Student Programs @ TUNL

TUNL REU Program

REU Projects for 2004 [photo album]

Construction and Characterization of High-Pressure 3He Gas Scintillators

Advisors: Werner Tornow, Gary Weisel, and J. Esterline

At TUNL a high-pressure (~50 atm) gas scintillator is needed to measure the asymmetry in polarized neutron scattering from 3He in the neutron energy range from En = 2 MeV to 10 MeV. Data for this reaction will help to interpret the long-standing analyzing power puzzle. At HIGS (High-Intensity Gamma-ray Source) 3He gas scintillators will be used to detect low-energy neutrons (En < 100 keV) produced by the gamma-ray induced breakup of deuterium. This reaction is important for Big-Bang nucleosynthesis and the determination of the baryon density in the early Universe.

Prototype 3He gas scintillators have been developed already and successfully used in experiments and a number of high-pressure cells have been constructed. The REU student will prepare the inner wall of the cells for maximum light output and best energy resolution by depositing reflector and evaporating wave length-shifter materials. Afterwards, the cells will be evacuated and filled with a mixture of Xe and 3He. The characterization of the 3He gas scintillators will be done at very low energies with a thermal neutron source and at higher energies (En = 4 to 10 MeV) by neutron elastic scattering from 3He using the 2H(d,n)3He reaction and the tandem accelerator at TUNL.

The High-Intensity Gamma-ray Source

Advisor: Ying Wu

The High Intensity Gamma Source (HIGS) is a world-class nuclear physics facility operated by the Triangle Universities Nuclear Laboratory (TUNL) and Duke Free Electron Laser Laboratory (DFELL). The high intensity, nearly monochromatic gamma beam is produced by colliding a high energy electron beam (from 270 MeV to 1.2 GeV) with the high power FEL light in the Duke storage ring via a process called Compton scattering. The production of the high quality gamma beams critically depends on the stability of the free electron laser system which employs one of longest laser cavities in the world at 54 meters. We have an going research program to study the stability of this laser cavity. Both vibrations and slow drifts will be carefully measured as functions of the varying environment so that their sources can be identified. An active feedback system has been developed to stabilize the laser cavity. The effectiveness of the feedback system is also subject to this study.

The REU student will will gain hands-on experience with advanced instruments such as the laser tracker. In addition, he/she will learn about the data acquisition system and advanced analysis software such as MATLAB. This student will also have the opportunity to participate in the Duke University storage ring upgrade project already under way.

3He Depolarization Study for the Neutron Electric Dipole Moment Experiment

Advisors: Dipangkar Dutta and Haiyan Gao

The electric dipole moment (EDM) of the neutron is a fundamental quantity which is predicted to be on the order of 10-31 e.cm by the Standard Model of electroweak interaction. This quantity is a direct probe of time-reversal symmetry violation. The current experimental limit on this quantity is 6 x 10-26 e.cm. A new search is underway aiming at a two orders of magnitude improvement over the current limit. This new experiment will have an important impact on the search of New Physics beyond the Standard Model.

Prof. Gao's Medium Energy Physics Group at TUNL is actively engaged in this new search by trying to measure and understand the effect of superfluid 4He at a temperature of 300 mK (the operating condition for the EDM experiment) on the relaxation time of polarized 3He nuclei. In the EDM experiment Polarized 3He nuclei will serve as a co-magnetometer and the neutron electric dipole moment can be determined by employing the spin-dependent nuclear reaction n + 3He --> p + T. Therefore, a precise knowledge of the relaxation rates of polarized 3He under the operating condition of the EDM experiment is very crucial.

At TUNL in the MEP lab such a study is underway using the spin-exchange optical pumping (SEOP) of 3He atoms and subsequent measurements of the 3He polarization using the nuclear magnetic resonance (NMR) technique. We invite REU students this summer to join us as we complete this very exciting and challenging set of measurements. In addition to learning about SEOP and NMR, students can expect to learn how to safely operate high power lasers, cryogenic vessels, ultra-high vacuum systems and get hands on experience in glass blowing techniques. Specific projects may involve:

Please visit our group's web page at http://www.tunl.duke.edu/~mep, which links to the web page of the EDM experiment as well as other experiments that our group is working on.

Construction of a Polarized 3He Target for HIGS

Advisors: Haiyan Gao and Kevin Kramer

here are a series of proposed experiments at the High Intensity Gamma-ray Source (HIGS) at the Duke Free Electron Laser Laboratory (DFELL) that require a polarized 3He target. A polarized target simply aligns the spins of the target material, in this case 3He gas. The target will be used to study neutron and 3He structure by studying the spin-related observables from gamma-ray scattering. This target is currently being designed, with the majority of the construcution and testing to occur in the late spring and summer.

There are many projects associated with this target that would be appropriate for an REU student. Below are two projects that the student will likely work on:

A polarized target is only useful if the polarization can be determined. The polarimetry system for this target will use measure nuclear magnetic resonance (NMR) of the 3He gas to determine the polarization. A set of control software for the polarimetry will have to be written and tested. Once the software is reliable, the system needs to be calibrated by extensive studying of the NMR signal of water.

The target is polarized by optical polarization of rubidium vapor which in turn polarizes the 3He gas through a hyperfine interaction. An array of diode lasers are used for the optical polarization process. This laser array will have to be assembled and aligned. This will include working with a new type of narrowed diode laser that will need extensive testing.

