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In Quest of a Quark:
ORNL's Role in the PHENIX Particle Detector

Just as violent wars have liberated oppressed peoples, scientists are hoping that violent collisions between gold nuclei racing around a circular track in opposite directions will free the building blocks of some protons and neutrons that make up the atomic nucleus. These building blocks are quarks, which combine in groups of three to form individual protons or neutrons (both of which are called nucleons).

Quarks are held together to form nucleons by particles called gluons, the carriers of the strong nuclear force. No one has succeeded in knocking a quark loose and seeing it in isolation as a free quark. It is believed that if the nucleons of heavy nuclei are compressed and excited enough by colliding nuclear beams, the quarks and gluons may be liberated temporarily from their nucleon prisons. In this process, called deconfinement, these quarks and gluons would roam freely in a volume much larger than that of a nucleus, forming an extremely hot soup called a quark-gluon plasma. Such a soup is thought to have existed during the latter part of the first 10 microseconds of the Big Bang, the explosion of an extremely dense blob of energy that formed our universe.

Terry Awes, Vince Cianciolo, Yuri Efremenko, Frank Plasil, Ken Read, Soren Sorensen, Paul Stankus, Glenn Young, and other ORNL physicists have been searching for free quarks since 1986. That's when they first conducted experiments using oxygen beams at the Super Proton Synchrotron at the European Laboratory for Particle Physics (CERN) near Geneva, Switzerland; later, also at CERN, they performed experiments with sulfur (1987-1992) and lead (1994-1996). In 2000 many of the scientists conducting experiments at CERN claimed they had observed deconfinement. But other scientists called this announcement premature. According to Young, "We felt the announcement was premature and did not address several expected signals of deconfinement."

Now, the ORNL physicists and scientists all over the world are pinning their hopes of freeing quarks and gluons on gold beams colliding at nearly the speed of light (100 billion electron volts per nucleon) at the $600-million Relativistic Heavy Ion Collider (RHIC) at the Department of Energy's Brookhaven National Laboratory. RHIC's first gold-gold (Au-Au) collisions were observed on June 13, 2000. In January 2001, at a conference at the State University of New York at Stony Brook, physicists involved with RHIC announced that Au-Au collisions had produced the densest matter ever created in a laboratory—the first step toward making a quark-gluon plasma. Such a particle soup in a very small volume would have a temperature of 2 trillion degrees Kelvin, 100,000 times hotter than the sun.

The ORNL physicists from the Physics Division and researchers from ORNL's Instrumentation and Controls Division are involved in the 420-person PHENIX collaboration at RHIC. The $100-million PHENIX is one of two large arrays of particle detectors at RHIC; the other large detector is STAR.

On April 16, 2001, scientists in the PHENIX collaboration—including Awes, Cianciolo, Efremenko, Plasil, Read, Sorensen, Stankus, and Young—published a paper entitled "Centrality Dependence of Charged Particle Multiplicity in Au-Au Collisions,"in the Volume 86, Number 16 issue of Physical Review Letters. According to Young, the paper provided evidence that the PHENIX detector works as planned and that 6000 particles are emitted per collision as predicted. "Most of these particles are produced by conversion of energy into matter during the collision," Young says. "These particles spraying in all directions are the result of smashing the 394 nucleons contained by two gold nuclei together at a total energy of 25 trillion electron volts."

ORNL physicists and engineers have developed two detectors and also electronic components for many of the 500,000 particle-detection channels of PHENIX. The goals of this work are to sort through the data on a selected subset of the 6000 particles emitted per Au-Au collision and to select the most meaningful collision events that are of greatest interest to the physicists. The STAR, on the other hand, is a gigantic digital camera, which has electronics for detecting and analyzing all 6000 particles emitted per collision.

"PHENIX is used to detect all the particles striking it, but the electronics decide which particle events should be stored in memory and analyzed in detail later by a parallel computer," Young says."Gold-nuclei collisions occur 1000 times a second. About 90% of the time the collisions are not interesting but 1 to 5% of the time they are very interesting.

"Of the 6000 subnuclear particles emitted after each collision event, which lasts only 10-22 second, about 90% are pions, which originally were thought to be the particles that hold the nucleus together. Our ORNL team is interested in studying mainly photons, electrons, and muons, a heavier partner of the electron." Pions are the lightest particles to feel the nuclear force and are thus most easily produced during the collision.

ORNL researchers have focused on the detector and electronics for two types of PHENIX detectors—a muon identifier and a calorimeter. The first detector detects muons and the second one detects electrons and photons. ORNL also produced electronics used in four other PHENIX detectors.

Muons are 210 times as massive as electrons. First discovered in cosmic rays by Nobel Laureate Carl Anderson, a muon is unstable; it decays in 2.2 microseconds into an electron plus two neutrinos (almost massless particles that can penetrate the earth). Of the 6000 particles emitted in a collision between two gold nuclei, only about 10 are muons. Even rarer is a muon pair (a positive muon paired with a negative muon), but if one is detected, it will be of interest to physicists.

View of all three PHENIX spectrometer magnets installed in the collision region. The long axis of the photograph spans some 50 feet. One of the central arm carriages has been retracted; the other can be glimpsed behind the central magnet. The octagonal magnets carry the muon spectrometers.

