe detector. The student will track muons at ground level using plastic scintillator detectors and record coincident events seen by the HPGe detector.
Measurements to Understand Neutron Events in the KamLAND Outer Detector
Advisors: Diana Markoff, Hugon Karwowski, Ryan Rohm, and Albert YoungThe Kamioka Liquid-scintillator Anti-Neutrino Detector, KamLAND, is a reactor neutrino experiment located in the Japanese alps. As part of the Japanese-American collaboration, the TUNL group is responsible for the Outer Detector region that is designed to detect muons so that we can distinguish possible muon-induced background events in the inner detector region. The Outer Detector consists of 225 large photomultiplier tubes (PMT) which detect Cerenkov light produced by muons as they pass through the surrounding water. The products of muon induced reactions, namely neutrons created in muon-induced spallation events, are a source of backgrounds that can mimic our neutrino signal of interest. It is therefore important that we can identify these muon spallation events and their products. To do this, we must be sure of our simulation codes and the parameters used in these calculations.
The REU student will be responsible for performing measurements of neutron events using a water-tank located at TUNL and two 20-inch PMT detectors like the ones mounted in the outer detector at KamLAND. This simplified outer-detector like environment will enable us to study the response of PMTs to neutron events in the water. In addition to the measurements, the student will be expected to run the GEANT Monte-Carlo code set up to simulate the test water-tank facility in order to compare the data with calculations.
The Supersonic Gas Jet Development
Advisors: Hugon Karwowski and Ed LudwigIn nuclear scattering experiments, a supersonic gas jet target is an exceptionally good alternative to overcome the problems of energy loss and backgrounds encountered while using solid targets or gas cells. Thus high precision measurements of cross sections and polarization observables in few-body systems at low energies are possible.
At TUNL we have built such a target. Higher target thickness (i.e higher gas flow) is required to get the desired statistical accuracy of the measurements. Thus a gas jet recirculation system throughout the target assembly is necessary. We are currently building such a system by which the exhaust gas will be recompressed and filtered.
The REU student will be involved mostly in the assembly and design of the control system for the recirculator. He/she will also gain experience in operating the TUNL gas jet target and learn relevant few-nucleon physics.
Position Profiling, Monitoring, and Characterizing a γ-Ray Beam at HIγS
Advisors: Henry Weller, RIchard Prior and the Low Energy Capture groupThe High Intensity Gamma-Ray Source (HIγS) at the Duke Free Electron Laser Laboratory produces nearly mono-energetic gamma-ray beams (photons) at energies from 2 to 225 MeV. This system produces intense gamma-rays of the order of 108 γ/sec (almost 1000 times more intense than at other facilities) that are 100% polarized. The polarized photon beam is important for experiments that cover a wide domain of interests in nuclear physics and in particular, the gamma rays from HIγS are already being used in a number of nuclear physics experiments.
In many experiments it is important to know the beam intensity (i.e., the number of gamma rays per second in the beam), the beam polarization, and the position profile of the beam that is hitting the target. In order to know the characteristics of the incoming gamma-ray beam, measurements are made with specifically designed devices that interact with the beam. Two REU projects are available that involve the design, building and testing of beam monitoring devices.
Position Profile Monitor (Project 1)
At the HIγS facility, the beam energies are reached by Laser Compton Back Scattering photons from relativistic electrons inside an optical cavity 53 m long. The gamma-ray beam is delivered ~70 m away from the optical cavity to the experimental area (called the gamma-vault) via beam pipe. Once the beam emerges in the gamma-vault, the position (horizontal and vertical) of the beam must be known in order to place the targets and related detectors at proper locations relative to the beam. Since the gamma-ray wavelengths are far from the visible region, the position of the beam must be inferred via interactions with some other position sensitive device.
Once such device is a Cathode-Strip multi-wire proportional Counter (CSC). The CSCs are composed of stripped cathode foils and anode wires placed at an angle with respect to one another and surrounded by a gas mixture (typically Argon and Isobutane). The anodes and cathodes are placed at some small distance, (e.g., 1/8 of an inch apart) and the anodes are held at a high voltage of about 3000 V. When a charged particle traverses the assembly, it ionizes the gas mixture and, due to the voltage on the anodes, the electrons and ions drift apart and cause further ionizations until the negative charge is collected at the anodes. The charge collection around the anodes is imaged on the stripped cathode foils and each strip is read out to find the amount of charge induced on it. Depending on where the charge particle traversed, a different set of cathode strips receive induced charge. Knowing the position of the cathode strips makes it possible to determine the position of the charged particle as it traverses the CSC.