Though these are the major tasks our group needs to complete, there are also a variety of other smaller tasks that are mostly hands-on and are focused on getting the target ready to run. A student helping to construct this target will get a lot of hands-on experience working in a laboratory setting and will hopefully get to see a working polarized target by the end of the summer.

Nuclear Data Evaluation of Neutron-Capture and Beta-Decay Data

Advisor: John Kelley

At TUNL, the Nuclear Data Evaluation Group is responsible for evaluating nuclear Structure properties in the atomic mass range of A = 2-20. Our group evaluates published data and provides summary reviews to the nuclear data user community. In this project, information from thermal neutron-capture and beta-decay reactions will be evaluated using standardized techniques in order to obtain the best structure parameters.

Activities will include searching databases of published articles for neutron-capture and beta-decay reaction data, familiarizing oneself with the information that can be obtained from these measurements, and evaluating the published data sets to deduce a set of "best values". The student will develop a web interface for presenting the information. As time permits, the student may choose to develop specialized discussion about topics such as parity violation, etc. In addition the student will become familiar with useful experimental procedures that are relevant to measurements with High-Purity Germanium gamma-ray detectors.

Nuclear & Nucleon Compton Scattering at the High Intensity Gamma-Ray Source (HIGS) and Commissioning of the Neutral Meson Spectrometer (NMS)

Advisors: Mohammad W. Ahmed, Henry R. Weller & the Capture Group at TUNL

In a simple, non-relativistic picture when an external electric and magnetic field is applied to a collection of charged particles, a charge-mass separation occurs in response to these fields. This process, commonly referred to as polarization, also occurs in nuclei (where protons and mesons act as charged particles) and nucleons (where quarks are massive charged objects). Different wavelengths of the applied electromagnetic field (or a photon) are required to study polarization in nuclei or nucleons.

The first-order study of polarization phenomenon consists of measuring electric and magnetic polarizabilities. The low-energy theories, such as Chiral Perturbation Theory (CPT), predicts them via energy expansion of the Compton scattering amplitude.

Scattering Amplitude = A0 + alpha F(omega) + beta G(omega),

Where A0 is a constant (depends on parameters such as charge and mass of the particle), alpha is the electric polarizability, and beta is the magnetic polarizability. The expansion is in terms of the energy based functions F and G.

At HIGS, we plan to perform precision measurements of alpha and beta. The experiment requires a polarized photon beam, a target (a nucleus or a nucleon), and a set of detectors to measure the scattered photon and the recoil nucleon(s). The beam will be provided by HIGS; various targets such as protons (liquid hydrogen), deuterons (liquid deuterium), and He-3 are in production; a spectrometer, the neutral meson spectrometer (NMS), is available to detect the scattered photons.

This REU project will focus on understanding the basics of the Compton Scattering on nuclei and nucleons (including calculations of count rates, and limits on experimental measurements), and commissioning of the NMS. The commissioning of the NMS will include testing of various detector elements, such as CsI and BGO scintillators using cosmic-ray background. The project will aid the student in developing strong skills in Compton Scattering theory, methods of photon detection, and hands-on experience in collecting, analyzing and organizing data from scintillating detectors.

Development of Polarized 3He Target Systems

Advisors: T. Katabuchi, T. Daniels, and T.B. Clegg

We seek a student to help in the coming summer with continuing development of optical pumping and target chamber systems to make nuclear-spin-polarized 3He targets for scattering and reaction experiments. At TUNL, atoms of 3He are polarized by spin-exchange interactions with polarized atoms of rubidium. The Rb atoms, in turn, are polarized by optical pumping with circularly polarized laser light. Having such targets is highly attractive because it provides control of the spin-magnetic moment of the 3He nucleus. This allows 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 us probe unique magnetic details of the tiny nuclear forces which govern their interaction during the collision.

TUNL/REU students Mark Fassler (of Colorado State) and Bryon Neufeld (of King College), respectively, worked in our research group in summers 2002 and 2003. Both had accomplished enough by the end of the program to make it possible to present their results at professional meetings.

Developing polarized 3He targets will require refinement and construction of several systems:

A. Refinement of Optical Pumping Hardware and Control Systems -
Our present optical pumping polarizer provides 3He at ~8 ATM pressure with a polarization of ~30%. The polarized gas flows in batches from the polarizer to the target cell where nuclear scattering occurs. When the 3He polarization is depleted there by wall-collisions and other effects which randomize the spins, the gas can be recovered for re-polarization. Work is needed to make these polarizer systems more efficient and user-friendly. Tasks foreseen include:

B. Investigations of the Feasibility of a High-Pressure Polarized 3He Target for Neutron Scattering -
Theoretical interest is high in measurements of the scattering of unpolarized and polarized neutrons from polarized 3He. Such nuclear interactions initiated by neutron beams are easier to model accurately than interactions initiated by charged particles like protons, because of the absence of the Coulomb interaction between the scattering species. Thus, we have begun investigating whether it might be possible to develop a high-pressure, 3He gas scintillator which could be used as a target cell for neutron scattering. Work last summer showed that this may be possible. http://www.physics.unc.edu/~clegg/RBNTgtCellPoster.ppt
Follow-up research is needed to determine whether the magnesium oxide powder needed for the interior walls of such a high-pressure scintillator really can support the long 3He spin-down lifetimes required for an experiment with incident neutrons. Studies are also needed to determine how much 3He polarization might be lost when the gas is compressed.

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