The LLNL Electron-Positron Beam Facility

The positron facility at LLNL is designed to produce positrons with a wide range of characteristics, and to deliver them to a number of experimental apparatuses. The electron linac can produce very intense pulsed beams of positrons, while the pelletron can produce DC beams at high energies with a very narrow energy distribution.

These beams are transported to experiments, including a positron trap for particle physics research and a positron microprobe for high-resolution materials science.

An overview of the positron facility is shown in Figure 1, with the accelerators and experiments also shown.

Figure 1. Overall view of the LLNL positron facility layout.

Positrons are produced at the facility in two ways: either from pair production in a target or from the decay of ²²Na.

Production in a target occurs as shown in Figure 2. A 100-MeV, 300-mA electron beam from the linac impinges on a target. As the electrons decelerate in the target, they produce an intense flux of bremsstrahlung (literally, "braking radiation") photons. In turn, these photons interact with the electric fields of the nuclei in the target to produce electron-positron pairs.

Figure 2. Positron production in a target. An electron beam impinges on a target, in which the electrons decelerate and produce a high flux of bremsstrahlung photons, which convert to electron-positron pairs in the field of the target nuclei. The positrons are moderated by vanes of tungsten and electromagnetically extracted into a low-energy beam.

The positrons and electrons emitted from the target are moderated by a set of tungsten vanes. Tungsten has a negative "work function" for positrons; positrons in the material are ejected from the surface with an energy distribution approximately equal to that of the electrons in the material. After the moderator vanes, the positrons are extracted into a beam by a combination of electric and magnetic fields.

This system produces the most intense beams of positrons in the world -- up to 10¹⁰ positrons per second. These positron beams are used for:

• Positron Annihilation Lifetime Spectroscopy
• Spectroscopy using Angular Correlation of Annihilation Radiation (ACAR)
• High-resolution defect mapping using the positron microprobe
• Plasma physics experiments

The electron beam used to produce the positrons is generated by the LLNL Electron Linac, built in 1972 for nuclear physics experiments. It is an S-band linear accelerator capable of generating electron beams with energies up to 150 MeV with currents up to 100 mA, or a current as high as 600 mA at a beam energy of 100 MeV.

Figure 3. Artists' conception of the LLNL electron linac.

The linac is used to produce the world's highest-intensity low-energy positron beams, as well as an electron beam device for studies of laser acceleration.

The Pelletron

The pelletron is a type of van de Graaf accelerator, in which a high-voltage terminal is charged by a moving chain of "pellets," or alternating links of metal and insulator. The LLNL pelletron can accelerate positrons or electrons to an energy of 3 MeV.

Figure 4. Photograph of the LLNL Pelletron.

To generate and accelerate positrons, a ²²Na source is placed inside the high-voltage terminal of the pelletron. This isotope, with a half-life of 2.6 years, emits positrons with an average energy of about 200 keV. The positrons emitted from the source are moderated by tungsten vanes to an energy width of about 2 eV. They are then accelerated to 3 MeV by the pelletron. Voltage ripple in the pelletron results in an energy width of about 100 eV, for a relative energy resolution of 3x10^-5. This system can produce a continuous beam of 3.5x10⁵ positrons per second. These positron beams are used for:

• Positron Annihilation Lifetime Spectroscopy
• Spectroscopy using Angular Correlation of Annihilation Radiation (ACAR)
• Positron channeling studies

For high-resolution materials science, a tightly focused positron beam with good energy resolution is required. To achieve this goal, a "positron microprobe" is being developed.

An overview of the microprobe apparatus is shown in Figure 5. Positrons produced in the linac target are captured into a "stretcher," which reduces their energy and intensifies the pulses. They are then sent through a series of bunchers, which shorten the pulses, and remoderators, which cool the pulses further.

Figure 5. Microprobe beam line.

The final beam on the target has a radius of 1 micron and a pulse length of about 100 picoseconds. The beam characteristics in each portion of the beam line are summarized in Table I.

The positron microprobe will be used for high-resolution defect mapping in materials.

Positron Trap

For plasma physics and particle physics experiments, we have built a trap for either positrons or electrons. The trap is a Penning-Malmberg trap that uses an axial magnetic field and a series of drift tube electrodes to confine charged particles.

Figure 6. Positron trap.

This page was created by Dave Knapp