Internucleon Reactions
With a polarized ion source and the Tandem Van de Graaff accelerator system, TUNL is well suited for measuring observables in the two-nucleon and three-nucleon systems to study the strong nucleon-nucleon (NN) interaction. Cross section measurements and spin-dependent observables provide needed data to compare with theoretical calculations in order to test models of the NN interaction. Specifically, TUNL measurements have concentrated on resolving the lingering discrepancies between experimental data and theoretical predictions based on NN potential model calculations and the contribution, if any, of three-body force terms to the NN interaction.
With the improvement of computers and complex computations, theorists are making great progress toward carrying out direct calculations of 4-nucleon systems. As a result, TUNL faculty are completing measurements in light nuclei to develop the necessary database with which to test these calculations.
Nuclear Astrophysics
The Nuclear Astrophysics program involving three TUNL faculty, seeks to measure the relevant reaction rates, transitions and processes important in star formation and stellar evolution, the creation of the elements in our solar system (nucleosynthesis) and in the production of neutrinos in the sun. Our understanding of the life of a star is limited by our knowledge of the interaction of the elements under stellar conditions, which are difficult to reproduce in the laboratory. The reactions occur at low-energies, and with very small probabilities. Since knowing the nuclear reaction rates is critical for explaining the observed elemental abundances, discrepancies remain between current observations and model predictions.
Two important low-energy, high-intensity beam facilities are available at TUNL. Experiments with the Low-Energy Beam Accelerator Facility (LEBAF) have significantly contributed to the knowledge of reaction rates that play an important role in nucleosynthesis and star formation. With the development of experiments at the new Low-Energy Nuclear Astrophysics (LENA) facility, we will extend the range of reactions that can be measured.
In addition to studying radiative capture reactions using stable beams produced at TUNL, the faculty take advantage of the new radioactive beams facilities to study reactions that were previously not able to be produced using combinations of stable beams and targets. The nuclear astrophysics group currently is involved with collaborations at the Oak Ridge National Laboratory (ORNL) in Tennessee and at Argonne National Laboratory (ANL) near Chicago, Illinois.
Rare Nuclear Processes
This TUNL group of three faculty are studying the rare process of double-beta decay (beta-beta-decay), with level half-lives on the order of 1019-1022 years. The rate of these rare decays has many implications for the mass of the neutrino (involved in single beta-decay processes) and the current Standard Model for particle physics. An improved coincidence experiment is currently underway to measure the beta-beta-decay lifetime of 100Mo, in which two previous measurements disagree. Simultaneously, the "neutrino-less" beta-beta-decay in 76Ge is being measured. Additional nuclei are being studied, namely the decay of 150Nd. These experiments are continually being monitored and improved to reduce the background contributions.
Sub-Nucleonic Degrees of Freedom
Currently two TUNL faculty are members of collaboration teams performing experiments at the Continuous Electron Beam Accelerator Facility (CEBAF) at the Thomas Jefferson National Accelerator Facility (TJNAF) in Virginia, to measure electromagnetic form factors of the neutron and proton. The group is involved in two experiments in the next two years to improve the available dataset for the electric form factor of the proton and both the electric and magnetic form factors of the neutron. These data will then be used to increase our understanding of the structure of nucleons. The group is also developing a Multi-Wire Proportional Counter to be used as a veto detector for the second Compton scattering experiment relevant to sub-nucleonic degrees of freedom. The only other experiment was performed in 1979.
Instrumentation Development
There are a number of instrumentation projects currently under development at TUNL. The LENA laboratory accelerator is nearly complete. This state-of-the-art facility is designed to produce intense beams of protons with energies up to 1 MeV for measurements of astrophysically interesting reactions. In 1999, an undergraduate worked on optimizing a detector configuration being proposed for this facility.
Toward our goal of producing a high-current of polarized beams, an ion source under development relies on a more efficient ionizer of spin-polarized deuterium atoms by charge exchange with hydrogen ions at ~1 eV. Initial test-bench measurements of an intense and highly collimated hydrogen-ion plasma jet from an Electron Cyclotron Resonance (ECR) source indicate our scheme to be promising. This design project is lead by one faculty member with the assistance of graduate and undergraduate students and a postdoctoral research associate.
TUNL has extensive experience in the production of cryogenically polarized targets for beam experiments. Current projects include an upgrade of the dilution refrigerator-based, dynamically polarized deuterium target and the development of a new deuteron frozen spin target that will be used at the DFELL facility.
The first data taken with the new high-density gas-jet target with nitrogen gas indicates a promising future for few-body interaction studies. While the basic system has been shown to work, improvements are needed for higher gas pressures, the recirculating system and to monitor the effective target thickness. Ultimately, gas targets of hydrogen, deuterium, and helium (including the expensive 3He gas) will be used in low-energy few-body interaction studies without the background effects caused by target windows, impurities, and metal target backings. Three TUNL faculty are working toward the completion of the gas-jet target, and undergraduates have contributed to the computer interface and control system for monitoring and diagnostics.
Several TUNL faculty have joined the KamLAND collaboration pursuing neutrino oscillation studies at the Kamiokande site in Japan. The TUNL group is in charge of the outer detector region to be used to veto detector pulses from reactions produced by particles other than the neutrinos of interest. Tests are underway on the detectors and calculational simulations are being developed to model the test set-up and the full detector configuration.