This muon identifier is designed to spot a muon by screening out other particles, including pions, that cannot cleanly penetrate the detector's 600 tons of steel walls as well as muons can. Instead, pions usually strike iron nuclei in the steel, creating reactions that stop the pions or events that destroy the particles. A muon loses just a bit of energy as it flies through these walls and strikes proportional counters interleaved with steel plates containing an electrically conducting wire and a mixture of two gasescarbon dioxide (CO₂) and isobutane (C₄H₁₀) The muons knock electrons loose from the gas mixture, ionizing it. These free electrons are attracted in an avalanche to the electric field of the wire where they cause a pulse of detectable current that indicates a muon got through. A team consisting of Vince Cianciolo and I&C members Bobby Ray Whitus, Tim Gee, Steve Hicks, and Miljko Bobrek created custom electronics boards, logic chips, and firmware to detect these pulses and record them for later analysis.

A set of precision proportional chambers, which measure position to 100 microns, is placed just before the muon identifier and inside a large electromagnet. These chambers, which are used to measure the muons' momenta, were built by a consortium headed by DOE's Los Alamos National Laboratory. Chuck Britton and Mike Emery (and earlier, Mark Musrock) of the I&C Division developed a preamplifier that senses, adds up, and amplifies the charges induced by the pulse on the wire, signaling the presence of a muon. This development in low-noise electronics is essential to the detector’s success in finding muons.

To detect photons and electrons emitted by the Au-Au collisions at RHIC, a calorimeter was built for PHENIX by a team of researchers from ORNL, Russia's Kurchatov Institute in Moscow, and the University of Münster in Germany. This calorimeter measures the energy of the photons and electrons coming into PHENIX. It is made of leaded glass similar to what is found in cut glass. The glass is 55% lead oxide. Because of the index of refraction of the glass, when a particle enters it, Cerenkov light is emitted and picked up by one of 25,000 photomultiplier tubes. These tubes amplify the light flash and convert it to 6 billion electrons, which form a brief (15 nanoseconds) pulse of electrical current caught by the electronics. The pattern of the Cerenkov light and its energy and time of arrival indicate whether the particle is an electron or photon on the one hand, or a charged pion, kaon (another type of meson), or proton on the other. An ORNL I&C team led by Alan Wintenberg and including Mike Cutshaw, Mike Emery, Shane Frank, Don Hurst, Gentry Jackson, Mark Musrock, Mike Simpson, David Smith, and Jim Walker designed custom circuits, chips, firmware, and circuit boards to read out data from these photomultipliers. Physics Division staffers providing assistance were Stankus, Awes, Efremenko, Plasil, Young, and postdoctoral scientist Sergei Belikov.

ORNL also designed and built custom electronics for a novel proportional chamber readout with small "pixels," a project handled by Bill Bryan, Usha Jagadish, and Melissa Smith. Alan Wintenberg and Shane Frank developed electronics, similar to those for the calorimeter, to read out a gas-filled Cerenkov counter used in PHENIX to tag electrons. Chuck Britton, Tony Moore, and Nance Ericson built a system used to detect the tiny (4 femtocoulomb) signals induced on 100-micron-wide strips of silicon diode, a detector used to count the particles hitting PHENIX.

ORNL has developed "trigger electronics" to decide what information to keep. If the 25,000 photomultiplier tubes of the calorimeter detect an interesting amount of energy, the electronics raise a red flag and ask for this collision event to be saved for later analysis. Similarly, if the 7000 elements of the muon identifier spot the pattern characteristic of a muon passing through, the flag is again raised.

View of the two PHENIX central arm carriages with their detector and electronics packages installed. The far one is in the process of being rolled into position. The cylindrical beam pipe threading the large central magnet (in green) is some 15 feet above floor level and carries the two counter-rotating beams of high-energy nuclei, be they protons, gold nuclei, or intermediate species.

"We keep up to 10% of the raw information on collision events that comes through," says Young. "Our electronics take 4 millionths of a second to decide whether to store a particle event in memory for later analysis or to not store it. A global triggered decision is made by electronics built by Iowa State University/DOE's Ames Laboratory. If the decision is to keep the 500,000 bytes of information from one collision, the event is digitized, sent along an optical fiber to data collection electronics prepared by Columbia University, and thence along another fiber to a magnetic tape drive for storage. Later it is analyzed by a powerful parallel computer provided by BNL."

About 35 ORNL researchers and technicians and some 20 University of Tennessee graduate students were involved, especially between 1995 and 1998, in designing and testing 2200 electronic-circuit boards for PHENIX. Other researchers in the electronic development collaboration are from Iowa State University; Columbia University; SUNY at Stony Brook; Brookhaven and Los Alamos national laboratories; Lund University in Sweden; and the KEK Laboratory, plus the universities of Tokyo, Waseda, Hiroshima, and Nagasaki, all in Japan. Overall, some 200 persons worked on the electronics for PHENIX.

"Without such a diverse collaboration and an amazing collider," Young says, "we could never hope to mimic the beginning of the universe."

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