The charged particle CSC is being proposed as a means for tracking the electrons and positrons produced by the gamma-ray beam via pair-production mechanism. At our gamma-ray energies, the pairs produced are very forward boosted due to the Lorentz effect. Therefore, measuring the position of the pairs is a direct measurement of the gamma-ray beam position.
The REU project includes the design, construction, and testing of the CSC device using the gamma-ray beam. The project will provide a familiarity with calculations of electric fields and potentials in a complex system and of induced charge distributions. Also, the analysis of the data collected during testing requires development of a pattern recognition code, hence providing an opportunity to learn analysis techniques in experimental physics.
Beam Intensity and Polarization Monitors (Project 2)
Currently, the intensity of the gamma-ray beam is measured by placing a gamma ray detector directly in the beam and counting the photons. The intensity of the available HIγS beam has been increasing as the facility develops and is now too large to measure directly because the count rate is greater than that which the detector can handle.
A new system has been designed to monitor the beam intensity. Basically, it consists of a thin metal foil, placed in the beam so that the beam passes through it. A small fraction of the beam will interact with the foil, producing recoiling electrons or electron-positron pairs. A thin plastic scintillation detector immediately behind the foil will detect these particles. The number of detected particles will be proportional to the total number of gamma rays passing through the foil. There may be other electrons and positrons in the vicinity of the beam, produced by collisions of the gamma rays with materials upstream from the foil. To reduce the background due to these particles passing through the foil and detector, a second thin scintillator will be placed just before the foil and operated so as to veto the counts in the second scintillator that are due to the background particles.
A second device is being built to determine the plane of polarization of the gamma-ray beam. This plane is nominally horizontal, but it may be rotated out of that plane by possible polarization-rotating effects in the laser mirrors. The polarimeter will consist of 8 detectors arranged symmetrically about the beam axis with a small target cylinder in the beam. Using the sensitivity of Compton scattering to polarization, the polarimeter will be able to determine the plane of the beam polarization.
An REU student will be responsible for assembling and testing these two devices during this summer. The procedures include mechanical assembly, testing the individual detectors, attaching the appropriate electronic modules to the detectors and connecting to the data acquisition computer. After this, they will be tested and calibrated using the HI&gama;S beam. Computer simulations of the setups may also be necessary to understand and confirm their operation.
Development of a polarized 3He target
Advisor: Tom CleggWe seek a student to help in the coming summer with development of optical pumping and target chamber systems to make a nuclear-spin-polarized 3He target for scattering and reaction experiments. Atoms of 3He will become polarized by spin-exchange interactions with polarized atoms of rubidium. The Rb atoms, in turn, will be polarized by optical pumping with circularly polarized laser light. Having such a target would be highly attractive because it provides control of the spin-magnetic moment of the 3He nucleus. This would allow detailed control of the interaction when, for example, spin-polarized proton beams are scattered from the 3He. One can think of this as controlling specifically the orientation of two nuclear bar magnets as they are brought together and interact. That lets one probe unique magnetic details of the tiny nuclear forces.
Developing a polarized 3He target will require the construction of several systems. The REU student would be primarily responsible for the design, fabrication and testing of :
- a shielded magnetic volume to contain the special non-magnetic target cell, so the polarized 3He atoms are not influenced by the presence of unwanted stray magnetic fields.
- a precisely designed and fabricated set of current carrying coils inside the magnetically shielded volume, so the orientation of the 3He magnetic moments will be precisely known.
We expect that the REU student who chooses this as his or her summer project will be collaborating closely with us and will be exposed to additional design activities including the construction of the target cell, (made of non-magnetic materials that can contain the helium during scattering experiments with an incident particle beam) the laser and optical systems, (to prepare and focus the circularly polarized light needed to optically pump the rubidium vapor) and the optical pumping cell (containing the Rb vapor polarized by laser light and the 3He gas polarized by spin-exchange with the Rb atoms). However, the student will have substantial control over, and will probably concentrate on the design, fabrication, and testing of the magnetic shielding and current coils mentioned in items 1 and 2 above. This is very likely to expose the student to computer interfacing of power supplies and magnetic field measuring systems, as well as to magnetic field mapping using a special computer-controlled robot. The programming language to be used will be LabView.