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Positron Beam: Activities

Performed are calculations needed for the development of the first stage of the high intensity positron beam at Jefferson National Laboratory (JLab). On the base of the calculations prepared is NSF MRI-R2 proposal for use of the existing Free Electron Laser accelerator at JLab to generate the world highest continuous current beam of low-energy positrons by pair production through electron deceleration in the target and subsequent positron thermalization on tungsten moderator.

We did needed calculations and other activities needed to prepare proposal to establish the world’s highest current and the brightest beam of medium and low-energy positrons at Jefferson National Laboratory (JLab). Our proposal combines state-of-the-art research and the development of infrastructure essential to address some of the most important questions in modern physics and material science. It takes advantage of a simultaneous confluence of needs in the scientific community and industry for the formation of a national center for fundamental research and materials studies using positron annihilation techniques to addresses simultaneously the goals of the highest priority in material research and characterization.

The Jefferson National Laboratory (Bogdan Wojtsekhowski and G. Neil), which is the consortium of the Southeastern Universities (SURA), in collaboration with researchers from the College of William and Marry (W. J. Kossler), North Carolina Central University (I. Bondarev, K. Kim, and B. Vlahovic), and U. Missouri-Kansas City (Jerry Jean) are proposing to modify existing Free Electron Laser (FEL) at JLab, to produce the most intense (>8x1010 e+s-1) and the brightest (for a factor more than 104 better than at any other facility) beam of slow positrons and to build the multi-user experimental program around this high current source of moderated positrons.

In addition to a great demand for such high intensity beam and experimental facility, this proposal is also attractive since it seeks to achieve the above goals for a relatively very low cost. The FEL facility that provides high quality 120 MeV electron beam with 1 mA current is already operational at JLab. It includes high beam diagnostic and sophisticated beam control and data acquisition equipment.

We studied modifications needed to produce slow positrons. It includes analysis of the building a vacuum beam line extension that will include a flywheel beam converter, a tungsten moderator, a solenoid to transport moderated positrons, beam bencher-stretcher, beam switchyard, and beam dump. We also considered which experiment will be performed in the facility and which the basic material characterization experimental equipment is needed for the end-users for the initial set of the experiments that will be used to commission the facility.

Since the early 1970’s considerable efforts were directed towards the development of the low-energy (< 100 keV) beams of monoenergetic positrons with intensities over 108 e+s-1 for applications ranging from material science to fundamental research. Intense positron beams are under development, or being considered at several laboratories. Present day radioactive source based slow positron beams are limited to intensities of 105-106 e+s-1. Only a few accelerator based, low brightness e+ beams exist to produce ~107-108 e+s-1, and only several laboratories in the world are aiming at high intensity, high brightness positron beams with intensities greater than 109 e+s-1 and current densities ~1013-1014 e+s-1cm-2. At present, world’s most intensive low energy positron beam (5x108 e+s-1) is built at the North Carolina State University. This project will be a significant upgrade by at least two orders of magnitude, which will open new experimental possibilities. However, for many experiments and applications the brightness of a positron beam is more important than the intensity of a beam or the flux. The proposed facility will have the brightness for more than four orders of magnitude better than NCSU or any other existing facility.

The particular aspects of the proposed facility that will be developed through the stage I of this project are: (1) the most intense (>1010 e+s-1) beam of slow positrons, a few eV to several tens of keV, for the spectroscopic study of surfaces, nanostructures, and thin film materials; (2) development of the basic experimental setup that will consist of an array of eight photomultipliers having BaF2 scintillators that will be used to collect lifetime spectra and two Ge Detectors for Doppler broadening spectroscopy and to measure age-correlation momentum spectra. The experimental facility will be later further develop through stage II in response to users’ need for basic and applied science driven by new opportunities opened by high intensity positron beam.

In collaboration with external users we considered science program and optimized it to address a wide range of research ranging from the academic intellectual curiosity to the real world applications. The proposed experiments will include, for example, Bose-Einstein condensate of positronium atoms and measurement of some of its optical properties. JLab high intensity positron beam will allow fundamental and applied research in materials characterization and basic materials science, solid state and surface physics, as well as in atomic physics. These range from the development of a new generation of materials to study of the nature, concentration, and spatial distribution of defects. This will be particularly applicable to nanotechnology and nanoscience, nanopores, nanostructures, optical materials and thin film semiconductors studies.