LBL EOS time projection chamber

February 28, 1992

By Judith Goldhaber

The EOS time projection chamber at the Bevalac -- the newest addition to LBL's sophisticated HISS detector system -- began operation on Jan. 31 with an experiment designed to study what happens when ordinary matter is subjected to extreme conditions of temperature and pressure, following collisions between energetic heavy ions.

This research is of fundamental interest and importance for understanding neutron stars and supernovas.

A time projection chamber (TPC) is a particle detector -- invented by LBL's David Nygren in 1974 -- that detects and identifies particles by translating position (the position of particle tracks in space) into time (the time it takes the signal to drift through a given distance in a gas).

EOS is an acronym for "equation of state" -- the response of matter to changes in temperature and pressure -- which is the principal focus of the detector's experimental program. It is also the name of the Greek goddess of the dawn. The name had originally been suggested by the late Howel Pugh, who for many years headed the Bevalac's scientific program in heavy-ion physics.

The EOS TPC will be capable of simultaneously detecting, measuring and identifying up to 200 particles flying out of collisions between heavy ions. This sensitivity is necessary to handle collisions involving the heaviest and highest energy projectiles available at the Bevalac. Collisions such as those between two gold nuclei can produce hundreds of charged particles.

Supplemented by other parts of the HISS system, EOS is able to observe and identify almost all the charged particles produced in collisions at the Bevalac.

The EOS TPC has a unique design in which virtually all the electronics are on the detector itself, drastically reducing the number of cables leading to the outside and permitting the entire detector to fit inside the Bevalac's HISS magnet. (Earlier generations of TPCs relied on large bundles of cables to carry the thousands of signals generated in the detector to racks of external electronics for processing.) To make such a detector work, the design team developed two state-of-the-art integrated- circuit chips that perform complicated electronic functions in less than a square inch of space (see Currents, June 21, 1991).

Thanks to this design, the EOS TPC requires only one exterior electronics rack to service 15,000 channels of information.

In the initial experimental run that began Jan. 31, ions of neon (atomic weight 20) were slammed into targets of gold (atomic weight 197). The new detector performed very well, and the research team is currently analyzing the data and studying the physics results.

Three experiments are planned for the initial phase of the EOS's experimental career. The first will be a general overview of the properties of the reactions, emphasizing the higher energies not well covered by previous detectors like the Plastic Ball and the streamer chamber. The second will focus on how the nuclei break apart as they approach a change in state -- whether they simply fission into two roughly equal parts or undergo a complex "multifragmentation," similar to the formation of droplets when water changes to steam. The third experiment will emphasize studies at the low-energy end of the Bevalac's spectrum.

EOS is a collaborative project between scientists at LBL, Kent State, Purdue, UC Davis, Texas A&M, and two European institutions, GSI in Germany and INFN Catania in Italy. The LBL part of the EOS team, all members of Nuclear Science Division's Relativistic Nuclear Collisions group, is headed by Hans-Georg Ritter and Howard Weiman.

The EOS detector begins its experimental operation -- originally slated for a three-to-five year run -- in a time of uncertainty about the future of the Bevalac, currently being debated in Washington. If LBL's venerable accelerator is shut down at the end of this fiscal year, EOS will log only about one thousand hours of active running time. If the Bevalac receives a reprieve through new funding sources, EOS will continue to operate as originally planned.